The speed, angular, and alignment distributions of S(1D2) atoms from the ultraviolet photodissociation of OCS have been measured by a photofragment imaging technique. From the excitation wavelength dependence of the scattering distribution of S(1D2), the excited states accessed by photoabsorption were assigned to the A′ Renner–Teller component of the  1Δ and the A″(1Σ−) states. It was found that the dissociation from the A′ state gives rise to high- and low-speed fragments, while the A″ state only provides the high-speed fragment. In order to elucidate the dissociation dynamics, in particular the bimodal speed distribution of S atoms, two-dimensional potential energy surfaces of OCS were calculated for the C–S stretch and bending coordinates by ab initio molecular orbital (MO) configuration interaction (CI) method. Conical intersections of 1Δ and 1Σ− with 1Π were found as adiabatic dissociation pathways. Wave packet calculations on these adiabatic surfaces, however, did not reproduce the low-speed component of S(1D2) fragments. The discrepancy regarding the slow S atoms was attributed to the dissociation induced by nonadiabatic transition from A′(1Δ) to A′(1Σ+) in the bending coordinate. This hypothesis was confirmed by wave packet calculations including nonadiabatic transitions. The slow recoil speed of S atoms in the nonadiabatic dissociation channel is due to more efficient conversion of bending energy into CO rotation than the adiabatic dissociation on the upper state surface. By analyzing the experimental data, taking into account the alignment of S(1D2) atoms, we determined the yield of the nonadiabatic transition from the A′(1Δ) to the ground states to be 0.31 in the dissociation at 223 nm. Our theoretical model has predicted a prominent structure in the absorption spectrum due to a Feshbach resonance in dissociation, while an action spectrum of jet-cooled OCS measured by monitoring S(1D2) exhibited only broad structure, indicating the limitation of our model calculations.

Reaction mechanisms of the ultrafast photoisomerization between cyclohexadiene and hexatriene have been elucidated by the quantum dynamics on the ab initio potential energy surfaces calculated by multireference configuration interaction method. In addition to the quantum wave-packet dynamics along the two-dimensional reaction coordinates, the semiclassical analyses have also been carried out to correctly estimate the nonadiabatic transition probabilities around conical intersections in the full-dimensional space. The reaction time durations of radiationless decays in the wave-packet dynamics are found to be generally consistent with the femtosecond time-resolution experimental observations. The nonadiabatic transition probabilities among the ground (S0), first (S1), and second (S2) excited states have been estimated by using the semiclassical Zhu-Nakamura formula considering the full-dimensional wave-packet density distributions in the vicinity of conical intersections under the harmonic normal mode approximation. The cyclohexadiene (CHD) ring-opening process proceeds descending on the S1(1 1B) potential after the photoexcitation. The major part of the wave-packet decays from S1(1 1B) to S1(2 1A) by the first seam line crossing along the C2-symmetry-breaking directions. The experimentally observed ultrafast S1-S0 decay can be explained by the dynamics through the S1-S0 conical intersection along the direction toward the five-membered ring. The CHD: hexatriene (HT) branching ratio is estimated to be approximately 5:5, which is in accordance with the experiment in solution. This branching ratio is found to be mainly governed by the location of the five-membered ring S1-S0 conical intersection along the ground state potential ridge between CHD and HT.

On-the-fly classical dynamics calculations combined with ab initio quantum chemical computations are carried out for two models of protonated Schiff base retinal in vacuo. The models are the 6pi system of 2-cis-penta-2,4-dieneimminium cation and the 12pi system in which two methyl groups are removed from the Schiff base of retinal. The CASSCF(6,6) level with the 6-31G basis set was employed for the quantum chemical part and the velocity Verlet algorism is utilized for time evolution of trajectories. The probabilities of nonadiabatic transition between the excited and ground state are estimated by the Zhu-Nakamura formulas. The 9-cis form product in addition to the all-trans one is generated in the present gas phase calculation for the 12pi model, despite the 9-cis generation being suppressed in protein. We have found that energy relaxation on the ground state occurs in two steps in the 12pi model. In the first step a metastable intermediate state is formed at approximately 100 fs after photoexcitation at the energy around 20-40 kcal/mol down from the excited potential energy surface, then it further relaxes to the energy around 60-80 kcal/mol from the excited surface, leading to the final state (second step). This relaxation pattern can be seen in all the three pathways to the all-trans, 9-cis, and (reverted) 11-cis form. Fourier transformation analysis reveals that the effective vibrational frequencies of the intermediate state are 1600-2000 cm(-1), which can be attributed to the conjugate CC bond frequencies in the electronic ground state. The two-step relaxation may be due to dynamical barriers. The two-step relaxation is not revealed in the smaller 6pi model. The crank-shaft motion of the C11C12 and C9C10 bonds is found in the isomerization, which indicates the motion is intrinsic in retinal, not due to the surrounding protein. The branching ratio is about 1:1:2 for the all-trans, 9-cis, and 11-cis form generation. The ratio is different from earlier works where Tully's fewest switching scheme was employed. The bond length and the dihedral angle at the transitions are also analyzed to investigate the transition mechanism.

