Radiochemical challenges in the study of endohedral fullerenes and MD simulation

The formation of atom-doped fullerenes has been investigated by using several types of radionuclides produced by nuclear reactions. From the trace of the radioactivities after high performance liquid chromatography (HPLC), it was found that formation of endohedral fullerenes (or heterofullerene) with small atoms (Be, Li), noble-gas atoms (Kr, Xe) and 4B–6B elements (Ge, As, Se, Sb, Te etc.) is possible by a recoil process following the nuclear reaction. In order to show the possibility of creating endohedral fullerenes by inserting a foreign atom with a suitably high kinetic energy into C₆₀, we have carried out large-scale ab initio molecular dynamics simulations on the basis of the all-electron mixed-basis approach with atomic orbitals and plane waves for Li, Be, N, O, Na, S, Cl, K, V, Cu, As, Se, Sb, Te, Kr, Xe. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the viability of small endohedral hydrocarbon cage complexes: X@C4H4, X@C8H8, X@C8H14, X@C10H16, X@C12H12, AND X@C16H16

Small hydrocarbon complexes (X@cage) incorporating cage-centered endohedral atoms and ions (X = H(+), H, He, Ne, Ar, Li(0,+), Be(0,+,2+), Na(0,+), Mg(0,+,2+)) have been studied at the B3LYP/6-31G(d) hybrid HF/DFT level of theory. No tetrahedrane (C(4)H(4), T(d)()) endohedral complexes are minima, not even with the very small hydrogen atom or beryllium dication. Cubane (C(8)H(8), O(h)()) and bicyclo[2.2.2]octane (C(8)H(14), D(3)(h)()) minima are limited to encapsulating species smaller than Ne and Na(+). Despite its intermediate size, adamantane (C(10)H(16), T(d)()) can enclose a wide variety of endohedral atoms and ions including H, He, Ne, Li(0,+), Be(0,+,2+), Na(0,+), and Mg(2+). In contrast, the truncated tetrahedrane (C(12)H(12), T(d)()) encapsulates fewer species, while the D(4)(d)() symmetric C(16)H(16) hydrocarbon cage (see Table of Contents graphic) encapsulates all but the larger Be, Mg, and Mg(+) species. The host cages have more compact geometries when metal atoms, rather than cations, are inside. This is due to electron donation from the endohedral metals into C-C bonding and C-H antibonding cage molecular orbitals. The relative stabilities of endohedral minima are evaluated by comparing their energies (E(endo)) to the sum of their isolated components (E(inc) = E(endo) - E(cage) - E(x)) and to their exohedral isomer energies (E(isom) = E(endo) - E(exo)). Although exohedral binding is preferred to endohedral encapsulation without exception (i.e., E(isom) is always exothermic), Be(2+)@C(10)H(16) (T(d)(); -235.5 kcal/mol), Li(+)@C(12)H(12) (T(d)(); 50.2 kcal/mol), Be(2+)@C(12)H(12) (T(d)(); -181.2 kcal/mol), Mg(2+)@C(12)H(12) (T(d)(); -45.0 kcal/mol), Li(+)@C(16)H(16) (D(4)(d)(); 13.3 kcal/mol), Be(+)@C(16)H(16) (C(4)(v)(); 31.8 kcal/mol), Be(2+)@C(16)H(16) (D(4)(d)(); -239.2 kcal/mol), and Mg(2+)@C(16)H(16) (D(4)(d)(); -37.7 kcal/mol) are relatively stable as compared to experimentally known He@C(20)H(20) (I(h)()), which has an E(inc) = 37.9 kcal/mol and E(isom) = -35.4 kcal/mol. Overall, endohedral cage complexes with low parent cage strain energies, large cage internal cavity volumes, and a small, highly charged guest species are the most viable synthetic targets.     

Be在富勒烯C74笼内稳定位置的理论研究

为了研究富勒烯金属包合物Be@C74的结构和电子性质,本文采用密度泛函理论B3LYP方法优化了Be@C74的结构,计算了它的势能面、LUMO-HOMO、电子亲和势、电子化能以及Mulliken集居数。结果表明:Be原子位于C74笼中心并且近似保持基态的电子构型;Be原子和C74笼之间是相互排斥作用;Be原子包入C74笼中心后,C74笼只有微小的变形;包合物Be@C74笼的给予和得到电力的能力与C74空笼几乎不变;Be与C74笼之间只有微小的杂化。     

Modifying the fullerene surface using endohedral noble gas atoms: density functional theory based molecular dynamics study of C70O3

We have performed a series of ab initio molecular orbital and molecular dynamics calculations to ascertain the influence of an endohedral noble gas atom on the reactivity of the surface of the model system C(70)O(3). Our simulations show that the minimum energy pathways for the ozone ring-opening reaction are influenced by the presence of the endohedral atom. The effect is isomer dependent, with the enthalpy of the reaction increasing for a,b-C(70)O(3) and decreasing for e,e-C(70)O(3) when doped with the heavy noble gas atoms Xe and Rn.     

