No abstract

The ab initio multiple spawning (AIMS) method is a time-dependent formulation of quantum chemistry, whereby the nuclear dynamics and electronic structure problems are solved simultaneously. Quantum mechanical effects in the nuclear dynamics are included, especially the nonadiabatic effects which are crucial in modeling dynamics on multiple electronic states. The AIMS method makes it possible to describe photochemistry from first principles molecular dynamics, with no empirical parameters. We describe the method and present the application to two molecules of interest in organic photochemistrysethylene and cyclobutene. We show that the photodynamics of ethylene involves both covalent and ionic electronic excited states and the return to the ground state proceeds through a pyramidalized geometry. For the photoinduced ring opening of cyclobutene, we show that the disrotatory motion predicted by the Woodward-Hoffmann rules is established within the first 50 fs after optical excitation.

/npsi/ctrl?action=rtdoc&an=12338313&lang=en http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=12338313&lang=fr READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/jsp/nparc_cp.jsp?lang=en Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

The photochemistry of stilbene is investigated using ab initio quantum chemistry with complete active space self-consistent field (CASSCF) and multireference perturbation theory (CASPT2) methods. We characterize photoisomerization pathways from both the cis and trans isomers, including a minimal energy conical intersection. Similarities to photoisomerization in ethylene are found and emphasized. In contrast to traditional one-dimensional models of stilbene photoisomerization, torsion and pyramidalization are required to reach the minimal energy conical intersection which is expected to dominate in quenching to the ground electronic state. This intersection is characterized as an interaction between charge transfer and covalent states. The present results suggest that the qualitative features of the photoisomerization dynamics elucidated for ethylene can also be expected to apply to stilbene, and call for reconsideration and refinement of the photoisomerization mechanism in stilbene.

We apply multireference ab initio quantum chemistry and microcanonical transition state (RRKM) theory with quantum energy flow corrections from local random matrix theory (LRMT) to determine the kinetics of trans-stilbene photoisomerization. With a single ab initio potential energy surface and no adjustable parameters, simultaneous agreement with experiment of the microcanonical isomerization rates for the d 0 , d 2 , d 10 , and d 12 isotopomers is obtained. We are also able to reproduce the pressure dependence of the thermal rate. Laser cooling effects on the isomerization rate are calculated and found to be quite small. The S 1 /S 2 energy gap at the transition state is found to be quite large (0.86 eV), suggesting that nonadiabatic effects are negligible. Using the ab initio results in a simple RRKM theory without corrections for finite quantum energy flow does not lead to agreement with experiment. We conclude that non-RRKM effects are essential to understand photoisomerization of trans-stilbene and that these can be predicted using LRMT.

Cytochrome c oxidase is a redox-driven proton pump which converts atmospheric oxygen to water and couples the oxygen reduction reaction to the creation of a membrane proton gradient. The structure of the enzyme has been solved; however, the mechanism of proton pumping is still poorly understood. Recent calculations from this group indicate that one of the histidine ligands of enzyme's CuB center, His291, may play the role of the pumping element. In this paper, we report on the results of calculations that combined first principles DFT and continuum electrostatics to evaluate the energetics of the key energy generating step of the model-the transfer of the chemical proton to the binuclear center of the enzyme, where the hydroxyl group is converted to water, and the concerted expulsion of the proton from delta-nitrogen of His291 ligand of CuB center. We show that the energy generated in this step is sufficient to push a proton against an electrochemical membrane gradient of about 200 mV. We have also re-calculated the pKa of His291 for an extended model in which the whole Fe(a3)-CuB center with their ligands is treated by DFT. Two different DFT functionals (B3LYP and PBE0), and various dielectric models of the protein have been used in an attempt to estimate potential errors of the calculations. Although current methods of calculations do not allow unambiguous predictions of energetics in proteins within few pKa units, as required in this case, the present calculation provides further support for the proposed His291 model of CcO pump and makes a specific prediction that could be targeted in the experimental test.

Using classical electrostatic calculations, earlier we examined the dependence of the protonation state of bovine cytochrome c oxidase (CcO) on its redox state. Based on these calculations, we have proposed a model of CcO proton pumping that involves His291, one of the Cu(B) histidine ligands, which was found to respond to redox changes of the enzyme Fe(a)(3)-Cu(B) catalytic center. In this work, we employ combined density functional and continuum electrostatic calculations to evaluate the pK(a)() values of His291 and Glu242, two key residues of the model. The pK(a) values are calculated for different redox states of the enzyme, and the influence of different factors on the pK(a)'s is analyzed in detail. The calculated pK(a)() values of Glu242 are between 9.4 and 12.0, depending on the redox state of the protein, which is in excellent agreement with recent experimental measurements. Assuming the reduced state of heme a(3), His291 of the oxidized Cu(B) center possesses a pK(a)() between 2.1 and 4.0, while His291 of the reduced Cu(B) center has a pK(a) above 17. The obtained results support the proposal that the His291 ligand of the Cu(B) center in CcO is a proton pump element.

Raman spectra from 50 to 3500 cm(-1) and 4-296 K are analyzed for molecular crystal powders of the explosives pentaerythritol tetranitrate (PETN), beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) and the inert naphthalene. Temperature-dependent Raman spectroscopy is utilized for its sensitivity to anharmonic couplings between thermally populated phonons and higher frequency vibrations relevant to shock up-pumping. The data are analyzed with anharmonic perturbation theory, which is shown to have significant fundamental limitations in application to real data. Fitting to perturbation theory revealed no significant differences in averaged anharmonicities among the three explosives, all of which exhibited larger averaged anharmonicities than naphthalene by a factor of 3. Calculations estimating the multiphonon densities of states also failed to correlate clearly with shock sensitivity. However, striking differences in temperature-dependent lifetimes were obvious: PETN has long lived phonons and vibrons, HMX has long lived phonons but short lived vibrons, while TATB has short lived phonons and vibrons at low temperature. Naphthalene, widely used as a model system, has significantly different anharmonicities and density of states from any of the explosives. The data presented suggest the further hypothesis that hindered vibrational energy transfer in the molecular crystals is a significant factor in shock sensitivity.