Hybrid QM/MM molecular dynamics simulations for an ionic S_N2 reaction in the supercritical water: OH⁻ + CH₃Cl → CH₃OH + Cl⁻

Abstract

A hybrid real space quantum mechanical/molecular mechanical (RS-QM/MM) method has been applied to an ionic S(N)2 reaction (OH- + CH3Cl --> CH3OH + Cl-) in water solution to investigate dynamic solvation effects of the supercritical water (SCW) on the reaction. It has been demonstrated that the approaching process of OH- to methyl group is prevented by water molecules in the ambient water (AW), while the reaction takes place easily in the gas phase. Almost the same solvation effect on the dynamics of OH- is observed in the SCW, though the bulk density of water is substantially reduced compared with that of the AW. It has been shown that the solvation of the SCW around the OH anion is locally identical to that of the AW due to the strong ion-dipole interactions between OH- and water molecules. At the transition state, the QM/MM simulations have revealed that the excess electron is quite flexible, and the charge volume, as well as the fractional charges on atoms, vary seriously depending on the instantaneous solvent configurations. However, it has been found that the solvation energy in the SCW can be qualitatively related to the HOMO volume of the system by Born's equation.

Kinetic and Equilibrium Study on Formic Acid Decomposition in Relation to the Water-Gas-Shift Reaction

“…It is desirable to perform a quantum-chemical calculation for the acid-catalyzed decarbonylation to get insight in the transition-state structure for the hydrothermal reaction. 11,[21][22][23] 3.2. Decarbonylation Equilibrium.…”

Hybrid quantum chemical studies for the methanol formation reaction assisted by the proton transfer mechanism in supercritical water: CH3Cl+nH2O→CH3OH+HCl+(n−1)H2O

“…The electronic structure for the QM subsystem is determined by the Kohn-Sham density functional theory 22 employing real-space grids. 10,17,18,[27][28][29][30][31][32][33] A detailed description of the RS-QM/MM method is written in Ref. 17.…”

“…͑2͒ as shown in the previous QM/MM simulations. 10 Since the charge volume of the excess electron of the solute complex (OH Ϫ ϩCH 3 Cl) in Eq. ͑2͒ increases drastically at the TS, the solute-solvent interaction is remarkably destabilized and the activation energy increases up to 20.6 kcal/mol in SCW ͑Ref.…”

“…As for the S N 2 reaction ͑2͒, we conducted simulations based on quantum chemistry in the previous paper and found that the increase in the charge volume at the transition-state ͑TS͒ configuration gives rise to the increase in activation energy for this reaction. 10 The activation free energy for the reaction was computed as 18.8 kcal/mol in SCW (T r ϭ1.066, ϭ0.33 g/cm 3 ͒. The path ͑2͒ is related to the wellknown fact that the ion product of water is raised in nearcritical water.…”

“…We carry out the RS-QM/MM simulations and compute the energetics for the water-catalytic reaction ͓path ͑1͔͒ to make a comparison with that for the ionic reaction ͓path ͑2͔͒ studied in former simulations. 10 In Sec. II, we give computational details and a brief review of the RS-QM/MM method developed in our laboratory.…”