Transition state looseness and alpha-secondary kinetic isotope effects
Transition state looseness and alpha-secondary kinetic isotope effects
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DOI：
10.1021/ja962733i
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发表时间：
1997-01-08
影响因子：
15
通讯作者：
Jensen, F
中科院分区：
文献类型：
--
作者：
Glad, SS;Jensen, F
用MP2/6-31++G(d，p)方法对X(-)+CH(3)X S(N)2三个等式反应进行了理论计算，研究了氢/氢二次动力学同位素效应与过渡态松散的关系。通过改变这些体系中的C-X距离，发现二次动力学同位素效应随着过渡结构疏松度的增加而增加。此外，研究还表明，松弛最好定义为C-X键长的增量延伸，其中参照态可以是孤立的反应物或离子-偶极络合物。因子分析表明动力学同位素效应的变化是由C-H弯曲振动决定的。C-H伸缩振动的贡献对动力学同位素效应的绝对值很重要，部分原因是C-H键长的变化。伸缩振动的二次动力学同位素效应随松散的变化方向与弯曲贡献相反，导致致密过渡结构的总动力学同位素效应趋于平缓。
The correlation between hydrogen/deuterium alpha-secondary kinetic isotope effects and transition state looseness has been investigated by MP2/6-31++G(d,p) calculations on three identity X(-) + CH(3)X S(N)2 reactions. By varying the C-X distances in these systems it is shown that the secondary kinetic isotope effect increases with increasing transition structure looseness. Furthermore, it is shown that looseness is best defined as a Delta-elongation of the C-X bond lengths, where the reference state can be either the isolated reactants or the ion-dipole complex. The variation of kinetic isotope effects is shown by factor analysis to be determined by C-H bending vibrations. The contributions from C-H stretching vibrations are important for the absolute values of the kinetic isotope effects, and are partly due to C-H bond length changes. The variation of the secondary kinetic isotope effect from the stretching vibrations with looseness is in the opposite direction as the bending contribution, causing a level-off in the total kinetic isotope effect for tight transition structures.