Infrared and ab initio study of the relative stability and geometry of the3-fluoropropene-hydrogen chloride van der Waals complexes

The mid-infrared spectra (4000-400 cm(-1)) of complexes between the cis and gauche conformers of 3-fluoropropene, CH2=CHCH2F, and HCl have been record ed in liquefied argon at temperatures ranging from 103 to 123 K. From this data, the complexation enthalpy for the complex with cis-3-fluoropropene wa s determined to be -9.6(3) kJ mol(-1), while that for the complex with gauc he 3-fluoropropene was estimated, from the observed HCl stretching frequenc y, to be -10.4(5) kJ mol(-1). The stoichiometry of these complexes was dete rmined from constant temperature, variable concentration experiments to be 1:1. A few weak bands in the spectra are assigned to higher order complexes . An ab initio study of the structures and vibrational frequencies of the c omplex was performed at the B3LYP/G-311++G(d,p) level. These calculations s how that HCl can form a van der Waals bond with either the carbon-carbon do uble bond or the fluorine atom. Comparison of the experimental frequencies with the calculated ones shows that all observed complex bands were due to complex formation with the fluorine atom.