Meredith Jordan

“Plug-and-Play” potentials: Investigating quantum effects in (H2)2–Li+–benzene

Publisher: AIP Publishing

Date: 21-08-2015

DOI: 10.1063/1.4928760

Abstract: Quantum and anharmonic effects are investigated in (H2)2–Li+–benzene, a model for hydrogen adsorption in metal-organic frameworks and carbon-based materials, using rigid-body diffusion Monte Carlo (RBDMC) simulations. The potential-energy surface (PES) is calculated as a modified Shepard interpolation of M05-2X/6-311+G(2df,p) electronic structure data. The RBDMC simulations yield zero-point energies (ZPE) and probability density histograms that describe the ground-state nuclear wavefunction. Binding a second H2 molecule to the H2–Li+–benzene complex increases the ZPE of the system by 5.6 kJ mol−1 to 17.6 kJ mol−1. This ZPE is 42% of the total electronic binding energy of (H2)2–Li+–benzene and cannot be neglected. Our best estimate of the 0 K binding enthalpy of the second H2 to H2–Li+–benzene is 7.7 kJ mol−1, compared to 12.4 kJ mol−1 for the first H2 molecule. Anharmonicity is found to be even more important when a second (and subsequent) H2 molecule is adsorbed use of harmonic ZPEs results in significant error in the 0 K binding enthalpy. Probability density histograms reveal that the two H2 molecules are found at larger distance from the Li+ ion and are more confined in the θ coordinate than in H2–Li+–benzene. They also show that both H2 molecules are delocalized in the azimuthal coordinate, ϕ. That is, adding a second H2 molecule is insufficient to localize the wavefunction in ϕ. Two fragment-based (H2)2–Li+–benzene PESs are developed. These use a modified Shepard interpolation for the Li+–benzene and H2–Li+–benzene fragments, and either modified Shepard interpolation or a cubic spline to model the H2–H2 interaction. Because of the neglect of three-body H2, H2, Li+ terms, both fragment PESs lead to overbinding of the second H2 molecule by 1.5 kJ mol−1. Probability density histograms, however, indicate that the wavefunctions for the two H2 molecules are effectively identical on the “full” and fragment PESs. This suggests that the 1.5 kJ mol−1 error is systematic over the regions of configuration space explored by our simulations. Notwithstanding this, modified Shepard interpolation of the weak H2–H2 interaction is problematic and we obtain more accurate results, at considerably lower computational cost, using a cubic spline interpolation. Indeed, the ZPE of the fragment-with-spline PES is identical, within error, to the ZPE of the full PES. This fragmentation scheme therefore provides an accurate and inexpensive method to study higher hydrogen loading in this and similar systems.

Path integral Monte Carlo simulations of H2 adsorbed to lithium-doped benzene: A model for hydrogen storage materials

Publisher: AIP Publishing

Date: 17-11-2015

DOI: 10.1063/1.4932940

Abstract: Finite temperature quantum and anharmonic effects are studied in H2–Li+-benzene, a model hydrogen storage material, using path integral Monte Carlo (PIMC) simulations on an interpolated potential energy surface refined over the eight intermolecular degrees of freedom based upon M05-2X/6-311+G(2df,p) density functional theory calculations. Rigid-body PIMC simulations are performed at temperatures ranging from 77 K to 150 K, producing both quantum and classical probability density histograms describing the adsorbed H2. Quantum effects broaden the histograms with respect to their classical analogues and increase the expectation values of the radial and angular polar coordinates describing the location of the center-of-mass of the H2 molecule. The rigid-body PIMC simulations also provide estimates of the change in internal energy, ΔUads, and enthalpy, ΔHads, for H2 adsorption onto Li+-benzene, as a function of temperature. These estimates indicate that quantum effects are important even at room temperature and classical results should be interpreted with caution. Our results also show that anharmonicity is more important in the calculation of U and H than coupling—coupling between the intermolecular degrees of freedom becomes less important as temperature increases whereas anharmonicity becomes more important. The most anharmonic motions in H2–Li+-benzene are the “helicopter” and “ferris wheel” H2 rotations. Treating these motions as one-dimensional free and hindered rotors, respectively, provides simple corrections to standard harmonic oscillator, rigid rotor thermochemical expressions for internal energy and enthalpy that encapsulate the majority of the anharmonicity. At 150 K, our best rigid-body PIMC estimates for ΔUads and ΔHads are −13.3 ± 0.1 and −14.5 ± 0.1 kJ mol−1, respectively.

