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Endohedral Fullerenes: N@C60, N@C70, and P@C60
Abstract
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Nitrogen in C60 is a paramagnetic atom with a magnetic moment corresponding to the S=3/2 electronic spin. The dipolar interaction of two NatC60 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 NatC60 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 NatC60 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 NatC60 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. .
FULLERENES with metals encapsulated within the carbon cage have been prepared recently1-6. Laser vaporization of a lanthanum-impregnated graphite rod yielded an air-stable, solvent-extractable species comprising a single La atom inside a C82 cage1, denoted La@ C82; a cluster containing two La atoms (La2@ C82) was later reported3. Similar techniques applied to graphite/yttrium rods have produced Y@ C82 and Y2@ C82 (refs 5, 6). Here we describe the preparation of scandium-containing C82 species, including an encapsulated scandium trimer, Sc3@C82. Mass spectrometry and ESR spectroscopy provide evidence that the Sc3, trimer is indeed trapped within the cage.
Well resolved EPR spectra of P@C60 in solution have been recorded, proving that the encased phosphorus atoms are in their quartet spin ground state. The isotropic hyperfine interaction is increased by a factor of 2.5 compared with the values measured for free atoms. An analysis of spin relaxation data reveals that fluctuating zero field splitting (ZFS) interaction induced by collision-induced deformations of the carbon shell constitutes the dominant relaxation mechanism. The variance of the time fluctuating ZFS interaction is about a factor of 10 larger than that observed for N@C60 under identical conditions. Values for the correlation time of the deformation of the fullerene cage range from 5ps to 32 ps in the temperature interval 190-300 K in toluene.
FULLERENES have internal cavities large enough to encapsulate atoms1,2. Recently, noble-gas atoms were introduced into about one in a million fullerene molecules3. It is now possible to achieve far greater yields of these noble-gas endohedral compounds4. Because 3He has a spin of 1/2 and is an excellent NMR nucleus5,6, it can be used as a probe for the magnetic shielding environment inside the fullerene cavity. This environment should reflect possible ring currents and hence the aromaticity in fullerenes7–15, an issue that measurements of magnetic susceptibility16,17 have not completely resolved. Here we present 3He NMR spectra of the endohedral compounds 3He@C60 and 3He@C70 (the @ symbol denotes a compound that is endohedral). We find that the 3He nuclei encapsulated in C6o and C70 are shielded by 6 and 29 parts per million respectively, relative to free 3He. These shieldings are unexpectedly large, indicating significant diamagnetic ring currents in C60 and very large ones in C70. Our results also show that, because of its small size and inertness, helium can serve as a useful probe of magnetic molecular properties. In addition, our work represents the first 3He NMR spectra of stable helium compounds.
and Lu@C82 were prepared by arc burning and subsequent HPLC purification. EPR spectra of Lu@C82 could be interpreted as arising from unresolved hyperfine interaction with the I=7/2 nuclear spin of 175Lu. At temperatures above 250K, athermally activated process, which is tentatively attributed to ahopping process of the
encapsulated ion or to time-dependent population of close-lying electronic states, leads to pronounced line broadening. For
the Ho@C82 sample, no EPR signals could be detected, indicating ahigh spin state of this molecule. Spin relaxation data of N@C60, which was prepared by ion bombardment, could be interpreted by assuming that collision-induced deformation of the carbon
shell leads to afluctuating zero-field splitting, sensed by the quartet spin state of the central encapsulated nitrogen atom.
N@C60 (atomic nitrogen in C60, produced by ion implantation) is the first member of a new class of endohedral fullerenes in which a highly reactive atom in its atomic ground state is enclosed in C60. Nitrogen in C60 is chemically inert and stable under ambient conditions. The atomic states of nitrogen in C60 can be slightly tuned by cage distortions caused by exohedral additions to N@C60. In this Letter, a new and simple production method for N@C60 is described and the effect of cage distortions on the atomic states of nitrogen is discussed.
Fullerenes can act as inert cages for highly reactive nitrogen atoms even at room temperature. Confinement in a cage of less than spherical symmetry as realized in C70 leads to a characteristic deformation of the atomic charge and spin distributions which can be sensed by magnetic resonance techniques. A quantitative analysis of the amount of orbital squeezing is possible by comparison with data of free nitrogen ions.
