The Formation of Cyanate Ion (OCN-) in Interstellar Ice Analogs

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