The effects of the CO wagging coordinate relaxation on the far-infrared torsional spectrum of acetone

The effect of the full relaxation of the molecular geometry during the double internal methyl rotation in acetone is analyzed. For this purpose, the potential energy function for the double rotation in acetone is recalculated and the spectrum simulated. It is seen that the relaxation of the out-of-plane wagging coordinate shifts the far-infrared frequency of the antisymmetric fundamental vibration, ν17, by 1.8 cm−1, but leaves invariant the Raman symmetric ν12. Furthermore, the infrared intensities are somewhat reduced especially in the vibrationally excited states.