The structures and vibrational spectra of the methyl halide dimers. An ab initio study

Four homodimers and six heterodimers of the methyl halides, CH3X (X = F, Cl, Br, I), have been examined by means of ab initio calculations at the second order level of Møller-Plesset perturbation theory and with an augmented correlation-consistent polarized valence triple-zeta basis set. Three families of dimers have been found. All four homodimers optimize as both cyclic doubly hydrogen-bonded CH … X species and as non-hydrogen-bonded van der Waals C … X aggregates. All six heterodimers also optimize with cyclic doubly bonded structures; in addition, three of them are non-hydrogen-bonded with C … X linkages, and the three containing CH3I are singly bonded, with a CH … I interaction. The properties normally regarded as distinguishing features separating red-shifted from blue-shifted hydrogen bond interactions (CH bond length changes, shifts of the CH stretching wavenumbers and infrared CH stretching band intensity ratios) have been determined, and their variations with respect to the structures of the adducts have been rationalized.