Charge transfer in the electron donor-acceptor complex BH3NH3

As a simple yet strongly binding electron donor-acceptor (EDA) complex, BH3NH3 serves as a good example to study the electron pair donor-acceptor complexes. We employed both the ab initio valence bond (VB) and block-localized wave function (BLW) methods to explore the electron transfer from NH3 to BH3. Conventionally, EDA complexes have been described by two diabatic states: one neutral state and one ionic charge-transferred state. Ab initio VB self-consistent field (VBSCF) computations generate the energy profiles of the two diabatic states together with the adiabatic (ground) state. Our calculations evidently demonstrated that the electron transfer between NH3 and BH3 falls in the abnormal regime where the reorganization energy is less than the exoergicity of the reaction. The nature of the NH3-BH3 interaction is probed by an energy decomposition scheme based on the BLW method. We found that the variation of the charge-transfer energy with the donor-acceptor distance is insensitive to the computation levels and basis sets, but the estimation of the amount of electron transferred heavily depends on the population analysis procedures. The recent resurgence of interest in the nature of the rotation barrier in ethane prompted us to analyze the conformational change of BH3NH3, which is an isoelectronic system with ethane. We found that the preference of the staggered structure over the eclipsed structure of BH3NH3 is dominated by the Pauli exchange repulsion.