VIBRATIONAL FREQUENCIES, RELATIVE STABILITY AND ANGULAR GEOMETRY OF SOME VINYL HALIDE HCL VAN-DER-WAALS COMPLEXES OBSERVED IN LIQUEFIED ARGON/

The mid-infrared spectra (4000 - 400 cm(-1)) of several vinyl halide/h ydrogen chloride mixtures, dissolved in liquefied argon at 105 K, were examined. In all spectra, evidence was found for the existence of a 1 :1 Van der Waals complex between CH2=CHX (X=Br, Cl, F) and HCl. At hig her concentrations of HCl, also absorption bands of a 1:2 species were observed. The stability of the 1:1 complexes was established using sp ectra recorded at several temperatures between 95 K and 115 K. The com plexation enthalpies derived were -5.75+/-0.30 kJ mol(-1) for CH2 CHF. HCl and -6.30+/-0.25 kJ mol(-1) for CH2=CHCl.HCl. Due to the low solub ility of vinyl bromide, no complexation enthalpy could be obtained for this compound. In addition, the complexation enthalpies of CH2=CHF(.H Cl)(2) and CH2=CHCl(.HCl)(2) were determined to be -9.85+/-0.34 kJ mol (-1) and -9.60+/-0.31 kJ mol(-1), respectively. A structural study, us ing ab initio calculations at the MP2/6-31+G* level, indicates that t he complexation can occur either via the halide atom (sigma complex) o r via the pi bond (pi complex). From a comparison of the experimental with the ab initio vibrational frequencies it was concluded that all o bserved bands of the 1:1 complexes are due to a s complex. Hence, no e vidence for the formation of a pi complex was found.