Kinetics of OCN⁻formation from the HNCO + NH₃solid-state thermal reaction

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 NH 3 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 + NH 3 → OCN − + NH + 4 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.

Hydrogenation at low temperatures does not always lead to saturation: the case of HNCO

“…If the dominant formation route of HNCO (NH + CO) were efficient (Garrod et al 2008), destruction routes would be required to explain this. As mentioned above, the formation of OCN − by reaction of HNCO with H 2 O and NH 3 are favourable routes, the reactions having activation barriers of 26 ± 2 kJ mol −1 (3130 ± 240 K; Theule et al 2011) and 0.4 ± 0.1 kJ mol −1 (48 ± 12 K; Mispelaer et al 2012), respectively. Irradiation of HNCO with vacuum ultraviolet (VUV) radiation has been shown to produce formaldehyde (H 2 CO), formamide (NH 2 CHO), and urea (H 2 NCONH 2 ) (Raunier et al 2004).…”

“…Jones et al (2011) contend that the hydrogenation of HNCO is insignificant as a route to formamide formation because of the low barrier to the reaction HNCO + NH 3 (Raunier et al 2003a;Mispelaer et al 2012). While it is true that NH 3 does react rapidly with HNCO, the flux of hydrogen atoms onto a grain will be superior to the quantity of NH 3 molecules in the ice mantle.…”

Diffusion measurements of CO, HNCO, H2CO, and NH3in amorphous water ice

“…On reason would be that there is a distribution of diffusion coefficients (corresponding to a distribution of sizes of cracks and pores) and not a single diffusion coefficient. Convoluting the diffusion coefficient with a distribution of pores on the basis of Mispelaer et al (2012) improves the adequacy between the theoretical and experimental curve, but will add one more parameter to fit, the shape factor of the distribution. But this does not necessarily give more physical insight into the diffusion problem.…”

Low-temperature surface formation of NH3 and HNCO: hydrogenation of nitrogen atoms in CO-rich interstellar ice analogues

“…Theule et al 2011 found an activation energy barrier for reaction (7) of 3127 K, which is too high to make this reaction important for the conditions and timescales typical for young stellar objects. In a follow-up study, Mispelaer et al 2012 determined a barrier of 48 K for reaction (6), indicating the latter pathway as the most promising one to form OCNand NH4 + . As stated before, OCNhas been observed, and NH4 + may have been identified in the solid state, but the focus here is to simulate dense molecular cloud conditions, well before thermal and energetic processing of ices become important.…”