Theory Investigation Of Bonding Character On C, B-N Clusters

The professor Xu Guangxian of our country defined clusters as"molecule or ion charactered by the framework of polyhedron orpolyhedron lack of angle with three or beyond three finite atomsstraightly bonding together",and classified numerous cluster intotransition metal cluster,main family cluster,pure metal cluster,andSo on.according to different property of the elements in clusters.Andthey also Can be classified into elemental clusters,binary clusters,tricolor clusters according to the kind of elements in clusters.Theexperimental and theoretical study of carbon clusters is a subject witha long history.Much of this history as well as the exciting more recentdevelopments surrounding the C60 species and other fullerenes arecovered in several recent reviews.The experimental work of severalgroups commencing in 1984 on C60 sparked a resurgence and expansion ofinterest in the stability,structure , and properties of carbonclusters.In 1989,Weltner and Van Zee provided an extersive review ofthe experimental gas-phase,matris-isolation,and theoretical state ofthe art for many Cn clusters as well as their positive and negative ions.For many of the Cn clusters,the energies of two or more geometricalisomers are similar enough to make the prediction of the truelowest-energy very difficult.Pitzer and Clementi concluded from earlyMolecular Orbital(MO)calculation that the smaller clusters should belinear,with the odd number species having an additional preference forlinearity.More recent work on even number clusters show that therelative energies of linear and monocyclic structures Can not bereliably predicted unless large atomic orbital basis sets are used andelectrons correlation at a reasonably high level is included foressentially all valence electrons.The experimental datas indicate for2≤n≤9,linear chains seem to be more stable,and for 10≤n≤29,monocyclic structures are the most stable.Cn fullerenes are thoseclosed polyhedral carbon clusters composed of exactly 12 pentagonalfaces and several hexagonal faces(strictly speaking n/2-10hexagonsfor all even values n≥20,apart from 22).For example,C60 is the justfullerene composed of 12 pentagonal faces and 20 hexagonal only of 12pentagons.Among numerous isomers of C20 cluster,the isomers of Ring,Bowl and Cage are lower energy and more stable than any otherstructures.However,the investigation of its stability and bondingcharacter does not go deep .Moreover,if there are other stablestructures。And what is character its isoelectronic molecule(BN)10 has.As the most important tool for analyzing aromaticity,nucleusindependent chemical shift(NICS)and molecule orbit(MO)methodscould well certify the experiment result.Despite without unique andprecise definition about the concept of aromaticity,many works triedto study the aromaticity.Recently,aromaticity has also been widelyused to explain the special stability of many complexes.Most of thearomatic compounds are related to benzene and derivants which havedefinirely conjugated π electrons(4n+2).These electrons Can movefreely on the conjugated orbit which leads to some special propertiesof the aromatic compounds,such as(1)the unique stability of thearomatic compounds.(2)scope of bond length and average bond lengthof isomer.The bond lengths of aromaticity are equalization.(3) thenumber of π;electrons and deloealized molecule orbits.Cyclicelectron delocalization enhances the stability of structure.Thearomatic clusters have special magnetic.The aromaticity was believedto be an extremely important factor for understanding the stabilityof some isomers.NICS was used to decide whether the structures havearomaticity or not.When a structure has minus NICS,this structurewas said to have aromaticity.On the contrary,it was said to beantiaromatic.However,there are few works using such method toanalysis C20 molecule So far.Such result not only could well explainthe stability of C20.This paper optimized 14 structures,of C20 molecule,includingRing,Bowl and Cage which have been found in experiment.At the samelevel , we got its isoelectronic(BN)10 structures . In order toinvestigate bonding character of carbon atom,boron and nitrogen,we study a series of annular carbon clusters,annular boron nitrogenclusters , linear carbon clusters and linear boron nitrogenclusters.In the process of study,we make the following work:(1)Besides ring , bowl , cage , 11 new isomers of C20 arepredicted.Optimized geometry reveals that some carbon atoms in thering,bowl and some other isomers exhibit super-valence phenomena.Thiskind of super-valence property contributes to the ring being stablestructure,even if it is antiaromatic.By molecular orbital theory,we find this super-valence property is attributed to the participationof 3s and 3p orbitals in bonding.In addition,aromaticity evaluatingwith NICS value is believed as another important factor for thestability of some C20 isomers,especially for the cage.(2)We optimized(BN)10 at the same level with C20.Bonding character,molecular orbital character and reaction activation are used to studythe stability and properties of these isomers.The calculated resultsshow that the most stable isomer of(BN)10 is ring structure and thestability of(BN)10 is not only relevant to the conformation but muchrelevant to the number of B-B and N-N bonds.Compared,with ring,bowl and cage structure of C20,isomers a,e and j has good stabilitybut feeble delocalization。In order to further research C20 and(BN)10clusters,we calculated a series of rings which either have differentnumber of(BN)units or have the same number of(BN)units but differentlocations,getting the conclusion that delocalized character of(BN)nC20-2n is affected by both the number and the locations of(BN)units.(3)A series of annular structures of Cn(3≤n≤31)are optimized andstudied.The calculated results show that odd structures and evenstructures have different bonding characters and properties.NBOanalyses indict that C6,C10,C14,C18,C22,C26 and C30 molecules havetwo big delocalized π-bonding and C8,C12,C16,C20,C24 and C28molecules have only one big delocalized π-bonding.C4 has aromaticityfor it has different conformation which makes it have special bondingcharacter.(4)Annular structures of(BN)x(2≤x≤19)clusters are optimized,NBOand MOs are calculated to investigate stability and property.Theanalyzed results show that(BN)x clusters have different bondingcharacter with corresponding C2x molecules.And annular(BN)x clustersalso have delocalized bonding although they are nonaromatic estimatedwith NICS values . (BN)2 has aromaticity for it has differentconformation which makes it have special bonding character.(5)A series of linear Cn(2≤n≤12)clusters are studied.Optimizingparameters show that the lowest frequencies fall gradually with nvalues,as well linear structures are apt to unstable to longerchain.From contrast of the gap of HOMO-LUMO,we find odd number carbonstake line as stable structures and even structures are general annularform.NBO analysis indicate that all atoms in even number structuresare SP hybridization and in the middle of odd structures chain haveone special carbon which does not have hybridization.