Hydrogenated polycyclic aromatic hydrocarbons have been proposed both as potential carriers of the unidentified infrared (UIR) bands and as catalytic sites for H2 formation in astrophysical environments. We report the infrared (IR) spectra for five monohydrogenated phenanthrene isomers (1-, 2-, 3-, 4-, and 9-HC14H10), generated by electron bombardment of phenanthrene (C14H10) codeposited with para-hydrogen onto a cryogenic substrate. Distinct absorption signatures intensified during extended dark storage of the matrix, while subsequent irradiation at 423, 380, 315, and 223 nm produced characteristic photochemical responses, enabling classification into five distinct groups. Assignments to individual isomers were supported by comparison with scaled harmonic vibrational wavenumbers and IR intensities calculated at the B3LYP/6-311++G(d,p) level of theory. Hydrogen addition was observed at all accessible nonbridging carbon sites. The resulting spectra exhibit intense features in the 11.5–14.5 μm region, indicating that these species are unlikely to represent major contributors to the UIR emission bands.
Feng et al. (Thu,) studied this question.