Computational Confirmation of the Carrier for the ``XCN'' Interstellar Ice Band: OCN- Charge Transfer Complexes

Recent experimental studies provide evidence that the carrier for the so-called XCN feature at 2165 cm^-1 (4.62 μm) in young stellar objects is an OCN^-/NH⁺₄ charge transfer (CT) complex that forms in energetically processed interstellar icy grain mantles. Although other RCN nitriles and RNC isonitriles have been considered, Greenberg's conjecture that OCN^- is associated with the XCN feature has persisted for over 15 years. In this work, we report a computational investigation that thoroughly confirms the hypothesis that the XCN feature observed in laboratory studies can result from OCN^-/NH⁺₄ CT complexes arising from HNCO and NH₃ in a water ice environment. Density functional theory calculations with HNCO, NH₃, and up to 12 waters reproduce seven spectroscopic measurements associated with XCN: the band origin of the asymmetric stretching mode of OCN^-, shifts due to isotopic substitutions of C, N, O, and H, and two weak features. However, very similar values are also found for the OCN^-/NH⁺₄ CT complex arising from HOCN and NH₃. In both cases, the complex forms by barrierless proton transfer from HNCO or HOCN to NH₃ during the optimization of the solvated system. Scaled B3LYP/6-31+G** harmonic frequencies for the HNCO and HOCN cases are 2181 and 2202 cm^-1, respectively.