hybrid Quantum Chemical Studies for the Reaction Mechanism and the Control of Chemical Processes in the Supercritical Water

(1)用密度泛函从头算方法研究了水分子在乙醇氧化反应中的催化作用：CH_3CH_2OH+NH_2O-&GT；CH_3CHO+H_2+NH_2O(n=0，1，2)。结果表明，当n=0时，反应势垒为88.0kcal/mol，当两个水分子(n=2)参与反应时，反应势垒降低约34kcal/mol。结果表明，反应速率常数增加到3.31×10~(-4)~(-4)~gt；S^~(-1)~(-1)，表明水分子在乙醇氧化反应中起着重要的催化作用。(2)采用实空间网格量子力学/分子力学(QM/MM)方法，通过质子转移机理，研究了水分子在乙醇氧化反应中的作用。计算中考虑了溶液的两种热力学条件：常压(AW)和超临界水(SCW)。QM/MM模拟表明，SCW中过渡态(TS)的溶剂化能大于反应态的溶剂化能，导致活化能降低3.7kcal/mol。而在AW中，能垒提高了7.2千卡/摩尔。径向分布函数表明，当AW中的络合物从反应物变为TS时，溶剂与参与反应的水分子之间的氢键严重崩溃，这表明附着在反应物上的紧密堆积的氢键网络干扰了质子的迁移。我们还研究了质子逐级移位的反应机制，发现与协调的反应机制相比，这种机制要小得多。

(1) Ab initio density functional theory calculations have been performed to investigate the catalytic role of water molecules in the oxidation reaction of ethanol : CH_3CH_2OH + nH_2O -> CH_3CHO + H_2 + nH_2O (n = 0, 1, 2 ). The results show that the potential energy barrier for the reaction is 88.0 kcal/mol in case of n =0, while it is reduced by 〜 34 kcal/mol when two water molecules are involved (n = 2 ) in the reaction. As a result, the rate constant increases to 3.31x10^<-4>s^<-1>, which shows a significant catalytic role of water molecules in the ethanol oxidation reactions.(2) A real-space-grid quantum mechanical / molecular mechanical (QM/MM) simulations have been carried out to investigate the role of the water solvent on the novel ethanol oxidation reaction catalyzed by two water molecules through proton transfer mechanism. We have considered two thermodynamical conditions of solutions for the calculations; ambient(AW) and supercritical water(SCW). The QM/MM simulations have revealed that the solvation energy for the transition state(TS) is larger than that of the reactant state in the SCW resulting in the reduction of the activation energy by 3.7 kcal/mol. While, in the AW, the energy barrier is raised by 7.2 kcal/mol. Radial distribution functions show that hydrogen bonding between the solvent and the water molecules that participate in the reaction seriously collapses when the complex is changed from the reactant to the TS in the AW, suggesting that the closely packed hydrogen bond network attached to the reactant disturbs the proton migration to take place. A reaction mechanism by stepwise proton translocations has also been examined and found to be minor as compared with the concerted one.