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 p redicted by ab initio calculations to accompany internal rotation in molecu les. The internal rotation kinetic energy coefficient F(tau) is found to va ry with tau, and to have an average value F-0 significantly less than the c onstant 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 approxim ate 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. T hus the predicted variation of F with tau could not be confirmed. The effec t of structural flexing on torsion-rotation interaction constants is also e xamined. (C) 2000 Elsevier Science B.V. All rights reserved.