Hybrid Quantum Mechanical And Molecular Mechanics Studies Of The Two Reactions Of CCl₄+OH^-and The Ring-opening Of8-OHGrad In Aqueous Solution At CCSD（T） Level

The bimolecular nucleophilic substitution reaction of CCl₄+OH and thering-opening of8-OHGrad in aqueous solution were investigated on the basis of acombined quantum mechanical and molecular mechanics method. A multilayeredrepresentation approach is employed to achieve high accuracy results at the CCSD（T）level of theory. The potential of mean force were calculated at the DFT level andCCSD（T） level of theory. The corresponding reaction barrier heights of CCl₄+OH were22.7and27.9kcal/mol, respectively, and the ring-opening of8-OHGrad,23.1and27.9kcal/mol. Both the solvation effects and the solvent-induced polarizationeffect have significant contributions to the reaction energetics, for example, thesolvation effect raises the saddle point of the two reactions by10.6kcal/mol and17.6kcal/mol. The calculated rate constant coefficient of CCl₄+OH is8.6×10^-28cm³molecule^-1s^-1 at the standard state condition, which is about17orders magnitudesmaller than that in the gas phase. Among the four chloromethanes （CH₃Cl, CH₂Cl₂,CHCl₃, and CCl₄）, CCl₄has the lowest free energy activation barrier for the reactionwith OH in aqueous solution, confirming the trend that substitution of Cl by H inchloromethanes diminishes the reactivity.