| Abstract
| - We present a quantum chemical investigation of the vibrational frequencies and Raman intensities of largepolycyclic 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 tocompute equilibrium structures, vibrational force fields, and Raman intensities. Molecules of different size(from 24 to 138 carbon atoms) and symmetry (D6h, D3h, D2h) were investigated. It is shown that the simulatedRaman spectra compare well with the available experimental spectra. The analysis of Raman activities interms of local contributions from internal coordinates shows that the PAHs can be grouped in essentially twoclasses related more strictly to their “benzenoid” character than to the shape of their periphery. The analysisof local contributions to polarizability derivatives combined with the shape of the molecular motion of theRaman active vibrations in the 1300 cm-1 region of PAHs supports the mechanism recently proposed by usto explain the appearance of the D band in disordered graphitic materials.
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