Rotational resonances in the H ₂ CO roaming reaction are revealed by detailed correlations

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

Date: 25-09-2020

DOI: 10.1126/SCIENCE.ABC4088

Abstract: The phenomenon of roaming in chemical reactions (that is, bypassing the minimum energy pathway from unlikely geometries) has attracted a great deal of attention in the chemical reaction dynamics community over the past decade and still demonstrates unexpected results. Using velocity-map imaging of state-selected H 2 products of H 2 CO photodissociation, Quinn et al. discovered the bimodal structure of rotational distribution of the other product fragment, CO. Quasiclassical trajectories showed that this bimodality originates from two distinctive reaction pathways that proceed by the trans or cis configuration of O–C–H⋯H, leading to high or low rotational excitations of CO, respectively. Whether such a mechanism is present in the many other chemical reactions for which roaming reaction pathways have been reported is yet to be determined. Science , this issue p. 1592

The energy dependence of CO(v,J) produced from H 2 CO via the transition state, roaming, and triple fragmentation channels

Publisher: AIP Publishing

Date: 16-05-2017

DOI: 10.1063/1.4983138

Abstract: The dynamics of CO production from photolysis of H2CO have been explored over a 8000 cm−1 energy range (345 nm–266 nm). Two-dimensional ion imaging, which simultaneously measures the speed and angular momentum distribution of a photofragment, was used to characterise the distribution of rotational and translational energy and to quantify the branching fraction of roaming, transition state (TS), and triple fragmentation (3F) pathways. The rotational distribution for the TS channel broadens significantly with increasing energy, while the distribution is relatively constant for the roaming channel. The branching fraction from roaming is also relatively constant at 20% of the observed CO. Above the 3F threshold, roaming decreases in favour of triple fragmentation. Combining the present data with our previous study on the H-atom branching fractions and published quantum yields for radical and molecular channels, absolute quantum yields were determined for all five dissociation channels for the entire S1←S0 absorption band, covering almost 8000 cm−1 of excitation energy. The S0 radical and TS molecular channels are the most important over this energy range. The absolute quantum yield of roaming is fairly constant ∼5% at all energies. The T1 radical channel is important (20%-40%) between 1500 and 4000 cm−1 above the H + HCO threshold, but becomes unimportant at higher energy. Triple fragmentation increases rapidly above its threshold reaching a maximum of 5% of the total product yield at the highest energy.

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

Photo-Tautomerization of Acetaldehyde to Vinyl Alcohol: A Potential Route to Tropospheric Acids

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

Date: 07-09-2012

DOI: 10.1126/SCIENCE.1220712

Abstract: Keto/enol tautomerization (HC−C=O→C=C−OH) plays a central role in the chemistry of carbonyl compounds in a solution in which solvent and catalytic acids or bases can facilitate the proton transfer from C to O and back again. In contrast, analyses of atmospheric chemistry tend to exclude enol structure, on the assumption that tautomerization does not proceed regularly in gas phase. Andrews et al. (p. 1203 , published online 16 August) used isotopic labeling to probe the photoisomerization pathway of gaseous acetaldehyde in the lab and discovered evidence for an enol. Subsequent modeling indicates that photogenerated enols could build up sufficiently in the troposphere to account for previously puzzling observations of organic acids in the atmosphere.

Phototautomerization of Acetaldehyde to Vinyl Alcohol: A Primary Process in UV-Irradiated Acetaldehyde from 295 to 335 nm

Publisher: American Chemical Society (ACS)

Date: 16-11-2012

DOI: 10.1021/JZ301701X

Abstract: The concentrations of organic acids, key species in the formation of secondary organic aerosols, are underestimated by atmospheric chemistry models by a factor of ∼2. Vinyl alcohol (VA, CH2═CHOH, ethenol) has been suggested as a precursor to formic acid, but sufficient tropospheric sources of VA have not been identified. Here, we show that VA is formed upon irradiation of neat acetaldehyde (CH3CHO) in the actinic ultraviolet region, between 295 and 330 nm. Besides the well-known photochemical products CO and CH4, we infer up to a 15% quantum yield of VA at 20 Torr acetaldehyde pressure and a photolysis wavelength of 330 nm. The experiments confirm a recent model predicting phototautomerization of acetaldehyde to VA and imply that photolysis of small aldehydes and ketones could provide tropospheric sources of enols sufficient to impact organic acid budgets. We also report absolute infrared absorption cross sections of VA.

