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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Mechanism and Thermodynamics of Reductive Cleavage of Carbon-Halogen Bonds in the Polybrominated Aliphatic Electrophiles
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Mechanism and Thermodynamics of Reductive Cleavage of Carbon-Halogen Bonds in the Polybrominated Aliphatic Electrophiles

机译：多溴代脂族亲电试剂中碳-卤素键还原裂解的机理和热力学

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Quantum-mechanical computations revealed that, despite the presence of electron-withdrawing and/or pi-acceptor substituents, the lowest unoccupied molecular orbitals (LUMO) of the polybromosubstituted aliphatic molecules R-Br (R-Br = C3Br2F6, CBr3NO2, CBr3CN, CBr3CONH2, CBr3CO2H, CHBr3, CFBr3, CBr4, CBr3COCBr3) are delocalized mostly over their bromine-containing fragments. The singly occupied molecular orbitals in the corresponding vertically excited anion radicals (R-Br center dot-)* are characterized by essentially the same shapes and show nodes in the middle of the C-Br bonds. An injection of an electron into the antibonding LUMO results in the barrierless dissociation of the anion-radical species and the concerted reductive cleavages of C-Br bonds leading to the formation of the loosely bonded {R-center dot center dot center dot center dot Br-} associates. The interaction energies between the fragments of these ion-radical pairs vary from similar to 10 to 20 kcal mol(-1) in the gas phase and from 1 to 3 kcal mol(-1) in acetonitrile. In accord with the concerted mechanism of reductive cleavage, all R-Br molecules showed completely irreversible reduction waves in the voltammograms in the whole range of the scan rates employed (from 0.05 to 5 V s(-1)). Also, the transfer coefficients alpha, established from the width of these waves and dependence of reduction peak potentials E-p on the scan rates, were significantly lower than 0.5. The standard reduction potentials of the R-Br electrophiles, ER-Br/R center dot+X-degrees and the corresponding R-center dot radicals, ER center dot/R-degrees were calculated in acetonitrile using the appropriate thermodynamic cycles. In agreement with these calculations, which indicated that the R-center dot radicals resulting from the reductive cleavage of the R-Br molecules are stronger oxidants than their parents, the reduction peaks' currents in cyclic voltammograms were consistent with the two-electron transfer processes.
机译：量子力学计算表明，尽管存在吸电子和/或pi受体取代基，但多溴取代的脂族分子R-Br（R-Br = C3Br2F6，CBr3NO2，CBr3CN，CBr3CONH2 ，CBr3CO2H，CHBr3，CFBr3，CBr4，CBr3COCBr3）大多在其含溴片段上离域。相应的垂直激发的阴离子自由基（R-Br中心点-）*中的单个占据的分子轨道的特征在于基本相同的形状，并在C-Br键的中间显示节点。将电子注入反键LUMO中会导致阴离子自由基物种无障碍解离以及C-Br键的一致还原性断裂，从而导致形成松散键合的{R中心点中心点中心点中心点中心点Br -}同事。这些离子-自由基对的片段之间的相互作用能在气相中大约为10至20 kcal mol（-1），而在乙腈中则为1至3 kcal mol（-1）。根据还原裂解的协调机制，在所用扫描速率的整个范围内（从0.05到5 V s（-1）），所有R-Br分子在伏安图中均显示完全不可逆的还原波。而且，由这些波的宽度和还原峰电位E-p对扫描速率的依赖性所建立的传递系数α显着低于0.5。使用适当的热力学循环，在乙腈中计算R-Br亲电体的标准还原电位ER-Br / R中心点+ X度和相应的R-中心点自由基，ER中心点/ R度。与这些计算结果一致，这表明由R-Br分子的还原裂解产生的R-中心点自由基比其母体具有更强的氧化剂，循环伏安图中的还原峰电流与两电子转移过程一致。

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