On the nature of the trimer, tetramer, and pentamer of ammonia borane

A stochastic search of stable configurations for ammonia borane trimers, tetramers, and pentamers affords very rich and complex potential energy surfaces. Structural preferences for three-dimensional, compact geometries for n >= 4 at the second-order perturbation theory level with an extended basis set, MP2/6-311++G(d, p), is inferred. Molecular units in the clusters interact via four different types of non-conventional intermolecular contacts: heteropolar N-H center dot center dot center dot H-B dihydrogen bonds (the main stabilizing factor), homopolar B-H center dot center dot center dot H-B dihydrogen bonds, and B center dot center dot center dot H-N and N center dot center dot center dot H-B hydrogen bonds. Despite the perceived weak nature of dihydrogen bonds, in the case of ammonia borane polymers studied here, they lead to unusually strong interactions, exceeding by more than 20 kcal mol(-1) the stabilization energies reported for water clusters of the same molecularities. We provide evidence to suggest that in all cases, the stabilizing dihydrogen bonds arise from charge transfer between orbitals of the general sigma(B-H) -> sigma(N-H)* type.