Quantum Chemical and Matrix-IR Characterization of CH₃CN-BCl₃3: A Complex Having Two Distinct Minima Along the B-N Bond Potential

Wrass, J. P.; Sadowsky, D.; Bloomgren, K. M.; Cramer, C. J.; Phillips, J. A.
Phys. Chem. Chem. Phys. 2014, 16, 16480 (doi:10.1039/c4cp01495b).

We have characterized the structural and energetic properties of CH₃CN-BCl₃ via computations and matrix-IR spectroscopy. We find two equilibrium structures of the complex via computations. At the MP2/aug-cc-pVTZ level, the global minimum energy structure has a B-N distance of 1.601 ang, and a binding energy of 12.0 kcal/mol. The secondary structure lies 7.1 kcal/mol higher in energy with a B-N distance of 2.687 ang and a binding energy of 4.9 kcal/mol. Computational scans of the B-N potential curve using both DFT and post-HF methods indicate that a significant barrier exists between these structures, and that it lies 1 to 2 kcal/mol above the secondary minimum at a B-N distance of about 2.2 ang. We also observed several key, structurally-sensitive IR bands for six isotopic forms of the complex in neon matrices, including: The B-Cl asymmetric stretching band (ν^a_BCl) at 792 cm^-1 and the C-N stretching band (ν_CN) at 2380 cm^-1 (for the primary isotopomer, CH₃C¹⁴N-¹¹BCl₃). These frequencies are consistent with computational predictions for the minimum-energy form of the complex. Energy decomposition analyses were conducted for CH₃CN-BCl₃ and also two related complexes, CH₃CN-BF₃ and CH₃CN-BH₃. These provide insight into the trend in Lewis acidity of the BX₃ acceptors toward nitriles. Furthermore, these analyses indicate that the barrier along the B-N potential of CH₃CN-BCl₃ results from Pauli repulsion between the π electrons on the nitrile moiety and the chlorine atoms in BCl₃, which is significant at relatively long distances, at which attractive bonding interactions fail to overcome it.