During experiments aimed at understanding the mechanisms by which long-chain carbon molecules are formed in interstellar space and circumstellar shells1, graphite has been vaporized by laser irradiation, producing a remarkably stable cluster consisting of 60 carbon atoms. Concerning the question of what kind of 60-carbon atom structure might give rise to a superstable species, we suggest a truncated icosahedron, a polygon with 60 vertices and 32 faces, 12 of which are pentagonal and 20 hexagonal. This object is commonly encountered as the football shown in Fig. 1. The C60 molecule which results when a carbon atom is placed at each vertex of this structure has all valences satisfied by two single bonds and one double bond, has many resonance structures, and appears to be aromatic.
The endohedral discandium fullerenes, Sc[sub 2]C[sub 74], Sc[sub 2]C[sub 82], and Sc[sub 2]C[sub 84], have been isolated for the first time from soot prepared by the arc burning of Sc[sub 2]O[sub 3]/graphite composite rods. The separation and isolation of the metallofullerenes from various hollow fullerenes have been realized by using a two-stage high-performance liquid chromatography. Laser-desorption time-of-flight mass analyses of the present samples confirm the isolation of the discandium fullerenes. The discandium fullerene, Sc[sub 2]C[sub 74], has been newly found in this study. The UV-vis absorption spectra of the isolated Sc[sub 2]C[sub 74], Sc[sub 2]C[sub 82], and Sc[sub 2]C[sub 84] reveal several salient features which are totally absent in those of the corresponding hollow fullerenes such as C[sub 76], C[sub 78], C[sub 82], and C[sub 84]. 17 refs., 4 figs.
The zero-field hyperfine frequencies in the ground state of atomic N14 have been measured in a hydrogen maser with sufficient accuracy to resolve the quadrupole coupling constant B. The result is B=+1.32+/-0.20 Hz.
The unambiguous identification of the various redox states of C60 is of importance in many areas of fullerene research, such as charge transfer studies between conducting polymers and C60, or the study of the distribution of charged states in solid-state-doped samples. This combined ESR and optical absorption spectroscopic study represents the most complete analysis of the anionic states of buckminsterfullerene, from the monoanion to the hexaanion, so far reported.
Buckminsterfullerene (Cââ) is conveniently prepared and isolated. Its chemical properties are being investigated with increasing interest. The fulleroids are inflated fullerenes where up to six carbon atoms, each bearing two substituents, are added systematically to Cââ. In this communication we report the preparation of the simplest, {open_quotes}parent{close_quotes} fulleroid formed by the reaction of fullerene Cââ with diazomethane. Diazomethane reacts with (Cââ) to give a thermally unstable compound, (CHâNâ)(Cââ) in 44% yield. It was shown that in fulleroid synthese, the intermediate pyrazoline can be isolated and characterized; the first step of fulleroid synthesis is a dipolar addition across the reactive 6-ring-6-ring (pyracyclene) junction; thermal nitrogen loss occurs concomitantly with rearrangement; and comparative ¹H NMR with a known bicyclic hydrocarbon incapable of sustaining an extended ring current indicates that there are no extended ring currents in the fulleroid {open_quotes}(pi){close_quotes} system. 16 refs., 1 fig.
By performing high-resolution electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) experiments on nitrogen atoms encapsulated in C60, the capability of the quartet spin system to sense small local fields at the site of the atom is demonstrated. Symmetry lowering induced by a phase transition in polycrystalline C60 at 258 K can easily be detected by the appearance of a zero-field splitting of axial symmetry. Freezing of cage rotation is observed via the magnetic dipole interaction with 13C nuclei of the carbon shell.
Electron-spin-resonance spectra of N, P, As, and H atoms trapped in matrices formed from rare-gas elements at 4.2°K have been studied. The isotropic hyperfine coupling of N and P increases with the atomic number of the matrix element whereas that of As decreases. The van der Waals interaction theory devised by Adrian to explain the matrix effects on the nitrogen hyperfine splitting was generalized and applied with good success to the heavier group-V atoms. The results indicate that these group-V atoms occupy substitutional lattice sites, whereas the H atoms occupy octahedral sites. Recalculation of the matrix perturbations on nitrogen with more recent analytical self-consistent-field wave functions significantly improved agreement between theory and experiment. The isotropic g factor of N, P, and As was found to be less than the free-spin g and to decrease with increasing atomic number of the matrix elements. A weak fine structure observed for As atoms trapped in the Kr matrix is attributed to a small fraction of the atoms trapped in crystalline faults in the matrix.
