【作者】 贾建峰；

【导师】 武海顺； 焦海军；

【作者基本信息】 山西师范大学 ， 无机化学， 2009， 博士

【摘要】 多面体(CH)n团簇一直以来受到广大化学工作者的关注,C60的发现更引起了人们对多面体团簇结构与稳定性的关注。对于Cn富勒烯团簇,人们发现它们的最稳定结构由五元环与六元环形成,并且五元环趋于最大分离。(BN)n多体面,可以看成C2n富勒烯的等电子体,最稳定结构由四元环与六元环形成,并且四元环也趋于分离。无论是Cn多面体,还是(BN)n多面体,最稳定结构都呈笼状构型。氢化的(BN)n结构,(HBNH)n多面体以及同类的(HGaNH)n多面体,虽然仍由四元环与六元环形成,但结构呈管状。四元环的分布与在(BN)n中的分布截然不同,它们中的四元环倾向于聚合,并分布在管的两端。(CH)2n、(HBNH)n、N2n和(BCO)2n可看成等价电子结构。稳定的Nn团簇也呈管状结构,它们由三元环、五元环与六元环形成。稳定的较大(BCO)n由三元环与六元环形成,也呈管状结构,三元环分布在管的两端。尽管(CH)n多面体团簇早就引起了人们注意,但对较小的(CH)n多面体,则更多关注它们的张力。对于(CH)60,研究的对象都是基于五元环最大分离的Ih对称性结构。我们提出的问题是最稳定的(CH)n是否会呈现出管状构型?(CF)n与(CH)n是否具有相同的结构?本论文中,我们运用密度泛函理论方法对(CH)n和(CF)n (n=4-60)及(SiH)60等的结构与稳定性进行了计算研究,对这些多面体团簇结构与稳定的关系进行了分析。同时我们还对笼外化学键对笼内原子的影响进行了初步探讨。本论文的主要内容及结论如下:1. C60F60笼状与管状结构的稳定性在密度泛函B3LYP/6-31G(d)计算水平下,对(CF)60系列异构体的结构及稳定性进行了理论研究。结果发现最稳定的异构体F4@C60F56为内含4个C—F键,并且两端存在聚合五元环的管状结构,其能量比已报道的最稳定的内含8个C—F键的笼状异构体(F8@C60F52)低94.6 kJ/mol。这与富勒烯的独立五元环规则相反。管状F4@C60F56的平均C—F键裂解能比C60F36, C60F48和氟化石墨的实验值都高。其平均每CF单元的能量比氟化石墨(CF)n的高15.5 kJ/mol,这一能量差值小于C60和石墨中C原子平均能量的差值(37-42 kJ/mol)。2. C60H60的管状和笼状异构体及与C60F60的比较有聚合五元环的管状C60H60异构体比完全独立五元环的笼状异构体更稳定。向管状异构体内引入内含式C—H键可使其结构更稳定,但有4个内含式C—H键的最稳定的管状结构比内含10个C—H键的笼状结构的能量高310.5 kJ/mol。我们还对管状和笼状结构的C60H60和C60F60进行了全面比较。3.氟化多面体碳团簇(CF)n的结构和稳定性在B3LYP/6-311++G(d,p)//B3LYP/6-31G(d,p)理论水平下对(CX)n多面体(n=偶数,4-60, X = F或H)计算后发现, (CH)n多面体比其相应(CF)n的张力小。对n = 6-24的异构体的计算表明,(CH)n和(CF)n具有相同的几何构型。对较大(CH)n和(CF)n (n = 26-60)的计算表明,存在聚合五元环的管状异构体比符合五元环最大分离的笼状异构体的热力学稳定性高。管状异构体的稳定性是由键角的扭曲程度(角张力)和非键F···F间的相互排斥力决定的。通过等键反应可以了解F···F间的相互排斥力。我们一共发现了四种类型的管状结构;与决定碳富勒烯的独立五元环规则相反,对(CX)n来说,聚合五元环数目越多管状结构稳定性越高。4.笼状和管状(SiH)60的结构与稳定性在1812个外接式(SiH)60富勒烯异构体中,最稳定的异构体是一个D5d对称性的管状构型,它的12个五元环聚合在管的两端。最稳定的笼状异构体可以容纳10-12个内含Si—H键(H10@Si60H50和H12@Si60H48),而且它们也代表了最稳定的(SiH)60异构体。我们发现(SiH)60的角张力和扭曲张力比(CH)60低很多。5. X@C60F ,X@C60F2(X=N,H)和(H@C60)2的结构与稳定性采用B3LYP密度泛函理论方法,对内含式化合物X@C60Fn(X=N,H;n=1,2)和(H@C60)2的结构与稳定性进行了计算。最稳定的N@C60F中有一个内含N—C键,而在N@C60F2结构中,氮原子位于笼的中心。在稳定的H@C60F和H@C60F2中,H原子倾向于与C60形成一个内含式C—H。最稳定的(H@C60)2二聚体结构中,在每个碳笼内形成一个内含式C—H键,在两个笼之间,形成一个C—C键。热力学计算表明,(H@C60)2二聚体在低于650K的温度下稳定,高于该温度就会分解为两个H@C60单体。更多还原

