Ball milling has been applied in numerous solvent-free carbon-carbon bond formations. In many cases, these transformations proved superior to the analogous reactions performed in solution. The reaction scope involves purely organic addition and coupling reactions, transformations involving metals (in stoichiometric and catalytic amounts) as well as asymmetric organocatalyses.

Solvent-Free Carbon-Carbon Bond Formations in Ball Mills

The low or lack of solubility of fullerenes, carbon nanotubes and graphene/graphite in organic solvents and water severely hampers the study of their chemical functionalizations and practical applications. Covalent and noncovalent functionalizations of fullerenes and related materials via mechanochemistry seem appealing to tackle these problems. In this review article, we provide a comprehensive coverage on the mechanochemical reactions of fullerenes, carbon nanotubes and graphite, including dimerizations and trimerizations, nucleophilic additions, 1,3-dipolar cycloadditions, Diels–Alder reactions, [2 + 1] cycloadditions of carbenes and nitrenes, radical additions, oxidations, etc. It is intriguing to find that some reactions of fullerenes can only proceed under solvent-free conditions or undergo different reaction pathways from those of the liquid-phase counterparts to generate completely different products. We also present the application of the mechanical milling technique to complex formation, nanocomposite formation and enhanced hydrogen storage of carbon-related materials.

Mechanochemistry of fullerenes and related materials

Molecular structures appear to be natural candidates for a quantum technology: individual atoms can support quantum superpositions for long periods, and such atoms can in principle be embedded in a permanent molecular scaffolding to form an array. This would be true nanotechnology, with dimensions of order of a nanometre. However, the challenges of realizing such a vision are immense. One must identify a suitable elementary unit and demonstrate its merits for qubit storage and manipulation, including input/output. These units must then be formed into large arrays corresponding to an functional quantum architecture, including a mechanism for gate operations. Here we report our efforts, both experimental and theoretical, to create such a technology based on endohedral fullerenes or 'buckyballs'. We describe our successes with respect to these criteria, along with the obstacles we are currently facing and the questions that remain to be addressed.

http://iopscience.iop.org/article/10.1088/0953-8984/18/21/S12/pdf

Towards a fullerene-based quantum computer

We present a scheme to perform universal quantum computation using global addressing techniques as applied to a physical system of endohedrally doped fullerenes. The system consists of an $\mathcal{ABAB}$ linear array of group-V endohedrally doped fullerenes. Each molecule spin site consists of a nuclear spin coupled via a hyperfine interaction to an electron spin. The electron spin of each molecule is in a quartet ground state $S=3/2.$ Neighboring molecular electron spins are coupled via a magnetic dipole interaction. We find that an all-electron construction of a quantum cellular automaton is frustrated due to the degeneracy of the electronic transitions. However, we can construct a quantum-cellular-automata quantum computing architecture using these molecules by encoding the quantum information on the nuclear spins while using the electron spins as a local bus. We deduce the NMR and ESR pulses required to execute the basic cellular automaton operation and obtain a rough figure of merit for the number of gate operations per decoherence time. We find that this figure of merit compares well with other physical quantum computer proposals. We argue that the proposed architecture meets well the first four DiVincenzo criteria and we outline various routes toward meeting the fifth criterion: qubit readout.

Quantum-cellular-automata quantum computing with endohedral fullerenes

We present a discussion of recent concepts for the construction of a spin quantum computer using endohedral fullerenes. The fullerene molecule is a static, room-temperature trap for atoms with slowly relaxing electron andnuclear spins. The fullerene "containers" can be used to arrange the spins in complex structures such as a linear chain, to form a spin quantum register. We discuss the probable properties of such registers and different strategies to use them in a quantum computer design, including gating and read-out methods.

Architectures for a Spin Quantum Computer Based on Endohedral Fullerenes

`Nitrogen doped' C60 dimers (N@C60–C60)

Nitrogen in C60 is a paramagnetic atom with a magnetic moment corresponding to the S=3/2 electronic spin. The dipolar interaction of two N@C60 at closest distance, i.e. 1 nm apart, reaches a maximum value of 1.85 mT or 52 MHz, more than three times larger than the hyperfine interaction with the nuclear spin. For diluted N@C60 in a C60 matrix, the distribution of interactions leads to a line broadening in EPR experiments. We report here on a EPR line width measurement for N@C60 in C60 with concentrations varying by a factor of 1000. For the first time a concentration in the percent range was reached by repeated HPLC enrichment. A surprising and completely unexpected result is that sharp lines appear superimposed on the broad lines. We attribute the sharp line to motional narrowing of N@C60 diffusing on the surface of C60 grains. It can be assumed that the diffusion seen here is a general phenomenon of C60, the doped fullerene being just the indicator.

Magnetic interaction in diluted N@C60

Introducing an endohedral atom in one of the cages of the dimer opens the possibility to get information about changes in the electronic structure of the molecule due to polymerization. Nitrogen is known to form a stable endohedral fullerene and can be used for further chemical processing. In this contribution we will present first EPR results of the dimer being ‘doped’ with nitrogen in one of the cages. Zero Field Splitting induced by cage distortion is significantly larger than that found in the monoadduct (C2v-N@C61(COOCH2CH3)2).

N@C120—the first step to endohedral polymers

Ideal trapping behavior was recently discovered for nitrogen incased in C60 and C70, although significant increase of Fermi contact interaction gave evidence for compression of the atomic spin distribution. Fullerene cages therefor are candidates for the study of atomic orbital interaction with inert confinements. Prolate deformation of the spin and charge distribution of nitrogen in C70 was detected by high resolution ENDOR spectroscopy. Magnetic shielding averaged over the extended electronic orbitals in poly anions of fullerenes was found to be similar but not identical to the values measured for helium atoms at the cage center of the cages.

Squeezing and shielding of atoms in fullerene traps

Dilute solutions of anion radicals produced from enriched endohedral N@C60 and N@C70 samples were prepared by reduction with lithium in tetrahydrofuran (THF). EPR spectra were measured at room temperature with a Bruker X-Band spectrometer. Although EPR signals from mono- to penta-anions of empty fullerenes gave characteristic signals, no signals could be detected for these reduction states originating from endohedral nitrogen. We believe that increased spin relaxation rates of states of higher spin multiplicity cause a line broadening impeding detection. Continuing reduction finally resulted in the diamagnetic hexaanions, detected by vanishing EPR signals of empty fullerenes and a reappearance of the characteristic three line spectrum of endofullerenes. The chemical shift was quite similar to those measured by the point-like 3He.

M. Waiblinger

Papers

`Nitrogen doped' C60 dimers (N@C60–C60)

Magnetic interaction in diluted N@C60

N@C120—the first step to endohedral polymers

Squeezing and shielding of atoms in fullerene traps

Synthesis and EPR studies of N@C60 and N@C70 radical anions