Ammonia borane in an external electric field: structure, charge transfer, and chemical bonding

Abstract

It is well known that ammonia borane (BH₃NH₃) is one of the simplest donor–acceptor complexes. The donor–acceptor bond (B–N bond) is formed by sharing lone pair electrons between BH₃ and NH₃ groups. In the present work, different strengths of external electric fields (E_ext) are applied along the Z-axis direction to investigate the electric field induced effect on the BH₃NH₃ structure and properties. Interestingly, we have found that the lone pair electrons can be gradually moved in the “channel” between the BH₃ and NH₃ groups by modulating E_ext. The donor–acceptor bond (B–N bond) is gradually elongated until it breaks when the E_ext ranges from 0.0000 to 0.0321 au. The B–N bond is the shortest at E_ext = −0.0519 au. Interestingly, the negative charge on BH₃ groups sharply decreases from 0.161 to 0.005 (for NH₃ groups decreases from 0.161 to 0.005) and the electron cloud of HOMO−2 exhibits an obvious transformation at “broking bonding” E_ext ranging from 0.0320 to 0.0321 au, indicating the electron movement induced by the electric-field is the main reason to change the structure and stability of BH₃NH₃. Further, atoms in molecules (AIM) analysis shows that the B⋯N interactions are similar to that of hydrogen bonds at E_ext = −0.0767 au and the iconicity of B–N bond in BH₃NH₃ is confirmed by its low electron density for B–N ρ_(B–N) (0.103–0.073) in the region of E_ext (0–0.025 au).