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Atomic Nitrogen Encapsulated in Fullerenes: Realization of a Chemical Faraday Cage
Abstract
Fullerenes, C60 and C70, are ideal containers for atomic nitrogen. We will show by electron paramagnetic resonance (EPR) experiments that nitrogen in C60 keeps its atomic ground state configuration and resides in the center of the cage. This is the first time that atomic nitrogen is stabilized at ambient conditions. The inert shell of the fullerene protects the highly reactive nitrogen from undergoing chemical reactions with the surroundings. The fullerene cage is the chemical analogue of the Faraday cage in case of electrical fields, i.e. it shields off the chemical reactivity. As for the free nitrogen atom, the spins of the three p-electrons of nitrogen in C60 are parallel (S = 3/2) and the atom has spherical symmetry. Due to the center position of nitrogen in C60, extremely sharp EPR lines are observed. This reflects the absence of a strong host–guest interaction and shows that the individuality of nitrogen in the fullerenes is preserved. Further evidence for the almost interaction-free suspension of nitrogen in the fullerene cages is provided by g-factor measurements. These investigations show that magnetic shielding of the host molecules can account for the observed differences between N@C60 and N@C70. The fullerene cage can be chemically modified without destroying the endohedral complex. The chemical modifications change the symmetry of the molecule which is observed through an additional fine structure in the EPR spectrum. Influences of the modifications on the stability of N@C60 will be discussed.
... D'après Lips et co. [172], la molécule perd sa stabilité à partir de 500 K. Il est donc important de vérifier par des mesures de transport que nous retrouvons les propriétés associées à la présence de l'azote et que nous pouvons donc écarter, sans aucun doute, le scénario B. ...
... Le seul bémol, c'est qu'aucune anisotropie n'est attendue dans ce système ou en tout cas pas une anisotropie dont les effets se prolongent presque jusqu'à 1 T . D'après les observations effectuées par d'autres méthodes, celle-ci n'excède pas quelques dizaines de mT [172,181,182]. 6.4.2 ...
The aim of molecular spintronic, a rapidly emerging field, is to gather the specific tools, both experimental and theoretical, of different themes : molecular magnetism, spintronic and molecular electronic. In order to implement my Phd project, we had first to create a transistor in which the central part is a single molecule. This was achieved through the use of the electromigration technique, a difficult choice as implemented in a dilution cryostat. Indeed, our goal was to perform experiments at few tens of milliKelvins, and under high magnetic field [0, ±9 T ]. Through experiments with fullerene molecules, also known as C60, we have studied the electronic interactions between a nano-sized object and metallic electrodes. Combining very low temperatures and molecular properties (degenerate electronic levels, high charging energy) allowed us to observe a quantum phase transition between two magnetic states with different symmetries. We also provided strong experimental evidences for the observation of an under-screened Kondo effect. The last part of this thesis focused on the study of electronic transport through the molecular magnet N@C60. It consists of a nitrogen atom located inside a fullerene. By performing different types of magnetic spectroscopies, we quantified interactions between this atom and the conduction electrons carried by the C60 cage.
... The endohedral fullerene 15 N@C 60 comprises a nitrogen atom encapsulated within a carbon cage [13]. The nitrogen, located at the cage centre, retains its ground state electronic configuration 4 S 3/2 [14], with spin quantum numbers S = 3/2 for the electron and I = 1/2 for the nucleus. The energy levels of this molecule in a magnetic field B 0 = B 0ẑ are described by the Hamiltonian ...
The endohedral fullerene has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock's projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.
The strong interaction between transition metal (TM) atoms and semiconductor surface atoms may diminish the magnetic moments of the TM atoms and prevent it from being used as single atom spin-based devices. A carbon cage that can encapsulate TM atoms and isolate them from interacting with surface atoms is considered to protect the magnetic moments of TM atoms. We have studied the magnetic moments of Fe, Co, and Ni atoms adsorbed inside the corner hole of Si(111)-(77) by using first-principles calculations based on the density functional theory. The results show that when Co and Ni atoms directly adsorbed inside the corner hole, the magnetic moments vanished. But when a C60 cage is used to encapsulate the atoms, the magnetic moments increase to 1.41μB and 0.21μB, respectively. The results show a clear protecting effect. For Fe with and without C60, the magnetic moments are 2.12μB and 2.01μB, respectively. The presence of C60 cage can also maintain the magnetic moment. Further analysis shows that the TM atoms possess magnetic moment when the conduction electrons are localized around it. An Anderson impurity model is proposed to well understand all the findings. Our results demonstrate that a carbon cage may effectively protect the magnetic moments of TM atoms. This provides a new strategy for developing single atom based spin devices on semiconductors.
