Kinetics of OCN − formation from the HNCO + NH 3 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 + NH$_3$ reaction can account for the observed abundances of solid OCN$^-$ and the HNCO non detection in young stellar objects.