An Infrared and Theoretical Study about the “XCN” Band Formation: Reactivity of HNCO with NH3Astrophysical Ice Laboratory Analogues and the Spontaneous Production of OCN-

Abstract

The reactivity of HNCO with molecular NH3 and with NH3 ices is investigated between 10 and 150 K with use of FT-IR spectroscopy and ab initio calculations. In argon matrix at 10 K, the formation of a 1/1 molecular complex between HNCO and NH3 is observed. Its structure determined by DFT calculations at the B3LYP/6-31G(d,p) level exhibits a strong hydrogen bond (1.825 Å) between the hydrogen donor (HNCO) and NH3. The warming up between 10 and 150 K of adsorbed HNCO on crystalline and on amorphous NH3 ices shows the formation of NH4+OCN- at 50 and 90 K, respectively. These results are different from the ones obtained when HNCO is embedded in a NH3 matrix. In this case, spontaneous formation of NH4+OCN- is observed at 10 K. Quantum calculations confirm this spontaneous character of the reaction. It occurs if HNCO is in an environment of four NH3 and if one of them is directly in interaction with HNCO via its electron lone pair. The crystal NH3 unit cell parameters were optimized by using theoretical calculations (DFT method combined with a plane wave basis set and nonlocal reciprocal space pseudopotential) and used in the cluster model representing the ice surface. Absorption energy of HNCO on the NH3 ice surface (−74.3 kJ/mol) is obtained with use of the DFT set. For the energy minimum, the cluster surface is modified and shows a strong hydrogen bond (1.662 Å) between the hydrogen of HNCO and a N atom of the surface as observed in argon matrix. HNCO lies flat on the surface and the oxygen of HNCO interacts with another neighboring NH group.