All-carbon molecules

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

Cyclocarbons All-Carbon Molecules through Rational Synthesis. Likely Intermediates in Fullerene Formation... 

Another fascinating all-carbon molecule, the molecular belt 21a, is an isomer of Cm and can be constructed from six non-planar TEE subunits (Figure 10).1491 Semiempirical calculations (AMI) indicate... 

F. Diederich, Y. Rubin, C. B. Knobler, R. L. Whetten, K. E. Schriver, K. N. Houk, Y. Li, All-Carbon Molecules Evidence for the Generation of Cydo [18] carbon from a Stable Organic Precursor , Science 1989,245,1088-1090. 

The absence of a formal nomenclature al this juncture is accompanied by a somewhat fuzzy chronology pertaining to the discovery and early research on ihe fullerenes. However, the isolation and confirmation of the CHI all-carbon molecule sans any dangling bonds, as first conjectured in 1985. was pivotal to subsequent research. 

The hollow interior of dodecahedrane and other organic cage compounds described in section 4.9 is much too small to envelop atoms, ions, or molecules. Tight closed-shell macromolecules have been obtained from vesicles by several research groups, by polymerization of amphiphiles possessing double or triple bonds within the membrane or at the head groups. Smaller, but well-defined, closed-shell containers have been obtained by two other methods described below, namely by directed synthesis and by formation of closed-shell all-carbon molecules in graphite vapor. 

The larger all-carbon molecules, Cn (n= 10-24), are predicted to have mono-cyclic structures (cyclo[/z]carbons), molecules with n— 14, 18, 22, being expected to be aromatic, with closed An + 2 electron shells.1,25,26,32 The smallest cyclocarbon for which a reasonable stability is predicted is cyclo[18]carbon, Ci8.366-368 Both in-plane and out-of-plane n systems are aromatic. In contrast, the larger molecules (n = 30-40), are very reactive, while more stable polycyclic structures predominate for higher values of n, culminating in fullerene structures for n > 60. 

Be91 R. D. Beck, P. St. John, M. M. Alvarez, F. Diederich and R. L. Whetten, Resilience of All-Carbon Molecules Ceo, C70, and Cs4 A Surface-Scattering Time-of-Flight Investigation, J. Phys. Chem. 95, 8402-8409 (1991). 

In a surprising recent development, Kratschmer et al. have shown that certain all-carbon molecules are produced in large quantities in the evaporation of graphite and can be isolated as soluble, well-defined solids. The major species was identiHed as molecular C ) through mass spectrometry and by comparison of the infrared spectrum with theoretical predictions for the celebrated truncated-icosahedron structure, which had earlier been proposed to account for cluster beam observations. The solid material, described as a new form of elemental carbon in a nearly pure state, has a disordered hep lattice of packed quasi-spherical molecules, but determination of the precise molecular structure awaits diffraction from well-ordered crystals. 

Kroto et al. have followed this announcement with a partial chemical separation of the soluble all-carbon molecules generated by the same procedure. They used mass spectrometric evidence to conclude that other air-stable C molecules are present (n = 62, 64, 66, 68, 70). They reported that chromatographic separation yields Qo and C70 in a 3 1 ratio, in contrast to the 2-10% of C70 estimated in ref 1. The reported C NMR spectrum of the Qo fraction, in particular, evidently confirms the existence of a species with all 60 carbon atoms chemically equivalent (proposed structures as shown in Chart 1),... 

Resilience of All-Carbon Molecules Cw, C70, and Cs4 A Surface-Scattering Time-of-Flight Investigation... 

Top. Curr. Chem. 1999, 201, 43-79. (d) Bunz, U.H.F. Rubin, Y. Tobe Y. Polyethynylated cyclic 7t-systems scaffoldings for novel two and three-dimensional carbon networks. Chem. Soc. Rev. 1999, 28(2., 107-119. (e) Tobe, Y. All-carbon molecules from small-ring propellanes. In Advances in Strained and Interesting Organic Molecules. Halton, B., Ed., JAI Press Stamford, 1999 Vol. 7, 153-184. (f) Tobe, Y. Wakabayashi, T. Carbon-rich compounds acetylene-based carbon allotropes. In Acetylene Chemistry Chemistry, Biology and Material Science. Stang, P.J. Diederich, F. Tykwinski, R., Eds. VCH Weinheim, 2005 387 426. 

The accessibility of the fullerenes [1] in macroscopic quantities [2] opened up the unprecedented opportunity to develop a rich three-dimensional chemistry of spherical and polyfunctional all carbon molecules. [3-8] A large multitude of fullerene derivatives like exohedral covalent addition products, salts, cluster opened and defined degradation products, heterofullerenes and endohedral derivatives can be imagined and numerous examples, especially of covalent adducts have been synthesized and characterized. [3-8] Within a few years the fullerenes became essential building blocks in organic chemistry. Most of the chemistry of fullerenes has so far been carried out with Ceo (1) with little work on C70 and few experiments with C76 and Cg4. This is simply due to the fact that Ceo... 

Due to the development of methods for the production of cold cluster beams containing species up to C200, spectroscopic data on all-carbon molecules are now widely available. No direct structural or energetic information is available experimentally, and this has stimulated the application of computational methods in order to evaluate the geometric and electronic structures of these molecules, as well as to predict properties such as ionization potentials and vibrational frequencies. 

Diederich F, Rubin Y, Knobler CB, Whetten RL, Schriver KE, Houk KN, Li Y (1989) All-carbon molecules evidence for the generation of cyclo[18]carbon fiom a stable organic precursor. Science 245(4922) 1088-1090... 

Molecular Crystals Fullerites. Organic crystals are usually prone to ionization damage and decompose very rapidly under electron irradiation they can thus be studied for only a short time (a few. seconds) and only with a very low electron beam intensity. Transmission electron microscopy has, therefore, seldom been applied to organic crystals. However, the all-carbon molecules Qo, C70. etc., (fullerenes) discovered at the end of the 1980s resist electron radiation fairly well. Early structural studies on the crystalline phases of ftil-lerenes (fullerites) were performed mainly by electron microscopy because only small quantities of sufficiently pure material were available. At room... 

Elegant chemical syntheses at UCLA have resulted in the unambiguous structural characterization of monocyclic all-carbon molecules. Cyclo(18) earbon and other species have been prepared and characterized. Neutral cycles satisfying the general formula 4n - - 2 atoms are predicted to be elosed-shell aromatic molecules with special stability. This new class of monocyclic molecules, which might well... 

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