THE STRUCTURES OF CO-CH3CCH AND N-2-CH3CCH - SPECTROSCOPIC MEASUREMENTS AND MODELING

Microwave and infrared measurements have been made for the van der Waa ls complexes N-2-CH3CCH and CO-CH3CCH, and their isotopomers. The infr ared spectra are complicated by the effects of a slightly hindered int ernal rotation of the propyne unit. Tunneling motions arising from the N-2 or CO molecules are not apparent in the infrared spectra, but in the case of N-2 are seen in the microwave spectra. Analysis of the spe ctral data shows that the dimers are near slipped parallel in structur e. Their dipole moments are almost parallel to the dimer b axis, with center-of-mass separations of 3.756(1) Angstrom for CO-CH3CCH and 3.70 8(2) Angstrom for N-2-CH3CCH. (The uncertainties shown in parentheses are two standard deviations.) A simple model based on electrostatic in teractions between rigid monomers gives dimer Structures in agreement with the experimental assignments, and in conjunction with an approxim ate model of dispersion energy allows estimation of barriers to intern al rotation.