Thermal reactivity of HNCO with water ice: an infrared and theoretical study

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

“…The HNCO monomer is synthetized from the thermal decomposition of the cyanuric acid polymer (Aldrich Chemical Co., 98%) at 650 0 C under a primary vacuum (Raunier et al 2003b). HNCO and H 2 O are inserted and outgassed in two separated vacuum lines using standard manometric techniques, and then they are co-deposited.…”

“…Isocyanic acid HNCO is characterized by strong bands located at 3373, 3233, 2251, and 858 cm −1 , but only the 2251 cm −1 band, assigned to the NCO antisymmetric stretch vibration, is not blended by the water bands, which is why it will be used to monitor the HNCO evolution. The 2251 cm −1 band has actually two components: one component at 2260 cm −1 corresponding to HNCO in a H 2 O environment (Raunier et al 2003b), and a component 2234 cm −1 corresponding to HNCO in a HNCO environment. The proportion of the two components in the 2251 cm −1 band depends on the ice sample deposition, and accordingly the position of the band may be shifted from one deposition to another by few wavenumbers.…”

“…Some molecules are in configurations that enable the total reaction, some molecules are not, like in clusters of pure HNCO for example. Only a limited set of HNCO molecules are in a favorable configuration, one HNCO molecule making a hydrogen bond with the electron lone pair of one H 2 O molecule and being activated enough (Raunier et al 2003b). We conclude that for the population of molecules that are statistically in a favorable configuration, the reaction is total, because the reverse reaction is not observed.…”

“…The formation of the solid-state cyanate ion OCN − can be explained by different processes: proton bombardment (Moore et al 1983), electron bombardment (Bennett et al 2010), vacuum ultraviolet photolysis of NH 3 :CO:H 2 O ices where the photochemical formation of isocyanic acid HNCO is followed by a proton transfer to ammonia NH 3 (d 'Hendecourt et al 1986;Grim et al 1989), or directly from a acid-base reaction of HNCO with solid NH 3 or H 2 O (Demyk et al 1998;Raunier et al 2003b;van Broekhuizen et al 2004). A good review of all the experiments carried out on OCN − and of the processes that lead to its formation is given in Table 1 of van Broekhuizen et al (2005) and a quantitative comparison of these different processes is given in van Broekhuizen et al (2004).…”

“…Moreover, the thermal deprotonation of HNCO by H 2 O is observed (Raunier et al 2003b), though it is less effective than the deprotonation of HNCO by NH 3 , which indicates a higher activation energy. However, whatever their activation energies are, these reactions are limited by the diffusion and the abundances of available reactants.…”