Electron Dynamics in Molecular Elementary Processes and Chemical Reactions

Abstract

This account places a particular emphasis on recent progress in the theory and its applications of nonadiabatic electron dynamics in chemical science. After a brief description of the fundamental relevance of the breakdown of the Born-Oppenheimer approximation, we show examples of our extensive and systematic application of electron dynamics to highlight the significance and necessity of beyond-Born-Oppenheimer chemistry. The chemical subjects presented herewith cover (1) characteristic phenomena arising from nonadiabatic dynamics, (2) flow of electrons during chemical reactions and ionization dynamics, (3) symmetry breaking and its possible control in chemical reactions emerging from multi-dimensional nonadiabatic interactions, a special example which can cause possible breakdown of molecular mirror symmetry, (4) physical mechanism of charge separation in organic compounds and biomolecules, (5) essential roles of charge separation and elementary chemical reaction mechanisms in catalytic cycles of Mn oxo complexes up to Mn4CaO5 in water splitting dynamics (2H2O → 4H+ + 4e− + O2), (6) chemical bonds and huge electronic state fluctuation in densely quasi-degenerate electronic manifolds, which make chemistry without the notion of potential energy surfaces, and so on. All these materials and issues have been chosen because they are not directly resolved by the method of energetics based on time-independent quantum chemistry. We thus have been exploring, developing, and cultivating a new chemical realm beyond the Born-Oppenheimer paradigm. This account is closed with a scope about the theory of simultaneous electronic and nuclear quantum wavepacket dynamics.

Time-dependent variational dynamics for nonadiabatically coupled nuclear and electronic quantum wavepackets in molecules

“…The cutting-edge electronic state theory should be capable of describing dynamical electrons as quantum wavepackets that are kinematically (nonadiabatically) coupled with nuclear motions. Therefore, we have been developing a theory of such nonadiabatic electron dynamics, in which electron wavepackets propagate in time along simultaneously generated nuclear "paths" [1][2][3][4][5][6][7]. These paths can naturally and smoothly branch into pieces at each significant nonadiabatic transition region as many as the number of adiabatic potential energy surfaces that commit the nonadiabatic avoided crossings and conical intersections.…”

“…Therefore, those paths are not classical (Newtonian) in general and driven by the so-called force matrix, in which nonadiabatic interactions are taken into account besides the adiabatic nuclear forces [2]. The advantages starting from the nonadiabatic electron wavepacket representation are (1) electron dynamics can be tracked directly and vividly as chemical reactions proceed, and (2) one can treat large molecules in size and in the number of involved electronic states, since there is no need to prepare global PES because of the on-the-fly nuclear dynamics [7]. More essential virtue of the nonadiabatic electron wavepacket dynamics is that it can cope with (1) highly quasi-degenerate electronic states, the fluctuation of which is huge and extremely frequent due to their mutual mixing by nonadiabatic coupling among them, and (2) the real-time dynamics of electron wavepackets that are nonadiabatically coupled with nuclei like protons as in charge separation in water splitting in Photo System II of plants, and so on.…”

“…Such one-dimensional semiclassical theory has been mathematically finalized by Nakamura and Zhu [36]. For further progress, we refer to reviews about the rich physical implications emerging from the realistic molecular nonadiabatic chemistry, including effects arising from the genuine multidimensional nonadiabatic interactions [5][6][7][36][37][38][39][40].…”

“…With the flux one can directly track the electron flow in molecules as chemical dynamics proceeds. [7] Moreover, even real-time ionization dynamics such as photoionization and autoionization can be tracked by making appropriate use of the electron flux. For instance, Matsuoka and Takatsuka have shown how electrons are pumped up to an ionization manifold by the assistance of nonadiabatic couplings in autoionization.…”

“…To address one of the ultimate goals of theoretical molecular science, we here propose to incorporate nuclear quantum wavepackets represented in the Cartesian Gaussian functions multiplied plane waves into nonadiabatic electron wavepacket dynamics we have long been developing [1][2][3][4][5][6][7] with use of our proposed quantum least action principle (time-dependent variational principle) [8]. In order to describe why the formulation of simultaneous nuclear and electronic dynamics is needed in current scientific status and to clearly distinguish the present theory from canonical and the traditional methods based on the Born-Huang expansion [10], we track our pathways to the goal starting from two early works with the Born-Huang dynamics; one is electron scattering by molecules as an electron dynamics in the fixed nuclei approximation and the a e-mail: kaztak@fukui.kyoto-u.ac.jp (corresponding author) other being direct observation of nuclear wavepacket dynamics through time-resolved photoelectron spectroscopy.…”