A quantum chemical approach to the study of a novel reaction path and its control in the supercritical water

本工作所得的结果可归纳为以下两个主题。一是提出了一种计算凝聚态化学反应自由能变化的新方法（QM/MM-ER）。二是通过QM/MM-ER模拟，探索了一种新的反应途径。有人认为，质子沿着水分子的氢键转移促进了丁二醇在热水中的脱水。结果的详细情况分列如下。(1)凝聚态自由能计算的量子化学方法为了计算水溶液中化学反应的自由能变化，我们将量子力学/分子力学方法与能量表示理论相结合。在能量表示理论中，溶质的溶剂化自由能是根据溶质-溶剂相互作用能的分布函数来描述的，而不是传统溶液理论中使用的空间分布函数。由于能量表示的方法不需要相互作用位置的概念，因此与量子化学处理的结合是直接的。(2)醇脱水反应的量子化学计算量子化学计算揭示了丁二醇在质子转移反应的过渡态（TS）形成双基电子结构。在气相中活化能极高（~ 70 kcal/mol），而在热水中活化能降至~ 50 kcal/mol。TS的电子态在极性溶剂中形成两性离子结构，与反应物相比，这导致了TS的稳定。由此推测，质子转移机制催化了乙醇在热水中的脱水反应。

The results obtained by present work can be summarized by the following two subjects. One is that a novel methodology (QM/MM-ER) has been developed to compute the free energy change associated with a chemical reaction in a condensed phase. The other is that a novel reaction pathway has been explored by means of the QM/MM-ER simulations. It has been suggested that the proton transfers along the hydrogen bonds of water molecules promote the dehyderation of butanediol in hot water. The details of the results have been itemized as follows.(1)Quantum chemical approach to the free energy calculation in condensed phaseIn order to compute the free energy change associated with a chemical reaction in aqueous solution, we have combined the quantum mechanical/molecular mechanical approach with the theory of energy representation. Within the theory of the energy representation, the solvation free energy of a solute is described in terms of the distribution functions of the solute-solvent interaction energy instead of the spatial distribution function used in the conventional theory of solution. Since the method of the energy representation does not need the concept of the interaction site, the combination with the quantum chemical treatment is straightforward.(2)Quantum chemical calculation for the dehydration reaction of alcoholQuantum chemical calculations have revealed that butanediol forms a biradical electronic structure at the transition state (TS) of the proton transferring reaction. The activation energy is extremely high (〜70 kcal/mol) in the gaseous phase, however, it reduces to 〜50 kcal/mol in hot water. The electronic state of the TS forms a zwitterionic structure in the polar solvent, which leads to the stabilization of the TS as compared to the reactant. Thus it has been suggested that the proton-transfer mechanism catalyzes the dehydration reaction of alcohol in hot water.