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

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(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.