The first water-soluble derivative of the paramagnetic endohedral fullerene N@C 60 has been prepared through the covalent attachment of a single addend containing two permethylated β-cyclodextrin units to the surface of...
Endohedral fullerene N@C60, which is formed by C60 plus an atomic nitrogen located in the centre of the fullerene cage, has attracted interest, due to its exotic properties, such as exceptionally long
spin relaxation time
. Its preparation and chemical functionalization are discussed in this chapter; those are the most important topics in N@C60 field, as preparation directly affects the availability of this new material, and chemical functionalization enables us to control and tune its properties. Herein, we will firstly introduce different synthesis, purification, and characterization methods of N@C60, then move on to N@C60 stability. At the end, a variety of N@C60-related chemical reactions are reviewed, with emphasis on how the properties of the molecule change after functionalization.
We proposed a nanoscale water switch composed of CNTs. A capped CNT with potassium ions in it may move under an external electric field, just like a piston. We can control the switch toggle between the open and closed states only by changing the direction of the external electric field. Molecular dynamics (MD) was used to simulate the performance of the switch. The results show that this device may act well as a switch. The response speed of the switch reaches the nanosecond level. The stability of the piston position is also investigated.
Paramagnetic endohedral fullerenes and phthalocyanine (Pc) complexes are promising building blocks for molecular quantum information processing, for which tunable dipolar coupling is required. We have linked these two spin qubit candidates together, and characterized the resulting electron paramagnetic resonance properties, including the spin dipolar coupling between the fullerene spin and the copper spin. Having interpreted the distance-dependent coupling strength quantitatively, and further discussed the antiferromagnetic aggregation effect of the CuPc moieties, we demonstrate two ways of tuning the dipolar coupling in such dyad systems: changing the spacer group and adjusting the solution concentration.
Two cyclopropane derivatives of N@C60 were synthesised by well-controlled Bingel reactions, in which 94% of the spin centres were retained, confirmed by a series of quantitative electron spin paramagnetic resonance (EPR) measurements. Further study on the influence of the DBU catalyst base revealed a spin loss mechanism through a fullerene-DBU diradical.
How the electronic properties and chemical reactivity of empty fullerenes can be changed upon endohedral metaldoping? How the dynamic behavior and position of encapsulated metal atoms can be controlled? How its functionality can be extended by chemical treatments? We have addressed all aspects of endohedral metallofullerenes, including the synthesis, isolation, structure, chemical properties, and theoretical considerations and their applications in the related fields to answer these questions.
The endohedral fullerene molecule, N@C60, is a candidate for molecular spin qubits (quantum bits) and spin probes owing to its exceptional electron spin properties. Advancements in the processing of N@C60 are key to obtaining samples of high purity on a reasonable timescale. We investigate enrichment by high throughput processing (flow rate of 18 L h(-1) and operating pressure of 1.5-2 MPa) using high performance liquid chromatography (HPLC) as a means of scaling N@C60 production. We use detection by electron paramagnetic resonance (EPR) spectroscopy to map N@C60 during processing, and through the reconstruction of the peak position in the chromatogram, we are able to determine the retention time and relative purity of N@C60 without the need for its isolation. Based on this, we establish a procedure for time-efficient, high throughput processing to isolate N@C60 in high purity.
Ion implantation was applied to the production of endohedral 133Xe-fullerene. Using an isotope separator, 133Xe ions were implanted into a fullerene target of C60 and C70 produced by vacuum evaporation on a Ni backing. An HPLC analysis following dissolution of the fullerene targets in o-dichlorobenzene corroborated the formation of 133Xe@C60 and 133Xe@C70, showing a strong correlation between C60/C70 and 133Xe. The observed tailing following 133Xe peaks in the elution curves suggests a possibility of the isolation of endohedral 133Xe-fullerene from empty fullerene.
The nitrogen atom encapsulated fullerene (N@C60) with relatively high yield has been synthesized by a plasma irradiation method. We have examined the relationship between optical emission spectra of a radio frequency (RF) discharge nitrogen plasma and the synthesis yield of N@C60. As a consequence, the increasing amount of nitrogen molecule ions (N2+) impinging on the sublimated fullerenes are found to enhance the synthesis of N@C60. Furthermore, it is clarified that there is an optimum condition of the nitrogen plasma for the high-yield synthesis of N@C60, which is generated under lower gas pressure and is irradiated to the larger amount of fullerenes.
