Infrared spectrum of hydrogenated corannulene rim-HCH isolated in solid para-hydrogen.

Hydrogenated polycyclic aromatic hydrocarbons have been proposed to be carriers of the interstellar unidentified infrared (UIR) emission bands and the catalysts for formation of H; spectral characterizations of these species are hence important. We report the infrared (IR) spectrum of mono-hydrogenated corannulene (HCH) in solid para-hydrogen (p-H). In experiments of electron bombardment of a mixture of corannulene and p-H during deposition of a matrix at 3.2 K, two groups of spectral lines increased with time during maintenance of the matrix in darkness after deposition. Lines in one group were assigned to the most stable isomer of hydrogenated corannulene, rim-HCH, according to the expected chemistry and a comparison with scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method. The lines in the other group do not agree with predicted spectra of other HCH isomers and remain unassigned. Alternative hydrogenation was achieved with H atoms produced photochemically in the infrared-induced reaction Cl + H (v = 1) → H + HCl in a Cl/CH/p-H matrix. With this method, only lines attributable to rim-HCH were observed, indicating that hydrogenation via a quantum-mechanical tunneling mechanism produces preferably the least-energy rim-HCH regardless of similar barrier heights and widths for the formation of rim-HCH and hub-HCH. The mechanisms of formation in both experiments are discussed. The bands near 3.3 and 3.4 µm of rim-HCH agree with the UIR emission bands in position and relative intensity, but other bands do not match satisfactorily with the UIR bands.