Isotopomer Fractionation in the UV Photolysis of N_2O: 3. 3D Ab Initio Surfaces and Anharmonic Effects

The wavelength-dependent isotopic fractionation of N_2O is calculated, extending our previous work, Parts 1 and 2, in several aspects: (1) the fully three-dimensional ab initio electronic potential and transition dipole moment surfaces of S. Nanbu and M. S. Johnson (J. Chem. Phys. A 2004, 108, 8905) are used to calculate the absorption cross sections, instead of a 2D surface and (2) the vibrational frequencies and wave functions with anharmonicity correction are used for the ground electronic state. The results for the absorption spectrum and for the isotopic fractionation of the different isotopomers are discussed. One difference between experiments measuring the absorption coefficient (von Hessberg et al. Atmos. Chem. Phys. 2004, 4, 1237) and the others that measure instead the photodissociation is also discussed. Experiments on the quantum yield for wavelengths longer than 200 nm (>50000 cm^(−1)) would be helpful in treating the observed difference

He*(23S) penning ionization of H2S. II. Formation of the SH+(A3∏) and H2S +(Ã2A1) ions

Emissions in the 200–750 nm region produced by the collision of He*(2 3S) with H2S were studied under single-collision conditions. The hydrogen Balmer lines and the SH+(A 3Π–X 3Σ-) and H2S+(A˜ 2A1–X˜ 2B1) bands were assigned. The total emission cross section (σem) was evaluated to be (1.7±0.3)×10-20 m2 at a collision energy of 150 meV. The σems of the SH+(A–X) and H2S+(A˜ –X˜ ) bands decreased with increase in the collision energy in the 115–200 meV range, indicating that attractive forces are effective for the incident channels with regard to the formation of these species. The rotational distribution of SH+(A 3Π,ν'=0) is represented by a Boltzmann temperature of 870±80 K. The H2S+(A˜ 2A1–X˜ 2B1) emission, which was assigned for the first time in the Penning ionization of H2S, primarily consists of the bending progressions. The internal populations of H2S+(A˜ ) were analyzed using the vibrational energies and Einstein's A coefficients calculated in this study. The details of the calculation and derived spectroscopic constants are reported in the accompanying paper, Paper I. The populations obtained for the bending vibration (ν' 2) of H2S+(A˜ ) show an inverted distribution with a peak at ν' 2=3. This distribution is shifted lower compared that with a peak at ν' 2=4 – 5 observed by He*(2 3S) Penning ionization electron spectroscopy and that with a peak at ν' 2=6 – 7 predicted by the theoretical Franck–Condon factors for the H2S(X˜ )–H2S1(A˜ ) ionization. The origin of the difference is discussed concerning the formation mechanism of H2S1(A˜ 2A1)

He*(23S) Penning ionization of H2S. I. Theoretical Franck-Condon factors for the H2S(X̃ 1A1, v′ = 0)→H2S+ (X̃2B1, Ã2A1) ionization and H2S+(Ã-X̃) transition

In order to elucidate the ionization dynamics, in particular the vibrational distribution, of H2S+(A˜ ) produced by the Penning ionization of H2S with He*(2 3S) atoms, the Franck–Condon factors (FCFs) were presented for the H2S(X˜ )!H2S+(X˜ ,A˜ ) ionization and the H2S+(A˜ –X˜ ) transition, and Einstein's A coefficients were presented for the latter transition. The FCFs were obtained by quantum vibrational calculations using the global potential energy surfaces (PESs) of H2S(X˜ 1A1) and H2S+(X˜ 2B1 ,A˜ 2A1 ,B˜ 2B2) electronic states. The global PESs were determined by the multireference configuration interaction calculations with the Davidson correction and the interpolant moving least squares method combined with the Shepard interpolation. The obtained FCFs exhibit that the H2S+(X˜ ) state primarily populates the vibrational ground state since its equilibrium geometry is almost equal to that of H2S(X˜ ), while the bending mode (ν 2) is strongly enhanced for the H2S+(A˜ ) state; the maximum in the population is around ν 2=6 – 7. In the same manner, the bending progressions are expected to consist of the great part of the H2S+(A˜ –X˜ ) emission. A detailed comparison with the experimental study for this system is reported in the accompanying paper, Paper II

Isotope effects in the dissociation of the B̃1A, state of SiH2, SiHD, and SiD2 using three-dimensional wave packet propagation

