Ab initio calculations of the structural, energetic, and vibrational properties of some hydrogen bonded and van der Waals dimers. Part 4. The acetonitrile dimer

The structures, interaction energies, Mulliken atomic charges, and vibrational spectra of five dimers of acetonitrile, CH₃CN, have been determined by means of ab initio molecular orbital theory, at the second order level of Møller–Plesset perturbation theory, using the 6-31++G(d,p) split-valence polarized basis set, augmented with diffuse functions. The structural features of the dimers are discussed, as well as their respective interaction energies, which have been corrected for basis set superposition error by the full counterpoise technique of Boys and Bernardi. The atomic charge reorganizations resulting from dimerization have also been discussed. The vibrational wavenumbers and infrared intensities of each dimer have been computed and, in the case of the most probable dimer, they have been used to determine the wavenumber shifts relative to the corresponding modes of the monomer. The shifts have been compared with those in the infrared and Raman spectra of acetonitrile molecules trapped in cryogenic matrices, previously reported in the literature. The computed and experimental results support the conclusion that the favored dimer structure is a centrosymmetric cyclic species of C_2h symmetry, stabilized by two weak CHN hydrogen bonds. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 226–240, 2001