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

Infrared and Laser-Induced Fluorescence Spectra of Sumanene Isolated in Solid para-Hydrogen

The para-hydrogen (p-H₂) matrix-isolation technique has been scarcely used to record electronic absorption and emission spectra. It is expected that its small matrix shifts due to diminished molecular interactions and the softness of the lattice might be advantageous to help identify the carriers of the diffuse interstellar bands. In this article, we present infrared, fluorescence excitation, and dispersed fluorescence spectra of sumanene (C₂₁H₁₂), a bowl-shaped polycyclic aromatic hydrocarbon and a fragment of C₆₀, isolated in solid p-H₂. The recorded vibrational wavenumbers from infrared and dispersed fluorescence agree with the scaled harmonic vibrational wavenumbers calculated with the B3PW91/6-311++G(2d,2p) and B3LYP/6-311++G(2d,2p) methods. The recorded fluorescence excitation spectra are consistent with the spectra of jet-cooled gas-phase C₂₁H₁₂ reported previously by Kunishige et al. We found a rather small matrix shift of 55 cm^-1 for the S₁-S₀ electronic transition origin located at 27 888 cm^-1. Vibrational wavenumbers associated with the S₁ state of C₂₁H₁₂ inferred from the experimental spectrum can be assigned mostly to fundamental normal modes; they are in satisfactory agreement with scaled harmonic vibrational wavenumbers calculated at the TD-B3PW91/6-311++G(2d,2p) level of theory. Significantly more vibrational modes of the S₁ state were identified as compared with those in the reported gas-phase work. The potential of p-H₂ matrix-isolation spectroscopy to provide electronic excitation spectra suitable for comparison to astronomical observations is discussed by comparing the spectra of C₂₁H₁₂ isolated in solid p-H₂ and in solid Ne, a matrix host commonly employed in astrochemistry.

Infrared Spectra of Monohydrogenated Aniline, ortho- and para-HC6H5NH2, Generated in Solid para-Hydrogen

The isomers of monohydrogenated aniline (HC₆H₅NH₂) are regarded as important intermediates in reduction reactions of aniline, but their spectral identification has been limited to electron paramagnetic resonance in an adamantane matrix. We report here infrared (IR) spectra of two least-energy isomers of HC₆H₅NH₂, produced on electron bombardment during the deposition of a matrix of aniline and para-hydrogen at 3.2 K. The intensities of IR lines of HC₆H₅NH₂ increased during maintenance of the electron-bombarded matrix in darkness for a prolonged period because of the neutralization of protonated aniline, H⁺C₆H₅NH₂, by trapped electrons and further reactions between aniline and the unreacted hydrogen atoms that were produced during electron bombardment. The observed lines were grouped according to their behaviors on secondary photolysis with light at 520, 465, and 375 nm. On comparison of experimental spectra with quantum chemically predicted spectra for four possible isomers of HC₆H₅NH₂, lines in one group were assigned to the most stable ortho-HC₆H₅NH₂ and those in the other group were assigned to the secondmost stable para-HC₆H₅NH₂. Their photolytic behaviors at varied wavelengths are consistent with predicted ultraviolet absorption bands. The mechanisms of formation of these isomers are discussed according to semiquantitative analysis.

Infrared spectrum of hydrogenated corannulene rim-HC20H10 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 H₂; spectral characterizations of these species are hence important. We report the infrared (IR) spectrum of mono-hydrogenated corannulene (HC₂₀H₁₀) in solid para-hydrogen (p-H₂). In experiments of electron bombardment of a mixture of corannulene and p-H₂ 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-HC₂₀H₁₀, 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 HC₂₀H₁₀ isomers and remain unassigned. Alternative hydrogenation was achieved with H atoms produced photochemically in the infrared-induced reaction Cl + H₂ (v = 1) → H + HCl in a Cl₂/C₂₀H₁₀/p-H₂ matrix. With this method, only lines attributable to rim-HC₂₀H₁₀ were observed, indicating that hydrogenation via a quantum-mechanical tunneling mechanism produces preferably the least-energy rim-HC₂₀H₁₀ regardless of similar barrier heights and widths for the formation of rim-HC₂₀H₁₀ and hub-HC₂₀H₁₀. The mechanisms of formation in both experiments are discussed. The bands near 3.3 and 3.4 µm of rim-HC₂₀H₁₀ agree with the UIR emission bands in position and relative intensity, but other bands do not match satisfactorily with the UIR bands.

Infrared spectra of 3-hydroxy-(1H)-pyridinium cation and 3-hydroxy-(1H)-pyridinyl radical isolated in solid para-hydrogen

As pyridine and its derivatives are regarded as building blocks of nitrogen-containing polycyclic aromatic hydrocarbons, spectral identifications of their protonated and hydrogenated species are important. The infrared (IR) absorption spectra of the 3-hydroxy-(1H)-pyridinium cation, 3-C₅H₄(OH)NH⁺, and the 3-hydroxy-(1H)-pyridinyl radical, 3-C₅H₄(OH)NH, produced on electron bombardment during deposition of a mixture of 3-hydroxypyridine, 3-C₅H₄(OH)N, and para-H₂ to form a matrix at 3.2 K were recorded. Intense IR absorption lines of trans-3-C₅H₄(OH)NH⁺ at 3594.4, 3380.0, 1610.6, 1562.2, 1319.4, 1193.8, 1167.5, and 780.4 cm^-1 and eleven weaker ones decreased in intensity after the matrix was maintained in darkness for 20 h, whereas lines of trans-3-C₅H₄(OH)NH at 3646.2, 3493.4, 3488.7, 1546.7, 1349.6, 1244.1, 1209.1, 1177.3, 979.8, and 685.2 cm^-1 and nine weaker ones increased. The intensities of lines of trans-3-C₅H₄(OH)NH decreased upon irradiation at 520 nm and diminished nearly completely upon irradiation at 450 nm, whereas those of trans-3-C₅H₄(OH)NH⁺ remained unchanged upon irradiation at 370, 450, and 520 nm. Observed vibrational wavenumbers and relative intensities of these species agree satisfactorily with the scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method. The observed 3-C₅H₄(OH)NH⁺ cation and 3-C₅H₄(OH)NH radical are predicted to be the most stable species among all possible isomers by quantum-chemical calculations.

Spectroscopy of prospective interstellar ions and radicals isolated in para-hydrogen matrices

The p-H₂ matrix-isolation technique coupled with photolysis in situ or electron bombardment produces protonated or hydrogenated species important in astrochemistry.

Infrared spectra of HSCS+, c-HSCS, and HCS2− produced on electron bombardment of CS2 in solid para-hydrogen

We report infrared spectra of HSCS⁺, c-HSCS, HCS₂⁻, and other species produced on electron bombardment of a mixture of CS₂ and para-hydrogen during deposition at 3.2 K.