Nitrogen-C60 compounds such as azafullerene (C59N) and nitrogen-atom-encapsulated fullerene (N@C60) are produced by implanting nitrogen ions into C60 thin films on a substrate immersed in an electron cyclotron resonance plasma under a mirror magnetic field. Each compound is found to be synthesized, depending on the ion energy provided by the potential difference between the substrate and the plasma. The optimum energy for C59N synthesis is approximately 40-50 eV, and the amount of C59N decreases in an ion energy range larger than 50 eV. In contrast, an ion energy larger than 20 eV is required for N@C60 synthesis.
The glow discharge at low pressure, under nitrogen atmosphere, is an easy method to synthesize nitrogen encapsulated fullerenes. We present here the results concerning our attempts to optimize the synthesis conditions by this method as well as basic characterization of the obtained products, namely N@C60 and N@C70 together with the molecular counterparts N2@C60 and N2@C70. Changes in pressure, voltage discharge and temperature indicated the optimum yield for N@C60 in our case, which was about 0.0025%, was obtained at 650°C, 1 Torr, 850 V and 1 to 2 mA discharge. In this paper we discuss the HPLC separation of these products and the behavior of N2 in the C60 cage.
The nature of the interaction of paramagnetic atoms A (4 N, 4 P, 3 O, 3 S with systems and C 60 in the A • • • C 6 H 6 complexes and endohedral fullerenes (A@C 60 has been investigated employing second-order Møller–Plesset perturbation level of theory and density functional theory calculations. The coupled cluster theory with single and double excitations, and with perturbed triplet excitations have also been carried out for the A • • • C 6 H 6 complexes. The calculated geometries indicate that the paramagnetic N and P atoms lie on the C 6 axis of benzene in the A • • • C 6 H 6 complex and at the center of the C 60 cage in A@C 60 . On the other hand, the O and S atoms are slightly shifted towards the C–C bond of benzene in the A • • • C 6 H 6 complex. A comparison of the calculated binding energies BEs of these paramagnetic complexes and the corresponding rare gas complexes like He • • • C 6 H 6 and He@C 60 indicate that the BEs of the former are much larger than those of the latter. For both the rare gas and paramagnetic atom complexes dispersive interactions dominate the BEs. The energy barriers of the penetration of the O atom through a 6,6 bond are lower than the penetration of the helium atom and higher than the penetration of a N atom. © 2002 American Institute of Physics.
Nitrogen radio frequency (rf) plasma processing of C60 in the formation of the nitrogen endohedral complex (N@C60) and C60N adducts was fully examined with spectroscopic methodology and molecular orbital calculations. N@C60 formation was achieved using an internal parallel plate electrode type of capacitively coupled nitrogen rf plasma,1 whereas this was impossible when using an external ring electrode type of capacitively coupled rf nitrogen plasma. This experimental observation suggests that the formation of N@C60 in the internal parallel plate plasma system stems from the steep charge gradient around the rf cathode electrode induced by the self-bias effect, which promotes accelerated collisions of N+ with C60. Another important nitrogen rf plasma-induced process is the formation of C60N adducts. The formation and relative stabilities of internal and external C60N adducts and N@C60 were fully examined with the aid of MO calculations at a semiempirical approximation level, and the calculation was further extended to the plasma processing of C70. The calculation suggested that a C60 polymer is abundantly formed in a nitrogen rf plasma in which the nitrogen plays an important role in the intermolecular cross-linking of C60, derived from the radical addition and/or recombination reaction of the C60N adduct. The polymer formation was clearly supported by the observation of the peaks attributable to the nitrogen-containing C60 dimers in the time-of-flight mass spectra observed on the carbonaceous deposits formed in the nitrogen rf plasma.
The endohedral nitrogen-atom-containing fullerenes, N@C60 and N@C70, were studied mass spectrometrically. The neutral endohedrals are thermally unstable. This instability can be followed by mass spectrometry as well as by EPR spectroscopy. The corresponding ions were studied by using mass-analyzed ion kinetic energy (MIKE) spectroscopy. Unlike metal-containing and noble-gas-containing endohedral fullerenes studied earlier, the ions N@C60+ and N@C70+ do not lose C2 groups unimolecularly but lose the nitrogen atom instead. This indicates that the activation energy for nitrogen-atom loss is lower than that for the C2 elimination from the cage. This was verified through the analysis of kinetic energy release distribution (KERD) measurements. The results are discussed in light of the thermal instability of nitrogen-atom-containing endohedral compounds compared to noble-gas-containing analogues.
