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

Abstract

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-C 32 H 15 + ) measured in a para-hydrogen (p-H 2 ) matrix at 3.2 K, generated by bombarding a mixture of ovalene and p-H 2 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.

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC+ and HBC2+

“…These three bands (6.2, 7.7, and 8.6 μm), which correlate well, therefore may be attributed to PAH cations. Recent studies, using PAH ions trapped in hydrogen matrices (Tsuge et al 2016) or measured in tagged ion dissociation studies (Knorke et al 2009;Ricks et al 2009), also show nice agreement with vibrational modes recorded for protonated PAH species. This is supported by observational studies that reveal that these bands grow in strength-relative to the CH bands characteristic for neutral PAHs-in regions characterized by high ionization parameters 4 ; e.g., close to the illuminating source (Rapacioli et al 2005;Berné et al 2007;Boersma et al 2013).…”

“…Experimentally, IR spectra of PAH neutrals, cations, and other derivatives have been measured in rare gas matrices (e.g., Szczepanski & Vala 1993a, 1993bHudgins & Allamandola 1995a, 1995bMattioda et al 2003;Bernstein et al 2007;Tsuge et al 2016). In the gas phase, IR emission spectra have been studied for neutral PAHs and PAH cations (Cook et al 1998;Kim et al 2001;Kim & Saykally 2002).…”

Low-temperature gas-phase formation of indene in the interstellar medium

“…Since Léger’s and Puget’s very first hypothesis of polycyclic aromatic hydrocarbons (PAHs)—organic molecules composed of fused benzene rings—as the missing link between small carbon molecules and carbonaceous nanoparticles (interstellar grains) ( 1 ), PAHs ( 2 ) along with their (de)hydrogenated ( 3 ), alkylated ( 4 , 5 ), protonated ( 6 ), and ionized ( 7 ) counterparts have been associated with the diffuse interstellar bands (DIBs) ( 8 )—discrete absorption features superimposed on the interstellar extinction curve ranging from the visible (400 nm) to the near-infrared (1.2 μm)—and the unidentified infrared (UIR) emission bands ( 9 , 10 ) in the 3- to14-μm range. Encompassing up to 20% of the carbon budget in our galaxy ( 11 , 12 ), the identification of PAHs in carbonaceous chondrites such as Murchison, Allende, and Orgueil ( 13 , 14 ) along with 13 C/ 12 C and D/H isotopic analyses ( 15 ) advocates a circumstellar origin of aromatics in carbon-rich asymptotic giant branch (AGB) stars and planetary nebulae as the descendants of AGB stars at elevated temperatures of a few 1000 K through molecular mass growth processes involving hydrogen abstraction–carbon addition (HACA) sequences ( 16 ).…”

Infrared Spectra of Isomers of Protonated Aniline in Solid para-Hydrogen

“…Although this method lacks mass selection, the electron bombardment of a p-H2 matrix containing aromatic species has been proven to produce protonated and hydrogenated species with negligible fragmentations; methods to distinguish protonated and hydrogenated products have been established. [29][30][31][32][33][34][35][36] Because the spectral shift from the gaseous phase is expected to be small in solid p-H2, a comparison with quantum-chemically predicted IR spectra of potential carriers provides definitive assignments of these species. In this work, we applied this method to identify unambiguously the IR spectra of para-, ortho-, and amino-H + C6H5NH2.…”