Laboratory spectroscopy and astronomical significance of the fully-benzenoid PAH triphenylene and its cation

Triphenylene (C18H12) is a highly symmetric polycyclic aromatic hydrocarbon (PAH) molecule with a ‘fully-benzenoid’ electronic structure. This confers a high chemical stability compared with PAHs of similar size. Although numerous infrared and UV-visible experimental spectroscopic and theoretical studies of a wide range PAHs in an astrophysical context have been conducted, triphenylene and its radical cation have received almost no attention. There exists a huge body of spectroscopic evidence for neutral and ionised PAHs in astrophysical sources, obtained principally through detection of infrared emission features that are characteristic of PAHs as a chemical class. However, it has so far not proved possible to identify spectroscopically a single isolated PAH in space, although PAHs including triphenylene have been detected mass spectrometrically in meteorites. In this work we focus on recording laboratory electronic spectra of neutral and ionised triphenylene between 220 and 780 nm, trapped in H2O ice and solid argon at 12 K. The studies are motivated by the potential for spectroscopic astronomical detection of electronic absorption spectra of PAHs in ice mantles on interstellar grains as discussed by Linnartz (2014), and were undertaken also in a cold Ar matrix to provide guidance as to whether triphenylene (particularly in its singly positively ionised form) could be a viable candidate for any of the unidentified diffuse interstellar absorption bands. Based on the argon-matrix experimental results, comparison is made with previously unpublished astronomical spectra near 400 nm which contain broad interstellar absorption features consistent with the predictions from the laboratory matrix spectra, thus providing motivation for the recording of gas-phase electronic spectra of the internally cold triphenylene cation

ELECTRON SPIN RESONANCE INVESTIGATION OF FORMATION MECHANISMS OF MATRIX ISOLATED H4+H_{4}^{+}

Author Institution: Department of Chemistry, Furman University, Greenville, SCHydrogen cluster ions are of interest as reactants in astrophysical processes and as simple models for theoretical calculations. In this work, the formation mechanism of $H_{4}^{+}$ and its deuterated isotopomers was investigated by varying the experimental conditions required to observe $H_{4}^{+}$ isolated in a neon matrix. The $H_{4}^{+}$ cluster was formed by mixing $H_{2}$, $D_{2}$, and HD gases with neon and depositing the mixtures onto a copper rod cooled by liquid helium. The resulting matrix was then x-irradiated at 60 keV for 30 minutes and electron spin resonance spectra were recorded. Previous studies conducted in our lab have indicated that hydrogen cluster cations can only be formed at extremely low temperatures (2.6 K) and are very sensitive to temperature change. In the current study, the local environment of the deposition region was characterized by investigating the allowable temperature range, the effect of sample gas flow rate, and the need for nearby cold surfaces

Electron Spin Resonance Investigation of the Matrix Isolated NHCH2+ Radical Cation

Methanimine (NHCH2) is the simplest imine, and is of astrophysical interest in interstellar dust clouds. In this study, NHCH2+ was generated by x-ray irradiation of a mixture of dinitrogen and methane, and their various isotopologues. Neutral species were trapped in an inert neon gas matrix by depositing the gas mixture onto a copper surface cooled to 4 K by liquid helium. The resulting matrix was then x-irradiated for varying lengths of time and power to optimize generation conditions. Electron spin resonance spectra of the irradiated matrix were recorded to give insight into the electronic structure and behavior of this radical cation in the neon matrix

Vibrational and Electronic Absorption Spectroscopy of 2,3-Benzofluorene and Its Cation

Benzofluorene (C17H12) has been studied in argon matrices via Fourier transform infrared and UV?visible absorption spectroscopy. The analysis of the infrared absorption spectra of neutral and cationic 2,3-benzofluorene was supported by density functional theory (DFT) B3LYP/6-311+G** calculations of the harmonic-mode frequencies. Extensive time-dependent DFT calculations of the electronic vertical excitation energies with BLYP/6-31++G** and B3LYP/6-31++G** functionals/basis sets and the Casida?Salahub asymptotic correction were performed to assign the observed electronic absorption bands of the neutral species. Although the observed low-energy absorption bands are predicted well by theory, the higher-energy bands (Sn ? S0 transitions, n ≥ 4) have been assigned only tentatively. However, the observed electronic absorption bands for the parent, singly dehydrogenated cationic and neutral species are in accord with TDDFT (BLYP/6-31G**) results. The possibility that the 2,3-benzofluorene cation contributes to the unidentified infrared (UIR) bands observed from interstellar space is discussed briefly. Benzofluorene (C17H12) has been studied in argon matrices via Fourier transform infrared and UV?visible absorption spectroscopy. The analysis of the infrared absorption spectra of neutral and cationic 2,3-benzofluorene was supported by density functional theory (DFT) B3LYP/6-311+G** calculations of the harmonic-mode frequencies. Extensive time-dependent DFT calculations of the electronic vertical excitation energies with BLYP/6-31++G** and B3LYP/6-31++G** functionals/basis sets and the Casida?Salahub asymptotic correction were performed to assign the observed electronic absorption bands of the neutral species. Although the observed low-energy absorption bands are predicted well by theory, the higher-energy bands (Sn ? S0 transitions, n ≥ 4) have been assigned only tentatively. However, the observed electronic absorption bands for the parent, singly dehydrogenated cationic and neutral species are in accord with TDDFT (BLYP/6-31G**) results. The possibility that the 2,3-benzofluorene cation contributes to the unidentified infrared (UIR) bands observed from interstellar space is discussed briefly

Vibrational and electronic spectroscopy of acenaphthylene and its cation

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ELECTRON SPIN RESONANCE INVESTIGATION OF MASS-SELECTED, MATRIX ISOLATED RADICAL CATIONS

Author Institution: Department of Chemistry, Furman University, Greenville, SC 29613Small mass-selected, radical cations were matrix isolated and investigated by ESR spectroscopy. Ions produced by electron bombardment were mass-selected in a quadrupole filter and co-deposited with neon on a 4 K copper surface. The unit resolution of the quadrupole mass filter enabled distinction of adjacent ions in the mass spectrum. Collisional fragmentation of selected ions was minimized by low ion energies (15-30 eV). Ion beam current ranged from 0.2 nA to 2 nA. During deposition, the matrix was neutralized by an electron beam perpendicular to the ion flow. Because positive and negative species were trapped in separate matrix sites, matrix neutrality was maintained without total cation elimination. Systems studied include various boron hydride cations, hydrocarbon cations of astrophysical interest, and the first observation of matrix isolated $^{16}$O$^{+}$ and $^{17}$O$^{+}$