Kinetics of the OCN- and HOCN formation from the HNCO + H2O thermal reaction in interstellar ice analogs

Aims. We study in the laboratory the kinetics of the low-temperature OCN- and HOCN formation from the purely thermal reaction of solid HNCO and H2O. The cyanate ion OCN- is an intermediate in the isomerization process of isocyanic acid HNCO into cyanic acid HOCN in water ice. Methods. We study the reaction, isomerization and desorption kinetics of the HNCO/OCN-/HOCN system using Fourier transform infrared spectroscopy. Results. Activation energies of 26 +/- 2 kJ mol(-1) (3127 K) and 36 +/- 1 kJ mol(-1) (4330 K) are found for the HNCO + H2O -> OCN- + H3O+ and OCN- + H3O+ -> HOCN + H2O reactions respectively. Desorption energies of 37 +/- 3 kJ mol(-1) (4450 K) and 40 +/- 3 kJ mol(-1) (4811 K) are measured for HNCO and OCN-, respectively. Conclusions. The present experiment has the important implication that the H2O + HNCO reaction alone cannot account for the observed abundances of solid OCN- in astronomical IR sources.