QUARTIC CENTRIFUGAL-DISTORTION CONSTANTS DERIVED FROM A FLEXIBLE MODEL FOR 3-METHYLTHIETAN

The one-dimensional flexible model treatment of observed large amplitude vibrational frequencies and effective rotational constants in nonlinear molecules has been extended to include quartic centrifugal distortion constants for different vibrational states. The importance of centrifugal distortion data to the assignment of vibrational levels in asymmetric double minimum systems, recently pointed out in the literature, is confirmed for the ring puckering states of 3-methylthietan. A comprehensive set of experimental data for this molecule, 22 vibrational frequencies, 18 rotational constants, and 18 centrifugal distortion constants, are reproduced by 9 physically meaningful model parameters describing the potential energy function and the structural relaxation associated with the ring puckering motion. Two additional excited states have been observed in the microwave spectrum as predicted by the model. © 1991.