60 (atomic nitrogen inside C60) is produced by ion implantation. Two different production methods are employed: Kaufman ion source and glow discharge. After
the bombarded material is dissolved in toluene or CS2 and is filtered, several milligrams of C60 containing N@C60 in a concentration of 10-4 to 10-5 are obtained. N@C60 gives a very clear hyperfine-split electron paramagnetic resonance signal. The most prominent features of N@C60 are: (i) Nitrogen in C60 keeps its atomic electronic configuration and occupies the on-center position. (ii) N@C60 is stable at ambient conditions, the thermal instability starts at 260 °C. (iii) The complex survives exohedral addition
reactions and is a sensitive detector of cage distortions caused by addends. (iv) C60 and N@C60 exhibit slightly different retention times in column chromatography, thus permitting an enrichment of N@C60 by this method.
We report on pulsed electron-spin-resonance (ESR) investigations of endohedral N@C60. The measurements were performed in X band (9.5 GHz) and W band (94 GHz). Applying a two-pulse sequence with a variable flip angle of the second pulse, we could not only separate the contributions of the central and the satellite transitions, but also monitor the phase transition in C60 at T≈250 K. Analysing the temperature dependence of the longitudinal relaxation rate T1−1, the hyperfine relaxation, which is caused by the damped quantum oscillator of the nitrogen atom inside C60, was found to play the most important role. From a comparison of the experimental data with our relaxation model, the oscillator energy was determined to be Eosc≈11(2) meV.
The structure of fullerene can be interpreted as a condensation of 20 cyclohexatriene rings [with single-and double-bond lengths of 1.450 (4) angstrom and 1.390 (4) angstrom, respectively] to form a highly symmetrical sphere. The addition of the bis(ethoxycarbonyl)methano group to a double bond prevents the known disorder of the unsubstituted fullerene molecule in the solid phase.
The formation of C60 dimers with a nitrogen atom in one of the C60 cages is studied. The dimers are produced by ball milling of a mixture of N@C60 with C60 and a suitable additive. The products are purified by high pressure liquid chromatography (HPLC) and identified by UV/Vis and IR spectroscopy. Electron paramagnetic resonance (EPR) measurements show that nitrogen remains in C60 during the dimerization and keeps its atomic configuration. A slight deformation of the electron shell reflecting the distortion of the cage is observed. The optimization of the dimer formation is described.
Endohedral chemical shifts of all isolated pentagon isomers of C76 and C78, as well as those of the most stable isolated pentagon isomers of C84, have been computed at the GIAO-SCF/DZP level using DFT optimized geometries (Becke88-perdew86/3-21G). The theoretical data support tentative assignments of recent experimental σ(3He) NMR data to the major components in a partly separated, 3He labeled mixture. The identification of at least one new fullerene isomer, C84D2d(4), is suggested, based on the endohedral chemical shift of −25.0 ppm (calc.) versus −24.4 ppm (exp.).
The reduction of 3He@C60 and 3He@C70 by lithium metal to give solutions of the corresponding hexaanions in THF-d8 has been achieved under gentle conditions, at subambient temperatures, without sonication, by capitalizing on the ability of corannulene (1) to serve as an efficient electron carrier between the lithium metal and the solid fullerenes, which are virtually insoluble in the reaction medium. The 3He inside the C60 hexaanion is found to be more strongly shielded (by nearly 20 ppm!) than any previously reported 3He in a fullerene [δ(3He@C606-) = −48.7 ppm, relative to dissolved 3He gas in the solution], whereas the 3He inside the C70 hexaanion is actually deshielded [δ(3He@C706-) = +8.3 ppm], resonating at nearly 15 ppm lower field than any previously reported 3He in a fullerene. These results stand in complete accord with earlier predictions that the magnetic properties of C60 and C70 would be altered dramatically, and in opposite directions, by reduction of the fullerenes to their hexaanions. The phenomenal ability of C606- to shield an endohedral 3He from a powerful external magnetic field provides the most compelling evidence to date for the ability of electrons to move freely about the surface of a spheroidal π-system.
