ON THE ETHENE HCL VAN-DER-WAALS COMPLEXES OBSERVED IN LIQUEFIED ARGONAND LIQUEFIED NITROGEN/

The mid-infrared (4000-400 cm(-1)) and far-infrared (300-10 cm(-1)) sp ectra of C2H4/HCl mixtures dissolved in liquefied argon (93-125 K) and in liquefied nitrogen (80-118 K) are discussed. In all spectra, exper imental evidence was found for the existence of 1:1 and 1:2 Van der Wa als complexes. Using spectra recorded at different temperatures, the c omplexation enthalpy Delta dH degrees for the 1:1 complex was determin ed to be -87(2) kJ mol(-1) in liquid argon and -6.4(4) kJ mol(-1) ill liquid nitrogen, while for the 1:2 complex a Delta H degrees of -18.3( 6) kJ mol(-1) was found. For all species studied, the vibrational freq uencies were obtained from ab initio calculations at the MP2/6-311+G* level. Using the SCRF/SCIPCM scheme to correct for the solvent influe nces. and using statistical thermodynamics to account for the zero-poi nt vibrational and thermal contributions, approximate values for the d issociation energy were calculated from the complexation enthalpies, T he resulting values, -12.7(5) kJ mol(-1) for the 1:1 complex and -26.9 (2) kJ mol(-1) for tile 1:2 complex are compared with the nb initio va lues. The transition dipole moment of the nu(2) ethene mode induced in the 1:1 complex is determined to be 0.13 D Angstrom(-1). A multipole analysis shows that the main contribution is due to the quadrupole mom ent of the HCl moiety, closely followed by that due to the dipole mome nt, with much smaller contributions due to higher multipoles, In the 1 :1 complex, the HCl stretching intensity is enhanced by a factor of 7. 7 in comparison with the monomer. This increase is interpreted in the equilibrium charge-charge flux model using nit initio atomic polar ten sors. (C) 1997 American Institute of Physics.