Scott Kable

Product state and speed distributions in photochemical triple fragmentations

Publisher: Royal Society of Chemistry (RSC)

Date: 2012

DOI: 10.1039/C2FD20015E

Abstract: The clearest dynamical signature of a roaming reaction is a very cold distribution of energy into the rotational and translational degrees of freedom of the roaming donor fragment (e.g. CO) and an exceptionally hot vibrational distribution in the roaming acceptor fragment (e.g. H2, CH4). These signatures were initially identified in joint experimental/theoretical investigations of roaming in H2CO and CH3CHO and are now used to infer the presence of roaming mechanisms in other photodissociation reactions. In this paper we construct a phase space theory (PST) model of triple fragmentation (3F) and show that the dynamical signature of 3F is similar to that of the roaming donor fragment. The PST model starts with a calculation of two-body fragmentation (2F) of a generic molecule, ABC into AB + C. Every AB fragment with sufficient energy to undergo subsequence spontaneous dissociation is allowed to dissociate and the PST distribution of energy into A + B products is calculated for every initial AB state. Using CH3CHO --> HCO + CH3 --> H + CO + CH3 as an ex le, we calculate that the energy disposal into the rotational and translational degrees of freedom of the 3F products is very low, and is similar to the dynamical signature expected for production of CO via a roaming mechanism. We compare the 3F PST model with published experimental data for photodissociation of CH3CHO and CH3OCHO at energies above the 3F threshold.

Photodissociation of acetone from 266 to 312 nm: Dynamics of CH3 + CH3CO channels on the S and T1 states

Publisher: AIP Publishing

Date: 24-01-2017

DOI: 10.1063/1.4974035

Abstract: The photodissociation dynamics of acetone (CH3)2CO, cooled in a molecular beam, have been explored over the wavelength range 266–312 nm. Nascent CH3 fragments were detected by resonance-enhanced multiphoton ionization, followed by mass-selected ion imaging. For photolysis at λ = 306 nm, the image shows a sharp ring, which, when converted to a translational energy distribution, reveals a narrow Gaussian peak with a maximum at 90% of the available energy. As the photolysis energy is increased, the distribution slowly broadens and shifts to higher recoil translational energy. The fraction of available energy in translation energy decreases in favour of internal energy of the CH3CO fragment. These observations are consistent with a dynamical model in which the energy of the exit channel barrier on the T1 surface evolves mostly into relative translational energy. Energy in excess of the barrier is partitioned statistically into all degrees of freedom. No evidence was found for any other dynamical pathway producing CH3 fragments, including reaction on S0 or S1, for dissociation between 306 and 266 nm. For λ & 306 nm, a diffuse, slow recoil component to the image appears. The translational energy distribution for this component is fit well by a statistical prior distribution of energy. We attribute this component to dissociation on the S0, ground state surface to our knowledge, this is the first direct observation of this channel. The appearance of S0 dynamics and the disappearance of the T1 component are consistent with previously inferred barrier height on T1 for the production of CH3CO + CH3. The possible atmospheric implications of our findings are discussed.

A Phase Space Theory for Roaming Reactions

Publisher: American Chemical Society (ACS)

Date: 25-07-2013

DOI: 10.1021/JP405582Z

Abstract: We describe a new, simple theory for predicting the branching fraction of products in roaming reactions, compared to the analogous barrierless bond dissociation products. The theory uses a phase space theory (PST) formalism to ide reactive states in the bond dissociation channel into states with enough translational energy to dissociate and states that may roam. Two parameters are required, ΔEroam, the energy difference between the bond dissociation threshold and the roaming threshold, and the roaming probability, Proam, the probability that states that may roam do roam rather than recombine to form reactants. The PST-roaming theory is tested against experimental and theoretical data on the dissociation dynamics of H2CO, NO3, and CH3CHO. The theory accurately models the relative roaming to bond dissociation branching fraction over the experimental or theoretical energy range available in the literature for each species. For H2CO, fixing ΔEroam = 146 cm(-1), the midpoint of the experimental bounds for the roaming threshold, we obtain Proam = 1. The best-fit value, ΔEroam = 161 cm(-1), is also consistent with the experimental bounds. Using this value, the relative roaming to dissociation branching ratios are predicted to be similar in D2CO and H2CO, consistent with experimental observation. For NO3, we fix ΔEroam = 258.6 cm(-1), the experimental threshold for NO + O2 production, and we model low-temperature experimental branching fractions using the experimental rotational and vibrational temperatures of Trot = 0 K and Tvib = 300 K. The best fit to the experimental data is obtained for Proam = 0.0075, with this very small Proam being consistent with the known geometric constraints to formation of NO + O2. Using Proam = 0.0075, our PST-roaming theory also accurately predicts the low-temperature NO yield spectrum and quantum yield data for room-temperature NO3 photolysis. For CH3CHO, we fix ΔEroam = 385 cm(-1), based on theoretical calculations, and obtain a best-fit value of Proam = 0.21, fitting to reduced dimensional trajectory calculations. These values of ΔEroam and Proam yield PST-roaming theory results that are also consistent with two experimental room-temperature data points. The combination of other kinetic theories and the PST-roaming theory will provide rate coefficients for roaming reactions.

Roaming Reaction Pathways Along Excited States

Publisher: American Association for the Advancement of Science (AAAS)

Date: 02-03-2012

DOI: 10.1126/SCIENCE.1218767

Abstract: The photodissociation of NO 3 into NO and O 2 , an important atmospheric reaction, has no transition state but proceeds via an O atom roaming around the NO 2 core.

Near threshold dynamics and dissociation energy of the reaction H2CO → HCO + H

Publisher: Elsevier BV

Date: 08-1993

DOI: 10.1016/0009-2614(96)00705-1

Electronic Spectroscopy of Jet-Cooled 1,2‘-Binaphthyl

Publisher: American Chemical Society (ACS)

Date: 05-2001

DOI: 10.1021/JP0101110

Unraveling the ùB₁ ← X̃¹A₁ Spectrum of CCl₂: The Renner−Teller Effect, Barrier to Linearity, and Vibrational Analysis Using an Effective Polyad

Publisher: American Chemical Society (ACS)

Date: 17-10-2008

DOI: 10.1021/JP806944Q

Abstract: We report studies aimed at unraveling the complicated structure of the CCl 2 A (1)B 1 <-- X (1)A 1 system. We have remeasured the fluorescence excitation spectrum from approximately 17,500 to 24,000 cm (-1) and report the term energies and A rotational constants of many new bands for both major isotopologues (C (35)Cl 2, C (35)Cl (37)Cl). We fit the observed term energies to a polyad effective Hamiltonian model and demonstrate that a single resonance term accounts for much of the observed mixing, which begins approximately 1300 cm (-1) above the vibrationless level of the A (1)B 1 state. The derived A (1)B 1 vibrational parameters are in excellent agreement with ab initio predictions, and the mixing coefficients deduced from the polyad model fit are in close agreement with those derived from direct fits of single vibronic level (SVL) emission intensities. The approach to linearity and thus the Renner-Teller (RT) intersection is probed through the energy dependence of the A rotational constant and fluorescence lifetime measurements, which indicate a barrier height above the vibrationless level of the X (1)A 1 state of approximately 23,000-23,500 cm (-1), in excellent agreement with ab initio theory.

Characterization of the Ã(1A″) state of HCF by laser induced fluorescence spectroscopy

Publisher: AIP Publishing

Date: 15-06-1999

DOI: 10.1063/1.479068

Abstract: An extensive experimental exploration of the Ã(1A″)←X̃(1A′) transition of supersonically cooled fluoromethylene has been performed using laser induced fluorescence spectroscopy. Several new bending vibrational bands are reported, which provide the harmonic frequency and anharmonicity constant for this vibration and lead to an estimate of the height of the barrier to linearity as 6400±500 cm−1. Assignment of the vibrational hot-band structure leads to the first measurement of the à state CF stretching frequency as ν3′=1260 cm−1 and tentatively the CH stretching frequency as ν1′=2852 cm−1. The A′ rotational constant increases strongly with increasing quanta of bending vibration, which indicates that the molecular structure is becoming more linear. Consideration of only the average bond angle, calculated from ab initio data for this state, is insufficient to account for the change in A′. The coupling of a-axis rotation with bending vibration must be included. A number of other dynamical effects were observed in the spectra, including lifetime shortening and disappearance of rotational transitions with K′⩾1. These were explained in terms of the Renner–Teller interaction between the X̃ and à states.

Atmospheric oxidation intermediates: Laser spectroscopy of resonance-stabilized radicals from p-cymene

Publisher: Elsevier BV

Date: 2015

DOI: 10.1016/J.CPLETT.2014.12.007

Identification of the Jet-Cooled 1-Indanyl Radical by Electronic Spectroscopy

Publisher: American Chemical Society (ACS)

Date: 09-2009

DOI: 10.1021/JP905831M

Abstract: The electronic spectrum of the jet-cooled 1-indanyl radical has been identified in the products of a hydrocarbon discharge in argon. Electronic excitation spectra were observed in the region 20800-22600 cm(-1) by resonant two-color two-photon ionization and laser-induced fluorescence spectroscopies. In addition to the new spectrum at m/z = 117, the spectrum of 1-phenylpropargyl was also observed strongly, as was an unidentified spectrum carried by m/z = 133. The origin band of the 1-indanyl A2A''-X2A'' band system was observed at 21159 cm(-1) with the ionization potential of the radical experimentally determined to be 6.578 +/- 0.001 eV from a photoionization efficiency spectrum. Single vibronic level fluorescence was dispersed to determine the ground state vibrational frequencies that were utilized to confirm the identity of the radical in comparison with quantum chemical calculations. The calculated ground state frequencies and ionization potential, along with a calculated dispersed fluorescence spectrum of the origin band for the 1-indanyl radical, all provide a positive chemical identification.

