A Multilayered Representation, Quantum Mechanical And Molecular Mechanics Study Of The Two S_N2Reaction Of CH₃F+OH^-Reaction And C₂H₅Cl+ClO^-Reaction In Aqueous Solution

The bimolecular nucleophilic substitution (SN2) reaction of CH3F+OH－in aqueoussolution were first investigated using a combined quantum mechanical and molecularmechanics approach. The SN2reaction of C2H5Cl+OCl—in water were first investigatedusing explicit solvent model. A multilayered representation approach of DFT/MM、ESP/MMand CCSD(T)/MM is employed to achieve high accuracy results at the CCSD(T) level oftheory. The reaction mechanism of the SN2reaction was explained scientifically at themolecular and atomic level.Reactant complex, transition state, and product complex along the reaction pathwaywere analyzed at the DFT/MM level in water. Note in water, there is no longer a complexwith F－attached to the OH group of CH3OH, and instead the product complex is like thetraditional C3VSN2complex with F－attached to the backside of CH3OH. This difference ismainly caused by the cage effect of solvent where the solvent hinders the large amplitudemotions that occurs in gas phase. The free energy profile and the potential of mean force wascalculated at the DFT level and CCSD(T) level of theory using saddle search、moleculardynamics simulation and NEB method. The free energy activation barrier for CH－3F+OHis18.3kcal/mol and17.7kcal/mol at CCSD(T)/MM and DFT/MM representation respectively.The data is13.2kcal/mol and11.7kcal/mol for C2H5Cl+ClO－reaction in water. The barrierheight under the DFT level of theory is lower than the one of CCSD(T), which is consistentwith previous studies that DFT systematically underestimates the reaction. The barrier heightinvestigated at the CCSD(T)/MM representation agree well with the experimental value. Boththe solvation effect and the solute polarization effect play key roles on raising the activationbarrier height in aqueous solution. Our results show that aqueous environment raises the freeenergy barrier and the reaction energy.This paper is organized as follows. The first chapter gives a brief introduction of the SN2 reaction of halogenated hydrocarbon. In the second chapter we show the theoretical methodsused in this paper. And then, The main work of this paper are listed in the third and the fourthchapter, we have discussed the reaction of the CH3F+OH－and C2H5Cl+ClO－in aqueoussolution respectively. At last, we briefly summarize the total subject and give an expectationfor the future work.