2005ApJ...626..909B

Polycyclic aromatic hydrocarbons (PAHs) are common throughout the universe. Their detection and identification are based on the comparison of IR observations with laboratory spectra. Polycyclic aromatic nitrogen heterocycles (PANHs) are heterocyclic aromatics, i.e., PAHs with carbon atoms replaced by a nitrogen atom. These molecules should be present in the interstellar medium, but have received relatively little attention. We present mid-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 astrobiological 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 potentially capable of hydrogen bonding with H₂O. Whereas the IR spectrum of phenanthridine in H₂O is similar to that of the same compound isolated in an argon matrix, quinoline absorptions shift up to 16/cm (0.072 µm) between argon and H₂O. Thus, astronomers will not always be able to rely on IR band positions of matrix-isolated PANHs to correctly interpret the absorptions of PANHs frozen in H₂O ice grains. Furthermore, our data suggest that 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.