The Radiation Chemistry of NH₃···CO Complex in Cryogenic Media as Studied by Matrix Isolation

Abstract

The NH3···CO complex can be considered an important building block for cold synthetic astrochemistry leading to the formation of complex organic molecules, including key prebiotic species. In this work, we have studied the radiation-induced transformations of this complex in Ar, Kr, and Xe matrices using FTIR spectroscopy. On the basis of comparison with the quantum chemical calculations at the CCSD­(T)/L2a_3 level of theory, it was found that the initial complex had the configuration with hydrogen bonding through the carbon atom of CO. Irradiation of the matrix isolated complex with X-rays at 6 K leads to the formation of a number of synthetic products, namely, HNCO (in all matrices), formamide NH2CHO, NH2CO, and HNCO-H2 (in argon and krypton). The matrix effect on the product distribution was explained by the involvement of different excited states of the complex in their formation. It was suggested that formamide results from the singlet excited states while other species mainly originate from triplet excited states. The latter states are efficiently populated through ion-electron recombination (in all matrices) and through intersystem crossing (particularly, in xenon). High yield of the recombination triplet states is a feature of the processes induced by high-energy radiation (in contrast to direct photolysis). NCO, CN, and NO were found as minor secondary products at high adsorbed doses. The astrochemical implications of the obtained results are discussed.

Astrochemically Relevant Radicals and Radical–Molecule Complexes: A New Insight from Matrix Isolation

“…[124] reporting the formation of H 2 NCO • and NCO • radicals under UV-photolysis of HNCO in a solid Xe matrix. In our recent FTIR spectroscopic study [125], it was shown that both these radicals were produced after X-ray irradiation of the NH 3 /CO/Ng (Ng = Ar, or Kr) deposited mixture at 5 K. It was concluded that the H 2 NCO • radical resulted from the NH 3 • • • CO intermolecular complex (the detailed mechanism is unclear), while the NCO • radical was formed though subsequent dehydrogenation of H 2 NCO • [125]. Synthetic processes of such kind may be generally important for understanding the mechanisms of the formation of COMs with amide moiety in mixed astrochemical ices subjected to ionizing radiation.…”

“…In this situation, the pre-existing building blocks consisting of two or more molecules should play a crucial role. In order to understand the basic mechanisms involved in the processes of cold synthetic transformations, we have introduced a general concept based on model studies of the radiation chemistry of 1:1 intermolecular complexes isolated in rigid inert media [94,95,112,117,125,134,[136][137][138][139][140][141]. It should be emphasized that the interaction in such complexes is typically rather weak (the computed interaction energy usually does not exceed 2-4 kcal/mol); however, it is more than sufficient to fix the pre-determined geometry in solid cryogenic matrices, and thus, to control further radiation-induced transformations.…”

10 Years of the ACS PHYS Astrochemistry Subdivision

“…Purely theoretical studies are providing characterization for other unique molecules including glycolic acid, pyridyl radicals, and diazirine along with its cyclic isomers . Additionally, the molecular structure and spectra of noncovalent interactions are also characterized through a mixture of theory and experiment for glycerol–water clusters, HCN and CH 3 Cl clusters, and complexes of NH 3 ···CO …”

“…16 Additionally, the molecular structure and spectra of noncovalent interactions are also characterized through a mixture of theory and experiment for glycerol−water clusters, 17 HCN and CH 3 Cl clusters, 18 and complexes of NH 3 •••CO. 19 The extreme conditions of various astrophysical environments also open novel chemistry. However, such reactions can only be interrogated through the use of emerging and cuttingedge experimental and theoretical techniques.…”