Vibronically resolved electronic circular dichroism spectra of (R)-(+)-3-methylcyclopentanone: a theoretical study.

The vibrationally resolved electronic circular dichroism (ECD) spectra of the two dominant conformers of (R)-(+)-3-methylcyclopentanone in gas phase are computed by density functional response theory, with a full account of Franck-Condon and Herzberg-Teller vibrational contributions at the harmonic level. Proper inclusion of the latter contributions was made possible by the recent implementation of effective-scaling computations of vibrational overlaps and of analytical gradients of time dependent DFT. The Coulomb-attenuated Becke three parameters Lee-Yang-Parr (CAM-B3LYP) functional reproduces both the position and the intensity of the experimental peaks, providing a remarkable improvement over the spectra obtained with the popular hybrid B3LYP functional, and allowing a confident assignment of the CD fine vibrational structure. Franck-Condon and Herzberg-Teller contributions are discussed in detail. The computed decrease of the CD intensity in the gas phase upon increase of the temperature of the sample follows the trend observed experimentally in different solvents.