Theoretical Study On The Design And Electronic Properties Of Planar Molecules

Planar molecules have attracted the extensive attentions because of their extraordinary geometries,electronic properties and unique physical properties.Inspired by Hoffmann’s efficient strategy for designing planar molecules proposed in1970,many groups have devote to this area,a large number of planar molecules have explored,synthesized and characterized in theory and experiment.Up to date,numerous great achievements of planar moleclues have been explored to planar hypercooridation of carbon,and other main-group atoms（such as Si,Ge,B,Al,O,N,et.al）and transition-metal atoms（such as Fe,Co,Ru,Cu,Ag,Au,Cd,Hg,Ta,Nb,et.al）.Meanwhile,the coordination number of central atoms were expanded to penta-,hexa-,hepta-coordinate and even deca-coordinate.The exploation of planar molecules not only push the bonding limit of atoms,but also tremendously enriches the family of planar molecules.Thus,it is of importance to design stable planar hypercoordinate molecues in theory,which can provide theoretical data for experimental detection.The main contents of this thesis are as follows:1.Avoide spin coupling:an unexpectedσ-σdiradical in global planar pentacoordinate carbon.We present a global planar pentacoordinate carbon（pp C）featuring a hitherto unreportedσ-σdiradical characteristic.Using the multi-reference approach combined with the CCSD（T）/aug-cc-p VTZ method,the pp C C₃Li₃^–was found to be an intriguing triplet ground state,in which the unpaired density is mostly located at three Li ligands.Chemical bonding analysis reveals that the 2p_zπelectrons of C₃Li₃^–are fully located at the C₃ ring formed by C-C multiple bonds,in contrast to the perfect2p_zπ-delocalization found in the well-known pp Cs.2.Planar pentacoordinate silicon and germanium atoms.The global minimum of XMg₄Y^-（X=Si,Ge;Y=In,Tl）and Si Mg₃In₂ contains a planar pentacoordinate atom of group 14 other than carbon.Its design is based on the“localization”approach,replacing one or two peripheral atoms in XMg₅^2-by more electronegative ones.This change diminishes the repulsion and leads to stronger covalent X-Y bonds,stabilizing the planar pentacoordinate atom species.