Showing papers by "Yu Harabuchi published in 2013"

Automated Search for Minimum Energy Conical Intersection Geometries between the Lowest Two Singlet States S0/S1-MECIs by the Spin-Flip TDDFT Method.

Abstract: Automated search for minimum energy conical intersection geometries between the lowest two singlet states (S0/S1-MECIs) was performed by combining the anharmonic downward distortion following (ADDF) method, the seam model function (SMF) approach, and the spin-flip (SF) TDDFT method. SMF/ADDF has been employed previously in automated searches for MECIs on potential energy surfaces (PESs) with expensive multireference methods. In this work, we adopt the SF-TDDFT method that enables efficient optimization of S0/S1-MECIs in the framework of TDDFT. To evaluate the performance of the present approach, it was applied to ethylene and 1,3-butadiene. The present method automatically gave unknown S0/S1-MECIs as well as all previously reported ones. Therefore, the present hybrid method of SMF/ADDF and SF-TDDFT is shown to be a promising approach to locate S0/S1-MECIs of large systems automatically with modest computational costs.

Photophysics of cytosine tautomers: new insights into the nonradiative decay mechanisms from MS-CASPT2 potential energy calculations and excited-state molecular dynamics simulations

Abstract: A comprehensive picture of the ultrafast nonradiative decay mechanisms of three cytosine tautomers (amino-keto, imino-keto, and amino-enol forms) is revealed by high-level ab initio potential energy calculations using the multistate (MS) CASPT2 method and also by on-the-fly excited-state molecular dynamics simulations employing the CASSCF method. To obtain a reliable potential energy profile along the deactivation pathways, the MS-CASPT2 method is employed even for the optimization of minimum energy structures in the excited state and conical intersection (CI) structures between the ground and excited states. In the imino (imino-keto) form, we locate a new CI structure involving the twisting of the imino group, and the decay pathway leading to this CI is found to be barrierless, suggesting a remarkably efficient deactivation of imino cytosine. In the keto (amino-keto) form, the MS-CASPT2 calculations exhibit an efficient decay path to the ethylene-like CI involving the twisting of the C–C double bond in the six-membered ring, with a barrier of ∼0.08 eV from the minimum of the 1ππ* state. In the enol (amino-enol) form, three types of CIs are identified for the first time. Among them, the ethylene-like CI with a similar molecular structure to the keto form provides the most preferred deactivation pathway in enol cytosine. This pathway exhibits a higher barrier of ∼0.22 eV and a higher energy of CI than those of keto cytosine. Nonadiabatic molecular dynamics simulations provide a time-dependent picture of the deactivation processes, including the excited-state lifetime of each tautomer. In particular, the decay time of the imino tautomer is predicted to be only ∼100 fs. Our computational results are in remarkably good agreement with the experimental findings of recent femtosecond pump–probe photoionization spectroscopy [J. Am. Chem. Soc., 2009, 131, 16939; J. Phys. Chem. A, 2011, 115, 8406], supporting the coexistence of more than one tautomer in the photophysics of isolated cytosine and that each tautomer exhibits a different excited-state lifetime.

A multireference perturbation study of the NN stretching frequency of trans-azobenzene in nπ* excitation and an implication for the photoisomerization mechanism

Abstract: A multireference second-order perturbation theory is applied to calculate equilibrium structures and vibrational frequencies of trans-azobenzene in the ground and nπ* excited states, as well as the reaction pathways for rotation and inversion mechanism in the nπ* excited state. It is found that the NN stretching frequency exhibits a slight increase at the minimum energy structure in the nπ* state, which is explained by the mixing of the NN stretching mode with the CN symmetric stretching mode. We also calculate the NN stretching frequency at several selected structures along the rotation and inversion pathways in the nπ* state, and show that the frequency decreases gradually along the rotation pathway while it increases by ca. 300 cm−1 along the inversion pathway. The frequencies and energy variations along the respective pathways indicate that the rotation pathway is more consistent with the experimental observation of the NN stretching frequency in nπ* excitation.