| Atomic-level,bimolecular nucleophilic substitution reaction mechanisms have been studied mostly in gas phase,but gas-phase results cannot be expected to reliably describe condensed-phase chemistry.As a novel,double-inversion mechanism has just been found for the F-+CH3Cl SN2 reaction in gas phase,here,using multi-level quantum mechanics methods combined with molecular mechanics method,we discovered a new,double-inversion mechanism for F-+CH3Cl and F-+CH3I reactions in aqueous solution.However,the structures of the stationary points along the reaction path show significant differences from those in gas phase due to the strong influence of solvent and solute interactions,especially due to the hydrogen bonds formed between the solute and solvent.More importantly,the relationship between the two double-inversion transition states is not clear in gas phase,but,here we revealed a novel intermediate complex serving as a“connecting link”between the two transition states of the abstraction-induced inversion and the Walden-inversion mechanisms.For F-+CH3I reaction,our calculated free energy barrier of the abstraction-induced inversion is 69.5 kcal/mol at the CCSD?T?/MM level of theory,which agrees with the one at 72.9 kcal/mol calculated using the Born solvation model and gas-phase data.In addition,contributions of solvent energy and solute-solvent interactions to the reaction pathway have been identified.As for the ring opening process of guanine damage by hydroxyl radical in aqueous solution,our calculated free-energy barrier height 31.6 kcal/mol at the CCSD?T?/MM level of theory agrees well with the one estimated based on gas-phase reaction profile and free energies of solvation.In addition,the reaction path in gas phase was also mapped using multi-level quantum mechanics theories,which shows a reaction barrier at 19.2 kcal mol-1at the CCSD?T?level of theory,agreeing very well with a recent ab initio calculation result at 20.8 kcal mol-1.We also investigated the water-assisted intramolecular proton transfer of8-OHGrad in aqueous solution,where an explicit SPC/E solvation model was used to describe the aqueous solution to describe the local rearrangements of water molecules.The water solution contributes-8.6 kcal/mol to the activation barrier,which means the water solution lowers the transition state in aqueous solution and has a catalytic effect for this proton-transfer reaction mechanism.In this article,we introduce the development of computational chemistry and basic concept in chapter 1;show the method in theory in chapter 2;study the double-inversion mechanism for F-+CH3Cl and F-+CH3I reactions and the back-side attack mechanism for Cl-+CH3I in chapter 3,study the ring opening process of 8-OHGrad and the water-assisted intramolecular proton transfer of 8-OHGrad in chapter 4.The fifth chapter is summary and outlook. |
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