Molecular orbital calculations of beryllium insertion in Cn fullerene cages: Be@Cn (n = 20, 60, 70, 80) by nuclear recoil after particle capture

Summary The molecular structure and some properties of Be @C_n (n = 20, 60, 70, 80) endohedral metallofullerenes were analyzed using the HyperChem 7.0. Computational Chemistry Model Building Program. The results were in agreement with previous calculations using the ab initio method based on an all-electron mixed-basis approach within the framework of the local-density approximation. In the case of ⁷Be, the ion is inside the fullerene cage and tries to make an electronic connection with a six-membered ring of the fullerene cage in order to improve its atomic orbital distribution in the valence layer. Due to the ion radius value of only 0.45 ? and electronic capture decay, ⁷Be appears to emerge as an ideal tool for studying radionuclide half-life variation in different hosts.     

M4 @Si28 (M=Al,Ga): metal-encapsulated tetrahedral silicon fullerene

It is known that silicon fullerenes cannot maintain perfect cage structures like carbon fullerenes. Previous density-functional theory calculations have shown that even with encapsulated species, nearly all endohedral silicon fullerenes exhibit highly puckered cage structures in comparison with their carbon counterparts. In this work, we present theoretical evidences that the tetrahedral fullerene cage Si(28) can be fully stabilized by encapsulating a tetrahedral metallic cluster (Al(4) or Ga(4)). To our knowledge, this is the first predicted endohedral silicon fullerene that can retain perfectly the same cage structure (without puckering) as the carbon fullerene counterpart (T(d)-C(28) fullerene). Density-functional theory calculations also suggest that the two endohedral metallosilicon fullerenes T(d)-M(4)@Si(28) (M=Al and Ga) can be chemically stable because both clusters have a large highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap ( approximately 0.9 eV), strong spherical aromaticity (nucleus-independent chemical shift value of -36 and -44), and large binding and embedding energies.     

ChemInform Abstract: Endohedral and Exohedral Metalloborospherenes: M@B40 (M: Ca,Sr) and M&B40 (M: Be,Mg).

A DFT study on the viability of the metalloborospherenes: endohedral M@B₄₀ (M: Ca, Sr) and exohedral M&B₄₀ (M: Be, Mg) is presented.     

内含式复合物X@(HAlNH)12 (X=Be,Mg, Ca,Zn, Al+, Ga+)的结构和稳定性

在研究闭式多面体(HAlNH)12簇合物几何构型及稳定性的基础上, 用DFT的B3LYP方法在6-31G(d)的水平上, 对其内含式复合物X@(HAlNH)12 (X= Be, Mg, Ca, Zn, Al+, Ga+)进行了构型优化和能量计算, 并讨论了稳定结构的几何构型、自然键轨道(NBO)、振动频率、能量参数及NMR数据与结构的关系. 用Gaussian 03的QST3方法确定了客体X通过笼面6-元环的迁移过渡态(TS)结构, 并用IRC方法对所得TS结构进行了验证. 最后得到内含式复合物X@(HAlNH)12结构在热力学和动力学上的稳定性信息, 其中复合物Ga+@(HAlNH)12的结构相对最稳定.     

The energetics of shooting ions into the dodecahedrane cage

The activation barriers and energy profiles along the inclusion coordinate for the penetration of H+, He, Li+, Be+, Be2+, and Mg2+ into dodecahedrane 1 vary considerably with the nature of the projectile. Using Hartree-Fock and MP2 derived structures and energies, the overall process of creating endohedral X n+@1 complexes is examined in detail.     

