Ab Initio Studies of the Structure and Vibrational Spectra of Methyl Isocyanide−Borane and Acetonitrile−Borane

Abstract

The SCF/DZP and MP2/DZP methods of ab initio quantum chemistry have been utilized to study the structure, vibrational spectra, binding energy, and barrier to internal rotation of methyl isocyanide−borane and acetonitrile−borane adducts. The eclipsed conformation of the complexes was predicted to be a minimum, and the staggered form is a transition state with a barrier height of about 10 cal/mol. The vibrational analyses of CH 3 NC−BH 3 and CH 3 CN−BH 3 and several of their isotopomers have been carried out by the GF matrix method. Computations have also been carried out for free CH 3 NC and CH 3 CN in order to investigate the changes in CH 3 NC and CH 3 CN as a result of their complex formation with BH 3 . To obtain an acceptable set of force constants, a recently proposed procedure “RECOVES” has been utilized. The increase in the N⋮C/C⋮N stretching force constant of CH 3 NC/CH 3 CN on adduct formation is interpreted with the help of Parr and Borkman's model. The binding energies for the two adducts have been determined taking basis set superposition error (BSSE) into consideration. The effect of the BSSE on structure, dipole moment, and vibrational frequencies of CH 3 CN and CH 3 NC is also evaluated. The predicted infrared band intensities for the two complexes are in good agreement with the experimentally observed features, and they have been utilized in the assignment of vibrational frequencies.