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

The reactivity of HNCO with molecular NH 3 and with NH 3 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 NH 3 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 NH 3 . The warming up between 10 and 150 K of adsorbed HNCO on crystalline and on amorphous NH 3 ices shows the formation of NH 4 + OCN - at 50 and 90 K, respectively. These results are different from the ones obtained when HNCO is embedded in a NH 3 matrix. In this case, spontaneous formation of NH 4 + 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 NH 3 and if one of them is directly in interaction with HNCO via its electron lone pair. The crystal NH 3 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 NH 3 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.