MECHANISMS OF ELIMINATION REACTIONS .11. THEORETICAL CALCULATIONS OF ISOTOPE EFFECTS IN ELIMINATION REACTIONS
MECHANISMS OF ELIMINATION REACTIONS .11. THEORETICAL CALCULATIONS OF ISOTOPE EFFECTS IN ELIMINATION REACTIONS
复制标题
DOI：
10.1021/ja01044a025
复制
发表时间：
1969-01-01
影响因子：
15
通讯作者：
SAUNDERS, WH
中科院分区：
文献类型：
--
作者：
KATZ, AM;SAUNDERS, WH
采用Wolfsberg-Schachtschneider FG矩阵程序计算了氢氧化物离子与乙基二甲基磺酸离子的E2反应的氘和硫同位素效应。探讨了同位素效应对过渡态力常数（包括用于实现零或虚反应坐标频率的非对角F矩阵元素）的依赖性。氘同位素效应在质子半转移时达到最大值，并且不受碳硫键减弱程度的强烈影响。同样，硫同位素效应随着碳硫键弱化程度的增加而增加，而与质子转移程度的关系不大。另外两个观察结果就不那么令人放心了。质子转移运动与其他原子运动的耦合程度（非对角线F矩阵元素）的变化会极大地改变氘同位素效应，而硫同位素效应可以保持很小，直到碳硫键的拉伸力常数下降到原来的一半以下。讨论了这些结果对利用同位素效应来确定过渡态结构的意义。
Deuterium and sulfur isotope effects have been calculated for E2 reactions of hydroxide ion with ethyldimethylsulfonium ion, usingthe Wolfsberg-Schachtschneider FG matrix program. Thedependence of the isotope effects on transition-state forceconstants (including off-diagonal F matrix elements used to achieve a zero or imaginary reaction coordinate frequency) was explored. The deuterium isotope effect goes through a maximum when the proton is half-transferred, and is not strongly affected by theextent of weakening of the carbon-sulfur bond. Similarly, the sulfur isotope effect increases with increasing extent of carbon-sulfur bond weakening, and does not depend much on the extent of proton transfer. Less reassuring are two other observations. The deuterium isotope effect can be changed drastically by changes in the degree of coupling (off-diagonal F matrix elements) of the proton-transfermotion with other atomic motions, and the sulfur isotope effect can remain very small until the stretching force constant of the carbon-sulfur bond has decreased to less than half of its original value. The implications of these results for the use of isotope effects in determining transition-state structures are discussed.