The enantiomers of syn-2,3-difluoro-4-aminobutyric acid elicit opposite responses at the GABA_Creceptor

Publisher: Royal Society of Chemistry (RSC)

Date: 2012

DOI: 10.1039/C1CC15816C

Abstract: The conformational behaviour and GABA receptor activity of the different stereoisomers of 2,3-difluoro-4-aminobutyric acid are described. Two enantiomeric GABA(C)-active ligands are identified, one of which is an agonist while the other is an antagonist. The results support an existing QSAR model of the bioactive geometry of GABA at GABA(C).

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

VARIATIONAL TRANSITION-STATE THEORY - A SIMPLE-MODEL FOR DISSOCIATION AND RECOMBINATION REACTIONS OF SMALL SPECIES

Publisher: American Chemical Society (ACS)

Date: 10-1991

DOI: 10.1021/J100175A050

Synthesis and Conformational Analysis of α,β‐Difluoro‐γ‐amino Acid Derivatives

Publisher: Wiley

Date: 20-01-2011

DOI: 10.1002/CHEM.201003320

Dynamics and quantum yields of H₂ + CH₂CO as a primary photolysis channel in CH₃CHO

Publisher: Royal Society of Chemistry (RSC)

Date: 2019

DOI: 10.1039/C8CP06412A

Abstract: A new ketene + H 2 channel in CH 3 CHO photolysis is not modelled by quasi-classical trajectories over the transition state.

Oxo-bridged isomers of aza-trishomocubane sigma (σ) receptor ligands: Synthesis, in vitro binding, and molecular modeling

Publisher: Elsevier BV

Date: 2010

DOI: 10.1016/J.BMCL.2009.11.019

Abstract: Isomeric oxo-bridged analogs of aza-trishomocubane sigma (sigma) receptor ligands were synthesized and shown to display a reduced affinity for the sigma receptor. In the case of phenethyl derivative 4, there was a concomitant introduction of high-affinity for the alpha(2C) adrenergic receptor, and moderate affinity for the dopamine transporter. Molecular modeling was undertaken to rationalize these results.

Method and basis set dependence of anharmonic ground state nuclear wave functions and zero-point energies: Application to SSSH

Publisher: AIP Publishing

Date: 02-02-2010

DOI: 10.1063/1.3276064

Abstract: One of the largest remaining errors in thermochemical calculations is the determination of the zero-point energy (ZPE). The fully coupled, anharmonic ZPE and ground state nuclear wave function of the SSSH radical are calculated using quantum diffusion Monte Carlo on interpolated potential energy surfaces (PESs) constructed using a variety of method and basis set combinations. The ZPE of SSSH, which is approximately 29 kJ mol−1 at the CCSD(T)/6-31G∗ level of theory, has a 4 kJ mol−1 dependence on the treatment of electron correlation. The anharmonic ZPEs are consistently 0.3 kJ mol−1 lower in energy than the harmonic ZPEs calculated at the Hartree–Fock and MP2 levels of theory, and 0.7 kJ mol−1 lower in energy at the CCSD(T)/6-31G∗ level of theory. Ideally, for sub-kJ mol−1 thermochemical accuracy, ZPEs should be calculated using correlated methods with as big a basis set as practicable. The ground state nuclear wave function of SSSH also has significant method and basis set dependence. The analysis of the nuclear wave function indicates that SSSH is localized to a single symmetry equivalent global minimum, despite having sufficient ZPE to be delocalized over both minima. As part of this work, modifications to the interpolated PES construction scheme of Collins and co-workers are presented.

