Matrix isolation and ab initio study on HCN/CO₂ system and its radiation-induced transformations: Spectroscopic...

Spectroscopic characteristics and X-ray induced transformations of the HCN⋯CO₂ complex in solid Ar and Kr matrices were studied by FTIR spectroscopy and ab initio calculations at the CCSD(T) level. The complex was prepared by deposition of the HCN/CO₂/Ng gas mixtures (Ng = Ar or Kr). The comparison of the experiment and calculations prove formation of a linear, H-bonded NCH⋯OCO complex with a substantial red shift of the C-H stretching band and a blue shift of the H-C-N bending band in respect to the monomer. This result is in contrast with the previous gas-phase observations, where only T-shape complex was found. Irradiation of deposited matrices leads to formation of CN radicals and HNC molecules and subsequent annealing results in appearance of H₂CN and trans-HCNH in both matrices plus HKrCN in the case of Kr. In the presence of CO₂, the strongest absorption of trans-HCNH radical demonstrates an additional blue-shifted (by 6.4 cm^-1) feature, which was assigned to the N-coordinated complex of this radical with CO₂ on the basis of comparison with calculations. To our knowledge, it is the first experimentally observed complex of this radical. No evidence was found for HKrCN⋯CO₂ complex, which was explained tentatively by steric hindrance.