Abstract:
Para-hydrogen (p-H2) matrix isolation has emerged as a wonderful technique for various applications in free-radical spectroscopy, atmospheric chemistry, and astrochemistry because of the unique characteristics associated with this quantum solid. We present two types of applications in astrochemistry using p-H2 in this talk.
(1) Production of protonated and hydrogenated species— Protonated polycyclic aromatic hydrocarbons (PAH) are possible carriers of the unidentified IR emission in the interstellar media. The electron-bombardment experiments in p-H2 provide an excellent tool to produce and identify these protonated species. The IR spectra of 1-quinolinium cation (C9H7NH+) and 1-quinolinyl radical (C9H7NH), produced upon electron bombardment during deposition of a mixture of quinoline (C9H7N) and p-H2 at 3.2 K, indicate that the protonation and hydrogenation occur mainly at the N-atom site. Additional experiments on irradiation of C9H7N/Cl2/p-H2 matrices at 365 nm were performed to induce the reaction H + C9H7N. We identified, in addition to C9H7NH, four radicals (3-, 4-, 7-, and 8-HC9H7N) with hydrogenation at the C-atom site.
(2) New concept of hydrogen reactions in astrochemistry—Hydrogen reactions play important roles in astrochemistry; H + methylamine (CH3NH2) serves as a good example. We performed this reaction in solid p-H2 and observed IR spectra of •CH2NH2 and CH2NH on irradiation and when the matrix was maintained in darkness. Observation of IR spectrum of •CH2NH2 clearly indicates that •CH2NH2, a precursor of glycine, can be formed from H + CH3NH2 in dark interstellar clouds. The observed dual-cycle mechanism containing two H-abstraction and H-addition steps chemically connects CH3NH2 and CH2NH and explains their quasi-equilibrium. We performed also reactions of H atoms with trans-NMF in p-H2 at 3.3 K and found that isomer cis-NMF, which has higher energy, increased continuously in darkness; H addition to CH3NCO produced only cis-radicals that led to cis-NMF. Furthermore, H-atom-induced fragmentation in darkness by breaking the C−C bond provides links between NMF and HCNO/CH2NH. These new types of reactions were overlooked previously in astrochemistry.