| Planar molecules with the non-traditional bonding mode have been receiving considerable attention because of their special geometric and electronic structures.In 1970,Hoffmann put forward the concept of "planar tetracoordinate carbon",which broke through the conventional cognition of tetrahedral structure of carbon.The strategy to combine the spacial constraint and electron bonding interaction can maintain the stable planar structure of molecules,which provides a way for the design of planar tetracoordinate structure.Both carbon and silicon are the elements of the fourth main group,and their compounds are important parts of organic and inorganic worlds,respectively.In recent years,a series of planar tetracoordinate carbon compounds have been successfully synthesized experimentally and investigated theoretically,which enrich the understanding of planar tetracoordinate carbon molecular systems.However,the chemistry of planar tetracoordinate silicon is still unknown,and the further study is highly required,both theoretically and experimentally.In this dissertation,novel molecules containing planar tetracoordinate silicon are designed and characterized,based on extensive theoretical calculations.The main results are summarized as follows:(1)Structures,stabilities and bonding features of planar tetracoordinate silicon molecules SiN4C8H8(Si-5555),SiN2C10H10(Si-5656),SiNC14H11(Si-5666)and SiN2C12H10(Si-4666)were studied by DFT calculations and MD simulations.These stable planar tetracoordinate structures are composed of four membered rings,five membered rings and six membered rings,and they generally have 16 π electrons.Taking Si-5555 as an example,the electronic structure and stability are discussed in detail.The computational results show that the central Si of Si-5555 adopts sp2d hybridization to form planar tetracoordinate structure(ptSi)with its surrounding four nitrogen atoms,and the bonding interaction of the 3pz unoccupied orbital(Si)with the occupied π*orbital on the ring results in the net 14π electrons,which meets the 4n+2 Hückel rule for Si-5555.Further NICS and ACID calculations show that the Si-5555 molecule had strong aromaticity.In addition,we also studied the electrophilic and nucleophilic properties of ptSi,as well as the structure and stability of bimolecular systems of Si-5555.(2)Extensive density functional theory calculations have been used to explore structures,bonding features,stabilities and spectroscopic properties of a new type of molecules with the planar tetracoordinate carbon(ptC),ptSi and ptGc.These compounds with the planar CB4,SiN4,GeN4,and GeB4 moieties are predicted to be stable,both structurally and electronically,according to the structure optimizations and frequency analysis.There is generally a delocalized π bond between the central atom and its surrounding four atoms in such stable planar tetracoordinate compounds,in which C and Si behave as the π donor and acceptor,respectively,while Ge bears the character of both π donor and acceptor.In the compounds of C20H16N4X(X=Si and Ge),ptSi and ptGe adopt sp2d hybridization to form σ bonds with the N atoms.Since there is relatively small s-p energy splitting for C without the d orbital,an approximate sp2 hybridization is responsible for its σ bonding with surrounding B atoms.(3)The planar tetracoordinate silicon SiN4C8H8(Si-5555)has a carbenoid-type amphoteric center,behaving as a strong electron donor and weak electron acceptor.The dimer structure of Si-5555 was constructed,and the adsorption interactions between the ptSi site and CO2 as well as other small molecules were explored by DFT calculations.The present results indicate that the ptSi center shows remarkable activity towards adsorption and activation of CO2,which mainly arises from the significant charge transfer interaction within ptSi…CO2. |
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