Theoretical Design Of Species With The Planar Hexacoordinate Carbon:D_3h CN₃X₃⁺（X=Mg,Ca,Si,Ge,Sn,Pb）

Since Hoffmann proposed the strategies of stabilizing planar tetracoordinate carbon (ptC), some molecules containing planar tetracoordinate carbon (ptC) and planar pentacoordinate carbon (ppC) had been successively designed. But only a few planar hexacoordinate carbon (phC) molecules had been proposed, because the size of the carbon atom is very small and due to the definition of phC, the carbon atom and all the other ligand atoms are forced in the same plane. when the coordination number increases, the repulsion between the central carbon atom and the ligand and that among the ligands become larger, the design of planar carbon species with high coordination number become more and moe difficult. In present thesis, we started from the phC molecules D3h CN3Be3+, and designed six new planar hexacoordinate Carbon molecules, D3h CN3Mg3+, D3h CN3Ca3+, D3h CN3Si3+, D3h CN3Ge3+, D3h CN3Sn3+, D3h CN3Pb3+by substituting the Mg atoms in D3h CN3Be3+by Mg, Ca, and Si-Pb, and systematically analyzed these systems. The results are as follows:(1)For CN3Mg3+, we found that although the structure containing phC is not global minimum on its potential energy surface, the studies of the molecular dynamics simulations and the lowest energy barrier ring-opening reaction indicated that the kinetic stability of contained phC structure is very excellent. While those isomers that are energetically lower than D3h CN3Mg3+are composed of two relatively independent molecules, N2and CNMg3+, and the bonding interactions between them are rather weak. Our ab initio Born-oppenheimer molecular dynamic simulations suggest that they will dissociate to a N2molecule and a various type CNMg3+subunit, i.e. they are dynamically unstable. Therefore, the D3h CN3Mg3+contained phC may be realized as a kinetically stable phC species.(2)For CN3Ca3+, after further optimizing isomers at high level which originated from the search of the potential energy surface, we found that the order of relative energies was much different from that of before optimization. To find the reason, we examined energy order of11selected isomers using the combination of four theoretical methods and five basis sets. The results suggested that the basis set employed was the main influencial factor to the energy orders of isomers. we also studied the proper theoretical level for PES exploration of this system.(3)For D3h CN3X3+(X=Si, Ge, Sn, Pb) phC species, we analyzed why the phC structure can be stabilized and found that using Si, Ge, Sn, Pb to substituted Be in D3h CN3Be3+is a "quasi-equivalent" replacement, because all these phC species has three lone pairs similar to singlet carbene. We predicted these molecules have potential applications in the field of molecular catalysis.The studies in this work enrich planar hexacoordination carbon molecules family, propose the possible targets for the future synthesis, and also provide necessary theoretical basis for the experiment realization.