The Infrared Spectrum of Protonated Ovalene in Solid Para-Hydrogen and its Possible Contribution to Interstellar Unidentified Infrared Emission

The mid-infrared emission from galactic objects, including reflection nebulae, planetary nebulae, proto-planetary nebulae, molecular clouds, etc, as well as external galaxies, is dominated by the unidentified infrared (UIR) emission bands. Large protonated polycyclic aromatic hydrocarbons (H⁺PAHs) were proposed as possible carriers, but no spectrum of an H⁺PAH has been shown to exactly match the UIR bands. Here, we report the IR spectrum of protonated ovalene (7-C₃₂H₁₅ ⁺) measured in a para-hydrogen (p-H₂) matrix at 3.2 K, generated by bombarding a mixture of ovalene and p-H₂ with electrons during matrix deposition. Spectral assignments were made based on the expected chemistry and on the spectra simulated with the wavenumbers and infrared intensities predicted with the B3PW91/6-311++G(2d,2p) method. The close resemblance of the observed spectral pattern to that of the UIR bands suggests that protonated ovalene may contribute to the UIR emission, particularly from objects that emit Class A spectra, such as the IRIS reflection nebula, NGC 7023.