Demonstration of a chemical transformation inside a fullerene. The reversible conversion of the allotropes of H2@C60

The interconversion of the two allotropes of the hydrogen molecule (para-H2 and ortho-H2) incarcerated inside the fullerene C60 is reported (oH2@C60 and pH2@C60, respectively). For conversion, oH2@C60 was adsorbed at the external surface of the zeolite NaY and immersed into liquid oxygen at 77 K. Equilibrium was reached in less than 0.5 h. Rapid removal of oxygen provides a sample of enriched pH2@C60 that is stable for many days in the absence of paramagnetic catalysts (half-life approximately 15 days). Enriched pH2@C60 is nonvolatile and soluble in organic solvents. At room temperature in the presence of a paramagnetic catalyst (dissolved O2 or the nitroxide Tempo) a slow back conversion into oH2@C60 was observed by 1H NMR. A bimolecular rate constant for conversion of pH2@C60 to oH2@C60 using Tempo of kTempo approximately 4 x 10-5 M-1 s-1 was observed, which is approximately 3 orders of magnitudes slower than that for dissolved pH2 in organic solvents which is not protected by the C60 shell.     

Icosahedral and ring-shaped allotropes of phosphorus

The existence of two new allotropic forms of phosphorus, icosahedral cages and ring-shaped chains, is predicted. The cages and rings are nanostructural modifications of the black and the red phosphorus, respectively. The icosahedral and ring-shaped allotropes are compared with the experimentally known allotropic forms of phosphorus by quantum chemical methods. Both the cages and the rings are thermodynamically favored over the white phosphorus, the rings being comparable to the Hittorf's violet phosphorus and to the recently discovered fibrous red phosphorus. The stabilities of the icosahedral cages increase as a function of their size, having structural resemblance with the rhombohedral black phosphorus. The high thermodynamic stability of the phosphorus nanostructures suggests their experimental synthesis to be viable.     

On the possibility of the existence of molecular nitrogen allotropes

The possibility of the existence of nitrogen molecules with an even number of atoms of composition N₄, N₆, N₈, and N₁₀ has been discussed with the use of the QCISD and G3 quantum-chemical methods. From these data, a conclusion has been made that three new nitrogen allotropes with an even number of atoms in a molecule can exist, namely, N₄ shaped as a rectangle and regular tetrahedron and N₆ with a shape remotely resembling an “open book.” The bond lengths and bond and torsion angles in each of these molecules have been reported.     

C2 C70F38 is aromatic, contains three planar hexagons, and has equatorial addends

Single crystal X-ray analysis shows the main (C(2)) isomer of C(70)F(38) to contain three planar delocalised aromatic hexagons (two equivalent and one centred on the C(2) axis), together with seven C[double bond, length as m-dash]C bonds (three pairs and one straddling the C(2) axis); C(70)F(38) is the first high addition level [70]fullerene derivative to be fully characterised, is the first to have equatorial addends, and is calculated to have high stability.     

Light-triggered reversible supramolecular transformations of multi-bisthienylethene hexagons

It is very challenging to realize well-controlled structural transformations in artificial supramolecules. Herein we report the construction of a novel family of multi-bisthienylethene hexagons with precise control of the shape and size as well as the specific number of photochromic units via coordination-driven self-assembly. These newly developed multi-bisthienylethene hexagons are highly sensitive and responsive to photostimuli, especially allowing for quantitative reversible supramolecular transformations triggered by light irradiation.     

Computational study of the Stone-Wales transformation in C36

The transition of the D6h neutral and charged isomers to D2d isomers of C36 via Stone-Wales transformation has been studied by means of the hybrid density functional method (B3LYP). The results show that the transition state (TS) and reaction pathway could be identified for the rearrangement from C36-D6h to C36-D2d on the potential energy surface. We found that the neutral and charged transition states all have C2 molecular point group symmetry with the two migrating carbon atoms remaining close to the fullerene surface. The other kind of possible TS with a carbene-like structure along the stepwise reaction path does not exist as a stationary point with the density functionals utilized here. The classical barriers are 6.23 eV through the neutral TS, 6.37 eV through the anionic TS, and 6.29 eV through the cationic TS at the B3LYP/6-31G level of theory.     

Icosahedral and ring-shaped allotropes of arsenic.

We predict the existence of two novel families of arsenic nanostructures: icosahedral cages and ring-shaped chains. Quantum chemical calculations on the cages, rings, and the experimentally known allotropes of arsenic suggest the nanostructures to be thermodynamically stable. The icosahedral cages are modifications of the gray allotrope of arsenic, while the ring-shaped chains are structurally related to the red allotrope of phosphorus. Comparisons between the analogous allotropes of arsenic and phosphorus show distinct differences. While phosphorus favors the ring-shaped chains over the icosahedral cages, large cages become favorable for arsenic. From the thermodynamical point of view, experimental preparation of the proposed families of arsenic nanostructures is expected to be viable.     

