THEORETICAL INFRARED-SPECTRA FOR POLYCYCLIC AROMATIC HYDROCARBON NEUTRALS, CATIONS, AND ANIONS

Calculations are carried out using density functional theory (DFT) to determine the harmonic frequencies and intensities of the neutrals and cations of 13 polycyclic aromatic hydrocarbons (PAHs) up to the size of ovalene. Calculations are also carried out for a few PAH anions. Th e DFT harmonic frequencies, when uniformly scaled to account primarily for anharmonicity, agree with the matrix isolation fundamentals to wi thin an average error of about 10 cm(-1). Electron correlation is foun d to significantly reduce the intensities of many of the cation harmon ics, bringing them into much better agreement with the available exper imental data. While the theoretical infrared spectra agree well with t he experimental data for all of the neutral systems and for many of th e cations, there still remain discrepancies with the experimental matr ix isolation data for some species that are difficult to rationalize e ntirely in terms of limitations in the calculations. In agreement with previous theoretical work, the present calculations show that the rel ative intensities for the astronomical unidentified infrared (UIR) ban ds agree reasonably well with those for a distribution of polycyclic a romatic hydrocarbon (PAH) cations but not with a distribution of PAH n eutrals. We also observe that the infrared spectra of highly symmetric al cations such as coronene agree much better with astronomical observ ations than do those of, for example, the polyacenes such as tetracene and pentacene. The total integrated intensities for the neutral speci es are found to increase linearly with size, while the total integrate d intensities are much larger for the cations and scale more nearly qu adratically with size. We conclude that emission from moderate-sized h ighly symmetric PAH cations such as coronene and larger could account for the UIR bands.