Laser-induced fluorescence excitation and dispersed fluorescence spectroscopy of the Ã(¹B₁)–X̃(¹A₁) transition of dichlorocarbene

Publisher: Royal Society of Chemistry (RSC)

Date: 2005

DOI: 10.1039/B413391A

Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation

Publisher: Proceedings of the National Academy of Sciences

Date: 02-09-2008

DOI: 10.1073/PNAS.0802769105

Abstract: Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An ex le of such a pathway, termed “roaming,” has been described in the photodissociation of H 2 CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of CH 3 CHO to CH 4 + CO. The CH 4 product is found to have extreme vibrational excitation, with the vibrational distribution peaked at ≈95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional potential energy surfaces reproduce these results and are used to infer that the major route to CH 4 + CO products is via a roaming pathway where a CH 3 fragment abstracts an H from HCO. The conventional saddle-point TS pathway to CH 4 + CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the CH 3 roaming mechanism is attributed to the fact that the CH 3 + HCO radical asymptote and the TS saddle-point barrier to CH 4 + CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as H 2 CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory.

Electronic spectroscopy of the B̃(0, 0, 0) ← X̃(0, 0, 0) transition of DCO and lifetimes and relative quantum yields of the B̃(0, 0, 0) state

Publisher: Elsevier BV

Date: 11-2011

DOI: 10.1016/J.JMS.2011.08.005

The dynamics of CO production from the photolysis of acetone across the whole S1 ← S absorption spectrum: Roaming and triple fragmentation pathways

Publisher: AIP Publishing

Date: 03-2022

DOI: 10.1063/5.0080904

Abstract: The unimolecular photodissociation dynamics of acetone spanning the entire S1 ← S0 absorption spectrum have been reinvestigated, with a focus on mechanisms that produce CO. At excitation wavelengths of λ & 305.8 nm, all photoproducts are formed on the S0 state after internal conversion. A roaming mechanism forming C2H6 + CO is active in the window λ = 311.2–305.8 nm. From λ = 305.8 to 262 nm, little or no CO is produced with the photochemistry dominated by the Norrish-type I C–C bond cleavage on the lowest excited triplet state, T1. At higher energy (λ & 262 nm), an increasing fraction of CH3CO radicals from the primary reaction have sufficient internal energy to spontaneously decompose to CH3 + CO. A new model is presented to account for the kinetic energy distribution of the secondary CH3 radical, allowing us to determine the height of the energetic barrier to CH3CO decomposition as 68 ± 4 kJ mol−1, which lies midway between previous measurements. The fraction of CO from triple fragmentation rises smoothly from 260 to 248 nm. We see no evidence of the return of roaming, or any other S0 reaction, in this higher energy region of the first electronic absorption band.

PAH Growth in Flames and Space: Formation of the Phenalenyl Radical

Publisher: American Chemical Society (ACS)

Date: 22-12-2021

DOI: 10.1021/ACS.JPCA.1C08310

Abstract: Polycyclic aromatic hydrocarbons (PAHs) are intermediates in the formation of soot particles and interstellar grains. However, their formation mechanisms in combustion and interstellar environments are not fully understood. The production of tricyclic PAHs and, in particular, the conversion of a PAH containing a five-membered ring to one with a six-membered ring are of interest to explain PAH abundances in combustion processes. In the present work, resonant ionization mass spectrometry in conjunction with isotopic labeling is used to investigate the formation of the phenalenyl radical from acenaphthylene and methane in an electrical discharge. We show that in this environment the CH cycloaddition mechanism converts a five-membered ring to a six-membered ring. This mechanism can occur in tandem with other PAH formation mechanisms such as hydrogen abstraction/acetylene addition (HACA) to produce larger PAHs in flames and the interstellar medium.

Near-threshold H/D exchange in CD3CHO photodissociation

Publisher: Springer Science and Business Media LLC

Date: 23-05-2011

DOI: 10.1038/NCHEM.1052

Abstract: Measuring the isotopic abundance of hydrogen versus deuterium atoms is a key method for interrogating reaction pathways in chemistry. H/D 'scrambling' is the intramolecular rearrangement of labile isotopes of hydrogen atoms and when it occurs through unanticipated pathways can complicate the interpretation of such experiments. Here, we investigate H/D scrambling in acetaldehyde at the energetic threshold for breaking the formyl C-H bond and reveal an unexpected unimolecular mechanism. Laser photolysis experiments of CD₃CHO show that up to 17% of the products have undergone H/D exchange to give CD₂H + DCO. Transition-state theory calculations reveal that the dominant mechanism involves four sequential H- or D-shifts to form CD₂HCDO, which then undergoes conventional C-C bond cleavage. At the lowest energy the molecule undergoes an average of 20 H- or D-shifts before products are formed, evincing significant scrambling of H and D atoms. Analogous photochemically induced isomerizations and isotope scrambling are probably important in both atmospheric chemistry and combustion reactions.

Infrared Spectra of Gas-Phase 1- and 2-Propenol Isomers

Publisher: American Chemical Society (ACS)

Date: 05-05-2017

DOI: 10.1021/ACS.JPCA.7B02323

Abstract: Fourier transform infrared spectra of isolated 1-propenol and 2-propenol in the gas-phase have been collected in the range of 900-3800 cm

A disconnect between staff and student perceptions of learning: An ACELL educational analysis of the first year undergraduate chemistry experiment 'investigating sugar using a home made polarimeter'

Publisher: Royal Society of Chemistry (RSC)

Date: 2011

DOI: 10.1039/C0RP90015J

First observation of the 3Πg3 state of C2: Born-Oppenheimer breakdown

Publisher: AIP Publishing

Date: 04-04-2017

DOI: 10.1063/1.4979293

Abstract: The 33Πg state of the dicarbon molecule, C2, has been identified for the first time by a combination of resonant ionization spectroscopy, mass spectrometry, and high-level ab initio quantum chemical calculations. This marks the discovery of the final valence triplet state of C2 spectroscopically accessible from the lowest triplet state. It is found to be vibronically coupled to the recently discovered 43Πg state, necessitating vibronic calculations beyond the Born-Oppenheimer approximation to reconcile calculated rotational constants with observations. The 33Πg state of C2 is observed to have a much shorter fluorescence lifetime than expected, possibly pointing to predissociation by coupling to the unbound d3Πg state.

Photodissociation dynamics of propanal and isobutanal: The Norrish Type I pathway

Publisher: AIP Publishing

Date: 26-04-2018

DOI: 10.1063/1.5019383

Abstract: The Norrish Type I photodissociation of two aliphatic aldehydes, propanal and isobutanal, has been investigated using velocity-map imaging. The HCO photoproduct of this reaction was probed using a 1+1 resonance-enhanced multiphoton ionization scheme via the 3p2Π Rydberg state. The velocity map images of HCO+ were collected across a range of photolysis energies for both species from 30 500 to 33 000 cm−1 (λ = 312–327 nm). The corresponding translational energy distributions show that the majority of the available energy goes into the translational motion of the products (55%–68%) with this fraction increasing as the T1 barrier is approached. Analysis of the translational energy distributions was also used to determine the aldehyde α C–C bond dissociation energies which were found to be 339.8 ± 2.5 and 331.2 ± 2.5 kJ/mol for propanal and isobutanal, respectively. These values were also found to be in good agreement with the computed dissociation energies using G4 and CCSD(T)/aug-cc-pVTZ//M062X/aug-cc-pVTZ levels of theory. Furthermore, these dissociation energies, combined with the known ΔfH (0 K) of the reaction products, provided the ΔfH (0 K) of propanal and isobutanal which were calculated to be −167.3 ± 2.5 and −184.0 ± 2.5 kJ/mol, respectively.

Triple-Resonance Spectroscopy Reveals the Excitation Spectrum of Very Cold, Isomer-Specific Protonated Naphthalene

Publisher: American Chemical Society (ACS)

Date: 21-10-2013

DOI: 10.1021/JZ401986T

Collision partner and level dependence of vibrational relaxation in S p-difluorobenzene. Stimulated emission pumping combined with single vibronic level fluorescence spectroscopy

Publisher: AIP Publishing

Date: 15-04-1988

DOI: 10.1063/1.454688

Abstract: Rate coefficients have been measured for vibrational relaxation in S0 p-difluorobenzene vapor under assault from eight collision partners. The dissipation of vibrational energy is monitored from each of four vibrational states with energies in the range ∼1500–3300 cm−1. Rate coefficients are found to vary between 1.5×10−10 cm3 molecule−1 s−1 for helium colliding with p-difluorobenzene in the lowest state explored, to 12×10−10 cm3 molecule−1 s−1 for p-difluorobenzene self-relaxation from the highest level studied. The dependence of collision partner on the vibrational relaxation efficiency for each initially prepared state is studied, revealing a dependence on the reduced mass of the collision pair, and/or the well depth of the system. In addition the dependence of the efficiency on the initially prepared state is investigated for any one collision partner using a modified version of the Parmenter–Tang propensity rule model. A comparison between these data for vibrational relaxation in the S0 state of p-difluorobenzene and previous data for vibrational relaxation in the S1 state of the same molecule is discussed.

