Kinetics of the OCN- and HOCN formation from the HNCO + H 2O thermal reaction in interstellar ice analogs

Abstract

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. © 2011 ESO.