Quantum chemical and matrix-IR characterization of CH₃CN-BCl₃: A complex with two distinct minima along the B-N bond potential

We have characterized the structural and energetic properties of CH₃CN-BCl₃via 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 Å, and a binding energy of 12.0 kcal mol^-1. The secondary structure lies 7.1 kcal mol^-1 higher in energy with a B-N distance of 2.687 Å and a binding energy of 4.9 kcal mol^-1. 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^-1 above the secondary minimum at a B-N distance of about 2.2 Å. 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 where attractive bonding interactions fail to overcome it.