Comparing the structural stability of PbS nanocrystals assembled in fcc and bcc superlattice allotropes

We investigated the structural stability of colloidal PbS nanocrystals (NCs) self-assembled into superlattice (SL) allotropes of either face-centered cubic (fcc) or body-centered cubic (bcc) symmetry. Small-angle X-ray scattering analysis showed that the NC packing density is higher in the bcc than in the fcc SL; this is a manifestation of the cuboctahedral shape of the NC building block. Using the high-pressure rock-salt/orthorhombic phase transition as a stability indicator, we discovered that the transition pressure for NCs in a bcc SL occurs at 8.5 GPa, which is 1.5 GPa higher than the transition pressure (7.0 GPa) observed for a fcc SL. The higher structural stability in the bcc SL is attributed primarily to the effective absorption of loading force in specific SL symmetry and to a lesser extent to the surface energy of the NCs. The experimental results provide new insights into the fundamental relationship between the symmetry of the self-assembled SL and the structural stability of the constituent NCs.     

C1 C70F38 contains four planar aromatic hexagons; the parallel between fluorination of [60]- and [70]fullerenes

The main C(1) isomer of C(70)F(38) is shown by single-crystal X-ray analysis to contain four planar aromatic hexagons and four isolated C=C bonds, has two fluorines on the equator, and is related to C(2) C(70)F(38) by means of three 1,3-fluorine shifts. The C(1) and C(2) isomers thus parallel the T and C(3)/C(1) isomers of C(60)F(36) in containing three and four aromatic rings, respectively, and in the fluorine shift relationship.     

Enumeration of conjugated hydrocarbons: Hollow hexagons revisited

The numbers of nonisomorphic hollow hexagons are expressed in terms of a generating function.     

Helices of boron-nitrogen hexagons and decagons. A theoretical study

Ab initio self-consistent field molecular orbital and density functional theory calculations have been performed on a series of helical boron-nitrogen structures comprised of fused hexagons and larger polygons. The presence of an even number N of rings in the boron-nitrogen [N]helicenes leads to the possibility of angular isomers. The electronic structure and stability of three isomeric nonhydrogenated boron-nitrogen helices were investigated at the HF/6-31G(d) and the B3LYP/6-31G(d) levels of theory. According to this study some of the initially assumed regular helical structures are unstable; two types of the isomeric structures convert to characteristically different equilibrium geometries. Electron density contours were calculated in order to interpret the existing bonding patterns.     

维生素C含量测定实验中果汁试样的保存条件探究

运用库仑滴定法,通过研究室温见光,冷藏避光,充氮除氧和加入EDTA、甲酸、EDTA-甲酸等不同稳定剂后,VC标准溶液和橙汁饮料中VC含量随时间变化的情况,来探究较长时间内储存VC溶液和橙汁饮料的最佳方法.研究表明在冷藏避光条件下向VC标准溶液和橙汁中加入分析实验室常规试剂EDTA作为稳定剂,能保证在1个月内VC的浓度变化在5%以内,可有效避免因VC稳定性引起的实验误差.     

Supramolecular isomerism in coordination compounds: nanoscale molecular hexagons and chains

A nanoscale supramolecular hexagon, 1, and its supramolecular isomeric chain structure, 2, have been prepared from self-assembly of 5-NO2-bdc and Cu(II) cations. The hexagon is neutral and soluble and has outer and inner diameters of 3.1 and 0.8 nm, respectively.     

Addition patterns in carbon allotropes: independence numbers and d-codes in the Klein and related graphs

The problem of predicting stoichiometries and patterns of chemical addition to a carbon framework, subject solely to the restriction that each addend excludes neighboring sites up to some distance d, is equivalent to determination of d-codes of a graph, and for d = 2 to determination of maximum independent sets. Sizes, symmetries, and numbers of d-codes are found for the all-heptagon Klein graph (prototype for "plumber's nightmare" carbon) and for three related graphs. The independence number of the Klein graph is 23, which increases to 24 for a related, but sterically relaxed, all-heptagon network with the same number of vertices and modified adjacencies. Expansion of the Klein graph and its relaxed analogue by insertion of hexagonal faces to form leapfrog graphs also allows all heptagons to achieve their maximum of 3 addends. Consideration of the pi system that is the complement of the addition pattern imposes a closed-shell requirement on the adjacency spectrum, which typically reduces the size of acceptable independent sets. The closed-shell independence numbers of the Klein graph and its relaxed analogue are 18 and 20, respectively.     

荔枝保鲜过程中维生素C的快速电位滴定

荔枝是岭南佳果中的精品,富含维生素C（VC）,是药食兼用之品。荔枝不易保存,其VC含量随着保存时间的延长而变化。本文测定了荔枝在保鲜过程中不同时间VC的含量,以期为荔枝保鲜技术的研究提供一定的理论数据。     

Kinetics of the 20°C phase transformation in polytetrafluoroethylene

The rate at which PTFE transforms from the triclinic to the hexagonal phase has been measured under isothermal conditions. Samples in a series of decreasing density and crystallinity show increased isothermal rates of transformation. Observed kinetic data are interpreted on the basis of a modified Avrami-Johnson-Mehl treatment. A model for the transformation in which planes of helix-hand reversal propagate through the lattice is shown to fit the experimental results. The transformation rate is observed to be proportional to the total (001) surface area in the polytetrafluoroethylene specimens, suggesting that nucleation of the transformation takes place at grain boundaries.     

