Theoretical Investigation On The Structures And Reaction Mechanism Of Several Kinds Of Multiply Bonded Compounds

In this thesis, the structures and stability of [XCN2] molecules (X=Ge, Sn, Pb), reaction mechanisms of Ge2 with N2, and catalytic nitrile-alkyne cross-metathesis (NACM) have been investigated by means of the quantum chemical methods. The thermodynamical and kinetic stability of various [XCN2] isomers, singlet and triplet pathways as well as the singlet-triplet tunneling of the Ge2+N2 reaction, and two main evolution channels of the NACM reaction have been provided. The calculation results agree well with available experiments. The results of this thesis may provide useful information for the further theoretical and experimental studies on related systems. The main results are summarized as follows:1. The CCSD(T)//B3LYP method was applied to study the structures and energetics of various isomers and transition states on the [XCN2] potential energy surface. Five linear forms XNCN, XCNN, XNNC, NXCN, NXNC and two cyclic forms X-cCNN and X-cNCN were located as energy minima. The results showed that linear isomers XNCN, XCNN and XNNC each possess good kinetic stability, and thus are promising for laboratory characterization. The molecular orbital, hydrogenation heat and bond dissociation energy analysis all indicated that XCNN is associated with a XC triple bond. Moreover, XCNN is the most stable isomer after XNCN in both thermodynamics and kinetics. Our designed triply bonded molecules differ from the traditional alkynes and belong to a novel type. Taking GeCNN as an example, we proposed that the transiton metal carbonyl coordination can provide considerable stabilization to the designed XCNN structure for the sake of laboratory detection. Our computational work might provide new insights for future study of the XC triply bonded molecules. 2. The CCSD(T)//B3LYP method was used to study the reaction of Ge2 with N2. A"singlet-triplet intersection mechanism"was disclosed. The starting reagent, triplet Ge2, initially reacts with the environmental gas N2 to form a triplet four-membered ring intermediate 3c-GeGeNN, which would undergo a singlet-triplet canonical intersection (CI) point to form a three-membered ring isomer 1Ge-cNNGe. Isomer 1Ge-cNNGe will further undergo a ring-opening to generate the singlet isomer 1GeNNGe as the enventual product. Thus, the main reaction channel can be written as: 3Ge2+N2 3c-GeNNGe CI 1Ge-cNNGe 1GeNNGe. Our computational results can reasonably explain the recent low temperature laser ablation experiment. By contrast, the careful IRC analysis showed that the two transiton states previously located are erroneously connected. Therefore, the previously proposed mechanism via a singlet reaction pathway ("singlet mechanism") to form the singlet GeNNGe is incorrect. The present results could provide useful information for future mechanistic studies on the analogous bimolecular reactions such as Si2/Sn2+N2.3. The catalytic nitrile-alkyne cross-metathesis (NACM) was studied by meansof the DFT method. A four-membered ring (4MR) mechanism was revealed, i.e., a ring-closure process is initially proceeded to form a 4MR intermediate that can easily undergo the bond-rearrangement to form another 4MR intermediate, which would take ring-opening to generate the eventual products. The two 4MR intermediates are kinetically unstable and have little likelihood to be observed in laboratory. In addition, since the rate-determining step (ring-closure) needs to overcome high barriers, the overall reaction should be slow. The main reaction channel is: R: [W]N+CH3CCCH3?IM1?IM2?[W]CCH3+CH3CN [W]=(CH3O)3W.In principle, the NACM reaction should coexist with the AM reaction in experiments, and the former can provide necessary reagents for the latter. NACM is associated with high barrier and low speed, while AM is associated with relatively low barrier and high speed. Moreover, one drawback of the AM reaction is that when the concentration of reagent is high, the alkynes and (RO)3W≡CR′would polymerize to deactive the WC triply bonded catalysis. Thus, the NACM controls the concentration of AM, which will lower the happening possibility of side reactions of AM. As a result, the overall reaction yield is raised. Our results are consistent with the observed experimental phenomena and couldbe helpful for future design of more effective catalysts.