The structures of CO–CH3CCH and N2–CH3CCH: Spectroscopic measurements and modeling.

Microwave and infrared measurements have been made for the van der Waals complexes N2–CH3CCH and CO–CH3CCH, and their isotopomers. The infrared spectra are complicated by the effects of a slightly hindered internal rotation of the propyne unit. Tunneling motions arising from the N2 or CO molecules are not apparent in the infrared spectra, but in the case of N2 are seen in the microwave spectra. Analysis of the spectral data shows that the dimers are near slipped parallel in structure. Their dipole moments are almost parallel to the dimer b axis, with center-of-mass separations of 3.756(1) Å for CO–CH3CCH and 3.708(2) Å for N2–CH3CCH. (The uncertainties shown in parentheses are two standard deviations.) A simple model based on electrostatic interactions between rigid monomers gives dimer structures in agreement with the experimental assignments, and in conjunction with an approximate model of dispersion energy allows estimation of barriers to internal rotation. [ABSTRACT FROM AUTHOR]