Shinkoh Nanbu

NONADIABATIC BENDING DISSOCIATION IN 16 VALENCE ELECTRON SYSTEM OCS

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 compone...

Ab initio nonadiabatic quantum dynamics of cyclohexadiene/hexatriene ultrafast photoisomerization.

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.

Nonadiabatic ab Initio Dynamics of Two Models of Schiff Base Retinal

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 Tullys fewest switching scheme was employed. The bond length and the dihedral angle at the transitions are also analyzed to investigate the transition mechanism.

Nonadiabatic ab Initio Dynamics of a Model Protonated Schiff Base of 9-cis Retinal

The dynamics of the photoisomerization of a model protonated Schiff base of 9-cis retinal in isorhodopsin is investigated using nonadiabatic molecular dynamics simulation combined with ab initio quantum chemical calculations on-the-fly. The quantum chemical part is treated at the complete-active space self-consistent field level for six electrons in six active pi orbitals with the 6-31G basis set (CASSCF(6,6)/6-31G). The probabilities of nonadiabatic transitions between the S(1) ((1)pipi*) and S(0) states are estimated in light of the Zhu-Nakamura theory. The photoinduced cis-trans isomerization of 9-cis retinal proceeds slower than that of its 11-cis analogue and at a lower quantum yield, confirming experimental observations. An energetic barrier in the excited state impedes the elongation and twist of the C(9)=C(10) stretch and torsion coordinates, respectively, resulting in the trapping of trajectories before transition. Consequently, the isomerization takes longer time and the transition more often occurs at smaller twist angle of =C(8)-C(9)=C(10)-C(11)=, which leads to regeneration of the 9-cis reactant. Thus, neither the smaller twist observed in the X-ray crystal nor the slower movement of nuclei in the transition region would be the main reason for the longer reaction time and lower yield. A well-known space-saving asynchronous bicycle pedal or crankshaft photoisomerization mechanism is found to be operational in 9-cis retinal. The simulation in vacuo suggests that the excited-state barrier and the photoisomerization itself are intrinsic properties of the visual chromophore and not triggered mainly by the protein environment that surrounds the chromophore.

Millimeter-wave spectroscopy of the internal-rotation band of the He-HCN complex and the intermolecular potential energy surface

Millimeter-wave absorption spectroscopy combined with a pulsed-jet expansion technique was applied to measure the internal-rotation band of He–HCN in the frequency region of 95–125 GHz. In total 13 rovibrational lines, split into nitrogen nuclear hyperfine structure, were observed for the fundamental internal-rotation band, j=1−0. The observed transition frequencies were analyzed including their hyperfine splitting to yield an intermolecular potential energy surface, as improved from the one given by a coupled-cluster single double (triple) ab initio calculation. The surface obtained has a global minimum in the linear configuration (He⋅⋅⋅H–C–N) with a well depth of 30.2 cm−1, and a saddle point located in the antilinear configuration (H–C–N⋅⋅⋅He) which is higher by 8.91 cm−1 in energy than the global minimum. The distance Rm between the He atom and the center of mass of HCN along the minimum energy path shows a strong angular dependence; Rm is 4.169 and 4.040 A in the linear and antilinear forms, respecti...

Theoretical studies of absorption cross sections for the C̃ B12-X̃ A11 system of sulfur dioxide and isotope effects

The C (1)B(2)-X (1)A(1) photoexcitation of SO(2) was studied to investigate excited-state dynamics and the effects of the initial vibrational state. Ultraviolet photoabsorption cross sections (sigmas) of seven isotopologues ((32)S (16)O(2), (33)S (16)O(2), (34)S (16)O(2), (36)S (16)O(2), (32)S(16)O(17)O, (32)S(16)O(18)O, (34)S(16)O(18)O) were computed using the wave packet propagation technique based on the three-dimensional potential energy surfaces of the X and C states, which were calculated using the ab initio molecular orbital configuration interaction method. Numerous wave packet simulations were carried out under the adiabatic approximation and used to calculate the sigmas of the seven isotopologues at 298 K; we concluded that the absorption spectrum of SO(2) can be reliably modeled within the adiabatic framework based on the analysis of the time evolution of the wave packet. The calculated sigmas are in reasonable agreement with the recent experiment in the 190-228 nm region, and the isotope shifts of the peaks for (33)S (16)O(2) and (34)S (16)O(2) relative to the corresponding peaks for (32)S (16)O(2) are in good agreement with the observed data. Relative to the sigma of (32)S (16)O(2), isotopic substitution shows a significant increment for those of (34)S (16)O(2) and (36)S (16)O(2) in the 190-228 nm region. This trend is consistent with the observed data.

