A Direct Comparison between Structure Correlations and Reaction Paths

Reaction paths for the formation of the donor−acceptor complexes HCN−BF 3 and H 3 N−BF 3 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 BF 3 complexes with amine and nitrile donors. Radial potential functions for the HCN and NH 3 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−BF 3, 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−BF 3, 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 BF 3 complexes with nitrogen donors.