An ab Initio Study of the Complex BF3·NF3

The structures and vibrational spectra of BF₃, NF₃, and the complex BF₃·NF₃ have been computed at the SCF and MP2 levels using the 6-31G* basis set. It is proposed that the basis set superposition error be calculated at the MP structure but using the HF energies (RHF//MP) to avoid the enhancement of BSSE arising from the spurious LUMO of the ghost orbitals. With this treatment the binding energies at the MP2/6-31G* and MP2/6-31+G* levels are ?8.58 and ?7.36 kJ mol^-1, respectively. The molecular dipole derivatives with respect to Cartesian displacements of the nuclei in a Cartesian framework, oriented with respect to the bond direction, have been evaluated. It is demonstrated that these are very informative monitors of electron displacements on complexation. The deviations of the dipole gradients from the expectations of the first-order bond moment model are rationalized in terms of through space electron cloud interactions. It is proposed that conventional bond parameter models cannot give realistic visualizations of the charge flow during molecular vibrations.