On the ethene/HCl Van der Waals complexes observed in liquefied argon and liquefied nitrogen

Abstract

The mid-infrared (4000–400 cm−1) and far-infrared (300–10 cm−1) spectra of C2H4/HCl mixtures, dissolved in liquefied argon (93–125 K) and in liquefied nitrogen (80–118 K) are discussed. In all spectra, experimental evidence was found for the existence of 1:1 and 1:2 Van der Waals complexes. Using spectra recorded at different temperatures, the complexation enthalpy ΔH° for the 1:1 complex was determined to be −8.7(2) kJ mol−1 in liquid argon and −6.4(4) kJ mol−1 in liquid nitrogen, while for the 1:2 complex a ΔH° of −18.3(6) kJ mol−1 was found. For all species studied, the vibrational frequencies were obtained from ab initio calculations at the MP2/6−311+G** level. Using the SCRF/SCIPCM scheme to correct for the solvent influences, and using statistical thermodynamics to account for the zero-point vibrational and thermal contributions, approximate values for the dissociation energy were calculated from the complexation enthalpies. The resulting values, −12.7(5) kJ mol−1 for the 1:1 complex and −26.9(2) kJ mol−1 for the 1:2 complex are compared with the ab initio values. The transition dipole moment of the ν2 ethene mode induced in the 1:1 complex is determined to be 0.13 D Å−1. A multipole analysis shows that the main contribution is due to the quadrupole moment of the HCl moiety, closely followed by that due to the dipole moment, 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 ab initio atomic polar tensors.