The formation of cyanate ion (OCN-) in interstellar ice analogs

Although the 2165 cm(-1) (4.62 mum) "XCN" IR absorption in interstellar ice s was first detected over 20 years ago, its assignment has remained controv ersial, and its mode of formation has seldom been studied. Here we report a n extensive laboratory investigation of this band's formation in interstell ar ice analogs. Ices with known or suspected interstellar molecules were pr oton-irradiated at 15-25 K to simulate interstellar energetic processing, a nd their IR spectra were recorded. Reactions for irradiated mixtures showin g an XCN spectral band have been developed based on results with chemically related systems and with over 60 ices examined here. Combined with previou s work, our new results leave no doubt that the band produced in the labora tory is due to OCN-, the cyanate anion. Tests of the reactions leading to O CN- are described, and independent methods of producing OCN- are reported. The results of all of these new experiments help reveal the chemistry under lying this ion's formation and establish some of the conditions under which OCN- might be found in interstellar ices. They also show that energetic pr ocessing is an efficient way to produce OCN- in interstellar ices and that temperature increases to promote acid-base chemistry are unnecessary.