Structure of the Photodissociation Products of CCl4, CBr4, and CI4 in Solution Studied by DFT and ab Initio Calculations

Abstract

Various molecular species that can be populated during the photoreaction of carbon tetrahalides CX(4) (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), CX(3), CX(2), C(2)X(6), C(2)X(5), C(2)X(4), X(2), and the isomer X(2)CX-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 CX(4) molecules. In the gas phase, the initial C-X bond rupture in CX(4) is followed by secondary C-X breakage in the CX(3) radical, leading to CX(2) and 2X, and the formation of C(2)X(6) or C(2)X(4) through bimolecular recombination of the CX(3) or CX(2) radicals is favored thermodynamically. In solution, by contrast, the X(2)CX-X isomer is formed via X-X binding, and two CX(3) radicals recombine nongeminately to form C(2)X(6), which then dissociates into C(2)X(4) and X(2) through C(2)X(5). The Raman intensities and the vibrational frequencies, as well as the absorption spectra and oscillator strengths of the Br(2)CBr-Br isomer in the gas phase and in various solvents were computed and the calculated absorption and Raman spectra of the Br(2)CBr-Br isomer in various solutions are in good agreement with the experimental data. The natural population analysis indicates that the Br(2)CBr-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 CX(2) radicals in the gas phase and in solution 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 CX(2) radicals. C(2)X(4), C(2)X(5), and C(2)X(6) (X = Br, I) in the gas phase and in solution were optimized at different computational levels, and the optimized geometric parameters of C(2)I(4) are in very good agreement with the experimental data.