IR spectra, relative stability and angular geometry of vinyl chloride[ndash ]HCl, vinyl bromide[ndash ]HCl and allyl chloride[ndash ]HCl van der Waals complexes observed in liquefied argon

Abstract

The mid-IR spectra (4000–400 cm^-1) of vinyl chloride–HCl, vinyl bromide–HCl and allyl chloride–HCl mixtures, dissolved in liquefied argon at 105 K, have been examined. In all spectra, evidence was found for the existence of a 1:1 van der Waals complex. At higher concentrations of hydrogen chloride, in the spectra of the vinyl chloride–HCl mixtures, absorption bands of a 1:2 species were also observed. Using spectra recorded at several temperatures between 95 and 120 K, the complexation enthalpies of CH₂CHCl·HCl and CH₂CHCl·(HCl)₂ were determined to be -6.5±0.2 kJ mol^-1 and -10.6±0.3 kJ mol^-1, respectively.

A structural study, using abinitio calculations at the MP2/6-31+G** level, indicates that the complexation between vinyl chloride and HCl can occur either via the chlorine atom or via the π-bond. From a comparison of the experimental with the abinitio vibrational frequencies it was concluded that all observed bands of the 1:1 complex are due to a species complexed via the chlorine atom. A similar conclusion applies to the vinyl bromide and allyl chloride complexes.