Structure of the photodissociation products of CCl₄, CBr ₄, and CI₄ in solution studied by DFT and ab initio calculations

Various molecular species that can be populated during the photoreaction of carbon tetrahalides CX₄ (X = Cl, Br, I) in the gas phase and in solution have been studied by ab initio and density functional theory (DFT) calculations. Geometries, energies, and vibrational frequencies of CX ₄, CX₃, CX₂, C₂X₆, C ₂X₅, C₂X₄, X₂, and the isomer X₂CX-X were calculated and transition states connecting these species were characterized. Spin-orbit DFT (SODFT) computations were also performed to include the relativistic effects, which cannot be neglected for Br and I atoms. The calculated potential energy surfaces satisfactorily describe the reactions of the photoexcited CX₄ molecules. In the gas phase, the initial C-X bond rupture in CX₄ is followed by secondary C-X breakage in the CX₃ radical, leading to CX₂ and 2X, and the formation of C₂X₆ or C₂X₄ through bimolecular recombination of the CX₃ or CX₂ radicals is favored thermodynamically. In solution, by contrast, the X ₂CX-X isomer is formed via X-X binding, and two CX₃ radicals recombine nongeminately to form C₂X₆, which then dissociates into C₂X₄ and X2 through C₂X ₅. The Raman intensities and the vibrational frequencies, as well as the absorption spectra and oscillator strengths of the Br₂CBr-Br isomer in the gas phase and in various solvents were computed and the calculated absorption and Raman spectra of the Br₂CBr-Br isomer in various solutions are in good agreement with the experimental data. The natural population analysis indicates that the Br₂CBr-Br isomer corresponds to the recently reported solvent-stabilized solvated ion pair (CBr ₃⁺//Br^-)_solv in the highly polar alcohol solvent. The singlet-triplet energy separations of the CX₂ radicals in the gas phase and in solutin were evaluated with high level computational methods, and the optimized geometric parameters are in good agreement with the experimental results. The geometric and energetic differences between the singlet and triplet states were explained by the electronic properties of the CX₂ radicals. C₂X₄, C ₂X₅, and C₂X₆ (X = Br, I) in the gas phase and in solution were optimized at different computational levels, and the optimized geometric parameters of C₂I₄ are in very good agreement with the experimental data.