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Mechanism of the Forbidden [3s,5s]-Sigmatrpic Shift: orbital Symmetry Influences Stepwise Mechanisms Involving Diradical Intermediates

机译:[3s,5s] -Sigmatrpic移位的机制:轨道对称性影响涉及双自由基中间体的逐步机制。

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The mechanisms of [3s,5s]-sigmatropic shifts of octa-1,3,7-triene and 7-methylenenona-1,3,8-triene have been elaborated using B3LYP and BPW91 density functional theory and CASPT2 methods. These orbital symmetry forbidden rearrangements are stepwise, involving diradical intermediates. A comparison with several [3,3]-sigmatorpic shifts of substituted hexadienes and of [5,5]-sigmatropic shifts that are allowed, but nevertheless folow stepwise paths, shows that the activiation barrier for the disallowed [3,5] shfit is significantly larger than that for the stepwise reactions that are orbital symmetry allowed. Cyclic diradicals that have an armoatic circuit of electrons including the two radical centers and conjugated #pi# or #sigma# bonds are stabilized as compared to cyclic diradicals with an antiaromatic circuit of elecrons. This applies to the transition stats leading to and from the diradicals and influences the activation energies of stepwise sigmatropic shfits. The magnitudes of these efects are small but will have a signficant influence on the rates of competing processes. This series of calculations has been used to assess the relative capabilities of the two functionals. We find that BPW91 underestimates the endothermicity of diradical formation and the barrier to diradical formation whereas B3LYP overestimates these quantities.
机译:利用B3LYP和BPW91密度泛函理论以及CASPT2方法,详细阐述了八--1,3,7-三烯和7-亚甲基壬基-1,3,8-三烯的[3s,5s]-σ位移机理。这些轨道对称的禁止重排是逐步的,涉及双自由基中间体。与被取代的己二烯的几个[3,3]-σ位移和允许的[5,5]-σ位移的比较,但遵循逐步路径,表明不允许的[3,5] shfit的激活障碍是明显大于允许的轨道对称性的逐步反应。与具有电子反芳香族电路的环状双自由基相比,具有包括两个自由基中心和共轭的#pi#或#sigma#键的电子的电枢电路的环状双自由基是稳定的。这适用于通向双基和从双基开始的过渡状态,并影响逐步sigmatropic shfit的激活能。这些影响的程度很小,但将对竞争过程的速率产生重大影响。这一系列的计算已用于评估两个功能的相对功能。我们发现BPW91低估了双自由基形成的吸热性和双自由基形成的障碍,而B3LYP高估了这些数量。

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