Theoretical Study On The Bimolecular Nucleophilic Substitution Reaction At Carbon

In my thesis the anion and ion-pair SN2 reactions at carbonhave been studied.In the gas phase there was an energy profile for the SN2 reaction represented by adouble well potential curve. All of calculations, including geometries optimization,vibrational frequencies analysis and natural population analysis (NPA) wereperformed at the same level of theory for the given reaction system. In anion SN2reactions at carbon all structures were completely optimized at the MPW1K level,but in the ion-pair at the CCSD(T)/6-311+G(d, p)//B3LYP/6-311+G (d, p) + ZPElevel. My thesis is divided into three parts as following: In chapter 2, the anion SN2 reactions Y- + CH3X CH3Y + X- Y, X = F, Cl,Br and I have been investigated and two theoretical methods, MPW1K and G2(+),were compared. Comparison of the results with G2(+) theory indicates that theMPW1K method can exhibit good performance in describing the potential energysurface. The kinetic and thermodynamic investigations predict that thenucleophilicity of X- in the SN2 at carbon follows the order: F- > Cl- > Br- >I- in gasphase, but the above order is reverse in aqueous solvent, i.e. F- < Cl- < Br- 四川大学硕士学位论文the anion SN2 reactions at carbon where inversion reaction pathway was muchmore favorable for all halogens. The fourth chapter covers theoretical investigation on the exchange reactionCH3SLi + CH3X LiX + CH3SCH3 (X = F, Cl, Br and I). There were also tworeaction channels, which proceed by the same complexation and differenttransitions of the retention or inversion mechanism. The stabilization energies fordipole-dipole complexes CH3X···LiSCH3 (X = F~I) were found to have a goodcorrelation with the electronegativity of X. The calculated central barriers andoverall barriers show good agreement with the predictions of Marcus equation andits modification, respectively. Further interesting feature of the non-identity ion pairSN2 reaction was a good correlation between inversion central barriers and thecomposite geometric looseness (%L).