Quantum chemistry studies of electronically excited nitrobenzene, TNA, and TNT.

The electronic excitation energies and excited-state potential energy surfaces of nitrobenzene, 2,4,6-trinitroaniline (TNA), and 2,4,6-trinitrotoluene (TNT) are calculated using time-dependent density functional theory and multiconfigurational ab initio methods. We describe the geometrical and energetic character of excited-state minima, reaction coordinates, and nonadiabatic regions in these systems. In addition, the potential energy surfaces for the lowest two singlet (S(0) and S(1)) and lowest two triplet (T(1) and T(2)) electronic states are investigated, with particular emphasis on the S(1) relaxation pathway and the nonadiabatic region leading to radiationless decay of S(1) population. In nitrobenzene, relaxation on S(1) occurs by out-of-plane rotation and pyramidalization of the nitro group. Radiationless decay can take place through a nonadiabatic region, which, at the TD-DFT level, is characterized by near-degeneracy of three electronic states, namely, S(1), S(0), and T(2). Moreover, spin-orbit coupling constants for the S(0)/T(2) and S(1)/T(2) electronic state pairs were calculated to be as high as 60 cm(-1) in this region. Our results suggest that the S(1) population should quench primarily to the T(2) state. This finding is in support of recent experimental results and sheds light on the photochemistry of heavier nitroarenes. In TNT and TNA, the dominant pathway for relaxation on S(1) is through geometric distortions, similar to that found for nitrobenzene, of a single ortho-substituted NO(2). The two singlet and lowest two triplet electronic states are qualitatively similar to those of nitrobenzene along a minimal S(1) energy pathway.

A quantum chemistry study of Diels-Alder dimerizations in benzene and anthracene.

There is considerable experimental evidence of covalent dimerization of aromatic compounds occurring under shock conditions. Because of their endothermicity, these reactions could play a large role in the shock initiation process of aromatic molecular explosives such as 2,4,6-trinitrotoluene and 1,3,5-triamino-2,4,6-trinitrobenzene by withdrawing energy from the shock compression. Very little is known about the energetics, however, and this knowledge is crucial for the design of empirical force fields that can treat shock-induced chemistry. We have employed ab initio electronic structure and density functional methods to study the Diels-Alder (DA) dimerizations of benzene and anthracene. The enthalpy of reaction for DA benzene dimerization is predicted to be +35.9 kcal/mol. The stepwise pathway to this dimer involves formation of a stable triplet intermediate that requires 71.8 kcal/mol of energy. Transition states along both the concerted and stepwise pathways were optimized and the energetics of the reaction pathways are detailed. The former is found to be the energetically preferred mechanism. Nine DA dimers of anthracene were found, with six predicted to have dimerization DeltaH(rxn)'s of 24-55 kcal/mol, two with dimerization energies near zero and one that is formed through an exothermic reaction. Twelve triplet dimers of anthracene, with DeltaH(rxn)'s ranging from 33-50 kcal/mol, are also described. Finally, the potential importance of these reactions in the context of shock compression of these materials is discussed.

Combined DFT and electrostatics study of the proton pumping mechanism in cytochrome c oxidase.

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.

Anharmonic vibrational properties of explosives from temperature-dependent Raman.

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.

DFT/electrostatic calculations of pK(a) values in cytochrome c oxidase.

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.

Vibrational spectrum, ab initio calculations, conformational equilibria and torsional modes of 1,3-dichloropropane.

Ab initio calculations are reported for three of four possible conformers of 1,3-dichloropropane. The fourth conformer, with Cs symmetry, has a predicted enthalpy difference of more than 1500 cm(-1) from the most stable conformer from each calculation regardless of the basis set used, so there is little chance of observing it. Thus, there is no evidence in the infrared or Raman spectrum of the presence of a fourth conformer. The order of stability given by the ab initio calculations is C2(GG)>C1(AG)>C2v(AA)>Cs(GG'), where A indicates the anti form for one of the CH2Cl groups and G indicates the gauche conformation for the other CH2Cl group relative to the plane of the carbon atoms. Almost every band observed can be confidently assigned to one or another of the conformers. Many observed bands proved to be of a composite nature, with several nearly coincident vibrations of different conformers contributing to the band contour. Nonetheless, a complete assignment of fundamentals is possible for the most stable C2 conformer, and 5 of the fundamentals of the C2v conformer and 13 those of the C1 conformer can be confidently assigned.

Mechanism and dynamics of azobenzene photoisomerization.

The excited-state dynamics of trans-azobenzene were investigated by femtosecond time-resolved photoelectron spectroscopy and ab initio molecular dynamics. Two near-degenerate pipi* excited states, S2 and S3,4, were identified in a region hitherto associated with only one excited state. These results help to explain contradictory reports about the photoisomerization mechanism and the wavelength dependence of the quantum yield. A new model for the isomerization mechanism is proposed.