A direct comparison between structure correlations and reaction paths

Reaction paths for the formation of the donor-acceptor complexes HCN-BF₃ and H₃N-BF₃ have been calculated by ab initio methods. The results are compared with the experimentally observed correlation between the B-N bond length and NBF angle for a series of BF₃ complexes with amine and nitrile donors. Radial potential functions for the HCN and NH₃ complexes exhibit a single minimum, confirming that the large gas-solid structure differences observed in these systems do not arise from distinct isomeric forms. In the case of HCN-BF₃, the ab initio results are combined with published vapor pressure data to obtain a value for the molar lattice energy of the crystal. Dipole moments are calculated as a function of B-N separation for both species, and for HCN-BF₃, the dipole-dipole interaction energy between adjacent monomers in the solid is shown to comprise a significant portion of the lattice energy. A simple model is described for reproducing the experimentally observed structure correlation among BF₃ complexes with nitrogen donors.