A semiclassical non-adiabatic theory for elementary chemical reactions

TL;DR

This paper introduces a semiclassical non-adiabatic theory for electron transfer reactions that incorporates covalent interactions and predicts the transition from electron transfer to covalent bond formation, extending the classical Marcus theory.

Contribution

The paper develops a new semiclassical framework that includes covalent interactions, improving upon Marcus theory for better modeling complex chemical reactions.

Findings

Energy barrier decreases with covalent interaction strength

Covalent bond formation can replace electron transfer

Standard Marcus theory is recovered when covalent interactions are neglected

Abstract

Electron Transfer (ET) reactions are modeled by the dynamics of a quantum two-level system (representing the electronic state) coupled to a thermalized bath of classical harmonic oscillators (representing the nuclei degrees of freedom). Unlike for the standard Marcus theory, the complex amplitudes of the electronic state are chosen as reaction coordinates. Then, the dynamical equations at non vanishing temperature become those of an effective Hamiltonian submitted to damping terms and their associated Langevin random forces. The advantage of this new formalism is to extend the original theory by taking into account both ionic and covalent interactions. The standard theory is recovered only when covalent interactions are neglected. Increasing these covalent interactions from zero, the energy barrier predicted by the standard theory first depresses, next vanish (or almost vanish) and for stronger covalent interactions, covalent bond formation takes place of ET. In biochemistry, the standard Marcus theory often fails to explain the enzymatic reactions especially those with non Arrhenius behavior which are barrierless and also dissipate little heat. We claim that this improved theory should yield an interesting tool for understanding them.