Path integrals with higher order actions: Application to realistic chemical systems

Publisher: AIP Publishing

Date: 16-02-2018

DOI: 10.1063/1.5000392

Abstract: Quantum thermodynamic parameters can be determined using path integral Monte Carlo (PIMC) simulations. These simulations, however, become computationally demanding as the quantum nature of the system increases, although their efficiency can be improved by using higher order approximations to the thermal density matrix, specifically the action. Here we compare the standard, primitive approximation to the action (PA) and three higher order approximations, the Takahashi-Imada action (TIA), the Suzuki-Chin action (SCA) and the Chin action (CA). The resulting PIMC methods are applied to two realistic potential energy surfaces, for H2O and HCN–HNC, both of which are spectroscopically accurate and contain three-body interactions. We further numerically optimise, for each potential, the SCA parameter and the two free parameters in the CA, obtaining more significant improvements in efficiency than seen previously in the literature. For both H2O and HCN–HNC, accounting for all required potential and force evaluations, the optimised CA formalism is approximately twice as efficient as the TIA formalism and approximately an order of magnitude more efficient than the PA. The optimised SCA formalism shows similar efficiency gains to the CA for HCN–HNC but has similar efficiency to the TIA for H2O at low temperature. In H2O and HCN–HNC systems, the optimal value of the a1 CA parameter is approximately 13, corresponding to an equal weighting of all force terms in the thermal density matrix, and similar to previous studies, the optimal α parameter in the SCA was ∼0.31. Importantly, poor choice of parameter significantly degrades the performance of the SCA and CA methods. In particular, for the CA, setting a1 = 0 is not efficient: the reduction in convergence efficiency is not offset by the lower number of force evaluations. We also find that the harmonic approximation to the CA parameters, whilst providing a fourth order approximation to the action, is not optimal for these realistic potentials: numerical optimisation leads to better approximate cancellation of the fifth order terms, with deviation between the harmonic and numerically optimised parameters more marked in the more quantum H2O system. This suggests that numerically optimising the CA or SCA parameters, which can be done at high temperature, will be important in fully realising the efficiency gains of these formalisms for realistic potentials.

Photophysical oxidation of HCHO produces HO2 radicals

Publisher: Springer Science and Business Media LLC

Date: 06-07-2023

DOI: 10.1038/S41557-023-01272-4

Photochemical formation of HCO and CH3on the ground S0(A1′) state of CH3CHO

Publisher: AIP Publishing

Date: 06-02-2009

DOI: 10.1063/1.3070517

Abstract: The dynamics of the photodissociation of CH3CHO into CH3+HCO products have been investigated at energies between 30 953 and 31 771 cm−1, spanning the threshold for radical production on the triplet (T1) surface. A barrierless pathway to CH3+HCO radical products formed on the ground state (S0) surface was discovered and established to be an important reaction channel in acetaldehyde photodissociation throughout this wavelength range. HCO laser induced fluorescence (LIF) spectra recorded from CH3CHO dissociated above and below the T1 barrier energy are quite different HCO produced on S0 yields a more congested LIF spectrum with sharp rotational transitions, while HCO formed on the T1 surface displays fewer, more intense, Doppler-broadened lines. These differences have been further explored in the populations of the HCO Ka=1 doublets. Despite the upper and lower levels being almost isoenergetic, HCO formed on T1 preferentially populates the upper Kc state due to the geometry of the T1 transition state structure. In contrast, HCO formed on S0 produces equal population in each of the upper and lower Ka=1 components. Product state distributions (PSDs) showed that HCO formed on S0 is born with an approximately statistical distribution of population in the available product states, modeled well by phase space theory. HCO formed on the T1 surface, in contrast, has a PSD that can be characterized as arising from “impulsive” dynamics. Previous discrepancies in the height of the T1 barrier are discussed following the observation that, once the T1 channel is energetically accessible, there is competition between the S0 and T1 pathways, with the dominance of the triplet channel increasing with increasing photolysis energy.

