Kong Qingyu, Wulff Michael, Bratos Savo, Vuilleumier Rodolphe, Kim Joonghan, Ihee Hyotcherl
European Synchrotron Radiation Facility, Grenoble Cedex 38043, BP 220, France.
J Phys Chem A. 2006 Sep 28;110(38):11178-87. doi: 10.1021/jp062919w.
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.
通过从头算和密度泛函理论（DFT）计算，研究了四卤化碳CX₄（X = Cl、Br、I）在气相和溶液中的光反应过程中可能出现的各种分子物种。计算了CX₄、CX₃、CX₂、C₂X₆、C₂X₅、C₂X₄、X₂以及异构体X₂CX - X的几何结构、能量和振动频率，并对连接这些物种的过渡态进行了表征。还进行了自旋轨道DFT（SODFT）计算以考虑相对论效应，对于Br和I原子而言，这种效应不可忽略。计算得到的势能面令人满意地描述了光激发CX₄分子的反应。在气相中，CX₄中初始的C - X键断裂之后是CX₃自由基中的二次C - X键断裂，生成CX₂和2X，并且通过CX₃或CX₂自由基的双分子重组形成C₂X₆或C₂X₄在热力学上是有利的。相比之下，在溶液中，X₂CX - X异构体通过X - X结合形成，两个CX₃自由基非成对重组形成C₂X₆，然后C₂X₆通过C₂X₅分解为C₂X₄和X₂。计算了气相和各种溶剂中Br₂CBr - Br异构体的拉曼强度、振动频率以及吸收光谱和振子强度，并且计算得到的各种溶液中Br₂CBr - Br异构体的吸收光谱和拉曼光谱与实验数据吻合良好。自然布居分析表明，在高极性醇溶剂中，Br₂CBr - Br异构体对应于最近报道的溶剂稳定化溶剂化离子对（CBr₃⁺//Br⁻）（solv）。采用高水平计算方法评估了气相和溶液中CX₂自由基的单重态 - 三重态能量间隔，优化后的几何参数与实验结果吻合良好。通过CX₂自由基的电子性质解释了单重态和三重态之间的几何和能量差异。在不同计算水平下对气相和溶液中的C₂X₄、C₂X₅和C₂X₆（X = Br、I）进行了优化，C₂I₄的优化几何参数与实验数据非常吻合。
