Ab initio insights into the face, edge, and vertex interactions of BH₄ ^1- with electron-accepting molecules

The ab initio calculations at the MP2/aug-cc-pvdz computational level were used to analyze the interactions of FCN, ClCN, BrCN, CF₃H, CF₃Cl, CH₃OH, HF, HCl, HCN, SH₂, SHF, SF₂, H₂O, HOCl, HOBr, CO, N_2, and H₂ molecules with BH₄ ^1-. On BH₄ ^1-, three sites were accessible for interactions with L molecules to form BH₄(L)^1- aggregates. The faces, edges, and vertices of BH₄ ^1- as electron donors, could interact with electron acceptor species. In addition, the BH₄ ^1- anion, through its σ-holes, could obtain electrons from interacting molecules. The significant preference of some molecules was interaction along the triangular faces, BH₄(L)_f ^1- (where L = ClCN, BrCN, FCN, CF₃Cl, CF₃H) whereas, for others, the vertices, BH₄(L)_v ^1- (where L = HOCl, HOBr, PF₃) or edges, BH₄(L)_e ^1- (where L = H₂O, HF, HCl) of BH₄ ^1- might be more suitable for interaction. Some molecules, such as CH₄ and H₂, despite their preferred facial interactions, could interplay with the vertex counterpart through an edge intermediate. It seems that accepting electrons (triel bonding) by BH₄ ^1- σ-holes had important roles in the face interactions for BH₄(L)_f ^1- adducts. Bader's Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) calculations were used to analyze optimized complexes. Noncovalent interaction (NCI) analysis was used for further determination of interactions in BH₄(L)^1- adducts.