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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 complexwas 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. [ABSTRACT FROM AUTHOR]