Potential Catalytic Role of Small Heterocycles in Interstellar H 2 Formation: A Laboratory Astrochemistry Study on Furan and Its Hydrogenated Forms

It is now well-accepted in astrochemistry that the formation of interstellar H is taking place on the surface of interstellar grains. It has also been suggested a long time ago that polyaromatic hydrocarbons (PAHs) can catalyze this process by subsequent H atom addition and H abstraction reactions. Recent quantum chemical computations suggested that small heterocycles can be better catalysts than PAHs. In this study, the reaction of H atoms with furan, 2,3- and 2,5-dihydrofurans, and tetrahydrofuran were studied in solid -H at 3.1 K. The reactions were followed by Fourier transform infrared (FTIR) spectroscopy. By the analysis of spectra, 2-hydrofuran-3-yl, 3-hydrofuran-2-yl, 2,3,4-trihydrofuran-5-yl, and 2,3,5-trihydrofuran-4-yl radicals were identified among the products. The experiments revealed that all the possible H atom addition and H abstraction cycles connecting furan and tetrahydrofuran proceed effectively in both directions at a low temperature. This indicates the possible important role of small heterocycles in interstellar H formation. Furthermore, it also indicates that, in the case of H atom excess, a quasi-equilibrium exists between the -C H O ( = 4-8) species, and the ratios of these species in an astrophysical object are determined by the rate of the different H atom addition and H abstraction reaction steps.