Photodissociation dynamics of CF3CHO: C–C bond cleavage

Publisher: AIP Publishing

Date: 23-11-2021

DOI: 10.1063/5.0073974

Abstract: The photodissociation dynamics of jet-cooled trifluoroacetaldehyde (CF3CHO) into radical products, CF3 + HCO, was explored using velocity mapped ion imaging over the wavelength range 297.5 nm ≤λ≤ 342.8 nm (33 613–29 172 cm−1) covering the entire section of the absorption spectrum accessible with solar actinic wavelengths at the ground level. After initial excitation to the first excited singlet state, S1, the radical dissociation proceeds largely via the first excited triplet state, T1, at excitation energies above the T1 barrier. By combining velocity-mapped ion imaging with high-level theory, we place this barrier at 368.3 ± 2.4 kJ mol−1 (30 780 ± 200 cm−1). After exciting to S1 at energies below this barrier, the dissociation proceeds exclusively via the ground electronic state, S0. The dissociation threshold is determined to be 335.7 ± 1.8 kJ mol−1 (28 060 ± 150 cm−1). Using laser-induced fluorescence spectroscopy, the origin of the S1 ← S0 transition is assigned at 28 903 cm−1. The S0 dissociation channel is active at the S1 origin, but the yield significantly increases above 29 100 cm−1 due to enhanced intersystem crossing or internal conversion.

Assessing the atmospheric fate of trifluoroacetaldehyde (CF3CHO) and its potential as a new source of fluoro- form (HFC-23) using box modelling

Publisher: Royal Society of Chemistry (RSC)

Date: 2023

DOI: 10.1039/D3EA00120B

A Rapid Radiochemical Bacterial Bioassay to Evaluate Copper Toxicity in Freshwaters

Publisher: Springer Science and Business Media LLC

Date: 30-09-2005

DOI: 10.1007/S00244-004-0179-9

Abstract: A rapid, highly sensitive bacterial bioassay to determine copper toxicity in freshwaters was developed based on the inhibition of cellular assimilation of radiolabeled glucose. The test used a copper-sensitive bacterium isolated from a freshwater stream. Employing sensitive radiochemical techniques enabled environmentally relevant concentrations of the test bacterium (10(5) cells mL(-1)) and a short incubation period (4 hours) to be used, which minimized the potential for changes in copper speciation during the test. The 4-hour median effective concentration (EC(50)) for inorganic copper at pH 7.5 in synthetic freshwater was 0.6 microg L(-1) (95% confidence limits 0.4 to 1.0 microg L(-1)). This compared well with chronic growth inhibition of this bacterium in minimal medium (48-hour EC(50) of 0.9 microg L(-1) [95% confidence limits 0.7 to 1.0 microg L(-1)]). MINEQL + software (Environmental Research Software) was used to calculate copper (II) ion concentrations in synthetic freshwater at pH 7.5, giving an EC(50) value of pCu(2+) 8.8. However, using nitrilotriacetic acid metal-ion buffers (Cu-NTA), 50% inhibition occurred at a pCu(2+) of 9.7, suggesting this bacterium was markedly more inhibited by copper in these Cu(2+)-buffered solutions. This may indicate that the Cu-NTA species was contributing to toxicity. The radiochemical bioassay was evaluated further using freshwater s les from both copper-impacted and pristine environments. Measured EC(50) values ranged from 3.4 to 34.0 microg L(-1)inorganic copper and were strongly correlated with dissolved organic carbon (DOC) concentrations (r = 0.88, p < 0.05).

CO product distributions from the visible photodissociation of HCO

Publisher: AIP Publishing

Date: 15-12-1992

DOI: 10.1063/1.463330

Abstract: The final state distribution of carbon monoxide produced in the photodissociation of the formyl (HCO) radical has been studied both experimentally and theoretically. Renner–Teller coupling between the excited HCO Ã state and the ground state leads to dissociation and yields H and CO. Vibrational and rotational distributions have been measured for CO produced after excitation to specific vibrational levels on the Ã←X̃ transition of HCO cooled in a supersonic expansion. The strongest transitions are for excitation to vibrational states with six to 16 quanta in the bending mode, and dissociation from these states produces inverted CO rotational distributions with average rotational quantum numbers & J≳ in the 22–33 range. The value of & J≳ increases monotonically with the vibrational quantum number describing the bend of the excited triatomic. Experiments involving excitation of one quantum of the C–H stretching motion have revealed that this vibration results in increased rotational excitation of the product CO with values of & J≳ as high as 41. In contrast, experiments indicate that the C–O stretching mode of HCO acts nearly as a spectator during the dissociation process. Excitation of HCO states with one quantum of C–O stretch yields vibrationally excited CO as the dominant dissociation product, but with a rotational distribution similar to that for CO(ν=0) produced following the excitation of HCO states without the quantum of C–O stretch. Classical trajectory calculations on an ab initio potential energy surface have modeled many of the experimental features and trends of the CO product distributions. There are, however, some discrepancies in the positions of rotational maxima and in the efficiency of the coupling of the C–O vibration of HCO to the dissociation coordinate. It is not clear whether these are due to approximations made in the modeling or inaccuracies in the potential energy surface.

Product distributions in the 157 nm photodissociation of CO2

Publisher: AIP Publishing

Date: 1992

DOI: 10.1063/1.462521

Abstract: The vibrational and rotational distributions of CO(1Σ+g) produced in the 157 nm photodissociation of CO2 have been determined by measuring vacuum-ultraviolet laser-induced fluorescence spectra of the CO photoproduct. The photodissociation of CO2 is known to occur via two pathways one yielding O(1D) and the other yielding O(3P). Spin conservation and previous experimental studies confirm that dissociation via the O(1D) channel is the dominant process. The available energy for this channel is sufficient to populate only the ground and first excited vibrational levels of CO. We measured the rotational distributions for CO in v=0 and v=1 and found them to be nonBoltzmann. In fact, a highly structured distribution with distinct peaks at J=10, 24, 32, and 39 is observed for CO in v=0. A less structured population is displayed by molecules in v=1. The relative vibrational population (v=0/v=1) was determined to be 3.7±1.2. Doppler spectra of in idual rovibronic transitions were also recorded. The profiles have widths in accord with the available translational energy, display the expected v⊥J correlation, and are best described by an isotropic distribution of the velocity vectors with respect to the polarization direction of the dissociation light.

Quantum chemical computation of the spectroscopic constants of the

Publisher: Elsevier BV

Date: 04-2005

DOI: 10.1016/J.CPLETT.2005.02.044

The electronic spectroscopy of jet-cooled m-difluorobenzene

Publisher: AIP Publishing

Date: 15-10-1995

DOI: 10.1063/1.470421

Abstract: The S1(1B2)–S0(1A1) electronic transition of m-difluorobenzene cooled in a supersonic free jet has been investigated in detail by use of laser induced fluorescence excitation and dispersed fluorescence spectroscopy. By analysis of over 50 vibronic transitions we have assigned 16 new S1 vibrational frequencies, and have confirmed seven previous assignments. This advances the total number of known S1 fundamental vibrational frequencies to 23 out of the possible 30. The assignments of two S0 frequencies have also been somewhat modified. Two different types of symmetry-forbidden transitions were identified: transitions involving one-quantum changes in b2 modes were found and transitions involving combinations of b1 vibrations were observed to be unusually active in the vibronic spectrum. The observation of these forbidden transitions in both the fluorescence excitation and dispersed fluorescence spectra are explored in terms of first- and second-order vibronic coupling.

A classical trajectory study of the photodissociation of T1 acetaldehyde: The transition from impulsive to statistical dynamics

Publisher: AIP Publishing

Date: 23-01-2006

DOI: 10.1063/1.2139672

Abstract: Previous experimental and theoretical studies of the radical dissociation channel of T1 acetaldehyde show conflicting behavior in the HCO and CH3 product distributions. To resolve these conflicts, a full-dimensional potential-energy surface for the dissociation of CH3CHO into HCO and CH3 fragments over the barrier on the T1 surface is developed based on RO-CCSD(T)/cc-pVTZ(DZ) ab initio calculations. 20 000 classical trajectories are calculated on this surface at each of five initial excess energies, spanning the excitation energies used in previous experimental studies, and translational, vibrational, and rotational distributions of the radical products are determined. For excess energies near the dissociation threshold, both the HCO and CH3 products are vibrationally cold there is a small amount of HCO rotational excitation and little CH3 rotational excitation, and the reaction energy is partitioned dominantly (& % at threshold) into relative translational motion. Close to threshold the HCO and CH3 rotational distributions are symmetrically shaped, resembling a Gaussian function, in agreement with observed experimental HCO rotational distributions. As the excess energy increases the calculated HCO and CH3 rotational distributions are observed to change from a Gaussian shape at threshold to one more resembling a Boltzmann distribution, a behavior also seen by various experimental groups. Thus the distribution of energy in these rotational degrees of freedom is observed to change from nonstatistical to apparently statistical, as excess energy increases. As the energy above threshold increases all the internal and external degrees of freedom are observed to gain population at a similar rate, broadly consistent with equipartitioning of the available energy at the transition state. These observations generally support the practice of separating the reaction dynamics into two reservoirs: an impulsive reservoir, fed by the exit channel dynamics, and a statistical reservoir, supported by the random distribution of excess energy above the barrier. The HCO rotation, however, is favored by approximately a factor of 3 over the statistical prediction. Thus, at sufficiently high excess energies, although the HCO rotational distribution may be considered statistical, the partitioning of energy into HCO rotation is not.

