Laboratory Infrared Spectra of Polycyclic Aromatic Nitrogen Heterocycles: Quinoline and Phenanthridine in Solid Argon and H2O

Polycyclic aromatic hydrocarbons (PAHs) are common throughout the universe. Their detection and identifi- cation are based on the comparison of IR observations with laboratory spectra. Polycyclic aromatic nitrogen heterocycles(PANHs)areheterocyclicaromatics,i.e.,PAHswith carbonatoms replacedby anitrogenatom.These moleculesshouldbepresentintheinterstellarmedium, buthavereceivedrelativelylittleattention.Wepresentmid- IR spectra of two PANHs, quinoline (C9H7N) and phenanthridine (C13H9N), isolated in solid argon and frozen in solid H2O at 15 K, conditions yielding data directly comparable to astronomical observations. Quinoline and phenanthridine have been detected in meteorite extracts, and in general these nitrogen heterocycles are of astro- biological interest, since this class of molecules includes nucleobases, basic components of our nucleic acids. In contrast to simple PAHs, which do not interact strongly with solid H2O, the nitrogen atoms in PANHs are potentiallycapableofhydrogenbonding withH2O.WhereastheIRspectrumofphenanthridineinH2Oissimilarto that of the same compound isolated in an argon matrix, quinoline absorptions shift up to 16 cm 1 (0.072 m) between argon and H2O. Thus, astronomers will not always be able to rely on IR band positions of matrix-isolated PANHstocorrectlyinterprettheabsorptionsofPANHsfrozeninH2Oicegrains.Furthermore,ourdata suggestthat relative band areas also vary, so determining column densities to better than a factor of 3 will require knowledge of the matrix in which the PANH is embedded and laboratory studies of relevant samples. Subject headinggs: infrared:ISM — ISM:clouds — ISM:linesandbands — ISM:molecules — line:formation — molecular data