Structure, Bonding, and Vibrational Frequencies of CH3CN−BF3: New Insight into Medium Effects and the Discrepancy between the Experimental and Theoretical Geometries

Abstract

We have conducted an extensive computational study of the structure, bonding, B−N potential energy curve, and vibrational frequencies of CH3CN−BF3, using MP2, B3LYP, and BWP91 methods with basis sets ranging from STO-3G to aug-cc-pVQZ. Two types of minimum energy structures were found; one group with B−N distances near 1.8 Å, another with distances near 2.3 Å. In most cases, longer bond length structures were found with basis sets lacking diffuse functions, whereas shorter bond length structures were found when these functions were included. The exception is the largest basis set (aug-cc-pVQZ), for which the equilibrium B−N distance was found to be 2.315 Å. Potential energy curves calculated for the B−N stretching coordinate are found to be remarkably flat, and this results from the occurrence of two competing minima corresponding to the two types of minimum energy structures. At the B3LYP/aug-cc-pVQZ level, an extremely flat region occurs near 1.93 Å on the B−N potential curve, which lies about 0.2 kcal/mol above the global minimum after accounting for the effects of basis set super position error (BSSE) and zero-point vibrational energy (ZPE). The results are nearly converged with respect to basis set at the B3LYP/aug-cc-pVQZ level; further attempts at increasing the size of the basis set were not successful. An AIM analysis indicates that the two minima in the B−N potential arise from distinctly different interactions, the longer being primarily an electrostatic interaction, the inner being a partial covalent bond. Given the flat, asymmetric nature of the potential, it is very likely that the equilibrium and vibrationally averaged structures differ significantly due to large amplitude motion in the intermolecular B−N stretching mode. Furthermore, a comparison of experimental and calculated vibrational frequencies leads to the tentative conclusion that the B−N distance is significantly shorter in an argon matrix than in the gas phase.