THE CALCULATION OF ACCURATE HARMONIC FREQUENCIES OF LARGE MOLECULES -THE POLYCYCLIC AROMATIC-HYDROCARBONS, A CASE-STUDY

The vibrational frequencies and infrared intensities of naphthalene ne utral and cation are studied at the self-consistent-field (SCF), secon d-order Moller-Plesset (MP2), and density functional theory (DFT) leve ls using a variety of one-particle basis sets. Very accurate frequenci es can be obtained at the DFT level in conjunction with large basis se ts if they are scaled with two factors, one for the C-H stretches and a second for all other modes. We also find remarkably good agreement a t the B3LYP/4-31G lever using only one scale factor. Unlike the neutra l polycyclic aromatic hydrocarbons (PAHs) where all methods do reasona bly well for the intensities, only the DFT results are accurate for th e PAH cations. The failure of the SCF and MP2 methods is caused by sym metry breaking and an inability to describe charge delocalization. We present several interesting cases of symmetry breaking in this study. An assessment is made as to whether an ensemble of PAH neutrals or cat ions could account for the unidentified infrared bands observed in man y astronomical sources. (C) 1997 Elsevier Science B.V.