THE STRUCTURE OF AR-C2H4 FROM HIGH-RESOLUTION INFRARED-SPECTROSCOPY AND ABINITIO THEORY - THE TWOFOLD BARRIER TO C2H4 INTERNAL-ROTATION

The optothermal detection method has been used to obtain high resolution infrared spectra for both the v9 and v11 vibrational bands of Ar-ethylene. Analysis of these spectra reveals that the argon atom lies essentially in the plane of the ethylene, situated next to the carbon-carbon double bond. Although the rigid structure that best fits the measured rotational constants actually places the argon atom 15' out of the ethylene plane, the experimental and theoretical results are more consistent with a planar equilibrium geometry and wide amplitude in-plane and out-of-plane motion of the ethylene sub-unit. In both spectra the transitions appear as doublets, due to the fact that the ethylene sub-unit undergoes internal rotation between equivalent frameworks. Of the possible paths, only internal rotation about the C=C axis is consistent with the data. The barrier to internal rotation is estimated from the tunneling splitting to be 76 cm-1 which is compared with the results of ab initio calculations.