Ammonia Borane Clusters: Energetics of Dihydrogen Bonding, Cooperativity, and the Role of Electrostatics.

Cluster formation of ammonia borane (NHBH) driven by noncovalent H···H dihydrogen interaction is investigated at the M06L/6-311+G(d,p) level of density functional theory. For clusters containing up to six monomers, ladder, cyclic, stacked, cross-stacked, end-on, mixed and hexagonal configurations have been screened for their energetic stability. In the dimer, 7.94 kcal/mol stabilization energy per monomer (E) is observed. Compared to ladder and cyclic configurations, a tetramer consisting of stacked dimer units is more stable by 3.0 kcal/mol whereas a hexamer composed of hexagonally arranged monomers promoting side-on H···H interaction is more stable than a stacked configuration by 2.5 kcal/mol. The hexagonal packing of cluster is repeated to obtain (NHBH), (NHBH), (NHBH), (NHBH), and (NHBH) clusters. The E 17.81 kcal/mol observed for (NHBH) is 2.24 fold higher than the dimer, suggesting strong cooperativity in cluster growth mechanism. The zwitterionic features of NHBH is characterized in terms of molecular electrostatic potential (MESP) features. During cluster formation, donation of electron density from negatively charged BH unit of a monomer to the positively charged NH unit of other interacting monomers occurs through H···H dihydrogen bonding. The extent of electron donation is revealed through the value of MESP minium (V) in every monomer. A strong linear correlation between the total value of V for a cluster (ΣV) and the total stabilization energy of the cluster (E) is established. Further, MESP at the nuclei of N (V) and B (V) are found to be very sensitive to the strength of H···H bonding. With respect to free NHBH, the total change in V (ΣΔV) as well as the total change in V (ΣΔV) in a cluster shows near-perfect linear correlation with E. Further, the magnitude of the three quantities, viz. ΣΔV, ΣΔV, and E is nearly same and indicates that the cluster formation of NHBH is almost effectively controlled by electrostatics of dihydrogen interactions. The extended network of dihydrogen interactions observed in large clusters and the significant positive cooperativity effect of such interactions support the use of ammonia borane as a potential hydrogen storage material.