Computational Studies on the Effect of External Electric Field on Hydrogen Atom Transfer and Proton Coupled Electron Transfer Reactions

Abstract

The effect of external electric field (EEF) on the activation energy barrier for two types of H-abstraction reactions, namely, Hydrogen Atom Transfer (HAT) and Proton Coupled Electron Transfer reaction was examined by means of Density Functional Theory (DFT). It has been found that the external electric field affects the PCET reaction significantly as compared to HAT. Two cases of PCET reactions (phenoxyl radical/phenol and p-methyl phenoxyl/phenol) have been studied which shows that the barrier for H-abstraction reaction decreases while the external electric field is applied on a preferred direction. Thus external electric field can be used to catalyse the PCET type reactions. On the other hand, three different cases of HAT reactions (methoxy/methanol, benzyl/toluene and p-nitro benzyl/toluene) have been studied in presence and absence of external electric field. It has been observed that the HAT barriers for all three reactions remain almost unaffected by the EEF and hence no catalysis is observed in presence of EEF. The Natural Bond Orbital (NBO) charge analysis and the Valence Bond State Correlation Diagram (VBSCD) have been used to rationalize the observations. Our finding of the external electric field catalyzing the PCET can have broader implications in enzymatic conditions where the H-abstraction reaction plays a vital role in various oxidation processes.