Dynamics of acetaldehyde dissociation at 308 nm: Rotational (N, Ka) and translational distributions of the HCO photoproduct

Publisher: American Chemical Society (ACS)

Date: 10-1994

DOI: 10.1021/J100093A021

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Publisher: Proceedings of the National Academy of Sciences

Date: 31-10-2006

DOI: 10.1073/PNAS.0604441103

Abstract: Product state distributions of the CO produced in the 308-nm photolysis of acetaldehyde show clear evidence of two dissociation mechanisms. One is attributed to the conventional transition state mechanism predicted by theory, with high rotational and translational energy of the CO and a pronounced v⊥J vector correlation. However, as much as 15% of the reaction flux proceeds via another pathway that produces low CO rotational and translational energy, very high CH 4 internal energy, and no correlation between the CO velocity and angular momentum vectors. The attributes of this channel are dynamically similar to the recently reported “roaming atom” mechanism in formaldehyde. We therefore speculate that the second pathway in acetaldehyde also occurs via a roaming mechanism in the CH 3 + HCO exit channel that decays into the CH 4 + CO channel.

The photodissociation dynamics of CFBr excited into the Ã(1A″) state

Publisher: AIP Publishing

Date: 22-06-1999

DOI: 10.1063/1.479122

Abstract: The dynamics of the photolysis reaction, CFBr+hν→CF+Br, have been investigated for photolysis energies in the range, ν̄=23 500–26 000 cm−1 (λ=385–435 nm). These energies correspond to excitation into the Ã(1A″) state of CFBr with 2500–5000 cm−1 of excess vibrational energy. Following dissociation of jet-cooled CFBr, the internal energy (Ω, Λ, J) of the nascent CF fragments (X 2Π, υ=0) was probed by laser induced fluorescence spectroscopy. Two distinct types of product state distributions were observed. At energies above T00+3360 cm−1 the populations of the Π1/22 and Π3/22 spin–orbit states of CF were equal, while A″ lambda doublet states were preferred over A′. These populations are consistent with a direct dissociation mechanism on the à state, over a barrier with a height of 3360 cm−1. The strong state mixing in the vicinity of the barrier ensures a statistical mixture of final spin–orbit states. The preference for the A″ lambda doublet states is consistent with the two lone electrons in in-plane orbitals pairing up in the final CF product, leaving one unpaired electron in an out-of-plane orbital, lying parallel to the J vector of the recoiling fragment. For excitation at energies below T00+3360 cm−1 the ground spin–orbit state of CF (2Π1/2) is preferred, while the lambda doublet populations are equal. The interpretation of these populations is that at these energies à state CFBr is stable with respect to dissociation over the barrier. The molecule crosses to either the X̃ or ã state where it encounters a deep attractive potential well. The subsequent slower dissociation rate allows the molecule to follow a more adiabatic pathway producing the lowest spin–orbit state of CF, and for any preference for lambda doublet states to be lost.

Measurement of propagation rate coefficients using pulsed-laser polymerization and matrix-assisted laser desorption/ionization mass spectrometry

Publisher: American Chemical Society (ACS)

Date: 11-1993

DOI: 10.1021/MA00076A064

Photodissociation dynamics of acetone at 193 nm: Photofragment internal and translational energy distributions

Publisher: AIP Publishing

Date: 15-12-1989

DOI: 10.1063/1.457274

Abstract: The photofragment internal and translational energy distributions resulting from the 193 nm photolysis of acetone have been measured. Vacuum-ultraviolet laser-induced fluorescence was used to probe the CO fragment, and multiphoton ionization time-of-flight mass spectrometry was used to probe the CH3. A Boltzmann distribution was observed to fit each degree of freedom with the following characteristic temperatures: CO: Tvib =2700 K, Trot =3000 K, Ttrans =3000 K CH3: Tvib =800 K, Trot =500 K, Ttrans =3500 K. No evidence was found for two distinct CH3 populations, as might be characteristic of a stepwise reaction. Energy partitioning between the fragments was fit well by a simple impulsive model in which the available energy is ided equally between the two dissociating C–C bonds, the two bonds cleaving in rapid succession on a time scale short enough to allow little redistribution of energy into the methyl degrees of freedom.

Vibrational relaxation induced by very low energy collisions in the S1(1B2u) state of naphthalene: a search for resonance enhancement of the cross section

Publisher: American Chemical Society (ACS)

Date: 06-1988

DOI: 10.1021/J100324A013

The 193-nm photodissociation of cyclobutanone: dynamics of the C2 and C3 channels

Publisher: American Chemical Society (ACS)

Date: 05-04-1990

DOI: 10.1021/J100370A055

Collision-free lifetimes of vibrational levels in S0 p-difluorobenzene: a view of IVR and an application of SEP-SVLF spectroscopy

Publisher: American Chemical Society (ACS)

Date: 02-1987

DOI: 10.1021/J100289A002

Photo-tautomerization of acetaldehyde as a photochemical source of formic acid in the troposphere

Publisher: Springer Science and Business Media LLC

Date: 03-07-2018

DOI: 10.1038/S41467-018-04824-2

Abstract: Organic acids play a key role in the troposphere, contributing to atmospheric aqueous-phase chemistry, aerosol formation, and precipitation acidity. Atmospheric models currently account for less than half the observed, globally averaged formic acid loading. Here we report that acetaldehyde photo-tautomerizes to vinyl alcohol under atmospherically relevant pressures of nitrogen, in the actinic wavelength range, λ = 300–330 nm, with measured quantum yields of 2–25%. Recent theoretical kinetics studies show hydroxyl-initiated oxidation of vinyl alcohol produces formic acid. Adding these pathways to an atmospheric chemistry box model (Master Chemical Mechanism) demonstrates increased formic acid concentrations by a factor of ~1.7 in the polluted troposphere and a factor of ~3 under pristine conditions. Incorporating this mechanism into the GEOS-Chem 3D global chemical transport model reveals an estimated 7% contribution to worldwide formic acid production, with up to 60% of the total modeled formic acid production over oceans arising from photo-tautomerization.

Mode-dependent intramolecular vibrational redistribution in the S1 state of jet-cooled p-difluorobenzene

Publisher: American Chemical Society (ACS)

Date: 07-1984

DOI: 10.1021/J150658A005

Educational analysis of the first year chemistry experiment ‘Thermodynamics Think-In’: an ACELL experiment

Publisher: Royal Society of Chemistry (RSC)

Date: 2007

DOI: 10.1039/B6RP90034H

Photodissociation dynamics of the reaction H2 CO→H+HCO via the singlet (S0) and triplet (T1) surfaces

Publisher: AIP Publishing

Date: 08-08-2007

DOI: 10.1063/1.2752161

Abstract: We have explored the photodissociation dynamics of the reaction H2CO+hν→H+HCO in the range of 810–2600cm−1 above the reaction threshold. Supersonically cooled formaldehyde was excited into selected JKa,Kc rotational states of six vibrational levels (1141, 51, 2261, 2243, 2341, and 2441) in the Ã(A21) state. The laser induced fluorescence spectra of the nascent HCO fragment provided detailed product state distributions. When formaldehyde was excited into the low-lying levels 1141, 51, and 2261, at Eavail& cm−1, the product state distribution can be modeled qualitatively by phase space theory. These dynamics are interpreted as arising from a reaction path on the barrierless S0 surface. When the initial states 2243 and 2341 were excited (Eavail=1120–1500cm−1), a second type of product state distribution appeared. This second distribution peaked sharply at low N, Ka and was severely truncated in comparison with those obtained from the lower lying states. At the even higher energy of 2441 (Eavail≈2600cm−1) the sharply peaked distribution appears to be dominant. We attribute this change in dynamics to the opening up of the triplet channel to produce HCO. The theoretical height of the barrier on the T1 surface lies between 1700 and 2100cm−1 and so we consider the triplet reaction to proceed via tunneling at the intermediate energies and proceed over the barrier at the higher energies. Considerable population was observed in the excited (0,0,1) state for all initial H2CO states that lie above the appearance energy. Rotational populations in the (0,0,1) state dropped more rapidly with (N,Ka) than did the equivalent populations in (0,0,0). This indicates that, although in idual rotational states are highly populated in (0,0,1), the total v3=1 population might not be so large. Specific population was also measured in the almost isoenergetic Kc and J states. No consistent population preference was found for either asymmetry or spin-rotation component.