The theoretical aspects of the magnetic susceptibility of Cââ have been controversial. The most recent contribution, due to Pasquarello, claims that the 5-membered rings of buckminster-fullerene exhibit strong paramagnetic currents, whereas the 6-membered rings support mild diamagnetic currents. The intriguing question of the influence of the Cââ sphere on NMR chemical shifts can be solved experimentally only by placing and spin-active probe in the vicinity of the fullerene. Since this cannot be done with Cââ itself, it is necessary to use properly functionalized derivatives. We report herein the first experimental evidence for the presence of ring currents in some methanofullerenes and fulleroids-Cââ. Compounds were prepared, according to our reported procedure, by addition of diazo compounds to Cââ, and their structures were determined mainly by NMR spectroscopy. This constitutes the first experimental evidence for the theoretically postulated strong paramagnetic currents associated with 5-membered rings in Cââ. 9 refs., 2 figs.
We have used classical hot-atom chemistry to put tritium atoms inside fullerene molecules. The tritium is generated in a nuclear reactor with the reaction 6Li(n,α)3H. The hot tritium atom slows down and can end up being thermalized inside a fullerene where it is trapped. The irradiated sample is dissolved, chromatographed, and counted in a scintillation counter, showing a small tritium activity. After some time, the sample is analyzed with a sensitive mass spectrometer, and 3He was found on heating above 400°C, showing that the tritium had decayed leaving the 3He trapped inside the fullerene.
Heating fullerenes at 650°C under 3000 atmospheres of the noble gases helium, neon, argon, krypton, and xenon introduces these atoms into the fullerene cages in about one in 1000 molecules. A “window” mechanism in which one or more of the carbon-carbon bonds of the cage is broken has been proposed to explain the process. The amount of gas inside the fullerenes can be measured by heating to 1000°C to expel the gases, which can then be measured by mass spectroscopy. Information obtained from the nuclear magnetic resonance spectra of helium-3-labeled fullerenes indicates that the magnetic field inside the cage is altered by aromatic ring current effects. Each higher fullerene isomer and each chemical derivative of a fullerene that has been studied so far has given a distinct helium nuclear magnetic resonance peak.
Line shapes of electron paramagnetic resonance spectra of manganous ion in various ligand environments have been examined at two microwave frequencies, 9.1 and 35 GHz. From the observed differences in line shape at two microwave frequencies and the theoretical expression for relaxation of sextet state ions, the correlation times for processes modulating the zero‐field splitting (zfs) interaction and the magnitude of the effective zfs were evaluated. For all complexes which were examined the correlation times were in the range 3×10−12−9×10−12sec at 300°K. Two types of zfs are distinguished, a transient zfs which is related to the relaxation processes and a static zfs. In this regard there appears to be no simple relation between the transient zfs as obtained from relaxation studies and the static zfs as obtained from solid‐state spectra. The large static zfs for MnEDTA gives quasi‐solid‐state features to the solution EPR spectra of this complex.
We describe here a new method for preparing cage molecules containing atoms which, in principle, is not limited to thermodynamic levels of incorporation. Beams of ions can be easily made at selected energies. On colliding with cage molecules, they have a chance of penetrating into the interior. If one exposes the molecules to the beam particles continually and if these collisions neither knock occupants out of the cages nor destroy occupied cage molecules, there is no upper limit to the fraction of occupation which can be achieved. 10 refs., 1 fig., 1 tab.
3He NMR spectrometry has been used to examine bisaddition to C60 containing an encapsulated 3He atom (3He@C60) using three types of reactions: (1) cyclopropanation with diethyl bromomalonate and base to give dicarbethoxymethanofullerenes (Bingel−Hirsch reaction), (b) addition of azomethine ylides to give N-methylfulleropyrrolidines (Prato−Wudl reaction), and (c) reduction to give C60H4. 3He NMR spectra of crude reaction mixtures in all three series showed well-separated resonances for each of the bisadducts, spread out over more than 2 ppm. The major isomeric bisadducts from the first two reactions were separated and characterized, and the 3He NMR spectra of the individual major bisadducts from 3He@C60 were obtained (five isomers from reaction 1, four isomers from reaction 2). Although the absolute chemical shifts in the two series of bisadducts differ, the relative chemical shifts are significantly but not perfectly correlated. Bisadducts with appended ligands on opposite hemispheres of C60 (7, 6, and 5) tend to have 3He NMR resonances downfield of the bisadducts with appended ligands on the same hemisphere (bisadducts 2 and 3). Unfortunately, the equatorial isomer 4, a major product from the Hirsch addition reaction, was not obtained in sufficient quantity for study from the Prato reaction. Well-separated resonances were also seen for the isomeric compounds of composition 3He@C60H4 from reaction of the fullerene with diimide, but the individual isomers were not separated. The very large differences in the 3He NMR chemical shifts of the isomeric bisadducts in all three reactions demonstrate that the magnetic field felt by the 3He atom due to the ring currents in the residual π-system is extremely sensitive to the pattern of ligation on the C60 surface. These results provide further support for the assertion that 3He NMR is a very sensitive probe of patterns of chemical addition to fullerenes, and suggest that this technique will prove to be generally useful in determining the ratio as well as the identity of isomeric fullerene bisadducts.
