Infrared spectra of ovalene (C32H14) and hydrogenated ovalene (C32H15˙) in solid para-hydrogen

Abstract

We report the infrared (IR) spectra of ovalene (C₃₂H₁₄) and hydrogenated ovalene (C₃₂H₁₅˙) in solid para-hydrogen (p-H₂). The hydrogenated ovalene and protonated ovalene were generated from electron bombardment of a mixture of ovalene and p-H₂ during deposition of a matrix at 3.2 K. The features that decreased with time have been previously assigned to 7-C₃₂H₁₅⁺, 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-C₃₂H₁₅˙) 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-C₃₂H₁₅˙ is discussed according to the observed changes in intensity and calculated energetics of possible reactions of H + C₃₂H₁₄ and isomerization of C₃₂H₁₅˙. The formation of 7-C₃₂H₁₅˙ is dominated by the reaction H + C₃₂H₁₄ → 7-C₃₂H₁₅˙, implying that, regardless of the presence of a barrier, the hydrogenation of polycyclic aromatic hydrocarbons occurs even at 3.2 K.