Radiation-Induced Transformation of CHF₃···CO to the CF₃···CO Complex: Matrix Isolation and Ab Initio Study

The X-ray-induced transformations of CHF₃/CO/Ar and CHF₃/CO/Kr systems were investigated by Fourier transform infrared (FTIR) matrix isolation spectroscopy at 6 K. It was found that addition of CO suppressed decomposition of fluoroform in an Ar matrix, probably because of trapping of matrix holes by CO and CHF₃···CO complexes. Considerable increase of the CF₃/CF₂ ratio with increasing CO content in the matrix was attributed to stabilization of the CF₃ radical with respect to further radiation-induced fragmentation because of its complexation with the CO molecule. The CF₃···CO complex generated from the CHF₃···CO precursor complex was characterized by FTIR spectroscopy and ab initio calculations at the CCSD(T) and MP2 levels of theory. To the best of our knowledge, it is the first experimentally observed complex of the CF₃ radical. The computed interaction energy was found to be 0.35 kcal/mol at the CCSD(T)/L2a_3 level (0.36 kcal/mol at the MP2/L2a_3 level), taking into account zero-point energy and basis set superposition error corrections. Despite the very weak intermolecular bonding, the complex is characterized by distinct features in the regions of C-F symmetric and antisymmetric stretching (CF₃) and CO stretching (the latter one was observed only in a krypton matrix). The geometrical structure of the radical-molecule complex is close to that of its molecular precursor.