Yoshiaki Amatatsu

Ab initio potential energy surfaces and trajectory studies of A‐band photodissociation dynamics: CH3I*→CH3+I and CH3+I*

Ab initio contracted spin–orbit configuration interaction (SOCI) calculations have been carried out to obtain potential energy surfaces of 3Q0 and 1Q1 excited states of methyl iodide as functions of all the geometrical parameters except for the three C–H stretches. The results are fitted to six‐dimensional diabatic potential functions and their couplings. Classical trajectory calculations have been performed using these potential functions. The rotation of the CH3 product in the I channel has been calculated to be perpendicular to the top axis and to have a peak at N=5 and extend up to N=8, whereas it is cold in the I* channel, in good agreement with recent experiments. The CH3 rotation is excited by the time trajectories arrive at the conical intersection region; this excitation is retained in the I‐channel product because the 1Q1 surface has a small bending force constant outside the conical intersection, whereas it is damped in the I* channel because 3Q0 still has a large bending force constant. The ca...

Full nine-dimensional ab initio potential energy surfaces and trajectory studies of A-band photodissociation dynamics: CH3I*→CH3+I, CH3+I*, and CD3I*→CD3+I, CD3+I*

The full nine‐dimensional potential energy surfaces (PESs) of the 3Q0 and 1Q1 states of CH3I have been calculated with the ab initio contracted spin–orbit configuration interaction method. The results are fitted to three diabatic potential terms and their couplings as functions of all the internal degrees of freedom. The transition dipole at the Franck–Condon region has also been calculated. Surface hopping quasiclassical trajectory calculations on these potential energy surfaces have been performed to examine the photodissociation dynamics of both CH3I and CD3I in the A‐continuum. The results are in general good agreement with the recent experimental findings. The reasonable I*/(I*+I) branching ratio can be obtained with these PESs when the contribution of direct transition to the 1Q1 state is considered. The rotational distribution of the CH3 and CD3 fragments and its I*/(I*+I)‐channel selectivity are determined by the shape of the PESs with respect to the bending angle outside the conical intersection ...

Ab initio potential energy surfaces and trajectory studies of A‐band photodissociation dynamics: ICN*→I+CN and I*+CN

The photodissociation reaction of ICN in the A continuum has been theoretically studied based on ab initio potential energy surfaces and classical trajectories. Ab initio contracted spin–orbit configuration interaction calculations have been carried out to obtain potential energy surfaces (PES’s) of 3Π1, 3Π0+ and 1Π1 excited states, where results are fit to five diabatic potential functions and their couplings as functions of all three internal degrees of freedom. The transition dipoles at the Franck–Condon region have also been calculated. All the PES’s involved in photodissociation are bent near the Franck–Condon region. Classical trajectory calculations performed on these potential surfaces have produced results that are in agreement with various experimental findings and provide a basis for their interpretation. The calculations indicate that the absorption is a mixture of parallel and perpendicular transition. A reasonable I/I* branching ratio can be obtained by considering the effect of initial bend...

Ab initio structure and wave packet dynamics of ICN photodissociation

We present a time‐dependent quantum mechanical calculation of ICN photodissociation in the A continuum, using the ab initio potential surfaces of Morokuma and co‐workers [S. Yabushita and K. Morokuma, Chem. Phys. Lett. 175, 518 (1990); Y. Amatatsu, S. Yabushita, and K. Morokuma, J. Chem. Phys. 100, 4894 (1994)]. Five excited state potential energy surfaces are included in this model, 3Π0+, 1Π1 (A’,A‘), and 3Π1 (A’,A‘), which are accessed, respectively, by parallel, perpendicular, and perpendicular transitions from the ground state. The calculated absorption spectrum, β parameters, the I/I* branching ratio, and the rotational product distribution are in good agreement with experiment. The I/I* branching ratio for photodissociation from vibrationally excited states of ICN has been calculated. The results are in good agreement with the recent measurements at different vibrational temperatures by Kash and Butler [P. W. Kash and L. J. Butler, J. Chem. Phys. 96, 8923 (1992)] at 249 nm but, interestingly, predi...

Ab initio study on the electronic structures of stilbene at the conical intersection

Abstract Ab initio complete-active-space self-consistent-field calculations have been performed to examine the electronic structures of stilbene at the conical intersection. At the conformation where only the central ethylenic bond is perpendicularly twisted, the energy gap between S 0 and S 1 is too large for stilbene to relax into S 0 . At the optimized geometry of the conical intersection, a covalent state with a diradical character is energetically close to a zwitter ionic state where the electron on one of the central ethylenic carbon atoms is transferred onto the other.