16 O) were computed using the wave packet propagation technique to explore the influence of excited-state dynamics, transition dipole surface, and initial vibrational state. Three-dimensional potential energy surfaces for the electronic states of N 2 O related to the experimentally observed photoabsorption between 170 and 220 nm were calculated using the ab initio molecular orbital configuration interaction method. The transition dipole moment surfaces between these states were also calculated. Numerous wave packet simulations were carried out and used to calculate the temperature-dependent photodissociation cross sections of the six isotopically substituted species. The photolytic isotopic fractionation constants determined using the calculated cross sections are in good agreement with recent experiments. The results show that, in addition to the effect of the changed shape of the ground-state vibrational wave function with isotopic substitution, photodissociation dynamics play a central role in determining isotopic fractionation constants.

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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═ (ϕ9) 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 ϕ9 and ϕ11 (═C10-C11═C12-C13═) and enhances the quantum yields. The opposite rotation of ϕ9 and ϕ11 ("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.

[1] We report measurements of the ultraviolet absorption cross-sections of 32 cross-sections and is in good agreement concerning fine structure and peak widths, with localized differences at the peak maxima when isotope effects are taken into account. SO 2 samples were produced in an identical process via combustion of isotopically enriched S 0 , eliminating effects due to variation in oxygen isotopic composition. Peak positions for the rare isotopologues are red shifted relative to the 32 SO 2 isotopologue. Starting at the origin the shift increases linearly through the band. A linear shift model based on the spectrum of 32 SO 2 was used to estimate the cross-sections of 33,34,36 SO 2 ; the average of the wavelength resolved absolute difference between the modeled and experimental spectra is 77.4, 107 and 139 ‰ respectively. While the peak-to-valley amplitude of 36 SO 2 tends to be smaller than the other isotopologues throughout the spectrum, integrated band intensities for all isotopologues are conserved to within 4% relative to 32 SO 2 . The cross-sections were used in a photochemical model to obtain fractionation constants to compare with photochemical chamber experiments. We conclude that planetary atmospheres will exhibit isotopic fractionation from both photoexcitation and photodissociation, and that experiments in the literature have isotopic imprints arising from both the B

Various novel acridinium ester derivatives having phenyl and biphenyl moieties were synthesized, and their optimal chemiluminescence conditions were investigated. Several strongly chemiluminescent acridinium esters under neutral conditions were found, and then these derivatives were used to detect hydrogen peroxide and glucose. Acridinium esters having strong electron-withdrawing groups such as cyano, methoxycarbonyl, and nitro at the 4-position of the phenyl moiety in phenyl 10-methyl-10λ-acridine-9-carboxylate trifluoromethanesulfonate salt showed strong chemiluminescence intensities. The chemiluminescence intensity of 3,4-dicyanophenyl 10-methyl-10λ-acridine-9-carboxylate trifluoromethanesulfonate salt was approximately 100 times stronger than that of phenyl 10-methyl-10λ-acridine-9-carboxylate trifluoromethanesulfonate salt at pH 7. The linear calibration ranges of hydrogen peroxide and glucose were 0.05-10 mM and 10-2000 μM using 3,4-(dimethoxycarbonyl)phenyl 10-methyl-10λ-acridine-9-carboxylate trifluoromethanesulfonate salt at pH 7 and pH 7.5, respectively. The proposed chemiluminescence reaction mechanism of acridinium ester via a dioxetanone structure was evaluated via quantum chemical calculation on density functional theory. The proposed mechanism was composed of the nucleophilic addition reaction of hydroperoxide anion, dioxetanone ring formation, and nonadiabatic transition due to spin-orbit coupling around the transition state (TS) to the triplet state (T) following the decomposition pathway. The TS which appeared in the thermal decomposition would be a rate-determining step for all three processes.