富勒烯结构B_(80)分子静电势的理论研究

在混合密度泛函B3LYP理论下,用6-31G*基函数对富勒烯结构B80分子的3个异构体(1个具有Ih对称性,2个具有Th对称性)构型进行优化和分子静电势计算.结果表明:3个异构体球内全部为正电势,球外五元环中心所对应的区域都为负电势,B80Ih,Th(A)和Th(B)球外静电势的最大负值分别对应于20个六元环中心的B原子,五元环中心和12个六元环中心的B原子周围,它们组成了化学反应中最可能的活性点.     

内嵌金属碳氮化物团簇富勒烯的稳定性与生成机理

对新结构富勒烯金属包合物的探索是富勒烯领域中的研究重点。本文从内嵌团簇与富勒烯碳笼尺寸匹配的角度出发,对基于金属碳氮化物团簇的新结构富勒烯金属包合物进行了研究。通过量子化学计算研究了M_3NC团簇(M=Y,La,Gd)内嵌在D_2(186)-C_(96)和D_2(35)-C_(88)分子中所形成包合物的稳定性和电子结构,发现富勒烯碳笼接受内嵌团簇转移的六个电子形成了稳定结构。结合文献已报道过的Sc_3NC@I_h(7)-C_(80)分子,阐明了M_3NC团簇与富勒烯碳笼之间的尺寸匹配效应,并发现D_2(186)-C_(96)、D_2(35)-C_(88)和I_h(7)-C_(80)三种富勒烯碳笼均具有五元环均匀分布的结构特点。我们对富勒烯之间的转变路径进行了研究,提出了不含Stone-Wales异构化过程的富勒烯直接生成机理,即可以通过增加碳原子的过程使五元环重排,在保持稳定性结构单元的同时转变为更大碳笼。     

金属氮化物富勒烯MSc_2N@C_(76)的计算研究

内嵌金属富勒烯是一类具有新奇性质的富勒烯基化合物.最近,含有七元环的非经典内嵌金属富勒烯被实验报道.然而,因结构的多样性和计算量的挑战性,目前尚无有关非经典内嵌金属富勒烯的系统计算研究.本文采用密度泛函理论方法和拓展的螺旋算法,系统考察了C_(76)的经典结构和非经典结构的内嵌金属氮化物富勒烯MSc_2N@C_(76)(M=Sc,Y,La).结果显示,非经典内嵌金属富勒烯与经典内嵌金属富勒烯是竞争性的,且它们之间可以通过Stone-Wales旋转或C_2插入、挤出而实现相互转化;非经典富勒烯在内嵌金属富勒烯的形成过程中发挥了重要的作用.     

富勒烯合成化学研究进展

富勒烯是一类由12个五元环和若干六元环组成的笼状分子, 自20世纪80年代中期被发现以来就以其独特的结构和新奇的性质而成为科学界研究的热点, 25年来, 无论在基础研究还是在实际应用领域都有了长足的进步, 人们在发展富勒烯合成新方法和寻找富勒烯新结构方面做了大量的工作。本文对富勒烯的各种宏量合成方法进行了回顾, 并概述了迄今已发表的60余种富勒烯新结构,包括各种富勒烯空笼、内嵌富勒烯、富勒烯笼外修饰衍生物及氮杂富勒烯等结构。     

富勒烯C70及Sc3N@C70的密度泛函理论研究

在混合密度泛函B3LYP理论下,用6-31G*基函数对富勒烯C70、它的阴离子及内掺Sc3N富勒烯Sc3N@C70两种同分异构体的几何结构和电子结构进行了研究。计算结果表明,在C70的两种异构体中,满足五元环分离规则(IPR)的C70(D5h)稳定,C70q-(#7854)(q=4,6)比C70q-(D5h)稳定;在Sc3N@C70两种异构体中有三对两两相邻五元环的Sc3N@C70(#7854)稳定,C70(#7854)易于形成富勒烯金属包合物。     

一种C50Cl10富勒烯氯化物新的生成机理的密度泛函理论计算

过去广泛接受#271C50Cl10是由#271C50空笼直接氯化得到.我们通过研究拓扑结构弄清了C50富勒烯之间的相互关系.利用密度泛函理论(DFT)计算从最稳定 C50富勒烯#270C50出发,通过氯化和 Stone-Wales (SW)转变获得#271C50Cl10.结果表明：氯化后最终产物是热力学最有利的,并且在有氯存在下, SW 转变的活化能垒会降低.这些结果可以解释目前的相关实验事实,暗示了#270C50空笼先氯化得到不同#270C50氯化物,再进行两次SW旋转的路径,由于活化能垒更低因而是一条更为可行的路线.     

