Uggerud Einar
Department of Chemistry, University of Oslo, P. O. Box 1033, Blindern, 0315 Oslo, Norway.
Chemistry. 2006 Jan 23;12(4):1127-36. doi: 10.1002/chem.200500639.
The potential energy profiles of 18 identity S(N)2 reactions have been estimated by using G2-type quantum-chemical calculations. The reactions are: X- + CH3-X --> X-CH3 + X- and XH + CH3-XH+ --> +HX-CH3 + XH (X = NH2, OH, F, PH2, SH, Cl, AsH2, SeH, Br). Despite the charge difference, the barrier heights and the geometrical requirements upon going from the reactant to the transition structure are surprisingly similar for X- and XH. The barrier heights decrease on going from left to right in the periodic table, and increasing ionization energy (of X- and XH) is correlated with decreasing barrier. The observed trends are explained in terms of substrates with stronger electrostatic character giving rise to lower energetic barriers due to decreased electron repulsion in the transition structure. On the basis of this study, the relationship between the kinetic concept of nucleophilicity and the thermodynamic concept of basicity has been analyzed and clarified. Since the trends in intrinsic nucleophilicity (only defined for identity reactions) and basicity are opposite, overall nucleophilicity (defined for any reaction) will be determined by the relative contribution of the two factors. Only for strongly exothermic reactions will basicity and nucleophilicity be matching.
通过使用G2型量子化学计算，估算了18个相同的S(N)2反应的势能面。这些反应为：X- + CH3-X --> X-CH3 + X- 以及 XH + CH3-XH+ --> +HX-CH3 + XH（X = NH2、OH、F、PH2、SH、Cl、AsH2、SeH、Br）。尽管存在电荷差异，但对于X-和XH而言，从反应物到过渡结构时的势垒高度和几何要求惊人地相似。在元素周期表中，从左到右势垒高度降低，并且（X-和XH的）电离能增加与势垒降低相关。观察到的趋势可以这样解释：具有更强静电特征的底物，由于过渡结构中电子排斥减少，导致能量势垒更低。基于这项研究，亲核性的动力学概念与碱性的热力学概念之间的关系得到了分析和阐明。由于内在亲核性（仅针对相同反应定义）和碱性的趋势相反，总的亲核性（针对任何反应定义）将由这两个因素的相对贡献决定。只有对于强放热反应，碱性和亲核性才会匹配。
