Studies Of Three Systems Of Different Sizes Using Quantum Mechanical And Molecular Mechanics Approach With A Multilayered-representation In Aqueous Solution

There are numerous chemical reactions in aqueous solution driving from collision ofmolecules. It is of great differences when operating a gas phase reaction in water for thepresence of the solvent shielding effect and the solvent caging effect. Amultilayered-representation, quantum mechanical and molecular mechanics approach withan explicit solvent module (SPC/E) is used in our research to get results of CCSD(T) level oftheory, those are CCSD(T)/MM, DFT/MM and ESP/MM. As a serious pollution source ofthe surface/ground water and atmosphere, halogenated hydrocarbons need to be researchedin water. So we studied the SN2reaction of CH3Cl+Fâ†'CH3F+Cl and then the biggersystem EtCl+ClOâ†'EtClO+Cl. we transferred our work to a larger reaction afterresearches of halogenated hydrocarbons, that is G8OH·to a formimidic acid radical inaqueous solution.The first chapter in my thesis is introduction contains background and theories of ourresearch.The second chapter talks about the reaction mechanisms of CH3Cl+Fâ†'CH3F+Cl inwater. The calculated reaction activation barrier under the CCSD(T)/MM representation is23.2kcal/mol which agrees well with the experimental value at26.9kcal/mol in water; andthe calculated reaction free energy is-8.0kcal/mol which agrees excellently with theestimated one obtained based on the gas-phase reaction energy, reactant and productsolvation energies. The solvation energy and polarization effect combined increasesactivation barrier by18.5kcal/mol, in which the solvent effect plays a dominant role inshaping the PMF.Chapter three gives us the reaction mechanisms of EtCl+ClOâ†'EtClO+Cl. We havealso calculated parameters mentioned above, and found the solvation energy and polarization effect play the same important role. In addition, the rate constant of title reaction is5.27×10-17cm3molecule-1s-1in aqueous phase, which is about7orders magnitude smaller than ingas phase, therefore, the water environment slows down the title reaction in solution phase.The recommended hybrid density functional/basis set is M08-SO/cc-PVTZ in our system.The forth chapter is about the ring opening of G8OH· to a formimidic acid radical. wehave getten the geometries, charge distribution, PMF and water contribution along withpolarization effect to free energy. We calculated the PMFs of the title system based on theB3LYP/6-31++G**results using several different methods recommended by Truhlar inconsideration of accuracy and efficiency, and found that M08-SO/cc-PVTZ is the mostappropriate method when relative large basis sets and functionals are permitted.The fifth chapter contains conclusions and expectations.