Infrared spectra of ovalene (CH) and hydrogenated ovalene (CH˙) in solid para-hydrogen.

We report the infrared (IR) spectra of ovalene (CH) and hydrogenated ovalene (CH˙) in solid para-hydrogen (p-H). The hydrogenated ovalene and protonated ovalene were generated from electron bombardment of a mixture of ovalene and p-H during deposition of a matrix at 3.2 K. The features that decreased with time have been previously assigned to 7-CH, the most stable isomer of protonated ovalene (Astrophys. J., 2016, 825, 96). The spectral features that increased with time are assigned to the most stable isomer of hydrogenated ovalene (7-CH˙) based on the expected chemistry and on a comparison with the vibrational wavenumbers and IR intensities predicted by the B3PW91/6-311++G(2d,2p) method. The mechanism of formation of 7-CH˙ is discussed according to the observed changes in intensity and calculated energetics of possible reactions of H + CH and isomerization of CH˙. The formation of 7-CH˙ is dominated by the reaction H + CH → 7-CH˙, implying that, regardless of the presence of a barrier, the hydrogenation of polycyclic aromatic hydrocarbons occurs even at 3.2 K.