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Guided by a notion of symmetry-breaking modulation or control of the so-called symmetry-allowed conical intersection by shining laser pulses [Arasaki, Y.; et al. Phys. Chem. Chem. Phys. 2010, 12, 1239], we here explore a possibility of the modulation of the symmetry-allowed conical intersection by chemical substitution with functional groups. As a first case study, we choose photoisomerization dynamics of s-trans-1,3-butadiene H2C=CH--CH=CH2 with one of the terminal hydrogen atoms being replaced by -CF3. The target here is not the control of the rate of nonadiabatic transition but to know which one of the double bonds is more frequently isomerized in the radiationless quenching process on the way back to the ground state. We analyze when and how the symmetry is broken by tracking ab initio molecular dynamics paths, the mean-field paths with use of the nonadiabatic electron wavepacket dynamics, and the associated branching paths. [ABSTRACT FROM AUTHOR]

Classical trajectory study of nuclear motion on the Born−Oppenheimer potential energy surfaces is now one of the standard methods of chemical dynamics. In particular, this approach is inevitable in the studies of large molecular systems. However, as soon as more than a single potential energy surface is involved due to nonadiabatic coupling, such a naive application of classical mechanics loses its theoretical foundation. This is a classic and fundamental issue in the foundation of chemistry. To cope with this problem, we propose a generalization of classical mechanics that provides a path even in cases where multiple potential energy surfaces are involved in a single event and the Born−Oppenheimer approximation breaks down. This generalization is made by diagonalization of the matrix representation of nuclear forces in nonadiabatic dynamics, which is derived from a mixed quantum-classical representation of the electron−nucleus entangled Hamiltonian Takatsuka, K. J. Chem. Phys.2006, 124, 064111. A manifestation of quantum fluctuation on a classical subsystem that directly contacts with a quantum subsystem is discussed. We also show that the Hamiltonian thus represented gives a theoretical foundation to examine the validity of the so-called semiclassical Ehrenfest theory (or mean-field theory) for electron quantum wavepacket dynamics, and indeed, it is pointed out that the electronic Hamiltonian to be used in this theory should be slightly modified. [ABSTRACT FROM AUTHOR]

A high energy atomic cluster undergoing frequent structural isomerization behaves like a liquid droplet, from which atoms or molecules can be emitted. Even after evaporation, the daughter cluster may still keep changing its structure. We study the dynamics of such an evaporation process of atomic evaporation. To do so, we develop a statistical rate theory for dissociation of highly nonrigid molecules and propose a simple method to calculate the absolute valueof classical phase-space volume for a potential function that has many locally stable basins. The statistical prediction of the final distribution of the released kinetic energy is also developed. A direct application of the Rice−Ramsperger−Kassed−Marcus (RRKM) theory to this kind of multichannel chemical reaction is prohibitively difficult, unless further modeling and/or assumptions are made. We carry out a completely nonempirical statistical calculation for these dynamical quantities, in that nothing empirical is introduced like remodeling (or reparametrization) of artificial potential energy functions or recalibration of the phase-space volume referring to other “empirical” values such as those estimated with the molecular dynamics method. The so-called dividing surface is determined variationally, at which the flux is calculated in a consistent manner with the estimate of the phase-space volume in the initial state. Also, for the correct treatment of a highly nonrigid cluster, the phase-space volume and flux are estimated without the separation of vibrational and rotational motions. Both the microcanonical reaction rate and the final kinetic energy distribution thus obtained have quite accurately reproduced the corresponding quantities given by molecular dynamics calculations. This establishes the validity of the statistical arguments, which in turn brings about the deeper physical insight about the evaporation dynamics. [ABSTRACT FROM AUTHOR]

The difference between quantum and classical survival probabilities for molecular dissociation dynamics in the time domain, which arises mainly from quantum mechanical tunneling, has interesting characteristics that are not noticed through the counterpart in energy domain. It is shown that the early stage undergoes a fast tunneling, while the later stage is characterized with a long-lasting slow tunneling. The mechanism of this behavior is analyzed in terms of a quasi-semiclassical theory featuring the geometrical distribution of the so-called tunneling points. In particular, the role of dynamical tunneling is discussed as a phenomenon that typifies the time dependence of tunneling dynamics. It is predicted that these tunneling characteristics will be reflected in the isotope effect and should be experimentally observable. [ABSTRACT FROM AUTHOR]