Quantum effects and anharmonicity in the H2-Li+-benzene complex: A model for hydrogen storage materials

Publisher: AIP Publishing

Date: 16-12-2013

DOI: 10.1063/1.4831715

Abstract: Quantum and anharmonic effects are investigated in H2-Li+-benzene, a model for hydrogen adsorption in metal-organic frameworks and carbon-based materials. Three- and 8-dimensional quantum diffusion Monte Carlo (QDMC) and rigid-body diffusion Monte Carlo (RBDMC) simulations are performed on potential energy surfaces interpolated from electronic structure calculations at the M05-2X/6-31+G(d,p) and M05-2X/6-311+G(2df,p) levels of theory using a three-dimensional spline or a modified Shepard interpolation. These calculations investigate the intermolecular interactions in this system, with three- and 8-dimensional 0 K H2 binding enthalpy estimates, ΔHbind (0 K), being 16.5 kJ mol−1 and 12.4 kJ mol−1, respectively: 0.1 and 0.6 kJ mol−1 higher than harmonic values. Zero-point energy effects are 35% of the value of ΔHbind (0 K) at M05-2X/6-311+G(2df,p) and cannot be neglected uncorrected electronic binding energies overestimate ΔHbind (0 K) by at least 6 kJ mol−1. Harmonic intermolecular binding enthalpies can be corrected by treating the H2 “helicopter” and “ferris wheel” rotations as free and hindered rotations, respectively. These simple corrections yield results within 2% of the 8-dimensional anharmonic calculations. Nuclear ground state probability density histograms obtained from the QDMC and RBDMC simulations indicate the H2 molecule is delocalized above the Li+-benzene system at 0 K.

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.

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.

Modelling the interaction of molecular hydrogen with lithium-doped hydrogen storage materials

Publisher: Elsevier BV

Date: 12-2008

DOI: 10.1016/J.CPLETT.2008.10.081

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.

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.

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.

Relating environmental effects and structures, IR, and NMR properties of hydrogen-bonded complexes: ClH:pyridine

Publisher: American Chemical Society (ACS)

Date: 15-05-2001

DOI: 10.1021/JP004409M

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

Two roaming pathways in the photolysis of CH₃CHO between 328 and 308 nm

Publisher: Royal Society of Chemistry (RSC)

Date: 2014

DOI: 10.1039/C4SC02266A

Abstract: We attribute the two product-state distributions previously seen in CH 3 CHO photodissociation to CH 3 -roaming and H-roaming, unifying all previous experimental results.

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.

Experimental and Theoretical Investigation of Triple Fragmentation in the Photodissociation Dynamics of H₂CO

Publisher: American Chemical Society (ACS)

Date: 06-08-2013

DOI: 10.1021/JP404895Y

Abstract: The photodissociation dynamics of H2CO molecules at energies bracketing the triple fragmentation threshold were investigated using velocity map ion imaging of the H-atom fragments. An algorithm was developed to model the experimental results as a two-step process: initially barrierless C-H bond fission on the S0 potential energy surface to form H + HCO, followed by secondary fragmentation of those HCO radicals with sufficient internal energy to overcome the small exit channel barrier on the HCO surface to form H + CO. Our model treats the first step using phase space theory (PST) and the second using a combined PST-impulsive model, with a tunneling correction. Experimentally, triple fragmentation reaches 25% of the radical (H + HCO) channel photochemical yield at energies about 1500 cm(-1) above the barrier for breaking the second bond. In addition, the triplet (T1) channel appears to reduce in importance after the barrier on the T1 surface is exceeded, slowly decreasing to 7000 cm(-1) of available energy.

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.

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.

Discovery Projects - Grant ID: DP0560020

Start Date: 04-2005

End Date: 03-2014

Amount: $195,000.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP0665065

Start Date: 01-2006

End Date: 12-2010

Amount: $640,000.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP180103715

Start Date: 2018

End Date: 12-2023

Amount: $391,774.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP160101792

Start Date: 2016

End Date: 03-2019

Amount: $497,900.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP190102013

Start Date: 2019

End Date: 06-2022

Amount: $520,000.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP220102466

Start Date: 07-2022

End Date: 06-2025

Amount: $508,322.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP130104326

Start Date: 2013

End Date: 12-2016

Amount: $420,000.00

Funder: Australian Research Council

Special Research Initiatives - Grant ID: SR0354583

Start Date: 06-2004

End Date: 12-2004

Amount: $10,000.00

Funder: Australian Research Council