INFRARED-SPECTROSCOPY OF MATRIX-ISOLATED POLYCYCLIC AROMATIC-HYDROCARBONS - 1 - PAHS CONTAINING 2 TO 4 RINGS

Matrix isolation techniques have been used to measure the mid-infrared spectra of the polycyclic aromatic hydrocarbons (PAHs) naphthalene, a nthracene, phenanthrene, 1,2-benzanthracene, chrysene, pyrene, tetrace ne, and triphenylene. The band positions and relative strengths are co mpared to previous laboratory studies, where available, and with avail able theoretical calculations.(1) Comparisons with theory indicate tha t density functional theory (DFT) does an excellent job of describing the majority of the infrared active fundamentals of the PAHs considere d here. Band positions typically match to within 5 cm(-1), with the wo rst mismatches usually no more than 15 cm(-1). Matches in band strengt hs are not as precise but, with the exception of the CH stretching ban ds, are generally good to better than 35% for most strong and moderate bands and to factors of 2 to 3 for weaker bands. Theory predicts CH s tretching band strengths that are about a factor of 2 times too strong . The laboratory spectra contain large numbers of bands due to overton e/combination modes that are not considered in the calculations, and w hile most of these bands are weak, some can be moderately strong. Fina lly, comparisons between the infrared spectra of matrix isolated PAHs and a common family of interstellar emission bands confirm that neutra l PAHs do not provide a good fit to the astronomical data. Neutral PAH s produce features having appropriate frequencies but generally inappr opriate strengths. Nevertheless, significant contributions from PAH ne utrals may be required to explain the spectra of a few less energetic astronomical environments.