Theoretical Investigations On The Structures And Stability For Several Important Planar Molecules

It is fascinating to see and to understand the chemical structure of molecules. The chemical structure is a consequence not only of their constituent elements but also of the way those atoms are connected to each other, and what their special arrangement is. Any change in the spatial distribution of the nuclei will modify the nature of the chemical bond and vice versa.That the tetracoordinate tetravalent carbon atom prefers a tetrahedral arrangement of its four ligands was first recognized independently by J. H. van't Hoff and J. A. LeBel in 1874. This contribution marks a milestone in understanding the structure and sterochemistry of carbon compounds. Almost a century after the publication of the van't Hoff-LeBel model, Hoffmann et al. suggested rules to stabilize molecules containing a planar tetracoordinate carbon (ptC) atom in 1970, challenging the second structural foundation of Organic Chemistry. There are two approaches, to achieving this goal. The electronic approach involves selecting substituents that preferentially stabilize a planar disposition of the carbon bonds over the normal tetrahedral arrangement. The alternative approach is based on using mechanical molecular strain forces exerted by the surrounding ligands to the carbon atom. Inspired on Hoffmann's ideas, several groups have successfully suggested and experimentally characterized molecules containing ptCs. The main results are summarized as follows:(1)Very recently, theoretical chemists have found that the hexa-atomic system [CAl₅]⁺ has a planar pentacoordinate carbon structure as the most stable form. In this paper, we attempt to further such topic. By means of the B3LYP and CCSD(T) (single-point) methods, we for the first time investigate the isomeric structures and thermodynamic stability of four hexa-atomic systems [MAl₅]⁺ (M=Si,Ge,Sn,Pb). The results show that although bearing the same 18-valence electrons as [CAl₅]⁺,all the [MAl₅]⁺ (M=Si, Ge, Sn, Pb) systems do not have a planar pentacoordinate M isomer. Instead, the global structure is a Cs-symmetrized butterfly isomer int1.This can be rationalized that the bulkier central atom M (Si, Ge, Sn, Pb) than C has significantly destructed the stability of the planar pentacoordinate structure of [MAl₅]⁺.(2)The isomeric structures and energetics of two Sn,Pb-containing penta-atomic planar tetracoordinate carbon molecules [CAl₃X] and [CAl₃X]^- (X=Sn, Pb) (with 17 and 18 valence electrons, respectively) were studied by using the B3LYP method combined with the mixed basis set 6-311+G(d)&LANl2dz for the first time. The results were compared with the previously reported [CAl₃X] and [CAl₃X]^- (X=Si, Ge). It was shown that the isomer int1 with the planar tetracoordinate carbon structure was the global minimum point. Relative to the 17e system [CAl₃X], achieving an additional electron to form the 18e system [CAl₃X]^- int1 becomes thermodynamically more stable. The present work would enrich the penta-atomic planar tetracoordinae carbon chemistry.(3)One of the focuses of molecular science is the theoretical and experimental investigations on the pentatomtic planar coordinated carbon (ptC) molecules. This paper for the first time reported the design of sandwich-like compounds based on a Si-containing ptC unit CAl₂Si₂. We found that due to the strong fusion interaction between CAl₂Si₂ decks, the assembly of CAl₂Si₂ with the alkali and alkaline earth metals(M=Li,Na, K, Be ,Mg,Ca)in form of the traditional"homo-deckered sandwich"scheme to form [(CAl₂Si₂)M(CAl₂Si₂)]^q+ is unstable. Instead, our recently proposed"hetero-decked sandwich"scheme can be effectively applied to CAl₂Si₂ to form [CpM (CAl₂Si₂)]^q+). Both the"hetero-decked sandwich"and the ionic interaction play important roles in the assembly and stabilization of CAl₂Si₂. During the assembly, the electronic and structural features of CAl₂Si₂ are well kept。Thus, the ptC molecule CAl₂Si₂ can act as a"building block"to construct large-scale compounds that contain planar coordinated carbon centers.