Theoretical Investigations On Planar Tetracoordinate Heavier Group 14 Compounds

New structural forms of molecules are always important and appealing in chemistry and molecular science.One nice example is the pioneering concept of ?planar tetracoordinate carbon?,which is breakthrough the long-held perception of tetrahedral carbon.Over the past few decades,scientists have also been working on this "anticonventional" chemical bond compound,and have proposed design strategies and rules to promote the stability of planar structures.In this thesis,using the quantum chemistry,Wecarried out theoretical investigations on planar tetracoordinate heavier group 14 compounds.Through the global structure search program,we obtained some novel planar tetracoordinate heavier group 14 compounds(pt M).Moreover,it was found that ?-type ligand skeleton plays an important role in the stability of planar tetracoordinate heavier group 14 compounds.The main achievements of this paper include the following aspects:1.New structural motif of 18 valence electron molecules with aplanar tetracoordinate heavier group 14 center: Uniquestabilization effect of a ?-type skeleton.Proposing new valence electron counting rules and new structural motifs are both very importantin chemistry.In this work,we unexpectedly found that by introducing a ?-type skeleton YCCY(Y=Al/Ga/In/Tl),a total of sixteen novel planar tetracoordinate heavier group 14 species,that is,pt M(M=Si/Ge/Sn/Pb)in neutral,can be designed as global minima.The bonding situation of these 16 pt M structures is in sharp contrast with the well-known 18 electron-pt C and the limited 18 electron pt M,because the ligands in the past have almost no multiple bond characteristics.Most important of all,each YCCY(Y =Al/Ga/In/Tl)ligands may be stable skeleton allheavier group 14 atoms in a planar tetracoordinate,the fact that strong how the universality of?-type skeleton.The present work firmly demonstrates thatintroducing the ?-type ligand skeleton can effectively enrich the planar tetracoordinate chemistrywith the heavier group atoms.2.Structures and bonding of C2X2Yq(X=Si/Ge/Sn/Pb;Y=C/Si/Ge/Sn/Pb;q=+1,0,-1): p-type templates for planar tetracoordinate heavier group 14 atoms.Contrasting the big family of the planar tetracoordinate carbon(pt C),species featuring the planar tetracoordinate heavier group element M(pt M)have been largely limited.Effective structural frameworks to accommodate such pt M centers are thus highly desired.In the present article,we report an extensive computational study on 60 pentatomic systems C2X2Yq(X=Si/Ge/Sn/Pb;Y=C/Si/Ge/Sn/Pb;q=+1,0,-1)covering both the low and high spin states.Up to 34 systems were shown to have the very low-lying singlet planar tetracoordinate heavier group 14(pt M with M=Si,Ge,Sn,Pb)structures bearing the 19(q=+1),20(q=0)and 21(q=-1)valence electrons(ve).Structural and bonding analysis confirmed the effectiveness of the inherent p-type ligand skeleton XCCX or XCCY that each have several sets of p-bonding orbitals to stabilize the pt M center.The structural and bonding motifs of these pt Ms differ greatly from the classic pt Ms,which have the ?-type ligand skeleton,smaller number of valence electrons(? 18ve),and the center?ligand p-delocalization.