The very recent synthesis of the [sup 3]He C[sub 60] species has created the exciting opportunity for measuring shielding in the interior of the C[sub 60] cluster. Here we report on the first rigorous electronic structure calculations on the NMR chemical shift of the endohedral [sup 3]He atom. Good agreement with the experimental data is obtained, and the differences among the previous theoretical results are resolved. The present calculations open the avenue for accurate predictions of NMR spectra of endohedral complexes that will aid in isolation and characterization of these truly unusual chemical systems. In order to assess the influence of the cage geometry on the computed shifts, two calculations were performed. The calculations reported here demonstrate that it is now feasible to accurately predict the NMR spectra of host molecules in endohedral complexes and other large supramolecular systems. Such theoretical predictions are bound to guide the experimentalists in their quest for isolation and characterization of endohedral complexes. 17 refs.
In 1987, Elser and Haddon predicted a vanishingly small pi-electron contribution to the magnetic susceptibility of the icosahedral C60 molecule (buckminsterfullerene). This result runs counter to intuition and was subsequently disputed on the basis of ab initio computations. Following the recent discovery of methods for preparing (and purifying) large quantities of C60, we here report measurements of the magnetic susceptibility, chi, of a solid sample of pure C60 by SQUID magnetometry. The obtained mass value, chi-g = -0.35 x 10(-6), is far below that of graphite or benzene, consistent with the Elser-Haddon picture of accidental cancellation of the diamagnetic and paramagnetic contributions to chi. An estimate of the unusually large paramagnetic contribution is in accord with recent measurements of C60's electronic excitations. The C70 molecule is also measured to have chi-g = -(0.59 +/- 0.05) x 10(-6).
Energy intervals in 75As, 31P and 53Cr ground state hyperfine multiplets have been measured using the techniques of atomic beam magnetic resonance. Values for the interaction constants have been found which are consistent with all measured intervals to within the experimental errors. The results are for 75As: A = -66.204(1) mc/s, B = -0.535(3) mc/s, gJ = -1.9965(8); for 31P: A = +55.061(10) mc/s, gJ = -2.00165(40); and for 53Cr: A = -82.5994(16) mc/s and B = +0.008(12) mc/s.
THE conjecture that atoms can be trapped inside closed carbon cages such as the fullerenes was first made by Kroto et al. Mass spectroscopic evidence obtained soon after suggested that lanthanum atoms were encapsulated in fullerenes prepared by laser vaporization of a lanthanum-impregnated graphite disk, and these results were later corroborated. Recently, helium atoms have been incorporated into fullerenes through collisions in the gas phase, and evidence has been obtained for the formation of metal-containing fullerenes during arc burning of composite graphite rods. All of these studies, however, have produced quantities too small for characterization using standard spectroscopic techniques. We report here the preparation of milligram quantities of lanthanum-containing C82, which can be solvent-extracted in yields of about 2% along with empty C60 and C70 cages. We have measured the electron paramagnetic resonance spectrum of this mixture, both in solution and in the solid state, which reveals that the lanthanum atom has a formal charge of 3+, and the C82a charge of 3-. This runs contrary to some expectations that the doubly charged fulleride anions would be the most stable species; it also reveals that the fullerene cages have the same formal charge as in the superconducting alkali-metal-doped phases.
Three series of regioisomeric bisadducts of C60, namely, C62(anisyl)4 and the mixed systems C62(anisyl)2-(COOEt)2, and C61(COOEt)2(NCOOEt), were synthesized starting from the 1,2-monoadducts C61(COOEt)2 (1), C61-(anisyl)2 (2), and C60(NCOOEt) (4) by using the Bingel and Bamford-Stevens reactions, and nitrene additions. In the case of C61(COOEt)2(NCOOEt) the complete series of nine possible regioisomers were isolated for the first time. For steric reasons the cis-1 isomers of C62(anisyl)4 and C62(anisyl)2(COOEt)2 were not formed. The transannular [6,6] bonds in the cis-1 isomer 42 of C61(COOEt)2(NCOOEt) are closed. The properties and regioselectivities of formation of these bisadducts and their monoadduct precursors were compared with those of the series C62-(COOEt)4 and C60(NCOOEt)2, which we synthesized previously. In the additions to 1, 2, and 4 the preferred positions of attack are e and trans-3 for sterically demanding addends (e.g., combinations of C(anisyl)2 and C(COOEt)2) and cis-1, e, and trans-3 for sterically less demanding addends (e.g., combinations of N(COOEt) and C(COOEt)2). A detailed analysis of the MO structures, the experimental and calculated geometries of monoadduct precursors, and the stabilities of reaction products leads to the conclusion that the addend-independent cage distortion itself is responsible for the observed regioselectivities of bisadduct formations.
