A 2-DIMENSIONAL ENERGY SURFACE FOR A TYPE-II SN2 REACTION IN AQUEOUS-SOLUTION
A 2-DIMENSIONAL ENERGY SURFACE FOR A TYPE-II SN2 REACTION IN AQUEOUS-SOLUTION
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DOI：
10.1021/ja00074a036
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发表时间：
1993-10-20
影响因子：
15
通讯作者：
XIA, XF
中科院分区：
文献类型：
--
作者：
GAO, JL;XIA, XF
通过结合量子力学和统计力学方法，研究了水溶剂化对氨和氯甲烷之间的S(N)2门秀金反应势能面的作用。在当前的模拟方法中，反应物分子采用半经验的AM1理论处理，而溶剂则由经验的TIP3P模型表示。在整个流体模拟过程中，通过哈特里 - 福克分子轨道计算来评估溶质 - 溶剂相互作用。在本文中，首先通过与高水平的从头算结果进行比较表明，这种混合量子力学和分子力学（QM/MM - AM1/TIP3P）模型能够对门秀金反应中溶质和溶剂之间的分子间相互作用提供充分的描述。然后通过统计微扰理论结合网格搜索算法确定水溶液中的自由能面。结果表明，溶剂效应强烈地稳定了过渡态和产物。计算得到的活化自由能（26千卡/摩尔）与先前的理论和实验估计值吻合良好。最显著的发现是过渡态明显向反应物偏移，C - N键伸长了0.30埃，C - Cl键缩短了0.15埃。这符合哈蒙德假说，并且与先前的理论研究一致。对模拟结果的分析表明，在当前的II型S(N)2反应过程中，电荷分离受到溶剂效应的促进，在过渡态时电荷转移完成了约65%。提供了对反应物和过渡态差异溶剂化的结构和能量性质的详细见解。
The role of aqueous solvation on the potential surface of the S(N)2 Menshutkin reaction between ammonia and methyl chloride has been examined by using a combined quantum mechanical and statistical mechanical method. in the present simulation approach, the reactant molecules are treated by the semiempirical AM1 theory, while the solvent is represented by the empirical TIP3P model. Solute-solvent interactions are evaluated through Hartree-Fock molecular orbital calculations throughout the fluid simulation. In this paper, it is first demonstrated, by comparison with high-level ab initio results, that this hybrid quantum mechanical and molecular mechanical (QM/MM-AM1/TIP3P) model can provide an adequate description of intermolecular interactions between the solute and solvent for the Menshutkin reaction. The free energy surface in aqueous solution is then determined via statistical perturbation theory with a grid search algorithm. The results suggest that the solvent effects strongly stabilize the transition state and products. The computed free energy of activation (26 kcal/mol) is in good agreement with previous theoretical and experimental estimates. The most striking finding is that the transition state is shifted significantly toward the reactants, with a lengthening of the C-N bond by 0.30 angstrom and a shortening of the C-Cl bond by 0.15 angstrom. This is in accord with the Hammond postulate and consistent with previous theoretical studies. Analyses of the simulation results indicate that the charge separation during the present Type II S(N)2 reaction is promoted by the solvent effect, with a charge transfer of about 65% complete at the transition state. Detailed insights in to the structural and energetic nature of the differential solvation of the reactants and transition state are provided.