Theoretical and Experimental Spectroscopy of the S₂ State of CHF and CDF: Dynamically Weighted Multireference Configuration Interaction Calculations for High-Lying Electronic States

Publisher: American Chemical Society (ACS)

Date: 19-01-2010

DOI: 10.1021/JZ900380A

Structural Causes of Singlet/triplet Preferences of Norrish Type II Reactions in Carbonyls

Publisher: American Chemical Society (ACS)

Date: 11-09-2020

DOI: 10.26434/CHEMRXIV.12941702.V1

Abstract: Photolysis thresholds are calculated for the Norrish Type II (NTII) intramolecular γ-hydrogen abstraction reaction in 22 structurally informative carbonyl species. The B2GP-PLYP excited state S 1 and T 1 thresholds agree well with triplet quenching experiments. However, many linear-response methods deliver poor S 1 energetics, which is explained by a S 1 / S 0 conical intersection in close proximity to the S 1 transition state. Multiconfigurational CASSCF calculations confirm a conical intersection features across all carbonyl classes. Structure–activity relationships are determined that could be used in atmospheric carbonyl photochemsitry modelling. This is exemplified for butanal, whose NTII quantum yields are too low when used as a ‘surrogate’ for larger carbonyls, since butanal lacks the γ-substitution that stabilises the 1,4- biradical. Reaction on T 1 dominates only in species where the S 1 thresholds are high — typically ketones. The α, β-unsaturated carbonyls cannot cleave the α–β bond, causing them to photoisomerise. A concerted S 0 NTII mechanism is calculated to be viable and may explain the recent detection of NTII photoproducts in the photolysis of pentan-2-one below the T 1 threshold.

Global overview of the management of acute cholecystitis during the COVID-19 pandemic (CHOLECOVID study)

Publisher: Oxford University Press (OUP)

Date: 02-05-2022

DOI: 10.1093/BJSOPEN/ZRAC052

Abstract: This study provides a global overview of the management of patients with acute cholecystitis during the initial phase of the COVID-19 pandemic. CHOLECOVID is an international, multicentre, observational comparative study of patients admitted to hospital with acute cholecystitis during the COVID-19 pandemic. Data on management were collected for a 2-month study interval coincident with the WHO declaration of the SARS-CoV-2 pandemic and compared with an equivalent pre-pandemic time interval. Mediation analysis examined the influence of SARS-COV-2 infection on 30-day mortality. This study collected data on 9783 patients with acute cholecystitis admitted to 247 hospitals across the world. The pandemic was associated with reduced availability of surgical workforce and operating facilities globally, a significant shift to worse severity of disease, and increased use of conservative management. There was a reduction (both absolute and proportionate) in the number of patients undergoing cholecystectomy from 3095 patients (56.2 per cent) pre-pandemic to 1998 patients (46.2 per cent) during the pandemic but there was no difference in 30-day all-cause mortality after cholecystectomy comparing the pre-pandemic interval with the pandemic (13 patients (0.4 per cent) pre-pandemic to 13 patients (0.6 per cent) pandemic P = 0.355). In mediation analysis, an admission with acute cholecystitis during the pandemic was associated with a non-significant increased risk of death (OR 1.29, 95 per cent c.i. 0.93 to 1.79, P = 0.121). CHOLECOVID provides a unique overview of the treatment of patients with cholecystitis across the globe during the first months of the SARS-CoV-2 pandemic. The study highlights the need for system resilience in retention of elective surgical activity. Cholecystectomy was associated with a low risk of mortality and deferral of treatment results in an increase in avoidable morbidity that represents the non-COVID cost of this pandemic.

Chemistry at the threshold: Unexpected products, unusual mechanisms, and generally weird things that happen near the energetic threshold for a reaction

Publisher: IEEE

Date: 08-2011

DOI: 10.1109/IQEC-CLEO.2011.6193613

TheS1(1A1)–S0(1A1) Electronic Transition of Jet-Cooledo-Difluorobenzene

Publisher: Elsevier BV

Date: 09-1998

DOI: 10.1006/JMSP.1998.7608

Abstract: A detailed study of the S1(1A1)-S0(1A1) transition of jet-cooled o-difluorobenzene has been completed using the two techniques of laser-induced fluorescence excitation and dispersed, single vibronic level fluorescence spectroscopy. Analysis of over 60 dispersed fluorescence spectra resulted in both the assignment of 22 excited state vibrational frequencies and the confirmation of 23 ground state frequencies. The spectrum is dominated by Franck-Condon activity in totally symmetric vibrations with long progressions in the ring-breathing mode, nu9. By analogy with benzene and the para- and meta-substituted isomers, two vibronic coupling mechanisms are postulated to be responsible for the wealth of weaker symmetry-forbidden structure that has been observed. Single quantum changes in b2 vibrations are postulated to appear due to first order vibronic coupling to a higher lying B2 electronic state. Combinations of b1 and a2 modes are postulated to appear from second order vibronic coupling to an A1 electronic state. This second order coupling causes a pronounced Duschinsky mixing among excited state b1 and a2 modes with respect to their ground state counterparts. Franck-Condon factors are calculated for the a1 progression-forming modes, anharmonic contributions are evaluated, one strong Fermi resonance is identified and analyzed, and the Duschinsky rotation matrix elements are evaluated for the most strongly affected modes, nu17 and nu18. Several transitions in the oDFB-oDFB van der Waals dimer and oDFB-Ar complex are also assigned in the spectrum. Copyright 1998 Academic Press.

Elective surgery system strengthening: development, measurement, and validation of the surgical preparedness index across 1632 hospitals in 119 countries

Publisher: Elsevier BV

Date: 11-2022

DOI: 10.1016/S0140-6736(22)01846-3

Observation of the predissociated, quasilinear B̃(A′1) state of CHF by optical-optical double resonance

Publisher: AIP Publishing

Date: 07-02-2007

DOI: 10.1063/1.2515273

Abstract: We report the first observation of the predissociative B̃ state of a halocarbene molecule. Rovibronic energy levels were measured in the B̃(A′1) state of CHF by fluorescence dip detected optical-optical double resonance spectroscopy via the à state. The origin was found to lie 30817.4cm−1 above the zero point level of the X̃ state. Rotational transitions within six purely bending states, and states involving one or two quanta of CF-stretch were observed, including the vibrational angular momentum components. Interpretation of the spectrum, with support of ab initio calculations, shows that CHF is quasilinear in the B̃ state with a small (−200cm−1) barrier to linearity which lies below the zero-point level. The rotational constant, B=1.04 to 1.09cm−1, depending on vibrational state, again in good agreement with theory. All observed B̃ state levels were predissociative, as evidenced by Lorentzian line broadening. Linewidths varied with initial state from 0.7–10.8cm−1, corresponding to excited state lifetimes of 0.5–8ps.

The Under-Explored Possibilities of Ground State Carbonyl Photochemistry

Publisher: American Chemical Society (ACS)

Date: 15-09-2020

DOI: 10.26434/CHEMRXIV.12950822.V1

Abstract: Carbonyls are among the most abundant volatile organic compounds in the atmosphere, and their C=O chromophores allow them to photolyse. However, carbonyl photolysis reactions are not restricted to the excited state: the C=O chromophore allows relaxation to, and reaction on, the ground state, following photon absorption. In this paper, the energetic thresholds for eight ground state reactions across twenty representative carbonyl species are calculated using double-hybrid density functional theory. Most reactions are found to be energetically accessible within the maximum photon energy available in the troposphere, but are absent in contemporary atmospheric chemistry models. Structure–activity relationships are then elucidated so that the significance of each reaction pathway for particular carbonyl species can be predicted based upon their class. The calculations here demonstrate that ground state photolysis pathways are ubiquitous in carbonyls and should not be ignored in the analysis of carbonyl photochemistry.

Dissociation dynamics of C3O2 excited at 157.6 nm

Publisher: AIP Publishing

Date: 02-1991

DOI: 10.1063/1.459958

Abstract: The dissociation of carbon suboxide by single photon absorption at 157.6 nm has been studied under the collisionless environment of a molecular beam. The primary products are 2CO+C [3P(97%) or 1D(3%) ]. The spin–orbit levels of the 3P carbon are statistically distributed. The CO rotational populations in the first four vibrational levels are found to be well described by Boltzmann distributions with temperatures 3430, 4120, 4670, and 2340 K for υ=0,1,2,3, respectively. A second low temperature component in the υ=0 rotational distribution is attributed to CO produced in coincidence with C(1D). Significant population is found in the first four vibrational levels with less than 3% estimated in the higher levels a vibrational temperature of 3700 K fits the distribution. Analysis of the Doppler profiles of the CO and carbon suggest that the dissociation is stepwise the first dissociation appears to be described by an anisotropy parameter near β=2, while the second appears to be isotropic. The mean CO fragment speeds were nearly constant for all rotational levels, though slightly faster for υ=1 than υ=0. From the translational energetics of the CO at least a small amount of stable C2O is inferred to exist. The overall energetics place the stable C2O quantum yield under 2% assuming that excited C2O is not radiatively stabilized. We were unable to detect C2O directly in any electronic state. The dissociation of C3O2 into C(3P)+2CO appears to be best described as a stepwise reaction that produces a nearly statistical partitioning into all fragment degrees of freedom. The best agreement is obtained for an intermediate C2O electronic state in the vicinity of the b̃ state (e.g., b̃, ã or Ã) a ground state C2O intermediate is unlikely. The singlet to triplet crossing most likely occurs in the C2O system on a time scale longer than a rotation (a few picoseconds).

