Kinetics of OCN⁻ formation from the HNCO + NH₃ solid-state thermal reaction

Aix-Marseille Univ, PIIM UMR 7345, CNRS, 13397 Marseille, France
e-mail: patrice.theule@univ-amu.fr

Received: 22 April 2011
Accepted: 27 January 2012

Abstract

Context. Solid-state features in infrared astronomical spectra can provide useful information on interstellar ices within different astrophysical environments. Solid OCN⁻ has an absorption feature at 4.62 μm, which is observed in star formation regions only with a large source-to-source abundance variation.

Aims. We aim to investigate the thermal formation mechanism of solid OCN⁻ from HNCO on the basis of kinetic arguments.

Methods. We experimentally studied the kinetics of the low-temperature OCN⁻ formation from the purely thermal reaction between HNCO and NH₃ in interstellar ice analogs using Fourier transform infrared spectroscopy. We used a rate equation approach, a kinetic Monte Carlo approach and a gamma probability distribution approach to derive kinetic parameters from experimental data.

Results. The kinetics can de divided into two-processes, a fast process corresponding to the chemical reaction, and a slow process that we interpret as the spatial orientation of the two reactants within the ice. The three approaches give the same results. The HNCO + NH₃ → OCN⁻ + NH₄⁺ reaction rate follows an Arrhenius law with an activation energy of 0.4 ± 0.1 kJ mol^-1 (48 ± 12 K) and a pre-exponential factor of 0.0035  ±  0.0015 s^-1.

Conclusions. The present experiment has the important implication that the HNCO + NH₃ reaction can account for the observed abundances of solid OCN⁻ and the HNCO non detection in young stellar objects.