衬底对N@C60分子电子自旋共振谱的影响

N@C₆₀内嵌富勒烯是一种在量子科技领域有较高应用前景的分子。科学家们设计了一系列以内嵌富勒烯分子为基本量子单元的量子计算机模型,而构筑这样的模型具有极高的挑战。其中,由于内嵌富勒烯分子阵列的制备通常需要合适的衬底,而衬底与分子之间的相互作用会影响甚至破坏内嵌N原子的自旋信号。因此研究和理解衬底与内嵌富勒烯分子的相互作用具有重要的意义。本文制备了高质量的N@C₆₀分子,并采用扫描隧道显微镜对其在Au(111)表面的结构及电子态进行表征。通过对比N@C₆₀分子在Au(111)、Si(111)、SiO₂表面的电子自旋共振(ESR)信号随时间及其抽真空处理的变化,表明Au原子的核自旋与内嵌N原子的电子自旋的耦合作用是Au(111)表面N@C₆₀单分子层的ESR谱中内嵌N原子的信号衰减的主要原因。     

稀土富勒烯与苯的气相加合

内嵌稀土元素的富勒烯化合物一稀土富勒烯Lit＠CZ。是一类新型的化合物．它具有独特的“超分子结构和巨大的潜在用途，将在未来的功能材料开发中起到不可估量的作用［‘］．目前关干稀土富勒烯的研究主要集中于稀土富勒烯的合成、分离、纯化、表征和理论方面．关于稀土富勒烯化     

Synthesis and reaction of fullerene C70 encapsulating two molecules of H2

New endohedral fullerene C(70) encapsulating one and two H(2) molecule(s) has been synthesized by organic reactions, the so-called "molecular surgery" method, and the first organic derivatization of H(2)@C(70) and (H(2))(2)@C(70) has been conducted. Although the interaction between inner H(2) and outer C(70) is rather weak, (H(2))(2)@C(70) exhibits smaller equilibrium constants in the Diels-Alder reaction with 9,10-dimethylanthracene than those of H(2)@C(70).     

Studies on the encapsulation of F- in single walled nanotubes of different chiralities using density functional theory calculations and Car-Parrinello molecular dynamics simulations

In this study, the encapsulation of F(-) in different nanotubes (NTs) has been investigated using electronic structure calculations and Car-Parrinello molecular dynamics simulations. The carbon atoms in the single walled carbon nanotube (CNT) are systematically doped with B and N atoms. The effect of the encapsulation of F(-) in the boron nitride nanotube (BNNT) has also been investigated. Electronic structure calculations show that the (7,0) chirality nanotube forms a more stable endohedral complex (with F(-)) than the other nanotubes. Evidence obtained from the band structure of CNT calculations reveals that the band gap of the CNT is marginally affected by the encapsulation. However, the same encapsulation significantly changes the band gap of the BNNT. The density of states (DOS) derived from the calculations shows significant changes near the Fermi level. The snapshots obtained from the CPMD simulation highlight the fluctuation of the anion inside the tube and there is more fluctuation in BNNT than in CNT.     

Structures of MAu16 (-) (M=Ag, Li, Na, and K): how far is the endohedral doping?

The structural and electronic properties of MAu16 (-) (M=Ag, Li, Na, and K) have been studied by the scalar relativistic all-electron density-functional calculations, in which particular attention is paid to the stability of the endohedral Au16 (-) cage doped by different dopant atoms. It is found that only the smaller atoms, such as Cu, Li, and Na, can be stably encapsulated in the Au16 (-) cage, while the addition of the larger Ag or K atom prefers to locate in the surface or outside of the cage, which is inconsistent with the previous hypothesis that the Au16 (-) cage could act as a container to hold an arbitrary heterometal atom. The stable endohedral Li@Au16 (-) and Na@Au16 (-) have a large energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital gap, indicating that they are chemically stable and may be used as potential building blocks for designing cluster-assembled materials.     

Structural elucidation and regioselective functionalization of an unexplored carbide cluster metallofullerene Sc2C2@C(s)(6)-C82

A Sc(2)C(84) isomer, previously assumed to be Sc(2)@C(84), is unambiguously identified as a new carbide cluster metallofullerene Sc(2)C(2)@C(s)(6)-C(82) using both NMR spectroscopy and X-ray crystallography. The (13)C-nuclei signal of the internal C(2)-unit was observed at 244.4 ppm with a 15% (13)C-enriched sample. Temperature-dependent dynamic motion of the internal Sc(2)C(2) cluster is also revealed with NMR spectrometry. Moreover, the chemical property of Sc(2)C(2)@C(s)(6)-C(82) is investigated for the first time using 3-triphenylmethyl-5-oxazolidinone (1) which provides a 1,3-dipolar reagent under heating. Regarding the low cage symmetry of this endohedral which contains 44 types of nonequivalent cage carbons, it is surprising to find that only one monoadduct isomer is formed in the reaction. Single-crystal X-ray results of the isolated pyrrolidino derivative Sc(2)C(2)@C(s)(6)-C(82)N(CH(2))(2)Trt (2) reveal that the addition takes place at a [6,6]-bond junction, which is far from either of the two Sc atoms. Such a highly regioselective addition pattern can be reasonably interpreted by analyzing the frontier molecular orbitals of the endohedral. Electronic and electrochemical investigations reveal that adduct 2 has a larger highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap than pristine Sc(2)C(2)@C(s)(6)-C(82); accordingly, it is more stable.     

