Theoretical Study Of Some Special Structures Formed By Carbon,Nitrogen And Oxygen

The valence-shell electron-pair repulsion(VSEPR)principle provides a simple model for predicting the shapes of classical molecules.Molecules with special structures which does not satisfy VSEPR model offer the supplements to structures of classical molecules,reflect the diversity of molecular structures,and enrich the chemical bond theory.Such molecules often have atypical geometric shapes and exotic electronic structures,and often exhibit exceptional electronic,magnetic,and optical properties,which have become the frontier of chemical research.It is importance theoretically to study molecules with special structures offering the potential for applications.Molecules with planar polycoordinate carbon,nitrogen,and oxygen are seminal examples of molecules with special structures.Such molecules have been studied by using the DFT method based upon the previous researches.Molecules with special structures formed by carbon in three-dimensional space were also studied in this work.The main contributions are as follows:1.Theoretical study on neutral molecules with square ptC(N)4 substructuresOrdinarily,every carbon atom forms four single bonds and exhibits the tetrahedral geometry according to the VSEPR model.The planar tetracoordinate carbon(abbreviated for ptC)is a special structure that does not satisfie the VSEPR model and is rarely formed generally.In 1970,Hoffmann et al.studied the bonding interactions of the simple ptC of planar methane and concluded that the instability of the ptC was caused by the pair of 2pz-? lone pair electrons on the carbon atom,which were in the nonbonding molecular orbital.The key to stabilize the ptC is to delocalize or disperse the 2pz-? lone pair electrons to the ligand.Based on the previous work,strategies to accomplish the ptC were proposed.One is "electronic" assistance that delocalization of the 2pz-? lone pair electrons may be achieved by attaching substituents to ptC which are good electron acceptors and enhancement of bonds between the carbon and attached atoms in ptC to use the ? electron donors which provide the electron to center carbon.Another is "mechanical”scheme which constrain the ptC in rigid cages to overcome the enormous strain of ptC from the planar to the tetrahedral structures.Based on the above strategies,many ptC species have been designed and studied,such as ptC(B)4,ptC(M)4(M=metal atom),and ptC(C)4,which is considered extremely challenging to achieve in the literature.Nitrogen is an electron-rich element with more valence electrons than valence orbitals,and is more electronegative than carbon.If atoms attached to ptC are unique four nitrogen atoms to achieve the ptC(N)4,the above“electronic" assistance strategy is disobeyed.The ptC(N)4 is different from the common ptC structure,which is composed of 18 valence electrons,while the ptC(N)4 structure contains 24 valence electrons.In addition,to achieve the square ptC(N)4 substructure,apart from to conquer the huge strain from the planar to tetrahedral configuration,the importance is to constrain the square ptC(N)4 arrangement in a proper system with the right symmetry(to make ptC(N)4 arrangement to be a plane square structure).Therefore,it is of great significance to the study and achieve the special structure of ptC(N)4.In order to study the special structure,based on the isopagodane structure,some neutral molecules with the square ptC(N)4 substructure have been designed and studied by using the DFT method.The computational results show that they are all real minima on the potential energy surfaces,and successfully achieved the square ptC(N)4 substructure in theoretically,which solves the difficulty of achieving the square ptC(N)4 substructure using unique four nitrogen atoms as the attached atoms.The bonding interactions among carbon and four nitrogen atoms in the square ptC(N)4 substructure were studied here.2.Theoretical study on systems with ptN(C)4 or ptN(N)4 substructuresThe planar tetracoordination structure of nitrogen(ptN)is another special structure.Up to now,the ligands are generally metals in the structures of ptN reported in the literature.As we know,the ptN(C)4 and ptN(N)4 have not been reported.Based on the very rigid structure of isopagodane,the square ptC(N)4 was been obtained.Can the ptN(C)4 and ptN(N)4 be achieved with such very rigid cage-structures?Because the number of electrons of nitrogen is odd,the singlet ionic species with ptN(C)4 were