【Abstract】 Regular polyhedral clusters (CH)n have long been the subject of scientific curiosity. The discovery of fullerene (C60) had stimulated tremendous activity in this area and the structure and stability of polyhedral clusters were concerned by many studies. For Cn polyhedral clusters, it was found that the most stable isomers are comprised of five- and six-membered rings and the five-membered rings trend to be isolated as greatly as possible. The (BN)n polyhedral clusters, the number of valence electrons being equal to C2n clusters, are comprised of four- and six-membered rings, and the four-membered rings trend to be isolated. Both stable Cn and (BN)n polyhedral have cage-like structure.The hydrogenated (BN)n, namely (HBNH)n, as well as their analogy, (HGaNH)n polyhedral clusters, also being comprised of four- and six-membered rings as (BN)n, however, have tube-like structure. The distribution of four-membered rings in (HBNH)n and (HGaNH)n is totally different from that in (BN)n. In (HBNH)n, six four-membered rings cluster to two groups and locate at the ends of tube. All of CH, (HBNH)1/2, N and BCO have same number of valence electrons. The most stable Nn polyhedral clusters are comprised of three-, five- and six-membered rings, and also have tube-like structure. Stable (BCO)n polyhedral clusters are comprised of three- and six-membered rings. They also have tube-like structure with three-membered rings reside at the ends of tube.(CH)n polyhedral had received considerable attentions for a long time. However, for small (CH)n (n=4-24), previous studies mainly focused on the strain in their cage-like structures. For (CH)60, nearly all studies are based on the Ih C60H60 structure with isolated five-membered rings. We are interesting to know whether most stable large (CH)n cluster, like C60H60, has tube-like structure; whether (CF)n and (CH)n have same stable structures?By means of density functional method, the structures and stabilities of (CH)n and (CF)n (n=4-60), as well as (SiH)60 have been calculated and the relationship between structure and stability for these polyhedral clusters are investigated in detail. We also take an initial study about how the outer chemical bonds affect the endo atom in a C60 cage. The main contents and results are as follows.1. The stability of tube and cage C60F60The structure and stability of a set of (CF)60 isomers have been computed at the B3LYP/6-31G(d) density functional theory level. The most stable isomer (F4@C60F56) has tube-like structure with four endo C—F bonds and fused five-membered rings at the end of the tube, while the reported most stable cage structure (F8@C60F52) with eight endo C–F bonds is higher in energy by 94.6 kJ/mol. This is in contrast to the isolated pentagon rule for the stability of fullerenes. The mean bond dissociation energy of tube-like F4@C60F56 is larger than those of the experimental known C60F36, C60F48 and graphite fluoride. The relative energy per CF unit of tube-like F4@C60F56 to graphite fluoride (CF)n is 15.5 kJ/mol, which is smaller than that of C60 fullerene per carbon to graphite (about 37-42 kJ/mol).2. Tube and Cage C60H60: A Comparison With C60F60Tube C60H60 with fused five-membered rings is more stable than the cage isomer with isolated five-membered rings. Introduction of endo C–H bonds into tube isomer results in further stabilization, but the most stable tube structure with four endo C–H bonds is higher in energy than the most stable cage structure with ten endo C–H bonds by 310.5 kJ/mol. A comprehensive comparison of C60H60 with C60F60 has been made.3. Polyhedral Fluorocarbon (CF)n– Structures and EnergiesThe structures and energies of selected (CX)n (n = even, 6-60, X = F or H) polyhedra, computed at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d,p) level show (CH)n isomers to be generally less strained than the corresponding (CF)n analogs. For n = 6-24, both (CH)n and (CF)n favor the same geometric patterns. The tube isomers (t-(CX)n) of large (n = 26-60) species with fused five-membered rings at the tube ends are thermodynamically more stable than the cage isomers (c-(CX)n), deduced from the most stable carbon cages. The stabilities of tube isomers are determined by the distortion of the bond angles (angle strain) and the non-bonded F···F repulsive interactions. The latter have been quantified by means of homodesmotic equations. Four structural types of t-(CX)n structures are built on the basis of the number of fused five-membered rings; and the thermodynamically more stable t-(CX)n structures have more fused five-membered rings, in opposition to the isolated pentagon rule governing the stability of carbon fullerenes.4. Structure and Stability of Tube and Cage Si60H60Among all 1812 all-exo (SiH)60 fullerene isomers, the most stable isomer has a tube-like configuration (D5d) with twelve five-membered rings fused and located at the ends of the tube. The most stable cage isomer can admit ten to twelve endo Si–H bonds (H10@Si60H50 and H12@Si60H48), and they represent the most stable (SiH)60 isomers. It is found that (SiH)60 isomers have much lower angle and torsion strains than (CH)60 isomers.5. The structure and stability of X@C60F ,X@C60F2(X=N,H) and (H@C60)2The structure and stability of endohedral X@C60Fn (X = N, H; n = 1, 2) and (H@C60)2 are computed at the B3LYP level of density functional theory. The most stable N@C60F has one endo N–C bond, while N@C60F2 favors nitrogen atom at cage center. Both H@C60F and H@C60F2 favor isomers with one endo C–H bond. The most stable dimer structure, (H@C60)2, has one inter-cage C–C bond and two endo C–H bonds, and is stable below 650 K; but dissociate above 650 K.更多还原