Ab Initio Characterization of [H₃N·BH₃]₂, [H₃N·AIH₃]₂, and [H₃N·GaH₃]₂

The dimers of [H₃N·BH₃]₂, [H₃N·AlH₃]₂, and [H₃N·GaH₃]₂ are characterized as head-to-tail complexes with intermolecular H - H hydrogen bonding. In the case of [H₃N·BH₃]₂, that hydrogen bonding is bifurcated resulting in four short (N)H - H(B) distances. For both [H₃N·AlH₃ and [H₃N·GaH₃]₂, there are only two short (N)H - H(Al/Ga) distances, although the four-atom linkage remains far from linear. The binding energies that are calculated at the MP2 level of theory using polarized-double-ζ basis sets are -15.1, -11.8, and -10.7 kcal/mol for [H₃N· BH₃]₂2, [H₃N·AlH₃]₂, and [H₃N·GaH₃]₂, respectively. These energies appear converged to within 1 kcal/mol with respect to larger basis sets and more complete accounting for electron correlation effects. Binding energies from density functional calculations are within 0.3 kcal/mol of the MP2 results. All-electron and effective-core-potential basis sets for gallium provide similar predictions.