碳F4F6多面体遵循独立四元环原则

为了研究由四元环和六元环构成的碳多面体(F₄F₆多面体)的结构和稳定性之间的关系, 本文采用密度泛函方法对所有C₈~C₆₀之间的F₄F₆多面体进行了系统的计算研究. 结果表明, 能量最低的异构体都满足独立四元环原则, 能量较低的满足四元环比邻惩罚原则. 这两条原则与经典富勒烯所遵循的独立五元环原则和五元环比邻惩罚原则具有同等地位, 能够使研究者仅仅从形貌上就可以对碳F₄F₆多面体的稳定性进行简单高效的判定. 结构分析表明, 四元环之间共用的顶点的锥化角大于其他顶点的锥化角并决定了相应分子的稳定性.     

有机富勒烯化合物的逆环加成反应

逆环加成反应是富勒烯衍生物的一种重要反应。在还原、氧化或加热条件下,许多富勒烯衍生物的加成基团会从富勒烯骨架上断开,得到富勒烯母体,是富勒烯环加成反应的逆反应。该反应与富勒烯环加成反应结合,已被应用于富勒烯化学中保护/去保护基团方法,对于选择性制备富勒烯及其衍生物具有重要意义。然而由于逆环加成反应的存在,引起了有机富勒烯化合物的不稳定,从而可能影响其在实际中的应用。最近,我们研究了一系列含有亚胺酯杂原子环结构的富勒烯噁唑衍生物在电化学还原下的稳定性,结果表明加成基团之间存在的分子内C-H…X（X = N,O）氢键对化合物的稳定性可能起着较为关键的作用。在此,我们将结合我们的工作对有机富勒烯化合物的逆环加成反应进行综述。     

nido-C2B9H112-碳硼烷异构体结构和稳定性的密度泛函研究

采用密度泛函方法对11顶点巢式碳硼烷C2B9H112-异构体进行了几何结构优化,分析了稳定性、电荷分布及分子轨道.结果表明,9个异构体都有对应的稳定构型,保持了巢式骨架结构.C取代开口五元环上B的异构体更稳定,且随取代数目增加和C原子间距增加而增加,C—C键和C—B键作用增强.C取代内层B使异构体稳定性降低,C—C键和C—B键长随之增长.负电荷主要集中在C原子上,开口五元环上的C原子上负电荷要比内层C原子更多,成为亲核取代反应中心.异构体分子前线轨道具有和η5-C5H5-相似的π键性质,ΔELUMO-HOMO反映的化学稳定性与结构能量稳定性趋势一致.     

内嵌金属富勒烯Sc2S@C86的结构和性质

金属硫化物富勒烯是一类结构新奇的化合物,阐释其结构和性质是当前的重要研究任务。本文采用密度泛函理论(DFT)方法,系统研究了质谱实验已经检测到的内嵌金属富勒烯Sc₂S@C₈₆的结构和性质。结果显示,能量最低的异构体是Sc₂S@C₈₆:63751(独立五元环规则(IPR)-9),该碳笼与已报道的Sc₂C₂@C₈₆的碳笼一样;其次是non-IPR Sc₂S@C₈₆:63376。自然键轨道(NBO)和分子中原子理论(AIM)分析显示,内嵌团簇与碳笼间存在电荷转移相互作用和共价作用。温度效应计算显示,高温时Sc₂S@C₈₆是多个异构体共存的。为了对将来实验结构测定提供参考,本文提供了能量最低的两个异构体的红外光谱图。     

A C78 fullerene precursor: toward the direct synthesis of higher fullerenes

A C78 fullerene related structure (of C78:1 and C78:4, the last undiscovered C78 IPR isomer) has been synthesized and investigated as a pyrolytic precursor. The pyrolysis of precursor containing all 78 carbon atoms in the required positions and 93 of the 117 C-C bonds, needed for fullerene formation, showed selectivity for C78 fullerene formation. In independent experiments it has been shown that the flash pyrolysis of C78 fullerene is not affected by Stone-Wales rearrangement and loss of C2 fragments and, thus, is very promising for the synthesis of individual isomers of higher fullerenes.     

