Using ‘designer’ coherences to control electron transfer in a model bis(hydrazine) radical cation: can we still distinguish between direct and superexchange mechanisms?

Abstract

We have simulated two mechanisms, direct and superexchange, for the electron transfer in a model Bis(hydrazine) Radical Cation, which consists of two hydrazine moieties coupled by a benzene ring. The computations, that are inspired by the attochemistry approach, focus on the electron dynamics arising from a coherent superposition of four cationic states. The electron dynamics, originating from a solution of the time dependent Schrödinger equation within the Ehrenfest method, is coupled to the relaxation of the nuclei. Both direct (ca. 15 fs dynamics) and superexchange (ca. 2 fs dynamics) mechanisms are observed and turn out to lie on a continuum depending on the strength of the coupling of the benzene bridge electron dynamics with the hydrazine chromophore dynamics. This contrasts with the chemical pathway approach where the direct mechanism is completely non-adiabatic via a conical intersection, while the superexchange mechanism involves an intermediate radical with the unpaired electron localized on the benzene ring. Thus, with the attochemistry-inspired electron dynamics approach, one can distinguish direct from superexchange mechanisms depending on the strength of the coupling of two types of electron dynamics: the slow hydrazine dynamics (ca. 15 fs) and the fast benzene linker dynamics (ca. 2 fs). In this model bis(hydrazine) radical cation, only when the intermediate coupler is in an anti-quinoid state, does one see the coupling of the bridge and hydrazine chromophore dynamics.

Controlling Electronic Coherences and the Curvature Induced by the Derivative Coupling at a Conical Intersection: A Quantum Ehrenfest (QuEh) Protocol for Reaction Path Following Application to “Channel 3” Benzene Photochemistry

“…But this gives little chemical insight. In another study, we have used some physical properties such as the spin density to follow the electron dynamics . Here we choose an orthogonal VB representation.…”

“…In another study, we have used some physical properties such as the spin density to follow the electron dynamics. 16 Here we choose an orthogonal VB representation. The individual VB configurations are quasi diabatic.…”

“…The second-order Ehrenfest method that we have used in other work to probe a simple reaction path, , is different from a naïve Ehrenfest approach because the full interstate gradients are computed. These interstate gradients are the derivative couplings that arise in the off-diagonal derivative matrix elements that occur between the components of the states that are coherently mixed at a conical intersection (see Section 2 for a full discussion).…”