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

TL;DR

This study investigates the low-temperature hydrogenation of solid HNCO, revealing that it does not produce formamide and that deuteration occurs rapidly, leading to chemical desorption of HNCO and DNCO into the gas phase in dense molecular regions.

Contribution

It provides new insights into the reaction pathways of HNCO on grain surfaces, challenging the assumption that hydrogenation always leads to saturation and formamide formation.

Findings

Hydrogenation of HNCO does not produce formamide as a major product.

Deuteration of HNCO occurs rapidly via cyclic reaction paths.

Chemical desorption causes loss of HNCO and DNCO into the gas phase.

Abstract

Context. It is generally agreed that hydrogenation reactions dominate chemistry on grain surfaces in cold, dense molecular cores, saturating the molecules present in ice mantles. Aims. We present a study of the low temperature reactivity of solid phase isocyanic acid (HNCO) with hydrogen atoms, with the aim of elucidating its reaction network. Methods. Fourier transform infrared spectroscopy and mass spectrometry were employed to follow the evolution of pure HNCO ice during bombardment with H atoms. Both multilayer and monolayer regimes were investigated. Results. The hydrogenation of HNCO does not produce detectable amounts of formamide (NH2CHO) as the major product. Experiments using deuterium reveal that deuteration of solid HNCO occurs rapidly, probably via cyclic reaction paths regenerating HNCO. Chemical desorption during these reaction cycles leads to loss of HNCO from the surface. Conclusions. It is unlikely that significant quantities of NH2CHO form from HNCO. In dense regions, however, deuteration of HNCO will occur. HNCO and DNCO will be introduced into the gas phase, even at low temperatures, as a result of chemical desorption.