A number of interstellar features have been
assigned to species trapped
in ices that can form as mantles on dust grains. It was suggested than an IR feature at 2165 cm–1
(4.62 μm) observed initially in 1984 by
Lacy et al. might be due to the
cyanate ion (OCN– ) as early as 1987 by
Grim & Greenberg.
However, a number of additional studies were needed to confirm this identification. Various experimental studies,
including work by
Demyk et al.,
Hudson et al., and
Novozamsky et al.,
indicated that cyanate can be formed when isocyanic acid
(HNCO) is trapped in an ice
matrix, even at the very cold temperatures of dense interstellar clouds. If a strong base such as
ammonia (NH3) is present in the proper proximity, a proton can transfer from HNCO
to yield cyanate.
The figure to the right shows the asymmetric OCN stretching mode of cyanate as it appears
within a cluster of 12 water molecules as
computed quantum chemically. Note the ammonium
ion that is coordinated to the N end of the anion. The calculations reproduced the stretching frequency as well
as shifts observed in experiments where all H, N, C, or O was substituted with less common isotopes.