Calculated CH-stretching overtone spectra of naphthalene, anthracene and their cations

We have calculated fundamental and overtone CH-stretching vibrational band frequencies and intensifies of neutral and cationic naphthalene and anthrac ene, the smallest of the polycyclic aromatic hydrocarbons. The calculations use a simple anharmonic oscillator local mode model with local mode parame ters obtained from scaled ab initio calculations and ab initio calculated d ipole moment functions. The ab initio calculations were performed at the se lf-consistent-field Hartree-Fock and the B3LYP hybrid density functional le vels of theory with the 6-31G(d) and 6-311+G(d,p)basis sets. The Vapor phas e overtone spectra of anthracene and both cations have not yet been observe d. We find that the cation frequencies are blue shifted and the overtone tr ansitions are of comparable intensities to those in the neutral molecules, contrary to the fundamental transitions which are significantly weaker in t he cations. The method presented here can provide an accurate estimate of C H-stretching overtone spectra of neutral and cationic polycyclic aromatic h ydrocarbons.