Internal Rotation and Vibration in CH2=CCl–CH2D

The infrared spectrum of CH2=CCl–CH2D in the region 4000–160 cm−1 is presented and assigned. From a comparison of the observed vibrational spectrum with normal-coordinate results based on the assumption of rotational isomers the methyl group is found to eclipse the double bond. A torsion–vibration calculation is set up and applied to the same molecule for comparison with the normal-coordinate results. Owing to the relatively high torsional barrier the vibrational fundamentals obtained from the two approaches differ by less than 1 cm−1. The results suggest that for more precise treatments of vibrational spectra the dependence of the kinetic-energy matrix on all internal coordinates should be considered.