ORCID Profile
0000-0002-7150-560X
Current Organisations
Rikkyo University
,
Bharathiar University
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Publisher: American Chemical Society (ACS)
Date: 11-02-2015
DOI: 10.1021/OM500829Y
Publisher: Royal Society of Chemistry (RSC)
Date: 2016
DOI: 10.1039/C6MB00033A
Abstract: Theoretical and experimental studies of the deazaflavin dependent nitroreductase (Ddn) enzyme bound cofactor F 420 demonstrate that the reduced cofactor is deprotonated in the holoenzyme form.
Publisher: American Chemical Society (ACS)
Date: 17-01-2012
DOI: 10.1021/CT200768B
Abstract: The applicability of time-dependent density functional theory (TD-DFT) is tested in describing (1)La and (1)Lb π-π* states in indole, azaindole, indene, and benzimidazole. Several density functionals including popular three hybrid functionals (B3LYP, PBE0, and mPW1PW91), two meta-GGA functionals (M06-L and M06-2X), and four long-range corrected (CAM-B3LYP, ωB97XD, LC-BLYP, and LC-ωPBE) density functionals have been considered for the present study. The 6-311+G(2d,p) basis set incorporated with two sets of Rydberg sp functions for carbon and nitrogen atoms is utilized. The range-separation parameters for the calculations with the long-range corrected density functionals were tuned by enforcing the DFT version of Koopmans' theorem, and the effect of this tuning on the accuracy of the results is also examined. Results show that all of the hybrid and meta-GGA functionals predict a wrong order of (1)La and (1)Lb π-π* states in indole and azaindole. Although all of the LC functionals correctly predict that (1)Lb is the lowest excited state in indole, the energy gap calculated between the (1)Lb and (1)La state is much smaller than the value observed in the experimental studies. In the case of azaindole, only LC-ωPBE and LC-BLYP functionals could manage to reproduce the correct order of states however, here too, the calculated energy gap between the two π-π* states is very small compared to the experimental value. Overall, the (1)Lb state excitation energies derived with all of the functionals are overestimated. In contrast, all of the nine selected functionals correctly reproduce the order of states in indene and benzimidazole. The origin of this differing performance is analyzed. Also in the study, oscillator strengths and dipole moments of the excited states are derived, and two other important states, π-σ* and n-π* states, that could play important role in the photochemistry of these molecules are examined.
Publisher: American Chemical Society (ACS)
Date: 24-03-2011
DOI: 10.1021/JP200956B
Abstract: This paper presents a combined experimental and theoretical study of the electronic spectrum of the natural adamantane-type polyarsenical Arsenicin A. Experiments reveal that this molecule strongly absorbs UV light in the absence of an obvious chromophore. The observed absorbance is supported by the time-dependent density functional (TD-DFT) calculations with B3LYP, M06-L, and M06-2X functionals combined with the 6-311+G(3df,2pd) basis set, as well as by symmetry-adapted cluster/configuration interaction (SAC-CI) theory. The theoretical investigations reveal that the absorption is facilitated by through-space and through-bond interactions, between the lone pairs on the arsenic and oxygen atoms and the σ-bonding framework of the molecule, that destabilize occupied and stabilize unoccupied molecular orbitals.
Publisher: AIP Publishing
Date: 23-11-2015
DOI: 10.1063/1.4935578
Abstract: Electronic structures of azaindole were studied using symmetry-adapted cluster configuration interaction theory utilizing Dunning’s cc-pVTZ basis set augmented with appropriate Rydberg spd functions on carbon and nitrogen atoms. The results obtained in the present study show good agreement with the available experimental values. Importantly, and contrary to previous theoretical studies, the excitation energy calculated for the important n–π∗ state agrees well with the experimental value. A recent study by Pratt and co-workers concluded that significant mixing of π-π∗ and n-π∗ states leads to major change in the magnitude and direction of the dipole moment of the upper state vibrational level in the 0,0 + 280 cm−1 band in the S1←S0 transition when compared to that of the zero-point level of the S1 state. The present study, however, shows that all the four lowest lying excited states, 1Lb π-π∗, 1La π-π∗, n-π∗, and π-σ∗, cross each other in one way or another, and hence, significant state mixing between them is likely. The upper state vibrational level in the 0,0 + 280 cm−1 band in the S1←S0 transition benefits from this four-state mixing and this can explain the change in magnitude and direction of the dipole moment of the S1 excited vibrational level. This multistate mixing, and especially the involvement of π-σ∗ state in mixing, could also provide a route for hydrogen atom detachment reactions. The electronic spectra of benzimidazole, a closely related system, were also investigated in the present study.
Publisher: American Chemical Society (ACS)
Date: 25-04-2011
DOI: 10.1021/JP1093004
Abstract: An unknown species has been detected in the analysis of the products in a pyrolysis of polycarbonate using Li(+) ion-attachment mass spectrometry (IAMS). The mass spectra exhibited a Li(+) adduct peak at m/z 233 that was tentatively assigned to bisphenol A (BPA) biradical. Experimentally, this assignment was supported by the observation that the production rate increased under an inert nitrogen atmosphere. To further confirm the assignment, the stability of the BPA biradical to intramolecular rearrangement reactions as well as unimolecular decomposition has been analyzed via density functional theory calculations [B3LYP/6-311+G(3df,2p)]. The results show that the bisphenol A biradical is an open-shell biradical singlet that is stable to unimolecular decomposition. Although some of the proposed intramolecular rearrangement products have lower energies than those of the BPA diradical, these pathways have large reaction barriers and the kinetic lifetime of the radical is expected to be of the order of hours under the conditions of the experiment. The calculations also reveal that the bisphenol A diradical has large Li(+) affinities supporting the fact that these Li(+) complexes could be detected in the Li(+) ion attachment mass spectrometry. On the basis of these results the Li(+) adduct peak at m/z 233 detected in the pyrolysis of polycarbonate is assigned to the bisphenol A biradical.
No related grants have been discovered for Arulmozhiraja Sundaram.