B-N distance potential of CH3CN-BF3 revisited: resolving the experiment-theory structure discrepancy and modeling the effects of low-dielectric environments.

We have re-examined the B-N distance potential of CH3CN-BF3 using MP2, DFT, and high-accuracy multicoefficient methods (MCG3 and MC-QCISD). In addition, we have solved a 1-D Schrödinger equation for nuclear motion along the B-N stretching coordinate, thereby obtaining vibrational energy levels, wave functions, and vibrationally averaged B-N distances. For the gas-phase, MCG3//MP2/aug-cc-pVTZ potential, we find an average B-N distance of 1.95 A, which is 0.13 A longer than the corresponding equilibrium value. In turn, this provides solid evidence that the long-standing discrepancy between the experimental (R(B-N) = 2.01 A) and theoretical (R(B-N) = 1.8 A or R(B-N) = 2.2-2.3 A) distances may be genuine, stemming from large amplitude vibrational motion in the B-N stretching coordinate. Furthermore, we have examined the effects of low-dielectric media (epsilon = 1.1-5.0) on the structure of CH3CN-BF3 by calculating solvation free energies (PCM/B97-2/aug-cc-pVTZ) and adding them to the gas-phase, MCG3 potential. These calculations demonstrate that the inner region of the potential is stabilized to a greater extent by these media, and correspondingly, the equilibrium and average B-N distances decrease with increasing dielectric constant. We find that the crystallographic structural result (R(B-N) = 1.63 A) is nearly reproduced with a dielectric constant of only 5.0, and also predict significant structural changes for epsilon values of 1.1-1.5, consistent with results from matrix-isolation-IR experiments.