Photofragment excitation spectroscopy of the formyl (HCO/DCO) radical: Linewidths and predissociation rates of the Ã(A″) state

Publisher: AIP Publishing

Date: 02-1991

DOI: 10.1063/1.459953

Abstract: Photofragment excitation (PHOFEX) spectra of the jet-cooled formyl (HCO and DCO) radical have been measured by monitoring laser-induced fluorescence of the CO fragment. The following Ã(A″)←X̃(A′) vibronic transitions were measured: (v1,v2,v3)←(0,0,0) where for HCO v2=6–16 with v1,v3=0 v2=7–12 with v1=1, v3=0 v2=9–12 with v1=0, v3=1 and for DCO v2=14–18 with v1,v3=0. The PHOFEX technique provides a powerful method for discriminating against strong transitions so as to allow assignment and measurement of many weaker Franck–Condon bands. The linewidths of the diffuse transitions lie in the 16–150 cm−1 (FWHM) range they increase strongly with K′ (the projection of the total angular momentum onto the a axis) and decrease slightly with v′2 (the number of HCO/DCO bending quanta). The linewidths are interpreted as radiationless transition rates and yield upper Ã-state lifetimes ranging from 70 to 700 fs. These data are in excellent agreement with a dynamical model of HCO/DCO curve crossing developed by R. N. Dixon [Mol. Phys. 54, 333 (1985)].

Optical-optical double resonance spectroscopy of the quasi-linear S2 state of CHF and CDF. I. Spectroscopic analysis

Publisher: AIP Publishing

Date: 12-09-2011

DOI: 10.1063/1.3633724

Abstract: In this work, we report on our full results of the spectroscopic analysis of the quasi-linear S2 state of the prototypical halocarbene, CHF, and its deuterated isotopomer CDF using optical-optical double resonance spectroscopy through the S1 state. A total of 51 S2 state vibrational levels with angular momenta in the range ℓ = 0–3 were observed for CHF, and 76 levels for CDF. Progressions involving all three fundamental vibrations were observed, and rotational constants were determined for each of these levels by measuring spectra through different intermediate J levels of the S1 state. Our experimental results are in excellent agreement with the predictions of vibrational calculations using the discrete variable representation method. The variational vibrational calculations were performed with an analytic potential energy surface fit to ab initio data by the method of interpolating moving least squares. The ab initio data are Davidson-corrected multi-reference configuration interaction calculations based on a state-averaged multiconfigurational self-consistent field reference incorporating a generalized dynamic weighting scheme.

Quantitative (υ, N, K_a) Product State Distributions near the Triplet Threshold for the Reaction H₂CO → H + HCO Measured by Rydberg Tagging and Laser-Induced Fluores

Publisher: American Chemical Society (ACS)

Date: 16-08-2008

DOI: 10.1021/JP8021826

Abstract: In this paper, we report quantitative product state distributions for the photolysis of H2CO --> H + HCO in the triplet threshold region, specifically for several rotational states in the 2(2)4(3) and 2(3)4(1) H2CO vibrational states that lie in this region. We have combined the strengths of two complementary techniques, laser-induced fluorescence for fine resolution and H atom Rydberg tagging for the overall distribution, to quantify the upsilon, N, and Ka distributions of the HCO photofragment formed via the singlet and triplet dissociation mechanisms. Both techniques are in quantitative agreement where they overlap and provide calibration or benchmarks that permit extension of the results beyond that possible by each technique on its own. In general agreement with previous studies, broad N and Ka distributions are attributed to reaction on the S0 surface, while narrower distributions are associated with reaction on T1. The broad N and Ka distributions are modeled well by phase space theory. The narrower N and Ka distributions are in good agreement with previous quasi-classical trajectory calculations on the T1 surface. The two techniques are combined to provide quantitative vibrational populations for each initial H2CO vibrational state. For dissociation via the 2(3)4(1) state, the average product vibrational energy (15% of E(avail)) was found to be about half of the rotational energy (30% of E(avail)), independent of the initial H2CO rotational state, irrespective of the singlet or triplet mechanism. For dissociation via the 2(2)4(3) state, the rotational excitation remained about 30% of E(avail), but the vibrational excitation was reduced.

Near threshold photochemistry of propanal. Barrier height, transition state structure, and product state distributions for the HCO channel

Publisher: American Chemical Society (ACS)

Date: 25-05-2002

DOI: 10.1021/JP025590X

Photodissociation of dicarbon: How nature breaks an unusual multiple bond

Publisher: Proceedings of the National Academy of Sciences

Date: 20-12-2021

DOI: 10.1073/PNAS.2113315118

Abstract: It has long been observed that the coma of a comet is often green while its tail is not. While the explanation for this must be that the molecules responsible for the green emission, C 2 , are photodissociated, the mechanism was, until now, unknown. We have observed the photodissociation of C 2 in the laboratory for the first time and, in doing so, have determined its bond dissociation energy with unprecedented precision. Invoking the observed mechanism, the calculated lifetime of cometary C 2 is found to be consistent with astronomical observations.

Observation of the dΠg3←cΣu+3 band system of C2

Publisher: AIP Publishing

Date: 18-12-2006

DOI: 10.1063/1.2408412

Abstract: A new band system of C2, dΠg3←cΣu+3 is observed by laser induced fluorescence spectroscopy, constituting the first direct detection of the cΣu+3 state of C2. Observations were made by laser excitation of cΣu+3(v″=0) C2, produced in an acetylene discharge, to the dΠg3(v′=3) level, followed by detection of Swan band fluorescence. Rotational analysis of this band yielded rotational constants for the cΣu+3(v″=0) state: B0=1.9218(2)cm−1, λ0=−0.335(4)cm−1 and γ0=0.011(2)cm−1. The vibrational band origin was determined to be ν3−0=15861.28cm−1.

Aliphatic hydrocarbon content of interstellar dust

Publisher: Oxford University Press (OUP)

Date: 18-06-2018

DOI: 10.1093/MNRAS/STY1582

Reassignment of the CH stretching frequency of CHF in the à electronic state

Publisher: AIP Publishing

Date: 10-02-2004

DOI: 10.1063/1.1641011

Abstract: The 20 000–24 000 cm−1 region of the Ã(1A″)←X̃(1A′) transition of supersonically cooled fluoromethylene has been remeasured using laser induced fluorescence spectroscopy. Several new bands involving the ν1 (C–H stretching) vibrational frequency in the excited state are reported, allowing for the first time an accurate determination of ν1′=2799.7 cm−1 and x12′=−34.8 cm−1. The value of x22′ has been recalculated to be −7.4 cm−1. Dispersed fluorescence spectroscopy was used to confirm the identity of the ν1 vibrational bands. Several sequence bands involving the ν3 (C–F stretching) vibrational frequency are also reported.

Ab initio potential energy surface and vibrational frequencies of Ã(1A″) HCF

Publisher: Elsevier BV

Date: 07-1998

DOI: 10.1016/S0009-2614(98)00657-5

Pulsed-Laser Polymerization Measurements of the Propagation Rate Coefficient for Butyl Acrylate

Publisher: American Chemical Society (ACS)

Date: 1996

DOI: 10.1021/MA950747N

POPTARTS: A New Method to Determine Quantum Yields in a Molecular Beam

Publisher: American Chemical Society (ACS)

Date: 30-11-2022

DOI: 10.1021/ACS.JPCA.2C06289

Abstract: A new technique is reported to determine absolute photodissociation quantum yields, ϕ

Molecular collision dynamics

Publisher: Royal Society of Chemistry (RSC)

Date: 2011

DOI: 10.1039/C1CP90049H

Is there resonance enhancement of the cross section for vibrational relaxation induced by very low energy collisions? The I2–He system revisited

Publisher: AIP Publishing

Date: 12-1988

DOI: 10.1063/1.455352

Abstract: Rate coefficients for state-to-field vibrational relaxation of I2 (B 3Π0u+, υ′=16) induced by collisions with He at translational temperatures of 2–12 K are measured using state-selected time-resolved dispersed fluorescence spectroscopy in a supersonic free jet expansion. These vibrational relaxation rate coefficients in the 2–12 K regime are found to be an order of magnitude smaller than comparable rate coefficients at 300 K. The rate data are compared with calculated classical I2–He encounter rates and with rates computed using quantum mechanical cross sections for I2–He calculated by Schwenke and Truhlar. The role of scattering resonances in determining vibrational relaxation rates at low translational temperatures is discussed.

Photodissociation dynamics of 3-cyclopentenone: using the impact parameter distribution as a criterion for concertedness

Publisher: American Chemical Society (ACS)

Date: 05-1992

DOI: 10.1021/J100190A018

Hydrogen-atom attack on phenol and toluene is ortho-directed

Publisher: Royal Society of Chemistry (RSC)

Date: 2016

DOI: 10.1039/C5CP07619F

Abstract: The reaction of H + phenol and H/D + toluene has been studied in a supersonic expansion after electric discharge.