The fullerenes have been established as new and versatile building blocks in organic chemistry. A large number of fascinating
fullerene derivatives, especially of the icosahedral buck-minsterfullerene C60, have been synthesized. The chemistry of C60 continues to be good for many surprises. However, based on present knowledge a series of reactivity principles can be deduced
which makes derivatization of this all carbon cluster more and more predictable. In this article first the geometric and electronic
properties of the parent molecule are analyzed. The bent structure of the carbon network C60 and the filling of its molecular orbital with 60 π-electrons dictate the chemical reactivity. A very important aspect that
was introduced with the investigation of fullerene chemistry is the shape dependence of reactivity.
Large piezoelectric d33 coefficients around 600pC/N are found in corona-charged non-uniform electrets consisting of elastically “soft” (microporous
polytetrafluoroethylene PTFE) and “stiff” (perfluorinated cyclobutene PFCB) polymer layers. The piezoelectric activity of
the two-layer fluoropolymer stack exceeds the d33 coefficient of the ferroelectric ceramic lead zirconate titanate (PZT) by more than a factor of two and that of the ferroelectric
polymer polyvinylidene fluoride (PVDF) by a factor of 20. Soft piezoelectric materials may become interesting for a large
number of sensor and transducer applications, in areas such as security systems, medical diagnostics, and nondestructive testing.
Do the chemical properties of the surface of a carbon sheet depend on its shape? This question addresses a criterion for chemical behaviour that has hardly been investigated previously. The current neglect of this question may be due to the fact that suitable model systems with easily distinguishable graphitic surfaces were essentially unknown until the discovery [1] and synthesis [2,3,4] of fullerenes, nanotubes and other related forms of carbon. In this study, we present the first systematic comparison of the chemical behaviour of the convex outer and the concave inner surfaces of C60 by analysing the results of semiempirical and DFT calculations on exohedral and endohedral complexes with H- and F-atoms as well as with the methyl radical. We show that such extremely reactive species are trapped by the extraordinary inert inner surface of C60 and do not undergo chemical reactions.
Polymerization of a mixture of C 60 /C 70 in rf plasma is reported. The electric dark‐current conductivity of the plasma‐polymerized mixture of C 60 /C 70 , which is approximately 10-7 S/cm in the atmosphere, does not depend on the applied voltage at least in the range of -25 to 25 V. A semiconductor‐type temperature dependence of the conductivity in the higher‐temperature domain was observed, and the band‐gap energy was estimated to be 2.1 eV. The conductivity increased with increasing temperature from 25 to 230 °C in vacuum, whereas in the atmosphere the conductivity increased upon decreasing the temperature below 80 °C. It is supposed that in this temperature domain the electric conductivity is enhanced by the existence of water molecules on the film, the surface of which is characterized by a high hydrophilicity. The surface morphology of the polymerized film was characterized by the presence of aggregates with diameters of about 300 Å and the surface was highly hydrophilic, polar, and heterogeneous. The surface has a completely amorphous molecular structure. When the C 60 molecules polymerize each other, it has been shown that the polymerization of C 60 molecules proceeds, at least primarily, by the formation of 1,2‐cyclobutane structures between the cyclohexatrienyl parts of neighboring C 60 molecules. The addition of hydrogens, hydroxyl groups, carbonyl groups, and aryl peroxide groups to the radical sites occurs but the free radicals of the order of magnitude 1017 spins/g were found to remain without quenching.
Using pulsed EPR techniques, the electron spin relaxation properties of nitrogen atoms encapsulated in C60 have been studied. Because of the high symmetry of the cage, most of the conventional relaxation mechanisms otherwise affecting electron spins in solution are absent, resulting in an exceptional homogeneous ERP linewidth of only 2.5 kHz. Apparently, solvent collision-induced deformations of the carbon shell which modulate the zero-field-splitting sensed by the quartet spin state of N@C60 are the dominant spin relaxation mechanism.
