| The concept and significance of chemical bonds is an important core of our whole chemistry study.The theory of covalent bond proposed by Lewis in 1916 is of landmark significance.At present,the two-center,two-electron(2c-2e)bonding mode in inorganic chemistry is still the mainstream.The lack of electrons in borane broadens the bonding mode between two centers and two electrons,which is much studied by researchers.As is known to all,strong carbon-carbon(C-C)bond is the basis of organic chemistry.The length of carbon-carbon single bond is 1.54A,which is the single bond length formed between sp3 hybridized carbon atoms and the longest of all common C-C bonds.These bonds are formed when two carbon atoms with unpaired electrons come close to each other(for example,the CH3 group acts to form ethane),and the bond length of the C-C bond is elongated due to their strong coulomb repulsion.Thus,many electron-transfer salts involving free radicals are stable and exhibit interesting electrical and optical properties.However,its bonding mode is still unclear and more theoretical and experimental methods are needed to explore.In this dissertation,we will use the density functional method(DFT)to analyze the electronic structure,bonding patterns,and stability of ?-[TCNE]22-,[C3(NH2)3]22+,and[C3(NH2)3]22+.The main studies are as follows:1.The nature of the multicenter bonding in ?-[TCNE]22-dimerThe dianion dimer of tetracyanoethylene(TCNE),?-[TCNE]22-,represents an unusual class of organic compounds,that possess exceptionally long C-C bonding interations(-2.96A).It is twice of conventional C-C bonds but shorter than the sum of van der Waals radii.Experimental and computational studies best characterize the intradimer bonding as a multi-center C-C bond.A number of theoretical studies indicate that ?-[TCNE]22-dimer exhibits long,four-centers two-electron(4c-2e)C-C bonds.However,there is still some disputations about the number of centers involved in the multicenter bonding.This work focuses on quantitative understanding of the nature of the long,multicenter bonding in the ?-[TCE]22-dimer.By chemical bonding analysis,it is found that-63%electrons in the long bond locate on the-CC-groups and the remains locate on the-CN groups.Herein,the-CN plays an important role in the bonding of ?-[TCNE]22-dimer.It is more reasonable that a long,20c-2e(12 carbon plus 8 nitrogen)bond in dimers of ?-[TCNE]22-Moreover,we found an abnormal charge redistribution from TCNE to TCNE-,and the charge distribution of?-[TCNE]22-is very similar to that of TCNE-,which benefits the formation of?-[TCNE]22-dimer.2.A Novel Covalent Dispersion Bond in TAC Cationic DimerLong distance bonds have been widely studied in supramolecular ?-dimers.Recently,a trisaminocyclopropenium(TAC)cationic dimer[C3(NH2)3]22+ in"staggered"conformation was experimentally synthesized,in which the interplanar interaction was thought to be dispersion attraction.However,it is doubt whether the dispersion force is strong enough to overcome the electrostatic repulsion between two cationic monomers.This work tells a new story for the nature of interplanar interaction in this cationic dimer.Chemical bonding analysis reveals that the[TAC]+monomer has a closed-shell electronic structure with a six center two electron(6c-2e)delocalized n orbital.Intrudingly,due to the special orbital symmetry,the anti-bonding orbital composed by the two ? orbitals partially overlaps("body to hands")in the"staggered"configuration.However,in its "eclipsed"configuration,there is neither such an orbital overlap,nor corresponding attractive interaction.Just as the instantaneous columbic attraction induced by charge dispersion between two nonpolar molecules,this weak attraction exhibits a kind of instantaneous covalency between two electronic closed-shell structures,despite the nonbonding character inside the dimer.Thus,this weak attraction is titled covalent dispersion bond,which further extends the range of covalent interaction and gives references for the further study of closed-shell interactions.3.Super ?-? Delocalized Bonding in the Cyclobutadiene DimerCyclobutadiene(C4H4)as the simplest cycloene,it is an extremely unstable hydrocarbon that can only exist in nature for five seconds.The bonding pattern in the ring is composed of alternating single and double bonds,and each carbon is sp2 hybridized,so that each carbon has one p electron,there is only four ? electrons not meeting the 4n+2 rule,and it is antiaromatic.We constructed cyclobutadiene dimer(C4H4)2,which is C8H8.C8Hg is known as cubic alkane with a cubic configuration.Each carbon is hybridized with sp3 and is connected to each other by a single bond.We are trying to stretch the distance between the two rings,found there is a metastable C8H8 from energy scan,each carbon adopt sp2 hybridization,at this time the whole system of eight remaining ? electronics,through chemical bond bonding analysis found that formed the two four center two electronic n and two eight center two electronic super ?-? bond,and the system also has the aromaticity. |
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