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

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 mu m, 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.