Laser control of reactions of photoswitching functional molecules

Laser control schemes of reactions of photoswitching functional molecules are proposed based on the quantum mechanical wave-packet dynamics and the design of laser parameters. The appropriately designed quadratically chirped laser pulses can achieve nearly complete transitions of wave packet among electronic states. The laser parameters can be optimized by using the Zhu-Nakamura theory of nonadiabatic transition. This method is effective not only for the initial photoexcitation process but also for the pump and dump scheme in the middle of the overall photoswitching process. The effects of momentum of the wave packet crossing a conical intersection on the branching ratio of products have also been clarified. These control schemes mentioned above are successfully applied to the cyclohexadiene/hexatriene photoisomerization (ring-opening) process which is the reaction center of practical photoswitching molecules such as diarylethenes. The overall efficiency of the ring opening can be appreciably increased by using the appropriately designed laser pulses compared to that of the natural photoisomerization without any control schemes.

Theoretical study of photophysical properties of bisindolylmaleimide derivatives.

The photophysical properties of two bisindolylmaleimide derivatives, 3,4-bis(3-indolyl)-1-H-pyrrole-2,5-dione (arcyriarubin A) and indolo[2,3-a]pyrrolo[3,4-c] carbazole-5,7-(6 H)-dione (arcyriaflavin A), are investigated by using ab initio molecular orbital (MO) and multireference perturbation theory. These compounds are suggested to exist as monovalent anions deprotonated from an indole NH group in aprotic polar solvents. The analysis of MOs shows that the electronic structures of the S(1) and S(2) states are described by the single- or double-electron excitation between the naturally localized MOs on an indole moiety and on the maleimide part. This indicates that the intramolecular charge transfer (ICT) transfer may occur by photoexcitation. The minimum-energy structure of the arcyriarubin A anion is twisted; the dihedral angles between the indole and maleimide rings are 83.4 degrees and 20.2 degrees for the S(1) and S(0) states, respectively. The analysis of the minimum energy path along the coordinate of the twist angle is performed to explore the emission process from the S(1) state. It has been shown that the magnitude of the Stokes shift increases with increasing the twist angle, but the oscillator strength decreases. It has been suggested that the experimentally observed fluorescence arises on the way toward the energy minimum of the S(1) state. The Stokes-shifted emission of arcyriaflavin A is contributed by the S(1)-S(0) electronic relaxation after the excitation in the S(2) state.

Carbonyl sulfide isotopologues: ultraviolet absorption cross sections and stratospheric photolysis.

Ultraviolet absorption cross sections of the main and substituted carbonyl sulfide isotopologues were calculated using wavepacket dynamics. The calculated absorption cross section of (16)O(12)C(32)S is in very good agreement with the accepted experimental spectrum between 190 and 250 nm. Relative to (16)O(12)C(32)S, isotopic substitution shows a significant enhancement of the cross section for (16)O(13)C(32)S, a significant reduction for (18)O(12)C(32)S and (17)O(12)C(32)S and almost no change for the sulfur isotopologues (16)O(12)C(33)S, (16)O(12)C(34)S, and (16)O(12)C(36)S. The analysis of the initial wavepackets shows that these changes can be explained in terms of the change in the norm of the initial wavepacket. Implications for our understanding of the stratospheric sulfur cycle are discussed.

Molecular switching in one-dimensional finite periodic nonadiabatic tunneling potential systems

A new idea of molecular switching is presented. The idea is based on the intriguing phenomenon of complete reflection, which occurs in a two-state potential curve crossing of the nonadiabatic tunneling type. Complete switching of transmission is theoretically possible in one-dimensional systems by introducing impurities in the system. The basic semiclassical theory is presented, and the phenomenon of complete reflection is clearly interpreted and numerically demonstrated. An idea of an energy filter to facilitate the switching efficiently is also introduced. The possibility of bound states in the continuum is also clarified. This new molecular switching is numerically realized by the wave packet propagation.