Takehiro Yonehara

Chemical theory beyond the Born-Oppenheimer paradigm : nonadiabatic electronic and nuclear dynamics in chemical reactions

Basic Framework of Theoretical Chemistry Nuclear Dynamics on Adiabatic Electronic Potential Energy Surfaces Breakdown of the Born-Oppenheimer Approximation Classic Theories of Nonadiabatic Transitions and Ideas Behind Direct Observation of the Wavepacket Bifurcation due to Nonadiabatic Transitions Nonadiabatic Electron Wavepacket Dynamics in Path Branching Representation Dynamical Electron Theory for Chemical Reactions Molecular Electron Dynamics in Laser Fields

Role of isomerization channel in unimolecular dissociation reaction H2CO→H2+CO: Ab initio global potential energy surface and classical trajectory analysis

We constructed a full dimensional potential energy function of H2CO that can describe both the dissociation and isomerization channels by the modified Shepard interpolation method. Ab initio calculations at the MP2/cc-pVTZ level were carried out to obtain the local potential functions at about 4700 points. The interpolant points were sampled by classical trajectory calculations and by the grid searches in the internal coordinate space. Classical trajectory calculations were performed to examine the intramolecular dynamics associated with the dissociation as well as the product state distributions. The time scale of intramolecular vibrational energy randomization was much faster than that of the dissociation reaction. The dissociation rate was obtained from the classical trajectory results and the effect of the isomerization channel on the dissociation was estimated. The calculated rate constants were compared with those by Rice–Ramsperger–Kassel–Marcus theory.We constructed a full dimensional potential energy function of H2CO that can describe both the dissociation and isomerization channels by the modified Shepard interpolation method. Ab initio calculations at the MP2/cc-pVTZ level were carried out to obtain the local potential functions at about 4700 points. The interpolant points were sampled by classical trajectory calculations and by the grid searches in the internal coordinate space. Classical trajectory calculations were performed to examine the intramolecular dynamics associated with the dissociation as well as the product state distributions. The time scale of intramolecular vibrational energy randomization was much faster than that of the dissociation reaction. The dissociation rate was obtained from the classical trajectory results and the effect of the isomerization channel on the dissociation was estimated. The calculated rate constants were compared with those by Rice–Ramsperger–Kassel–Marcus theory.

Quantum dynamics study on multichannel dissociation and isomerization reactions of formaldehyde

We study quantum dynamics of the multichannel reactions of H(2)CO including the molecular and radical dissociation channels as well as the isomerization ones, H(2)CO-->trans-HCOH and trans-HCOH-->cis-HCOH. For this purpose, the previously developed potential energy function [T. Yonehara and S. Kato, J. Chem. Phys. 117, 11131 (2002)] is refined to give accurate transition state energies and to describe the radical dissociation channel. The cumulative reaction probabilities for the molecular dissociation and two isomerization channels are calculated by using the full Watson Hamiltonian. We also carry out wave packet dynamics calculations starting from the transition state region for the molecular dissociation. A contracted basis set for the angular coordinates is constructed to reduce the size of dynamics calculations. The intramolecular vibrational relaxation dynamics is found to be fast and almost complete within 300 fs. Using the energy filtered wave functions, the time propagation of HCOH population is obtained in the energy range from 81 to 94 kcal/mol. The branching ratio of the radical product is estimated by calculating the time dependent reactive fluxes to the molecular and radical dissociation products.

Path-Branching Representation for Nonadiabatic Electron Dynamics in Conical Intersection

Path-branching representation (or phase-space averaging and natural branching method (PSANB) as its approximation) of nonadiabatic electron wavepacket dynamics is now known to work well for avoided crossings in many dimensional nonadiabatic transitions [Yonehara, T.; Hanasaki, K.; Takatsuka, K. Chem. Rev. 2012, 112, 499]. In this paper we examine feasibility of the path-branching representation in the theoretical studies of conical intersection (CI). The most characteristic feature of CI is the Herzberg-Longuet-Higgins phase (or the Berry phase) arising from the electronic part of the total wave function, and accordingly quantum phases of both electronic and nuclear dynamics should be taken into account in a balanced manner. We first show the PSANB can well capture the essential feature of the phase dynamics of CI. However, the nuclear phases, the wavelength of which is far shorter than that of the electronic phases, make the computation of nonadiabatic transition extremely oscillatory, resulting in very slow convergence with respect to the number of sampling paths. A similar difficulty quite often takes place in theoretical chemical dynamics. To cope with this situation, we devise a simple and tractable approximation in the application of PSANB resting on the fact that a small number of PSANB paths already reproduce accurate nonadiabatic transition probability.

Electron wavepacket dynamics in highly quasi-degenerate coupled electronic states: a theory for chemistry where the notion of adiabatic potential energy surface loses the sense.

We develop a theory and the method of its application for chemical dynamics in systems, in which the adiabatic potential energy hyper-surfaces (PES) are densely quasi-degenerate to each other in a wide range of molecular geometry. Such adiabatic electronic states tend to couple each other through strong nonadiabatic interactions. Technically, therefore, it is often extremely hard to accurately single out the individual PES in those systems. Moreover, due to the mutual nonadiabatic couplings that may spread wide in space and due to the energy-time uncertainty relation, the notion of the isolated and well-defined potential energy surface should lose the sense. On the other hand, such dense electronic states should offer a very interesting molecular field in which chemical reactions to proceed in characteristic manners. However, to treat these systems, the standard theoretical framework of chemical reaction dynamics, which starts from the Born-Oppenheimer approximation and ends up with quantum nuclear wavepacket dynamics, is not very useful. We here explore this problem with our developed nonadiabatic electron wavepacket theory, which we call the phase-space averaging and natural branching (PSANB) method [T. Yonehara and K. Takatsuka, J. Chem. Phys. 129, 134109 (2008)], or branching-path representation, in which the packets are propagated in time along the non-Born-Oppenheimer branching paths. In this paper, after outlining the basic theory, we examine using a one-dimensional model how well the PSANB method works with such densely quasi-degenerate nonadiabatic systems. To do so, we compare the performance of PSANB with the full quantum mechanical results and those given by the fewest switches surface hopping (FSSH) method, which is known to be one of the most reliable and flexible methods to date. It turns out that the PSANB electron wavepacket approach actually yields very good results with far fewer initial sampling paths. Then we apply the electron wavepacket dynamics in path-branching representation and the so-called semiclassical Ehrenfest theory to a hydrogen molecule embedded in twelve membered boron cluster (B(12)) in excited states, which are densely quasi-degenerate due to the vacancy in 2p orbitals of boron atom [1s(2)2s(2)2p(1)]. Bond dissociation of the hydrogen molecule quickly takes place in the cluster and the resultant hydrogen atoms are squeezed out to the surface of the cluster. We further study collision dynamics between H(2) and B(12), which also gives interesting phenomena. The present study suggests an interesting functionality of the boron clusters.

A quantum dynamics method for excited electrons in molecular aggregate system using a group diabatic Fock matrix

We introduce a practical calculation scheme for the description of excited electron dynamics in molecular aggregate systems within a local group diabatic Fock representation. This scheme makes it easy to analyze the interacting time-dependent excitation of local sites in complex systems. In addition, light-electron couplings are considered. The present scheme is intended for investigations on the migration dynamics of excited electrons in light-induced energy transfer systems. The scheme was applied to two systems: a naphthalene-tetracyanoethylene dimer and a 20-mer circle of ethylene molecules. Through local group analyses of the dynamical electrons, we obtained an intuitive understanding of the electron transfers between the monomers.