Life on earth is based on compounds that have carbon frames and backbones. Today, chemists have added to the world of biomolecules and biopolymers approximately 107 different synthetic molecules and polymers, the structures of which also depend on the formation of strong, stable carbon–carbon bonds. Although the stability of carbon–carbon bonds has been recognized for more than a century, the two natural modifications graphite and diamond were, until recently, the only allotropic forms of carbon on earth that were available in macroscopic quantities and were structurally well characterized. With the synthesis of macroscopic quantities of buckminsterfullerene (C60) and the higher fullerenes (C70, C76, C78, etc.) and the exploration of the fascinating properties of these all-carbon spheres, this situation has completely changed. In the coming decades, the design, preparation, and study of novel molecular and polymeric allotropic forms of carbon will be a central topic in chemistry. Research in this area will dramatically advance the fundamental knowledge on carbon-based matter and, as already illustrated by the ongoing work on C60, generate unprecedented technological perspectives. This review surveys synthetic organic-chemical approaches toward the preparation and study of all-carbon molecules and polymers that differ from the familiar networks of graphite and diamond as well as from the fullerenes. We will also discuss the ongoing research on fullerenes with a particular focus on the synthetic approaches to these all-carbon spheres and their transition metal complexes.

Synthetic approaches toward molecular and polymeric carbon allotropes

During the course of studying the dehydrogenation reactions of hydrocarbons by titanium atoms, ions, and clusters, an exceptionally stable and abundant cluster which contains 8 titaniums and 12 carbons was discovered "Titration" reactions with ND3 reveal the uptake of eight molecules, pointing to the fact that the titanium atoms are at exposed positions of similar coordination A dodecahedral structure of Th point group symmetry is proposed to account for the unusual stability of this molecular cluster The Ti8C12+ dodecahedron has 12 pentagonal rings and each of the rings is formed by two titanium and three carbon atoms, where each titanium is bound to three carbons Based on the model, it is expected that neutral Ti8C12 would be a stable metallo-carbododecahedral molecule and may comprise one member of a new class of molecules, namely metallo-carbohedrenes

https://science.sciencemag.org/content/sci/255/5050/1411.full.pdf

Ti8C12+-Metallo-Carbohedrenes: A New Class of Molecular Clusters?

It is demonstrated that fullerenes, prepared via the standard method (an arc between graphite electrodes in a partial pressure of helium), on heating to high temperatures release (4)He and (3)He. The amount corresponds to one (4)He for every 880,000 fullerene molecules. The (3)He/(4)He isotopic ratio is that of tank helium rather than that of atmospheric helium. These results convincingly show that the helium is inside and that there is no exchange with the atmosphere. The amount found corresponds with a prediction from a simple model based on the expected volume of the cavity. In addition, the temperature dependence for the release of helium implies a barrier about 80 kilocalories per mole. This is much lower than the barrier expected from theory for helium passing through one of the rings in the intact structure. Amechanism involving reversibly breaking one or more bonds to temporarily open a "window" in the cage is proposed. A predicted consequence of this mechanism is the incorporation of other gases while the "window" is open. This was demonstrated through the incorporation of (3)He and neon by heating fullerene in their presence.

https://science.sciencemag.org/content/sci/259/5100/1428.full.pdf

Stable compounds of helium and neon: he@c60 and ne@c60.

Structural aspects of fullerene chemistry--a journey through fullerene chirality.

Carceplexes and Hemicarceplexes

Endohedral Cluster Compounds: Inclusion of Helium within C 60•⊕ and C 70•⊕ through Collision Experiments

A semiempirical MNDO method was used in calculations on monosubstituted neutral and cationic compounds of the [CH 3 −CO−R] type with a wide range of substituents (R=H, F, Cl, CH 3 , NH 2 , OH, OCH 3 , NHCH 3 , CH=CH 2 ). The results obtained are interpreted with respect to the effect of the individual substituents on the geometry and electron distribution in the systems studied and allow the conclusion that the ratios of partial charges in the individual substructures in a radical cation correspond to those of the respective peak intensities in the mass spectrum

Mndo study of fragmentations in mass spectrometry. Part II. Substituent effects in the ioinization of [CH3COR] compounds and their enol tautomers

MNDO calculations of [C 3 H 6O] +. predict the parallel existence of both structures of radical cations of acetone (1) and propen-2-ol (2) in electron ionization spectra. The calculated heats of formation of 1 +. (△H f MNDO =783.2 kJ mol −1 and of 2 +. (△H f MNDO =649.8 kJ mol −1 ) are in very good agreement with the experimental results. A comparison with the results of ab initio calculations (3-21G and &-#!G) and experimental data for the individual structures of the main fragment [C 2H3O] + demonstrates a sufficient accuracy of MNDO results, suggesting the possibility of applying the method also in other cationic systems, especially in larger ones

MNDO study of fragmentations in mass spectrometry. Part I. The geometry and electron structure of [C3H6O] + ˙ systems and [C2H3O]+ ions

The accuracy of the diatomics-in-molecules (DIM) model for the krypton ionic trimer is examined in a series of ab initio calculations. In the C2v
 symmetry, the ground states of irreducible representations B2 and A1 were calculated using partially spin restricted open-shell coupled cluster method with perturbative triple connections (RHF-RCCSD-T), the relativistic effective core potential (RECP) and an extended basis set of atomic orbitals. Internally contracted multireference configuration interaction method (icMRCI) with the extended and restricted basis set was used to generate the potential energy surfaces (PESs) of the nine electronic states of Kr
 3
 +
 corresponding to Kr(1S) + Kr(1S) + Kr+(2P) dissociation limit in a wide interval of nuclear geometries. The overall agreement of the accurate ab initio PESs and the diatomics-in-molecules PESs confirms the quality of the DIM Hamiltonian for the Kr
 3
 +
 clusters and justifies its use in dynamical and spectroscopic studies of the Kr
 +
 clusters. Inclusion of the spin–orbit coupling into the ab initio PESs through a semi-empirical scheme is proposed.

Ab initio excited states calculations of Kr 3 + , probing semi-empirical modelling

The MNDO calculations of protonated polyfluorobenzenes [Ph-F n ]H + indicate the possibility of a relatively free migration of the hydrogen proton with energy barriers of 125-145 kJ mol −1 . At a higher degree of substitution (n) the protonation of the ipso carbon atom occupied by fluorine becomes energetically feasible, along with analogous migrations of fluorine, which, however, are energetically the most advantageous (ΔE a ∼230 kJ mol −1 ). In addition to bridged fluoronium ions, relatively stable cyclic intermediates were also found, which make a possible a rearrangement to the difluoromethylenecyclopentadienyl cation and thus the elimination of CF 2 observed in collision-induced dissociation mass spectra

Ring opening reaction of protonated polyfluorinated benzenes in the gas phase. A theoretical MNDO study

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