A Multilayered Representation, Quantum Mechanical And Molecular Mechanics Study Of CH₃X+Y^-�'CH₃Y+X^-reaction In Aqueous Solution

The experimental study of the chemical reaction has been done for many years inthe aqueous solution. But the exploration for reaction mechanism in theory is from thebeginning of last century. Compared with the gas phase, the existence of solvent canbring more reaction degrees of freedom, more statistical sampling. So, the singlequantum mechanics calculation of reaction system is not enough. On the other hand,the existing method of molecular mechanics has their own advantage on thecomputing efficiency, but it can not correctly describe key fracture in the process ofreaction, as well as the new key generation process, this is the biggest flaw. In the70s,Warshel puts forward the opinion, quantum mechanics and molecular mechanics canbe mixed together, and established the method of quantum mechanics/molecularmechanics, which makes the reaction in aqueous solution and macromolecularcompound system is developing rapidly. In the work of this paper, using quantummechanics and molecular mechanics method (QM/MM method), the first study wasdone theoretically for halogenated hydrocarbonCHY3X+�'CH3Y+X (X=Br,Cl; Y=OH, CN) nucleophilic substitution reaction in aqueous solution. We introducethree kinds of representation, ESP/MM, DFT/MM, and (CCSD (T)/MM, begin withlower precision but high efficiency of ESP/MM representation, using the energydifference between the representation, then transform, high precision of (CCSD (T)level of theory results would be achieved.Firstly, we find the structure of reactants in the gas phase, then put it in the water.Here, we adopt the water molecules of SPC(E) model, and the standards field of amber. Under the DFT/MM representation，we optimize the system, and get theinitial reactants. Next, we simulate the chemical key fracture and generating, findproducts and optimize it. Using NEB (nudged elastic band), set up the initial pathwayof reaction, the highest point is suspected to be transition state, after numericalcalculation of frequency, saddle point search, and a true transition state structure isaffirmed. The free energy profile and the potential of mean force was calculated at theDFT level and CCSD(T) level of theory using saddle search、molecular dynamicssimulation and NEB method. Under the DFT/MM and (CCSD (T)/MM representation,the barrier for reaction CH3Br+OH－in aqueous solution is19.2kcal/mol, and22.8kcal/mol, respectively; barrier of the reaction CH3Br+CN－in aqueous solution is16.5kcal/mol and19.1kcal/mo; barrier of the reaction CH3Cl+CN－in aqueoussolution are18.3kcal/mol and20.3kcal/mol, compared with the existingexperimental results, the above value are all very close. We also can acquire the DFTtheory for the calculation of activation energy barrier is lower, the (CCSD (T)results are more accurate in this article. Aqueous phase environment’s contribution tothe reaction barrier root in the salvation effect and polarization effect, the solventeffect is the main.This article is divided into six chapters, the first chapter briefly introduces theresearch progress of halogenated hydrocarbon reaction and basic concept; the secondchapter show the method in theory and relevant theoretical knowledge; third, fourthand fifth chapters study the nucleophilic substitution reaction of CH3Br+CN－,CH3Br+OH－and CH3Cl+CN－reaction mechanism in aqueous solution,respectively. The sixth chapter is summary and outlook.