METHYL TORSIONAL INTERACTIONS IN ACETONE

The ground-state potential surface for methyl torsional interactions in acetone has been obtained by ab initio, semiempirical, and empirical methods. Frequencies acquired from recent Rydberg spectroscopy measurements on the a2 ground-state fundamental and overtone torsional vibrations and infrared measurements on b2 fundamentals in acetone-h6 and acetone-d6 are well simulated by purely theoretical ab initio HF/6-31G(d,p) and scaled MP2/6-31G(d,p) (i.e., semiempirical) fully-relaxed model potentials. This model incorporates skeletal flexing during the methyl rotation, causing the CCC angle to vary by 3°. The single unique empirical surface obtained from the above mentioned measured frequencies is in good agreement with the ab initio and scaled ones. The four empirically determined constants in the two (equivalent C3v) top potential function, eq 3, are V3 = 370 cm-1, V33 = 136 cm-1, V′33 = -156 cm-1 and V6 = 0 cm-1. The torsional barrier, Veff, is found to be 240 cm-1, significantly lower than the microwave value. We are unable to predict potential constants that simulate the fre quency-generated empirical ones at any level of ab initio calculation [i.e., HF/DZ, HF/6-31G(d,p), and MP2/6-31G(d,p)] within the rigid-frame model (i.e., geometry parameters at equilibrium are held fixed during the methyl top rotation). © 1990, American Chemical Society. All rights reserved.