Evolution spectrum of C60 isomers in buffer gas

The energy spectrum of C60 nonclassic fullerenes with single heptagon defects calculated by Brenner empirical potential is found to submerge into the spectrum of classic fullerenes. Geometry analysis indicates that these nonclassic fullerene isomers can be more attainable than classic fullerenes at higher Stone-Wales (SW) stacks. Molecular dynamic simulations of the C60 isomer evolution in He buffer gas at 2500 K demonstrate that nonclassic fullerenes, especially those with heptagon defects, play an important role in the dynamics of C60 annealing, and that the Stone-Wales stack-by-stack transition mainly occurs at lower SW stacks. A non-SW multistep rearrangement is first observed in the simulation with its transition sequence and intermediate state presented in detail.     

Nonclassical fullerenes with a heptagon violating the pentagon adjacency penalty rule

Nonclassical fullerenes with heptagon(s) and their derivatives have attracted increasing attention, and the studies on them are performing to enrich the chemistry of carbon. Density functional theory calculations are performed on nonclassical fullerenes C_n (n = 46, 48, 50, and 52) to give insight into their structures and stability. The calculated results demonstrate that the classical isomers generally satisfy the pentagon adjacency penalty rule. However, the nonclassical isomers with a heptagon are more energetically favorable than the classical ones with the same number of pentagon–pentagon bonds (B₅₅ bonds), and many of them are even more stable than some classical isomers with fewer B₅₅ bonds. The nonclassical isomers with the lowest energy are higher in energy than the classical ones with the lowest energy, because they have more B₅₅ bonds. Generally, the HOMO–LUMO gaps of the former are larger than those of the latter. The sphericity and asphericity are unable to rationalize the unique stability of the nonclassical fullerenes with a heptagon. The pyramidization angles of the vertices shared by two pentagons and one heptagon are smaller than those of the vertices shared by two pentagons and one hexagon. It is concluded that the strain in the fused pentagons can be released by the adjacent heptagons partly, and consequently, it is a common phenomenon for nonclassical fullerenes to violate the pentagon adjacent penalty rule. These findings are heuristic and conducive to search energetically favorable isomers of C_n, especially as n is 62, 64, 66, and 68, respectively. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Further test of the isolated pentagon rule: Thermodynamic and kinetic stabilities of C84 fullerene isomers

Thermodynamic and kinetic stabilities of 73 C₈₄ fullerene isomers were estimated from the MM3 heats of formation and the recently defined bond resonance energies (BREs), respectively. The BRE represents the contribution of a given π bond in a molecule to the topological resonance energy (TRE). All π bonds shared by two pentagons turned out to be highly reactive without exceptions. C₈₄ fullerene isomers with such π bonds must be incapable of survival during harsh synthetic processes. Thus, the isolated pentagon rule (IPR) proved to be applicable to such large fullerene cages. For sufficiently large fullerenes like C₈₄, some isolated-pentagon isomers are also predicted to be very unstable with highly antiaromatic π bonds. © 1996 by John Wiley & Sons, Inc.     

Stability of Fullerene Cages： C30 to C70 and Their Dependence on Shared Pentagon Bonds

1 INTRODUCTION Since the discovery of buckministerfullerene (C60)[1], experimental and theoretical studies of the structure and properties of fullerenes have spread worldwide. Many experimental studies appeared on their syntheses[2], isolation[3], and characterization[4]. Theoretically, except studies on their chemical pro- perties[5], most of the work was concentrated on their interesting geometric[6～8] and electronic structures[9, 10]. For example, the ground state of C48 has C2 symme…     