Structural and electronic properties of S-doped fullerene C58: where is the S atom situated?

Structural and electronic properties of S-doped fullerene C58 were calculated systematically via Hartree-Fock self-consistent field (SCF) and density functional B3LYP levels of theory with 6-31G(d) basis set. The most stable C58S represents an open cage structure with a nine-member ring orifice, which provides a large hole for large atoms or small molecules to pass through into the cage. The most stable endohedral S@C58 has the S atom seated near the center of the C58 cage. The calculated highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps of the isomers lie in the range of 1.42-2.50 eV. The electron affinity and the ionization potential were also presented as an indicator of the kinetic stability. Our results may aid in the design of experimental methods for controlling the nature of fullerene cages (for example, doping, opening, and reclosing them).     

Chemical reactivity of D3h C78 (metallo)fullerene: regioselectivity changes induced by Sc3N encapsulation

We report here for the first time a full comparison of the exohedral reactivity of a given fullerene and its parent trinitride template endohedral metallofullerene. In particular, we study the thermodynamics and kinetics for the Diels-Alder [4 + 2] cycloaddition between 1,3-butadiene and free D3h'-C78 fullerene and between butadiene and the corresponding endohedral D3h-Sc3N@C78 derivative. The reaction is studied for all nonequivalent bonds, in both the free and the endohedral fullerenes, at the BP86/TZP//BP86/DZP level. The change in exohedral reactivity and regioselectivity when a metal cluster is encapsulated inside the cage is profound. Consequently, the Diels-Alder reaction over the free fullerene and the endohedral derivative leads to totally different cycloadducts. This is caused by the metal nitride situated inside the fullerene cage that reduces the reactivity of the free fullerene and favors the reaction over different bonds.     

新型内嵌混合金属氮化物原子簇富勒烯的合成、结构与性质*

内嵌混合金属氮化物原子簇富勒烯的发现极大地扩展了内嵌富勒烯家族。内嵌混合金属氮化物原子簇富勒烯是一类新型的内嵌富勒烯,其内嵌物为由2-3种不同的金属组成的氮化物原子簇。本文首先介绍了新型内嵌混合金属氮化物原子簇富勒烯的发现、合成和分离方法,并对目前所分离出来的内嵌混合金属氮化物原子簇富勒烯进行了分类。然后总结了目前所报导的内嵌混合金属氮化物原子簇富勒烯的结构表征手段,对于不同的内嵌混合金属氮化物原子簇富勒烯的分子结构分别进行了阐述。最后着重讨论了内嵌混合金属氮化物原子簇富勒烯的特殊电子性质以及物理和化学性质。本文还对内嵌混合金属氮化物原子簇富勒烯潜在的应用前景作了展望,在内嵌具有不同物理性质的两到三种金属原子的基础上,所形成的内嵌混合金属氮化物原子簇富勒烯有可能兼具不同金属原子各自的性质,从而成为多功能综合的功能材料。     

Salvaging Reactive Fullerenes from Soot by Exohedral Derivatization

The awesome allotropy of carbon yields innumerable topologically possible cage structures of molecular carbon. This field is also related to endohedral metallofullerenes constructed by metal‐atom encapsulation. Stable and soluble empty fullerenes and endohedral metallofullerenes are available in pure form in macroscopic amounts from carbon arc production or other physical processes followed by extraction and subsequent chromatographic separation. However, many other unidentified fullerene species, which must be reactive and insoluble in their pristine forms, remain in soot. These “missing” species must have extremely small HOMO–LUMO gaps and may have unconventional cage structures. Recent progress in this field has demonstrated that reactive fullerenes can be salvaged by exohedral derivatization, which can stabilize the reactive carbon cages. This concept provides a means of preparing macroscopic amounts of unconventional fullerenes as their derivatives.