designed and studied firstly.Then,the singlet neutral molecules were designed and achieved by using the charge-compensation of the BH2 group.In order to answer the question of achieving the square ptN(N)4 substructure using unique four nitrogen atoms as the attached atoms to bond with center nitrogen,the singlet cation species with square planar N(N)4 substructures were proposed and investigated.The computational results show that the special structures of ptN(N)4 could be achieved in the singlet cation species.It could be seen that the "mechanical"approach of very rigid cage-structure is the key strategy to design and achieve the ptN(C)4 and ptN(N)4 substructures.For understanding the bonding interactions among the atoms in these special substructures,the bonding interactions among the atoms in the ptN(C)4 and ptN(N)4 substructures were analysed according to the group theory method,and the nature of C-N and N-N bonds in these substructures were characterized by using the quantum mechanical theory of atoms in molecules.The C-N and N-N bonds in the nonclassical structures are all typical covalent bonds.3.Theoretical study on systems with ptO(B)4 or ptO(C)4 substructuresThe special structures of ptC(N)4?ptN(C)4 and ptN(N)4 were achieved with very rigid cage-structures in chapter 3 and 4.Could the method be used to achieve the special structures of planar tetracoordination oxygen(ptO)attached with nonmetal elements?To date,the special structures of ptO(X)4(X=nonmetal elements)have not been reported in the literature.Based on the structures of isopagodane or pagodane,several neutral molecules with planar O(B)4 and O(C)4 substructures have been designed out by using the“electronic" and "mechanical”strategies,and studied by using the DFT method.The computational results verify that the molecules with the ptO(B)4 and ptO(C)4 substructures could be achieved theoretically.The bonding interactions among atoms in ptO(B)4 substructure have been analyzed with the D2d symmetry,and used the results to show that the 2pz-? electrons of the lone-pair of ptO are in the bonding molecular orbital.The nature of all bonds among atoms in the square ptO(B)4 and the ptO(C)4 substructures were analyzed in detail by using the quantum theory of atoms in molecules,and the characterization of the O-B and O-C bonds are strong polar bonds with some ? bonding interactions.4.Theoretical study on the molecular allotropes of carbon:endohedral fullerene Cn@C60Apart from the two-dimensional special structures with planar polycoordination of carbon,nitrogen and oxygen,the three-dimensional special structures formed by carbon are another interesting and valuable task to study.Thus,the molecular allotropes of endohedral fullerene(C=C=C=C)@C60 and C8@C60 formed by the C=C=C=C and C8 groups in the C60 cages and bonding to carbon atoms on the framework of C60 were designed out and studied by using the DFT method.Although the carbon atoms on the framework of C60 bonding with the embedded groups in the C60 cage also form four bonds in these molecules,the bonding modes of the carbons are different from that of carbon in methane,which seriously deviate from the regular tetrahedron,and the structures of such carbon atoms are the special structures in three dimensionsThe computational results show that they are all real minima on the potential energy surfaces and can be prepared experimentally.Their geometry,IR spectrum,NMR diagram,heat of formation,and the bonding interactions,which provide the theoretical evidences to identify them in the future experimentally,have been studied in detail.Since fullerenes are a large family of the molecular allotropes of carbon,the researches on fullerene-endohedral allotropes of carbon will open a wide area for allotropes of carbonIn summary,some special structures,namely,some novel two-dimensional structures with planar tetracoordination substructures of carbon,nitrogen,and oxygen have been designed out and studied by using the DFT method.The three-dimensional special structures formed by carbon in the molecular allotropes of endohedral fullerene(C=C=C=C)@C60 and Cs@C60 also have been designed and studied.The conditions of formation and bonding interactions of such special structures of carbon,nitrogen and oxygen are analyzed in this work.The investigation of this work will broaden the chemical structures and offer a method theoretically to look for the systems containing such special structures possessing unusual electrical,magnetic,and optical properties.