Infrared spectroscopy of matrix isolated polycyclic aromatic hydrocarbons. 1. PAHs containing two to four rings

Matrix isolation techniques have been used to measure the mid-infrared spectra of the polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, phenanthrene, 1,2-benzanthracene, chrysene, pyrene, tetracene, and triphenylene. The band positions and relative strengths are compared to previous laboratory studies, where available, and with available theoretical calculations.(1) Comparisons with theory indicate that density functional theory (DFT) does an excellent job of describing the majority of the infrared active fundamentals of the PAHs considered here. Band positions typically match to within 5 cm(-1), with the worst mismatches usually no more than 15 cm(-1). Matches in band strengths are not as precise but, with the exception of the CH stretching bands, 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 stretching band strengths that are about a factor of 2 times too strong. The laboratory spectra contain large numbers of bands due to overtone/combination modes that are not considered in the calculations, and while most of these bands are weak, some can be moderately strong. Finally, comparisons between the infrared spectra of matrix isolated PAHs and a common family of interstellar emission bands confirm that neutral PAHs do not provide a good fit to the astronomical data. Neutral PAHs produce features having appropriate frequencies but generally inappropriate strengths. Nevertheless, significant contributions from PAH neutrals may be required to explain the spectra of a few less energetic astronomical environments.