By performing high-resolution EPR and ENDOR experiments on nitrogen atoms encapsulated in C60, the capability of the quartet spin system to sense small local fields at the site of the atom is demonstrated. Such symmetry lowering can either be induced by chemical modification of the cage or by a phase transition in polycrystalline C60. Additional line splittings in the EPR spectrum indicate the presence of a non-vanishing zero-field-splitting. Freezing of cage rotation can be observed via the magnetic dipole interaction with 13C nuclei of the carbon shell resulting in broadening of ENDOR transitions. Fluctuating magnetic fields originating from additional paramagnetic species in solution can also be detected by their influence on the spin relaxation times.
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Positive muons injected into solid C60, K4C60, and K6C60 form vacuumlike muonium (μ+e-) with a (6–12)% probability. Observation of coherent spin precession of muonium establishes that all three materials are nonmagnetic and nonconducting at low temperatures. From the temperature dependence of the signals we estimate the electronic band gaps in K4C60 and K6C60 to be considerably smaller than in C60. The similarity of the muonium centers supports a model in which a muonium atom is caged inside the C60 molecule in pure C60 or the C60x molecular ion in KxC60.
The synthesis and EPR spectroscopic investigations of a family of six endohedral fullerenes, namely, N@C-60 (1), N@C-61(COOC2H5)(2) (2), N@C-66(COOC2H5)(12) (3), N@C-66(COOC2D5)(12) (4), N@C-61(COOC2D5)(2) (5), and N@C-70 (6), containing atomic nitrogen in the S-4(3/2) ground state is described. The parent systems N@C-60 (1) and N@C-70 (6) were synthesized by nitrogen ion implantation. The syntheses of the C-2 upsilon-symmetric monoadducts 2 and 5 and the Ti, symmetric hexaadducts 3 and 4 exhibiting well-defined cage distortions were accomplished via cyclopropanation with the corresponding malonates, With respect to these additions the reactivity of 1 is indistinguishable from that of empty C-60 The quartet electronic spin of the encapsulated N-atoms is a very sensitive probe for cage modifications. In the monoadducts 2 and 5 a permanent zero field splitting (ZFS) tensor with rigidly aligned axes was revealed reflecting the intrinsic droplet like cage distortion. In contrast, no fine structure due to intrinsic distortions was monitored in the ESR spectra of the highly symmetric hexaadducts 3 and 4. In these cases only matrix-induced distortions of the cage lead to ZFS interactions. In solution fluctuations of the ZFS tensor are the major source of spin relaxation. The root-mean-square value of this collision-induced fluctuating ZFS interaction as estimated from relaxation data fur N@C-60 (1) and the hexaadduct 3 is in the range of the ZFS interaction measured for the monoadduct 2.
The concave inner face of C60 is so inert with respect to the formation of covalent bonds, that even atomic nitrogen in the quartet ground state is stable as an encapsulated guest (shown on the right). Semiempirical calculations and ESR investigations offer an explanation for this unprecedented behavior.
A formulation within the London approximation of magnetic ring currents is applied to C60, C70, and their hexa-anions. Contrary to previous approaches, this formulation does not require the identification of closed loops and can therefore be applied to these molecules despite their topological complexity. Knowledge of the ring currents provides a better understanding of their magnetic ring-current susceptibilities. Probe dipoles are used to investigate local spatial variations of the magnetic field induced by the ring currents. In an attempt to evaluate how the carbon chemical shifts are affected by the ring currents, we separate the current into a component circulating above and one below the molecular surface. Assuming a displacement of the negatively charged electronic clouds towards the outside of the molecule, we are able to understand the relative positions of the carbon NMR lines of C60 and C6-60.
Synchrotron-x-ray powder-diffraction and differential-scanning-calorimetry measurements on solid C60 reveal a first-order phase transition from a low-temperature simple-cubic structure with a four-molecule basis to a face-centered-cubic structure at 249 K. The free-energy change at the transition is approximately 6.7 J/g. Model fits to the diffraction intensities are consistent with complete orientational disorder at room temperature, and with the development of orientational order rather than molecular displacements or distortions at low temperature.
