A computational study of the Raman spectra of large polycyclic aromatic hydrocarbons: Toward molecularly defined subunits of graphite

We present a quantum chemical investigation of the vibrational frequencies and Raman intensities of large polycyclic aromatic hydrocarbons (PAHs) as models for nanosized graphitic domains in carbon materials. The gradient corrected BLYP functional, in the framework of density functional theory, was employed to compute equilibrium structures, vibrational force fields, and Raman intensities. Molecules of different size (from 24 to 138 carbon atoms) and symmetry (D-6h, D-3h, D-2h) were investigated. It is shown that the simulated Raman spectra compare well with the available experimental spectra. The analysis of Raman activities in terms of local contributions from internal coordinates shows that the PAHs can be grouped in essentially two classes related more strictly to their benzenoid character than to the shape of their periphery. The analysis of local contributions to polarizability derivatives combined with the shape of the molecular motion of the Raman active vibrations in the 1300 cm(-1) region of PAHs supports the mechanism recently proposed by us to explain the appearance of the D band in disordered graphitic materials.