Electronic spectroscopy and ab initio quantum chemical study of the Ã(1A″)−X̃(1A′) transition of CFBr

Publisher: AIP Publishing

Date: 08-08-1998

DOI: 10.1063/1.476790

Abstract: The Ã(1A″)−X̃(1A′) electronic transition of jet-cooled CFBr has been investigated by spectroscopic and ab initio theoretical methods. Laser induced fluorescence (LIF) excitation spectroscopy was used to explore the rovibronic levels in the à state, and dispersed fluorescence spectroscopy was used to study ground-state vibrations. Analysis of these spectra yielded gas-phase vibrational frequencies and anharmonicity constants in both electronic states. The computed ab initio vibrational frequencies in both X̃ and à states are in good accord with the experimental values. The Ã-state fluorescence lifetimes varied between 100 ns and 3 μs as a function of excited vibronic state. The highest lying levels displayed a shortened fluorescence lifetime, and some vibronic states that involved ν1 (the CF stretch) exhibited shortened lifetimes (300–500 ns) irrespective of the vibrational energy. Vibronic structure in the LIF spectrum disappeared for vibrational energy in excess of 2957 cm−1. Calculations of the Ã-state potential-energy surface show that it has a small barrier to dissociation to CF+Br with a barrier height in good accord with observed termination of fluorescence. The predicted photochemical pathway to production of CF+Br fragments was proven experimentally by detection of CF fragments. The photofragment excitation spectrum showed strong, increasingly broad vibronic structure at higher energies than the LIF spectrum. At lower energy, sharp but weaker vibronic structure was still evident, overlapping the LIF spectrum. There appears to be two photochemical mechanisms to produce CF+Br, one direct and one indirect. We estimate the height of the barrier to direct dissociation to lie 3250±150 cm−1 above the zero-point level of the à state. The asymptotic thermochemical dissociation limit is estimated to lie ⩾1100 cm−1 lower. The thermochemical bond dissociation energy for the C–Br bond in CFBr was thereby estimated to be Ediss⩽23 180 cm−1, which led to an estimate of the heat of formation for CFBr, ΔfH2980⩾86 kJ mol−1.

Experimental and theoretical investigation of the dispersed fluorescence spectroscopy of HC4S

Publisher: AIP Publishing

Date: 18-05-2006

DOI: 10.1063/1.2196886

Abstract: A high-resolution single vibronic level emission study from the ÃΠ3∕22 state of the HC4S radical is reported. Ground state density functional theory frequencies have been used to assign ground state vibronic levels involving three stretching modes ν2, ν3, and ν5 in the region of 0–3250cm−1, while the frequency of ν4 remains speculative. Tentative assignments are given for the complicated structures arising from Renner-Teller and spin-orbit interactions within the bending energy levels. From analysis of the dispersed emission spectra, Fermi resonances involving pairs of bands have been identified in the ÃΠ3∕22←X̃Π3∕22 laser induced fluorescence spectrum.

A new role of curcumin: as a multicolor photoinitiator for polymer fabrication under household UV to red LED bulbs

Publisher: Royal Society of Chemistry (RSC)

Date: 2015

DOI: 10.1039/C5PY00661A

Abstract: Curcumin can act as a multicolor photoinitiator in free radical photopolymerization under air upon exposure to household LED bulbs.

Zero-point energy conservation in classical trajectory simulations: Application to H 2 CO

Publisher: AIP Publishing

Date: 21-05-2018

DOI: 10.1063/1.5023508

Abstract: A new approach for preventing zero-point energy (ZPE) violation in quasi-classical trajectory (QCT) simulations is presented and applied to H2CO “roaming” reactions. Zero-point energy may be problematic in roaming reactions because they occur at or near bond dissociation thresholds and these channels may be incorrectly open or closed depending on if, or how, ZPE has been treated. Here we run QCT simulations on a “ZPE-corrected” potential energy surface defined as the sum of the molecular potential energy surface (PES) and the global harmonic ZPE surface. Five different harmonic ZPE estimates are examined with four, on average, giving values within 4 kJ/mol—chemical accuracy—for H2CO. The local harmonic ZPE, at arbitrary molecular configurations, is subsequently defined in terms of “projected” Cartesian coordinates and a global ZPE “surface” is constructed using Shepard interpolation. This, combined with a second-order modified Shepard interpolated PES, V, allows us to construct a proof-of-concept ZPE-corrected PES for H2CO, Veff, at no additional computational cost to the PES itself. Both V and Veff are used to model product state distributions from the H + HCO → H2 + CO abstraction reaction, which are shown to reproduce the literature roaming product state distributions. Our ZPE-corrected PES allows all trajectories to be analysed, whereas, in previous simulations, a significant proportion was discarded because of ZPE violation. We find ZPE has little effect on product rotational distributions, validating previous QCT simulations. Running trajectories on V, however, shifts the product kinetic energy release to higher energy than on Veff and classical simulations of kinetic energy release should therefore be viewed with caution.

Nascent state distribution of the HCO photoproduct arising from the 309 nm photolysis of propionaldehyde

Publisher: American Chemical Society (ACS)

Date: 08-1995

DOI: 10.1021/J100034A004

Multihydroxy‐Anthraquinone Derivatives as Free Radical and Cationic Photoinitiators of Various Photopolymerizations under Green LED

Publisher: Wiley

Date: 20-04-2018

DOI: 10.1002/MARC.201800172

Abstract: Multihydroxy-anthraquinone derivatives [i.e., 1,2,4-trihydroxyanthraquinone (124-THAQ), 1,2,7-trihydroxyanthraquinone (127-THAQ), and 1,2,5,8-tetrahydroxyanthraquinone (1258-THAQ)] can interact with various additives [e.g., iodonium salt, tertiary amine, N-vinylcarbazole, and 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine] under household green LED irradiation to generate active species (cations and radicals). The relevant photochemical mechanism is investigated using quantum chemistry, fluorescence, cyclic voltammetry, laser flash photolysis, steady state photolysis, and electron spin resonance spin-trapping techniques. Furthermore, the multihydroxy-anthraquinone derivative-based photoinitiating systems are capable of initiating cationic photopolymerization of epoxides or inyl ethers under green LED, and the relevant photoinitiation ability is consistent with the photochemical reactivity (i.e., 124-THAQ-based photoinitiating system exhibits highest reactivity and photoinitiation ability). More interestingly, multihydroxy-anthraquinone derivative-based photoinitiating systems can initiate free radical crosslinking or controlled (i.e., reversible addition-fragmentation chain transfer) photopolymerization of methacrylates under green LED. It reveals that multihydroxy-anthraquinone derivatives can be used as versatile photoinitiators for various types of photopolymerization reactions.

Structural Effects on the Norrish Type I α-Bond Cleavage of Tropospherically Important Carbonyls

Publisher: American Chemical Society (ACS)

Date: 11-2019

DOI: 10.1021/ACS.JPCA.9B05534

Abstract: Norrish Type I (NTI) α-bond cleavage is the dominant photolysis mechanism in small carbonyls and is an important source of radicals in the troposphere. In nonsymmetric species two cleavages are possible, NTI

The halocarbenes: model systems for understanding the spectroscopy, dynamics and chemistry of carbenes

Publisher: Informa UK Limited

Date: 07-2009

DOI: 10.1080/01442350903087792

Intramolecular hole-transfer in protonated anthracene

Publisher: Royal Society of Chemistry (RSC)

Date: 2023

DOI: 10.1039/D3CP02922K

Abstract: Triple-resonance dissociation spectra of protonated and deuteronated anthracene reveal signatures of an intramolecular charge-transfer event that occurs on the excited state surface.

Laser-induced fluorescence and dispersed fluorescence spectroscopy of jet-cooled 1-phenylpropargyl radical

Publisher: AIP Publishing

Date: 13-04-2009

DOI: 10.1063/1.3110682

Abstract: The D1(A2″)−D0(A2″) electronic transition of the resonance-stabilized 1-phenylpropargyl radicalooled discharge of 3-phenyl-1-propyne, has been investigated in detail by laser-induced fluorescence excitation and dispersed single vibronic level fluorescence (SVLF) spectroscopy. The transition is dominated by the origin band at 21 007 cm−1, with weaker Franck–Condon activity observed in a′ fundamentals and even overtones and combinations of a″ symmetry. Ab initio and density functional theory calculations of the D0 and D1 geometries and frequencies were performed to support and guide the experimental assignments throughout. Analysis of SVLF spectra from 16 D1 vibronic levels has led to the assignment of 15 fundamental frequencies in the excited state and 19 fundamental frequencies in the ground state assignments for many more normal modes not probed directly by fluorescence spectroscopy are also suggested. Duschinsky mixing, in which the excited state normal modes are rotated with respect to the ground state modes, is prevalent throughout, in vibrations of both a′ and a″ symmetry.

Jet-Cooled Spectroscopy of ortho-Hydroxycyclohexadienyl Radicals

Publisher: American Chemical Society (ACS)

Date: 12-10-2018

DOI: 10.1021/ACS.JPCA.8B07603

Abstract: The electronic spectra of the ortho-hydroxycyclohexadienyl radical have been observed following the supersonic expansion of the electric discharge products of phenol and water. Hydrogen atoms, split from water, add to the phenol ring at the ortho position, generating syn and anti rotamers with respect to the hydroxyl group. The D

Quantum-Induced Symmetry Breaking in the Deuterated Dihydroanthracenyl Radical

Publisher: American Chemical Society (ACS)

Date: 16-07-2019

DOI: 10.1021/ACS.JPCA.9B04561

Abstract: The hydrogen-atom adduct with anthracene, 9-dihydroanthracenyl radical (C

The eΠg3 state of C2: A pathway to dissociation

Publisher: AIP Publishing

Date: 14-07-2017

DOI: 10.1063/1.4985882

Abstract: The lowest 13 vibrational levels, v = 0–12, of the eΠg3 state of the C2 molecule have been measured by laser-induced fluorescence of new bands of the Fox-Herzberg system. The newly observed levels, v = 5–12, which span the eΠg3 electronic state up to and beyond the first dissociation threshold of C2, were analyzed to afford highly accurate molecular constants, including band origins, and rotational and spin-orbit constants. The spin-orbit coupling constants of the previously published lowest five levels are revised in sign and magnitude, requiring an overhaul of previously published molecular constants. The analysis is supported by high level ab initio calculations. Lifetimes of all observed levels were recorded and found to be in excellent agreement with ab initio predicted values up to v = 11. v = 12 was found to exhibit a much reduced lifetime and fluorescence quantum yield, which is attributed to the onset of predissociation. This brackets the dissociation energy of ground state XΣg+1 C2 between 6.1803 and 6.2553 eV, in agreement with the Active Thermochemical Tables.

