Contrasting behaviour of hydrogen fluoride and hydrogen chloride in the formation of weak complexes with methane

Abstract

A theoretical study of weak molecular complexes of methane interacting with hydrogen fluoride and hydrogen chloride has been carried out. Geometries were optimized at the MP2/6-311G (d,p) level of MO theory while binding energies were determined at MP4/6-311 + + G (d,p). Calculated results show the formation of a non-conventional (HCH 3…HCl) complex in which methane acts as a proton acceptor (as in the earlier-discussed (HCH 3…HCN) system) versus a more conventional (H 3CH…FH) dimer in which methane plays the role of a proton donor. These results are in agreement with those derived from both vibrational and rotational spectrometries. An attempt to rationalize this contrasting behaviour of HF and HCl in complex formation with methane is presented in the framework of an electrostatic model. The non-conventional structure of (CH 4, HCl) or (CH 4, HCN) is attributable to the anisotropy of the short-range forces which allows a closer approach in (HCH 3…HX) than in (H 3CH…XH).