A new paramagnetic defect in solid C60 was produced by nitrogen implantation in solid C60. The hyperfine splitting and the isotope effect unambiguously show that the paramagnetic center contains one nitrogen nucleus. The hyperfine interaction is isotropic, its value is comparable to that of the free nitrogen atom, and the spin of the electron system is S = 3/2, as in atomic nitrogen. The complex responsible for this center is soluble in toluene and CS2 and is stable. We suggest that the complex consists of nitrogen inside C60.
Chem. Rev. 2000, 100, 1075−1120 Discrete Fulleride Anions and Fullerenium Cations Christopher A. Reed* and Robert D. Bolskar Department of Chemistry, University of CaliforniasRiverside, Riverside, California 92521-0403 Received June 22, 1999 Contents I. Introduction, Scope, and Nomenclature II. Electrochemistry A. Reductive Voltammetry B. Oxidative Voltammetry III. Synthesis A. Chemical Reduction of Fullerenes to Fullerides i. Metals as Reducing Agents ii. Coordination and Organometallic Compounds as Reducing Agents iii. Organic/Other Reducing Agents B. Electrosynthesis of Fullerides C. Chemical Oxidation of Fullerenes to Fullerenium Cations IV. Electronic (NIR) Spectroscopy A. Introduction B. C 60 n- Fullerides C. C 70 and Higher Fullerenes D. Fullerenium Cations E. Diffuse Interstellar Bands V. Vibrational Spectroscopy A. Infrared Spectroscopy B. Raman Spectroscopy VI. X-ray Crystallography A. Introduction B. [PPN] 2 [C 60 ] and Related C 602- Structures C. C 60 - Structures D. C 603- Structures E. Comparison of Discrete and Extended Structures VII. Magnetic Susceptibility and Spin States VIII. NMR Spectroscopy A. Introduction B. 13 C NMR Data in Solution C. Interpretation of Solution NMR Data D. 13 C NMR Data in the Solid State E. Knight Shift in A 3 C 60 F. 3 He NMR of Endohedral Fullerides G. 13 C NMR of Derivatized Fullerenes H. 13 C NMR of Fullerenium Cations IX. EPR Spectroscopy A. Introduction B. Features of the C 60 - Spectrum i. The Low g Value ii. Temperature Dependence of the Line Width (∆H pp ) X. XI. XII. XIII. iii. Anisotropy iv. Problem of the Sharp Signals v. Origins of Sharp Signals vi. The C 120 O Impurity Postulate vii. The Dimer Postulate C. Features of the C 602- EPR Spectrum D. Features of the C 603- EPR Spectrum E. Features of C 604- and C 605- EPR Spectra F. EPR Spectra of Higher Fullerides G. EPR Spectra of Fullerenium Cations Chemical Reactivity A. Introduction B. Fulleride Basicity C. Fulleride Nucleophilicity/Electron Transfer D. Fullerides as Intermediates E. Fullerides as Catalysts F. Fullerides as Ligands G. Fullerenium Cations Conclusions and Future Directions Acknowledgments References I. Introduction, Scope, and Nomenclature The electron-accepting ability of C 60 , the archetypal fullerene, is its most characteristic chemical property. It is a natural consequence of electronic structure and was anticipated in early molecular orbital calcula- tions 1 which place a low-lying unoccupied t 1u level about 2 eV above the h u HOMO: 2-4 Early in the gas-phase investigations of fullerenes, the electron affinity of C 60 was measured and found to be high (2.69 eV). 5-7 When the macroscopic era of C 60 chemistry began in 1990, this property was soon found to translate into the solution phase. 8 In a rather remarkable cyclic voltammogram (see Figure 1), the reversible stepwise addition of up to six electrons was soon demonstrated electrochemically. 9,10 10.1021/cr980017o CCC: $35.00 © 2000 American Chemical Society Published on Web 02/16/2000
A new formulation of the current within the London approximation allows the calculation of ring currents in topologically complex molecules. Application of this theory to C(60) demonstrates the existence of remarkable pi electron ring currents. Paramagnetic currents, in size comparable to the ones in benzene, flow within the pentagons, whereas weaker diamagnetic currents flow all around the C.(60) molecule. The overall vanishing ring-current magnetic susceptibility results from a cancellation of diamagnetic and paramagnetic contributions. The presence of ring currents significantly affects chemical shifts as measured in nuclear magnetic resonance experiments. In contrast to the magnetic susceptibility, which is a property of the molecule as a whole, chemical shifts are sensitive to the local magnetic field and the effect of ring currents does not vanish.