Dissociations after the A˜ 1B1→B˜ 1A1 photoexcitation of SiH2, SiHD, and SiD 2 were studied to investigate excited-state dynamics and effects of the initial vibrational state. The cross section (σ) for the photodissociation relative to SiH2(B˜)→Si( 1D)+H2 and the rovibrational population of the H2 fragment were computed using the wave packet propagation technique based on the three-dimensional potential energy surfaces (PESs) of the A˜ and B˜ electronic states and the transition dipole surfaces, which were reported in our previous paper [J. Chem. Phys. 122, 144307 (2005)]. The photodissociation spectrum consists of a broadband and a number of sharp peaks. For SiH2 and SiD2, the sharp peaks correspond to the resonance structure of the vibrational levels of the B˜ state and the broadbands are nearly independent of the photon energy. The broadband for SiHD increases steeply with the photon energy above 30 000 cm-1. The flux leaving the computational grid for SiH2 and SiD2 consists of at least two components, whereas that for SiHD consists of only a faster component. These large isotope effects were discussed based on the valley to the dissociation channel on PES and the difference in the position of the initial wave packet for three isotopomers

Vibrational energies for the X̃1 A1, Ã1 B1, and B̃1 A1 states of SiH2/SiD2 and related transition probabilities based on global potential energy surfaces

Transition probabilities were evaluated for the X˜ 1A1-A˜ 1B1 and A˜ 1B1-B˜ 1A1 systems of SiH2 and SiD2 to analyze the X˜→A˜→B˜ photoexcitation. The Franck–Condon factors (FCFs) and Einstein's B coefficients were computed by quantum vibrational calculations using the three-dimensional potential energy surfaces (PESs) of the SiH2(X˜ 1A1 ,A˜ 1B1 ,B˜ 1A1) electronic states and the electronic transition moments for the X˜ -A˜, X˜ -B˜, and A˜ -B˜ system. The global PESs were determined by the multireference configuration interaction calculations with the Davidson correction and the interpolant moving least-squares method combined with the Shepard interpolation. The obtained FCFs for the X˜ -A˜ and A˜ -B˜ systems exhibit that the bending mode is strongly enhanced in the excitation since the equilibrium bond angle greatly varies with the three states; the barrier to linearity is evaluated to be 21 900 cm-1 for the X˜ state, 6400 cm-1 for the A˜ state, and 230–240 cm-1 for the B˜ state. The theoretical lifetimes for the pure bending levels of the A˜ and B˜ states were calculated from the fluorescence decay rates for the A˜ -X˜, B˜ -A˜, and B˜ -X˜ emissions

QM/MM trajectory surface hopping approach to photoisomerization of rhodopsin and isorhodopsin: The origin of faster and more efficient isomerization for rhodopsin

The photoinduced cis-trans isomerization dynamics of rhodopsin and isorhodopsin are studied using a newly developed hybrid QM/MM trajectory surface hopping MD scheme based on the Zhu-Nakamura theory for nonadiabatic transitions. Rhodopsin and isorhodopsin have 11-cis and 9-cis forms of retinal as chromophore and the two proteins are isomerized to bathorhodopsin enclosing the all-trans form. The simulation reproduced faster and more efficient isomerization in rhodopsin than in isorhodopsin. In the excited state, rhodopsin shows a straightforward dynamics, whereas isorhodopsin dynamics is rather complicated and in a back-and-forth manner. The latter complicated dynamics would be mainly due to a narrow space near the active dihedral angle =C8-C9=C10-C11= (ø9) created by Thr 118 and Tyr 268 in opsin. Rhodopsin gives bathorhodopsin only while isorhodopsin yields a byproduct. The rigorous selectivity in rhodopsin would be another reason why rhodopsin is selected biologically. Comparison with our previous opsin-free investigations reveals that opsin tends to confine the twist of the active dihedral to only one direction and funnels transitions into the vicinity of minimum energy conical intersections (MECI). The twist-confinement totally blocks simultaneous twisting of ø9 and ø11 (=C10-C11=C12-C13=) and enhances the quantum yields. The opposite rotation of ø9 and ø11 ( wring-a-wet-towel motion) takes place upon photoexcitation, which also does without opsin. The wring-a-wet-towel motion is dynamically enhanced in comparison with the one expected from locations of the MECI. The present simulation reveals that the Weiss-Warshel model for cis-trans photoisomerization is not applicable for rhodopsin because the branching ratio after transition is crucial. © 2012 American Chemical Society