Study of proton-coupled electron transfer (PCET) with four explicit diabatic states at the ab initio level

Proton-coupled electron transfer (PCET) broadly exists in chemical and biological processes and can be intuitively described in terms of diabatic (resonance) states. The conventional molecular orbital (MO) based methods, however, have the difficulty in rationally defining diabatic states which are strictly electron-localized. Here we used the block-localized wavefunction (BLW) method, which is the simplest variant of valence bond (VB) theory, to define four major diabatic states and compare the concerted and the sequential transfers of electron and proton between formamide and its radical form. Our preliminary computations with partial breathing orbitals (BOs) show that the BLW method can locate the intermediate state in the stepwise mechanism, though the barriers are overestimated with references to high-level MO results, indicating the importance of dynamical electron correlation at transition states and the need for full BOs. But all computations at all levels concur that the concerted mechanism is preferred over the stepwise mechanism in the system studied in this work. (C) 2017 Elsevier B.V. All rights reserved.