A hidden rule in metal sulfide fullerenes: A case study of Sc2S@C88

To elucidate the structure of a compound is a necessary step for its practical applications. To study the structure and properties of metal sulfide fullerene Sc₂S@C₈₈ detected by mass spectrometry, 11 194 isomers of C₈₈ and 33 isomers of Sc₂S@C₈₈ were systematically examined by density functional theory calculations. The calculations show that the two lowest‐energy isomers are Sc₂S@C₈₈:81 738 (IPR‐35) and Sc₂S@C₈₈:81 735 (IPR‐32), followed by Sc₂S@C₈₈:81 729 (IPR‐26), Sc₂S@C_88:81 712 (IPR‐9), and Sc₂S@C_88:81 733 (IPR‐30). Structural analysis shows that the first two energetically favored isomers are bridged by the third and fifth energetically favored isomers, which can transfer into each other via direct Stone–Wales rotation. The calculations of temperature effect show that the first two favored isomers become dominant forms of Sc₂S@C₈₈ with decreasing temperature and may coexist in the soot. This structural convertibility among favored isomers of Sc₂S@C₈₈ suggest a hidden rule that birds of a feather flock together in metal sulfide fullerenes. This rule may decrease the range of candidate cages for the structural identification of a metal sulfide fullerene. IR spectra are simulated for helping the future experimental identification of Sc₂S@C₈₈.     

The way of stabilizing non-IPR fullerenes and structural elucidation of C(54)Cl(8)

Recently, a new non-IPR chlorofullerene C(54)Cl(8) was isolated experimentally (Science 2004, 304, 699). To explore the ways to stabilize non-IPR fullerenes, the authors studied all of the possible isomers of C(54) fullerene and some of the C(54)Cl(8) isomers at PM3, B3LYP/3-21G, and B3LYP/6-31G* levels. Combined with analysis of pentagon distributions, bond resonance energies, and steric strains, C(54):540 with the least number of 5/5 bonds was determined to be the thermodynamically best isomer for the C(54)Cl(8). Based on C(54):540, the most probable structure of the experimental C(54)Cl(8) was elucidated. The results suggested one of the necessary conditions of stabilizing non-IPR fullerenes: chemical derivatizations of either endohedral complexation or exohedral addition need to sufficiently stabilize all of the kinetically unstable 5/5 bonds of the cages.     

Comparison studies of fullerenes C72, C74, C76 and C78 by tight-binding Monte Carlo and quantum chemical methods

From C₇₂ to C₇₈, the top 20 low-energy isomers screened out from all isomers of each fullerene are optimized and computed by tight-binding Monte Carlo (TBMC), semi-empirical PM3, and ab initio B3LYP/6-31G*//HF/3-21G methods. The comparison results show that the TBMC method can efficiently optimize the structures and correctly predicate the low energy isomers. The relative energies computed by TBMC are in good agreement with the high-lever B3LYP calculation results. Our TBMC and B3LYP results show that the most energetically favorable structure of C₇₂ is not an isomer satisfying the isolated pentagon rule (IPR), which is different with the result by PM3. The symmetry of the most stable IPR isomer tends to low as the fullerene becomes large and several non-isolated-pentagon structures are found to have low symmetries and low energies close to the most stable isomer.     

Stable Non‐IPR C60 and C70 Fullerenes Containing a Uniform Distribution of Pyrenes and Adjacent Pentagons

The most abundant fullerenes, C₆₀ and C₇₀, and all the pure carbon fullerenes larger than C₇₀, follow the isolated‐pentagon rule (IPR). Non‐IPR fullerenes containing adjacent pentagons (APs) have been stabilized experimentally in cases where, according to Euler’s theorem, it is topologically impossible to isolate all the pentagons from each other. Surprisingly, recent experiments have shown that a few endohedral fullerenes, for which IPR structures are possible, are stabilized in non‐IPR cages. We show that, apart from strain, the physical property that governs the relative stabilities of fullerenes is the charge distribution in the cage. This charge distribution is controlled by the number and location of APs and pyrene motifs. We show that, when these motifs are uniformly distributed in the cage and well‐separated from one other, stabilization of non‐IPR endohedral and exohedral derivatives, as well as pure carbon fullerene anions and cations, is the rule, rather than the exception. This suggests that non‐IPR derivatives might be even more common than IPR ones.