Theoretical description of the overtone spectra of acetaldehyde using the local mode approach

The overtone spectra of the C–H bonds of the methyl group of acetaldehyde are analyzed using a coupled oscillator local mode theory. The higher energy absorption band of each overtone region is assigned to the in-plane C–H methyl bond while the lower energy absorption band is assigned to the two out-of-plane methyl C–H bonds. We conclude that the two out-of-plane methyl group C–H bonds are weaker and have longer bond lengths than the corresponding in-plane C–H bond. This observation is in agreement with predictions based on the trans effect theory. The calculations exemplify the problem of assigning overtone spectra based on relative intensities. In particular, the lower energy band of the methyl C–H group is calculated to be more intense than the higher energy band irrespective of assignment. This theoretical prediction is associated with the importance of oscillator coupling via the kinetic energy matrix element.