Spectroscopy of the Ã(B21)-X̃(A11) transition of jet-cooled fluorobenzene: Laser-induced fluorescence, dispersed fluorescence, and pathological Fermi resonances

Publisher: AIP Publishing

Date: 04-09-2007

DOI: 10.1063/1.2759931

Abstract: A detailed study of the S1(B21)-S0(A11) electronic transition of jet-cooled fluorobenzene has been carried out using laser-induced fluorescence and dispersed fluorescence (DF) spectroscopies. Analysis of over 40 single vibronic level DF spectra resulted in the assignment of 16 fundamental frequencies in the excited electronic state. Progressions in totally symmetric modes, particularly in the ring-breathing mode ν9, feature in both types of fluorescence spectrum. There is also significant activity in non-totally-symmetric modes, with activity in Franck-Condon (FC)-allowed overtones, FC-forbidden combinations induced by Duschinsky mixing, and symmetry-forbidden transitions induced by the same Herzberg-Teller vibronic coupling mechanism that induces the benzene S1-S0 transition. Fermi resonances (FRs) are extensive throughout the spectrum, especially in the important FC-active a1 modes. A consequence of these extensive FRs is that several important previous assignments are shown to be incorrect and have been reassigned here. Ab initio and density functional theory calculations have also been performed to support the experimental assignments.

Photodissociation dynamics of the reaction CF2Br2+hν→CF2+ 2Br. Energetics, threshold and nascent CF2 energy distributions for λ=223–260 nm

Publisher: Royal Society of Chemistry (RSC)

Date: 2000

DOI: 10.1039/B000203H

HCO (N,Ka,Kc,J) distributions from near-threshold photolysis of H2CO (J,Ka,Kc)

Publisher: AIP Publishing

Date: 22-02-1998

DOI: 10.1063/1.475736

Abstract: The dynamics of the reaction H2CO+hν(λ≈330 nm)→H+HCO have been studied following excitation of formaldehyde into the Ã(1A2) state, just above the dissociation threshold of the X̃(1A1) state. Formaldehyde was excited via specific J, Ka, Kc rotational states and the ensuing rotational distribution of HCO measured by fully resolving N, Ka, Kc, and J=N±S of the fragment. When only the N and Ka quantum numbers of both formaldehyde and the formyl radical are considered, the distributions are generally modeled well by phase space theory (PST). Within ≈10 cm−1 of the threshold, however, the PST predictions consistently exceed the experimental populations. This was accounted for by the inclusion of a centrifugal barrier in the PST model. The attractive part of the effective centrifugal potential was modeled by a dipole-induced dipole plus dispersion interaction. The barrier is weak and long range (& Å). Resolution of Kc in the reaction, in both parent and product, gave large deviations from the PST model. The HCO population distributions separate according to whether Kc was the upper- or lower-energy state. Additionally, the upper/lower preference was sensitive to the choice of Kc in the parent. Insufficient data are currently available to quantify this observation. The product state distribution was also found to be independent of the spin–rotation state of HCO.

Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries

Publisher: Oxford University Press (OUP)

Date: 20-04-2023

DOI: 10.1093/BJS/ZNAD092

Abstract: Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres. This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries. In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling reducing use of anaesthetic gases and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices reducing use of consumables and reducing the use of general anaesthesia. This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries.

The dΠg3-cΣu+3 band system of C2

Publisher: AIP Publishing

Date: 04-12-2007

DOI: 10.1063/1.2805090

Abstract: A two-dimensional fluorescence (excitation/emission) spectrum of C2 produced in an acetylene discharge was used to identify and separate emission bands from the dΠg3←cΣu+3 and dΠg3←aΠu3 excitations. Rotationally resolved excitation spectra of the (4←1), (5←1), (5←2), and (7←3) bands in the dΠg3←cΣu+3 system of C2 were observed by laser-induced fluorescence spectroscopy. The molecular constants of each vibrational level, determined from rotational analysis, were used to calculate the spectroscopic constants of the cΣu+3 state. The principal molecular constants for the cΣu+3 state are Be=1.9319(19)cm−1, αe=0.01855(69)cm−1, ωe=2061.9cm−1, ωexe=14.84cm−1, and T0(c−a)=8662.925(3)cm−1. We report also the first experimental observations of dispersed fluorescence from the dΠg3 state to the cΣu+3 state, namely, dΠg3(v=3)→cΣu+3(v=0,1).

Rotational energy transfer cross sections in methane (13CD4) from infrared double resonance measurements

Publisher: Elsevier BV

Date: 08-1985

DOI: 10.1016/0009-2614(85)85333-1

Quantum-Induced Symmetry-Breaking in the Deuterated Dihydroanthracenyl Radical

Publisher: American Chemical Society (ACS)

Date: 15-05-2019

DOI: 10.26434/CHEMRXIV.8123648.V1

Abstract: The hydrogen-atom adduct with anthracene, 9-dihydroanthracenyl radical (C 14 H 11 ), and its deuterated analogue, have been identified by laser spectroscopy coupled to time-of-flight mass spectrometry, supported by time-dependent density functional theory calculations. The electronic spectrum of 9-dihydroanthracenyl radical exhibits an origin band at 19115 cm -1 and its ionization energy was determined to be 6.346(1) eV. The spectra reveal a low-frequency vibrational progression corresponding to a mode described by a butterfly-inversion. In the deuterated analogue, a zero-point-energy imbalance along this coordinate is found to lead to a doubling of the observed spectral lines in the progression. This is attributed to quantum-induced symmetry breaking as previously observed in isotopologues of CH 5 + .

The electronic spectroscopy of jet-cooled difluorocarbene (CF2): The missing Ã-state stretching frequencies

Publisher: AIP Publishing

Date: 15-09-1995

DOI: 10.1063/1.470728

Abstract: The Ã(1B1)←X̃(1A1) electronic transition of difluorocarbene (CF2) has been studied by laser-induced fluorescence spectroscopy in a supersonic free jet and by ab initio theoretical methods. The radical was formed by pyrolysis of C2F4 at ∼1000 °C with a heating chamber at the tip of the nozzle just prior to the gas expansion. Fairly complete rotational cooling, but incomplete vibrational cooling allowed the identification of several new hot band transitions. Ab initio calculations for the X̃(1A1), ã(3B1), and Ã(1B1) electronic states were performed using the CASSCF method in conjunction with Dunning’s cc-pVTZ basis set, as well as two smaller sets. The calculations allow us to distinguish between several possible assignments of the observed transitions, and hence determine the vibrational frequencies of the two previously unassigned Ã-state stretching frequencies: ν3′=1180±2 cm−1 and ν1′=1011±2 cm−1. The ordering of these modes is different from that in the ground state (ν1″≳ν3″) and mechanisms for the switching are discussed.

Structural Evolution in a Hydrothermal Reaction between Nb₂O₅ and NaOH Solution:  From Nb₂O₅ Grains to Microporous Na₂Nb₂O₆

Publisher: American Chemical Society (ACS)

Date: 28-01-2006

DOI: 10.1021/JA056301W

Abstract: Niobium pentoxide reacts actively with concentrate NaOH solution under hydrothermal conditions at as low as 120 degrees C. The reaction ruptures the corner-sharing of NbO(7) decahedra and NbO(6) octahedra in the reactant Nb(2)O(5), yielding various niobates, and the structure and composition of the niobates depend on the reaction temperature and time. The morphological evolution of the solid products in the reaction at 180 degrees C is monitored via SEM: the fine Nb(2)O(5) powder aggregates first to irregular bars, and then niobate fibers with an aspect ratio of hundreds form. The fibers are microporous molecular sieve with a monoclinic lattice, Na(2)Nb(2)O(6).(2)/(3)H(2)O. The fibers are a metastable intermediate of this reaction, and they completely convert to the final product NaNbO(3) cubes in the prolonged reaction of 1 h. This study demonstrates that by carefully optimizing the reaction condition, we can selectively fabricate niobate structures of high purity, including the delicate microporous fibers, through a direct reaction between concentrated NaOH solution and Nb(2)O(5). This synthesis route is simple and suitable for the large-scale production of the fibers. The reaction first yields poorly crystallized niobates consisting of edge-sharing NbO(6) octahedra, and then the microporous fibers crystallize and grow by assembling NbO(6) octahedra or clusters of NbO(6) octahedra and NaO(6) units. Thus, the selection of the fibril or cubic product is achieved by control of reaction kinetics. Finally, niobates with different structures exhibit remarkable differences in light absorption and photoluminescence properties. Therefore, this study is of importance for developing new functional materials by the wet-chemistry process.