A precessing rotor model for structural flexing during torsional motion:: is there evidence from the internal rotation kinetic energy coefficient F?

A precessing rotor model is used to mimic the structural flexing which is predicted by ab initio calculations to accompany internal rotation in molecules. The internal rotation kinetic energy coefficient F(tau) is found to vary with tau, and to have an average value F-0 significantly less than the constant F-rig calculated for a rigid model. Analysis of published torsional data for a range of substituted toluenes shows that F-0 obs/F-rig approximate to 0.97, consistent with the occurrence of precession with a tilt angle theta approximate to 1.5 degrees. In fitting experimental spectra, it was found that the Fourier coefficients F-3 and F-6 are highly correlated with the potential constants V-3 and V-6, and cannot be separately determined. Thus the predicted variation of F with tau could not be confirmed. The effect of structural flexing on torsion-rotation interaction constants is also examined. (C) 2000 Elsevier Science B.V. All rights reserved.