The Intermolecular Force Field of Acetylene-HCl: The Normal Coordinate and Centrifugal Distortion Analysis of Two Weakly Bound Rigid Rods

In order to determine the intermolecular force field of C2H2⋅⋅HCl, a normal coordinate analysis appropriate for two weakly bound rigid rods is developed. An exact centrifugal distortion analysis is then carried out on the microwave spectrum of this T\u27\u27-shaped hydrogen-bonded π complex. The distortion constants and published matrix-isolated vibrational frequencies are employed in the normal coordinate analysis to obtain the intermolecular force field and refinements in the structural parameters. The following van der Waals force field is found to reproduce the existing spectroscopic constants of C2H2⋅⋅H35Cl … . This study permits the first opportunity to ascertain the effect of complexation on monomer properties in C2H2⋅⋅HX/DX where X = F,Cl. A more accurate description of the quadrupole coupling constant acknowledging the effects of vibrational averaging of the projection operator and changes in (∂2V/∂z2) indicate that hydrogen bonding decreases (∂2V/∂z2) by 7.8% for Cl in C2H2⋅⋅HCl and 10% for deuterium in C2H2⋅⋅DF. In the course of this study, DHF and rHF, the spin-spin coupling constant and the H-F bond length, were obtained for the complex. Comparison with rHF in free HF indicates that complexation causes an increase in bond length by 1%