Coupled‐channel scattering calculations of ICN(ÖX̃) photodissociation using ab initio potentials

We report coupled‐channel scattering calculations of the ICN(A–X) photodissociation using the recent ab initio potential energy surfaces, diabatic coupling, and transition moments of Morokuma and co‐workers [Y. Amatatsu, S. Yabushita, and K. Morokuma, J. Chem. Phys. 100, 4894 (1994)]. We focus on transitions from the ground electronic state to the coupled 1Π1(5A’) and 3Π0+(4A’) states, the 4A‘ component of the 1Π1 state, and to the 3A’ component of the 3Π1 state, for ICN in the ground and first excited bend vibrational states. Total photodissociation cross sections to form I(2P3/2) and I*(2P1/2) are calculated over a large range of energies, and compared to experiment. Rotational distributions, i.e., partial cross sections, for the associated CN fragments are also calculated for numerous energies within the absorption profile, and compared with experiment, and the quasiclassical trajectory calculations of Amatatsu et al.

Theoretical design of a fluorene-based light-driven molecular rotary motor with constant rotation.

A fluorene-based light-driven molecular rotary motor with constant rotation has been designed by means of ab initio molecular orbital calculations. A model molecule is obtained by a chemical modification of 9-(5-methyl-2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene (MPCPF) which we reported recently. Despite that MPCPF has a great advantage that the thermal helical inversion proceeds with a much lower energy barrier than those in previous model molecules, a small energy difference between the M- and the P-helical isomers is possible to cause a fast equilibration between them after electronic relaxation around the conical intersection (CIX), and therefore, a backward rotation from the M-helical isomer is not always suppressed effectively. In order to overcome this defect of MPCPF, we modified MPCPF by a bridge of a pentamethylene chain between the 2 position of the phenyl group and the psesudoaxial position of the C(5) atom in the 2-cyclopenten-1-ylidene ring. The modified molecule with a pentamethylene bridge (denoted by M5-PCPF) energetically destabilizes a conformation in the M-helical region and so passes through the M-helical region without any trap in the full rotary process, which leads to direct conversion from a stable P-helical isomer to another stable P-helical isomer via CIX. Therefore, M5-PCPF is expected to be a light-driven molecular rotary motor with constant rotation speed as well as unidirectionality.

Theoretical design of a light-driven molecular rotary motor with low energy helical inversion: 9-(5-methyl-2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene.

A light-driven molecular rotary motor of 9-(5-methyl-2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene (MPCPF) has been designed by means of ab initio complete active space self-consistent field and its second order multireference Møller-Plesset perturbation methods. In the present model molecule of MPCPF, 9H-fluorene (as a stator) and 5-methyl-2-phenyl-2-cyclopenten-1-ylidene (as a rotor) are directly linked with each other by a C═C double bond. Even by a substitution of phenyl group, MPCPF comes to have a stable P-helical MPCPF and a metastable M-helical MPCPF, and exhibits unidirectionality around the C═C double bond. In addition, interchange of the helicity can proceed with a low energy barrier through a floppy phenyl torsional motion. This is in contrast to previous light-driven molecular rotary motors where the unidirectionality is ensured by rigid and sterically overcrowded rotors. In the full rotary process of MPCPF, therefore, constancy of the rotation speed is expected to be much more improved as well as unidirectionality.

A theoretical study on the photochemical reaction of ICN in liquid Ar

Abstract ICN photochemical reaction in liquid Ar has been studied theoretically based on molecular dynamics simulation. The interaction potential functions between Ar and ICN in excited states as well as in the ground state were newly developed from ab initio calculation. Three types of reactions have been found for photodissociation starting from the 3 Π 0 + state, i.e. (i) a substantial fraction of ballistic dissociation to give mostly I ∗ ( 2 P 1 2 )+ CN , (ii) isomerization to INC, an isomer of ICN, and (iii) recombination to ICN. In the last two types of reactions, it has been found that non-adiabatic transition takes place repeatedly in the cage.

Ab initio study on the photochemical behavior of 4-dimethylamino,4′-cyanostilbene

Abstract Ab initio complete active space self-consistent field (CASSCF) and the second order perturbation (MRMP2) calculations have been performed to examine the photochemical behavior of 4-dimethylamino,4 ′ -cyanostilbene (DCS) in the low-lying excited states. The potential energy curves with respect to four torsional angles at the CASSCF level is possible to give a reasonable interpretation on the experimental findings and the MRMP2 calculation well reproduces the experimental observable such as the excitation energy. The S 1 state in the Franck–Condon region is the internal charge transfer (ICT) state where the electron is partially transferred from the 4-dimethylanilino into the 4-cyanostyryl groups. Only the potential energy curve in S 1 with respect to the torsional motion of the ethylenic double bond has a small barrier to the perpendicularly twisted conformation leading to the cis–trans photoisomerization, though the others becomes energetically much unstable. DCS in S 1 in polar solvent is, on the other hand, stabilized by the torsional motion of the 4-dimethylanilino group.