Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow 119334, Russia.
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow 119334, Russia.
J Chem Phys. 2019 Dec 7;151(21):214503. doi: 10.1063/1.5126337.
The geminate recombination kinetics of the triplet pairs of neutral radicals formed as a result of hydrogen atom transfer from 4-methylphenol to 4,4'-dimethylbenzophenone in the triplet excited state was studied by laser flash photolysis in the temperature range of 263-323 K. The pair included aromatic ketyl and 4-methylphenoxyl radicals produced in a viscous (∼10 P) solution of the initial reagents in glycerol. The geminate recombination kinetics demonstrate a "tail" c(t) ∼ 1/t, with contribution of magnitude A, which increases with temperature. The two-state (TS) model, which takes into account the potential well caused by the interaction between radicals at short distances and the radicals that escaped from a well but returned to it after free diffusion outside the well, was used to fit the data. The application of an external magnetic field (MF) (0.2 T) gave rise to the retardation of the geminate recombination and to the increase of fraction of radicals in the solvent bulk. The bell-shaped dependence of the magnetic field effect (MFE) value upon temperature (with a maximum near 300 K) was found. This dependence correlates with A and its dependence on the MF. At temperatures lower than 270 K, both the contribution of the nonexponential part of geminate recombination kinetics and the MFE were found to be small. The TS model can quantitatively describe the effects of solvent viscosity and MF on the kinetics of geminate recombination of triplet RP.
用激光闪光光解法在 263-323 K 的温度范围内研究了中性自由基对在三重激发态中从 4-甲基苯酚向 4,4'-二甲基二苯甲酮转移氢原子而形成的三重态对的复合动力学。该对包括在初始试剂在甘油中的粘性（约 10 P）溶液中产生的芳基酮基和 4-甲基苯氧自由基。复合动力学表现出“尾巴”c(t)∼1/t，其贡献幅度为 A，A 随温度增加而增加。采用双态（TS）模型对数据进行拟合，该模型考虑了由于自由基在短距离处的相互作用而引起的势阱以及从势阱中逃脱但在自由扩散到势阱外后又返回势阱的自由基。施加外磁场（MF）（0.2 T）会导致复合的延迟，并增加自由基在溶剂主体中的分数。发现磁场效应（MFE）值随温度的钟形依赖性（在 300 K 附近有最大值）。这种依赖性与 A 及其对 MF 的依赖性相关。在低于 270 K 的温度下，均发现复合动力学的非指数部分的贡献和 MFE 都很小。TS 模型可以定量描述溶剂粘度和 MF 对三重态 RP 复合动力学的影响。
