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

¹ Université de Provence, laboratoire de Physique des Interactions Ioniques et Moléculaires, Centre de St-Jérôme, Avenue Escadrille Normandie-Niémen, 13397 Marseille France
e-mail: patrice.theule@univ-provence.fr
² Academia Sinica Institute of Astronomy and Astrophysics Astronomy-Mathematics Building, NTU, Taipei, ROC

Received: 3 November 2010
Accepted: 21 February 2011

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.