Theoretical Study On The Reaction Mechanism Of Carbanions With N₂O In Gas Phase

Base-induced bimolecular elimination (E2) and bimolecular nucleophilic substitution (S_N2) reactions are two fundamental organic reactions in synthesis. They play an important role in the development of modern mechanistic physical organic chemistry . In many cases, E2 and S_N2 pathways are usually competitive processes, hence, in some reactions, Sn2 works as main reaction, while in others the main reaction is E2. Recently, several studies have focused on reactions of nucleophile with hydrocarbon, such as ClO^-,BrO. Villano et al investigated the reactions of ClO^- with CH₃CH₂Cl and found ClO^- + CH₃CH₂Cl via E2 and S_N2 channels. Then they studied the reactions of BrO^- with MeCl, EtCl, and i-PrCl, they found the reactions also produce minor amounts of Br-, these results is rather unexpected. Because we can not detect the neutral products in experiments, we can not provide accurate decision for the reaction mechanism. In order to explain the exist of Br-,they state that the reaction of BrO^- with CH₃Cl likely proceeds initially through a traditional S_N2 transition state, followed by an additional reaction pathway in the S_N2 additionally proceed, while they do not make more work for the additional step. On the basis of predecessors′work, further theory study on the mechanism for the gas-reactions of the XO^-(X=Cl,Br,I) with CH₃Cl is developed.In the paper, on the basis of the molecular orbital theory, the tradition transition state theory as well as quantum chemistry theory, the selected systems have been investigated using Moller-Plesset perturbation theory (MP2) and the Natural Bond Orbital analysis. The structures of the intermediates and the transition states along the reaction paths were fully optimized. The thermodynamic datum were all used to obtain the potential surface. The information of orbitals was also used to explain the reaction mechanism.The whole paper consists of five chapters. Chapter 1 is mainly about the development and application of quantum chemistry and the characteristics of organic gas-phase anion molecule reaction are also concerned.In Chapter 2, we will introduce the theory and computation methods that we based on.In Chapter 3, the mechanism for the ion-molecule reaction of BrO^- with CH₃Cl has been characterized in detail. Based on aug-cc-pVDZ basis set, using the second-order M?ller–Plesset perturbation theory (MP2), the single-point energies have also been refined at the CCSD(T)/ 6-311++G(3d,3p) level to get more accurate energies using the MP2/aug-cc-pVDZ optimized geometries. Computational results indicated that BrO^- + CH₃Cl initially proceed a traditional S_N2 mechanism, generating an energy-enriched intermediate IM2 (BrOCH₃···Cl). Then followed by decomposition of IM2 produce P₁(BrOCH₃ + Cl) in S_N2 mechanism. IM2 can also occur an elimination reaction between carbon and oxygen atoms. Further substitution reaction take place on oxygen atom . The former primary generated P₂(CH₂O + HCl + Br- ). The latter primary produced P₃(CH₃OCl + Br- ). The channel of traditional S_N2 reaction is the main reaction channel, while the minor channels of S_N2-induction elimination reactions and substitution reactions are the competitive reaction channels.In Chapter 4, the mechanism for the ion-molecule reaction of ClO^- with CH₃Cl has been characterized in detail. Based on aug-cc-pVDZ basis set, using the second-order M?ller–Plesset perturbation theory (MP2). Computational results indicated that ClO^- with CH₃Cl initially proceed a traditional S_N2 mechanism. Then followed by an elimination reaction between carbon and oxygen atoms. and substitution reaction on oxygen atom. The channel of traditional S_N2 reaction is the main reaction channel, while the minor channels of S_N2-induction elimination reactions and substitution reactions are the competitive reaction channels.In Chapter 5, the mechanism for the ion-molecule reaction of IO^- with CH₃Cl has been characterized in detail. Based on aug-cc-pVDZ basis set, using the second-order M?ller–Plesset perturbation theory (MP2).