B-N Distance Potential of CH₃CN-BF₃ Revisited: Resolving the Experiment-Theory Structure Discrepancy and Modeling the Effects of Low-Dielectric Environments

Phillips, J. A.; Cramer, C. J.
J. Phys. Chem. B 2007, 111, 1408.

We have re-examined the B-N distance potential of CH₃CN-BF₃ using MP2, DFT, and high-accuracy multi-coefficient 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 potential, we find an average B-N distance of 1.95 ang, which is 0.13 ang longer than the corresponding equilibrium value. In turn, this indicates that the long-standing discrepancy between the experimental (R(B-N)=2.01 ang) and theoretical (R(B-N)=1.8 or R(B-N)=2.2-2.3 ang) 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 (ε = 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 ang) is nearly reproduced with a dielectric constant of only 5.0, and also predict significant structural changes for ε values of 1.1 to 1.5, consistent with results from matrix isolation-IR experiments.

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