Spectroscopic characterization of the complex of vinyl radical and carbon dioxide : Matrix isolation and ab initio study

We report on the preparation and vibrational characterization of the C2H3 center dot center dot center dot CO2 complex, the first example of a stable intermolecular complex involving vinyl radicals. This complex was prepared in Ar and Kr matrices using UV photolysis of propiolic acid (HC3OOH) and subsequent thermal mobilization of H atoms. This preparation procedure provides vinyl radicals formed exclusively as a complex with CO2, without the presence of either CO2 or C2H3 monomers. The absorption bands corresponding to the nu(5)(C2H3), nu(7)(C2H3), nu(8)(C2H3), nu(2)(CO2), and nu(3)(CO2) modes of the C2H3 center dot center dot center dot CO2 complex were detected experimentally. The calculations at the UCCSD(T)/L2a level of theory predict two structures of the C2H3 center dot center dot center dot CO2 complex with C-s and C-1 symmetries and interaction energies of -1.92 and -5.19 kJ mol(-1). The harmonic vibrational frequencies of these structures were calculated at the same level of theory. The structural assignment of the experimental species is not straightforward because of rather small complexation-induced shifts and matrix-site splitting of the bands (for both complex and monomers). We conclude that the C-1 structure is the most probable candidate for the experimental C2H3 center dot center dot center dot O-2 complex based on the significant splitting of the bending vibration of CO2 and on the energetic and structural considerations. Published by AIP Publishing.Peer reviewe

Mechanisms of Radiation-Induced Degradation of CFCl₃ and CF₂Cl₂ in Noble-Gas Matrixes: An Evidence for “Hot” Ionic Channels in the Solid Phase

The X-ray-induced transformations of simple chlorofluorocarbons (CFCl₃ and CF₂Cl₂) in solid noble-gas matrixes (Ne, Ar, Kr, and Xe) at 7 K were studied in order to elucidate basic mechanisms of the radiation–chemical degradation with possible implications for stratospheric and extraterrestrial ice chemistry. The decomposition of parent molecules and formation of products were monitored by FTIR spectroscopy, and the identification was supported by <i>ab initio</i> calculations at the CCSD­(T) level. It was shown that the ionic reaction channels were predominating in most cases (except for CF₂Cl₂/Xe system). The primary radical cations (CFCl₃^+• and CF₂Cl₂^+•) are either stabilized in matrixes or undergo fragmentation to yield the corresponding secondary cations (CFCl₂⁺, CCl₃⁺, CF₂Cl⁺) and halogen atoms. The probability of fragmentation through different channels demonstrates a remarkable matrix dependence, which was explained by the effect of excess energy resulting from the exothermic positive hole transfer from matrix atoms to freon molecules. A qualitative correlation between “hot” ionic fragmentation at low temperatures and gas-phase ion energetics was found. However, dissociative electron attachment leads to formation of neutral radicals (CFCl₂^• or CF₂Cl^•) and chloride anions. One more possible way of dissociative electron attachment in the case of CF₂Cl₂ is formation of CF₂^•• and Cl₂^–•. A general scheme of the radiation-induced processes is proposed

Matrix Isolation and Ab Initio Study on the CHF₃···CO Complex

Intermolecular complexes between CHF₃ and CO have been studied by ab initio calculations and IR matrix isolation spectroscopy. The computations at the MP2 and CCSD­(T) levels of theory indicated five minima on the potential energy surface (PES). The most energetically favorable structure is the C­(CO)–H­(CHF₃) coordinated complex (<i>C_s</i> symmetry) with the stabilization energy of 0.84 kcal/mol as computed at the CCSD­(T) level (with ZPVE and BSSE corrections). This is the only structure experimentally found in argon and krypton matrixes, whereas the weaker non-hydrogen-bonded complexes predicted by theory were not detected. The vibrational spectrum of this complex is characterized by a red-shift of the CF₃ asymmetric stretching, splitting of the C–H bending mode, and blue-shifts of the C–H and C–O stretching vibrations as compared to the monomer molecules. The observed complexation-induced shifts of CHF₃ and CO fundamentals are in good agreement with the computational predictions. It was shown that both MP2 and CCSD­(T) calculations generally provided a reasonable description of the vibrational properties for the weak intermolecular complexes of fluoroform