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Structure, Bonding, and Energetic Properties of Nitrile-Borane Complexes: RCN-BH3

Smith, E. L.; Sadowsky, D.; Cramer, C. J.; Phillips, J. A.
J. Phys. Chem. A 2011, 115, 1955 (doi:10.1021/jp106391c)

The structural and energetic properties of CH3CN-BH3, HCN-BH3, FCH2CN-BH3, and F3CCN-BH3 have been examined via density functional theory and post-Hartree-Fock calculations. The B-N distances in these systems are notably short, less than 1.6 Å, and the binding energies are substantial, about 20 kcal/mol. The properties of these systems do vary as a result of the nitrile substituent, but surprisingly, more electronegative substituents result in shorter B-N distances. For example, the B-N distance for F3CCN-BH3 is 1.576 Å via MP2/aug-cc-pVTZ, while that for CH3CN-BH3 is 1.584 Å. However, the binding energies vary as expected, from 17.4 kcal/mol in the case of F3CCN-BH3 to 22.6 kcal/mol for CH3CN-BH3 (via MP2/aug-cc-pVTZ). The extent of charge transfer and the degree of covalent character in the B-N bonds were explored by a Natural Bond Orbital analysis, and the Atoms in Molecules formalism, respectively, and do provide some rationale for the substituent effects. Frequency calculations indicate that BH3-localized vibrational modes do shift appreciably upon complex formation, especially the BH3 asymmetric stretch. For CH3CN-BH3, experimental and calculated frequency shifts compare well for the asymmetric BH3 bending mode, but the observed shift for the BH3 asymmetric stretch, the most structurally-sensitive mode, are 20-30 cm-1 larger than the predictions. While this may suggest a very slight contraction of the B-N bond upon formation of solid CH3CN-BH3 (for which experimental data are available) the balance of evidence indicates that no significant medium effects occur in these complexes. For one, the B-N potential of HCN-BH3 shows no unusual flatness along the inner wall, and furthermore, the B-N distance for HCN-BH3 was predicted to contract only very slightly in dielectric media (ε=2 to 20). Finally, we present a direct comparison between the nitrile BH3 systems and their BF3 analogs. The underlying reasons for the markedly different structural properties of HCN-BH3 and HCN-BF3 were explored through an energy decomposition scheme, and the results indicate that less Pauli repulsion of the electrons on each respective subunit is the most significant factor that favors the overall stability of the BH3 complex.