Magnetic excitations in Cu6 and Mn6 hexagons embedded in D3d-symmetric polyoxotungstates

In this article we reconsider the discussion of the magnetic measurements for the two novel polyoxotungstates, (n-BuNH(3))(12)[(CuCl)(6)(AsW(9)O(33))(2)].6H(2)O and (n-BuNH(3))(12)[(MnCl)(6)(SbW(9)O(33))(2)].6H(2)O, which have been synthesized and characterized by Yamase et al. (Inorg.Chem. 2006, 45, 7698). Analysis of the magnetic susceptibility and magnetization for Cu(6)(12+) and Mn(6)(12+) hexagons based on the exact diagonalization of isotropic exchange Hamiltonian shows that the best-fit first-neighbor coupling parameters are J = 35 and 0.55 cm(-1), respectively, while the second-neighbor interactions are very small. These values exceed considerably those obtained by Yamase et al. (J = 8.82 and 0.14 cm(-1)) on the basis of the Kambe-Van Vleck formula that is inappropriate for six-membered rings. We also got perfect fits to the experimental data for the field dependence of magnetization at 1.8 K. The results imply the importance of axial anisotropy, which is shown to be especially pronounced for the Mn(6)(12+) cluster. We discuss also the symmetry assignments of exchange multiplets to the exact SGamma terms (full spin, S, and irreducible representation, Gamma, of the point group) and correlate the results with the selection rules for the anisotropic magnetic contributions. The antisymmetric exchange is shown to appear in orbitally degenerate multiplets as a first-order perturbation and gives rise to an easy axis of magnetization along the C(6) axis. Evaluation of the Zeeman levels shows that the field applied in the plane of the hexagon fully reduces the effect of the antisymmetric exchange.     

Synthesis of a new family of hexakisferrocenyl hexagons and their electrochemical behavior

The design and synthesis of two new hexakisferrocenyl hexagons has been achieved via coordination-driven self-assembly wherein the size and relative distribution of six ferrocene moieties has been precisely controlled. Insight into the structure and electronic properties of these supramolecules was obtained through electrochemical studies.     

Multiple Adducts of C60 by Tether-Directed Remote Functionalization and synthesis of soluble derivatives of new carbon allotropes Cn(60+5)

A comprehensive series of multiple adducts of C₆₀ was prepared by tether-directed remote functionalization. When the tether-reactive-group conjugates 2 and 10 were attached to methano[60]fullerenecarboxylic acid ( = cyclopropafullerene-C₆₀-I_h-carboxylic acid) and C₆₀, respectively, the e-bis-adducts 4 and 9 (Schemes 1 and 2) were obtained with complete regioselectivity as predicted by semi-empirical PM3 calculations (Fig. 2). Attachment of the anchor-tether-reactive-group conjugate 13 to C₆₀ by Bingel reaction, followed by double intramolecular Diels-Alder cycloaddition afforded the tris-adduct 12 (Scheme 3). Starting from 12 , a series of selective e-additions led to the tetrakis-adducts 16 and 19 (Scheme 4), pentakis-adducts 20 – 23 (Scheme 5), and, ultimately, to hexakis-adducts 24 and 25 (Scheme 6), and 29 and 30 (Scheme 7) with a pseudo-octahedral addition pattern on the fullerene core. Oxidative cyclization of diethynylmethanofullerene 30 under Eglinton-Glaser conditions afforded the trimeric and tetrameric acetylenic macrocycles 26 , with three, and 27 , with four appended C₆₀ moieties, respectively (Scheme 8). These multinanometer-sized compounds are the first soluble derivatives of C₁₉₅ and C₂₆₀, two members of a new class of fullerene-acetylene hybrid C-allotropes with the general formula C_{n(60 + 5)}. The matrix-assisted laser-desorption time-of-flight mass spectra of 26 and 27 showed a remarkable fragmentation; the sequential loss of fullerene spheres led to the formation of ions corresponding to mono-fullerene adducts of the cyclocarbons cyclo-C₁₅ and cyclo-C₂₀ (Fig. 4). Large solvent effects were observed in the Bingel addition of 2-bromomalonates to higher adducts of C₆₀, with the use of polar solvents enhancing the reaction rate without loss of regioselectivity. Experimental evidence for the enhanced reactivity of e_face over e_edge bonds was obtained, which had previously been predicted in computational studies. The correlated series of mono- to hexakis-adducts of C₆₀ allowed identification of the changes in reactivity and physical properties that occur, when the conjugated π-electron chromophore of the fullerene is reduced as a result of increasing functionalization; this analysis is the subject of the directly following paper.