Density functional theory prediction of the vibrational spectra of polycyclic aromatic hydrocarbons: effect of molecular symmetry and size on Raman intensities

The ground state structure, vibrational force fields and Raman intensities of several polycyclic aromatic hydrocarbons are computed with the help of density functional theory (DFT) calculations. The DFT computed vibrational force fields and Raman intensities are analyzed and compared for molecules with different size and shape. The computed Raman spectra are in good agreement with the experimental data and it is found that the largest computed intensities correspond to vibrations whose content of ideal coordinates A and R is large. An analysis of local contributions to Raman intensities shows that the Raman active modes related to the A ideal coordinate contribute to Raman intensities mainly through polarizability variations associated with CC stretches. Conversely, both local CC stretching and CCC bending components contribute to the Raman activities of modes related to the R ideal coordinate. This analysis suggests a mechanism to explain the appearance of the D band in graphitic materials. (C) 2001 Elsevier Science B.V. All rights reserved.