Molecules trapped inside fullerenes exhibit interesting quantum behavior, including quantization of their translational degrees of freedom. In this study, a theoretical framework for predicting quantum properties of nonlinear small molecules in nonsymmetric open-cage fullerenes (OCFs) has been described along the lines of similar theories which treat small molecules inside C(60) and clathrate cages. As an example, the coupled translational-rotational energy structure has been calculated for the case of CH(4) inside a known OCF. The calculated energy levels have been used to calculate the equilibrium fraction of incorporated CH(4) as well as the translational heat capacity for the encapsulated molecule. The heat capacity shows an anomalous maximum at 239 K for CH(4) and 215 K for CD(4) which are not present in free methane.
La spintronique moléculaire, domaine en pleine expansion, se propose de réunir les outils expéri- mentaux et théoriques propres à trois thématiques : le magnétisme moléculaire, l'électronique de spin, et l'électronique moléculaire. Le projet de ma thèse ne pouvait se réaliser sans la création préalable d'un transistor dont l'élé- ment central est une molécule unique. Cet objectif a été atteint grâce à l'utilisation de la technique d'électromigration, choix délicat puisque mis en œuvre dans un cryostat à dilution. Notre objectif était en effet de pouvoir effectuer des mesures à quelques dizaines de milliKelvins, et sous fort champ magnétique [0, ±9 T ]. A travers les expériences menées sur des molécules de fullerène, aussi appelées C60, nous avons étudié les interactions électroniques entre un objet de taille nanométrique et des électrodes métal- liques. La combinaison de mesures à très basse température et des propriétés des molécules (niveaux électroniques dégénérés, énergie de charge élevée) a permis d'observer une transition de phase quan- tique entre deux états magnétiques de symétries différentes. Nous avons également apporté de solides preuves expérimentales quant à l'observation d'un effet Kondo sous-écranté. La dernière partie de cette thèse a porté sur l'étude du transport électronique à travers l'aimant moléculaire N@C60. Celui-ci est constitué d'un atome d'azote placé à l'intérieur d'un fullerène. En effectuant différents types de spectroscopies magnétiques, nous avons quantifié les interactions entre cet atome et les électrons de conduction portés par la cage de C60.
Endohedral fullerenes N C60 and N C70 were dissolved in a liquid crystal and investigated by electron paramagnetic resonance EPR . Sharp, liquid solution like spectra are observed. However, contrary to the usual solution spectra, the lines are split, indicating that the electron shell of the enclosed nitrogen atom is deformed and that an alignment with respect to the magnetic field axis is achieved. The present results are of practical interest for controlling the interaction of endohedral fullerenes in a quantum computer
In connection with the realization of a fullerene quantum computer, the construction of linear chains of endohedral fullerenes and their orientation with respect to the external magnetic field is required. We show here that the orientation of these chains can be achieved by embedding them in a liquid crystal matrix. The orientation is demonstrated for the elongated C70 endohedral fullerene.
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 and nuclear 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.
A study of spherical aromaticity of fullerenes was carried out. The fullerenes form a unique class of spherical molecules containing π system. The classical criteria for aromaticity of planar, unsaturated compounds were applied to fullerenes and their derivatives. The nature of fullerene aromaticity considering count rules involving the whole π-electron system and the presence of two-dimensional Hückel-aromatic substructures were also discussed.
Atomic nitrogen was encapsulated inside a vaporized C60 molecule in rf-plasma, which was confirmed by ESR.
N@C60 has a lower photochemical reactivity toward disilirane than C60, although N@C60 does not differ from C60 in its thermal reactivity; theoretical calculations reveal that N@C60 and C60 have the same orbital levels and that N@3C60* has a shorter lifetime than 3C60*.
Unimolecular decomposition of C70(+) and its endohedral cation N@C70(+) were studied by high-resolution mass-analyzed ion kinetic energy (MIKE) spectrometry. Information on the energetics and dynamics of these reactions was extracted. C70(+) dissociates unimolecularly by loss of a C2 unit, whereas N@C70(+) expels the endohedral N atom. Kinetic energy release distributions (KERDs) in these reactions were measured. By use of finite heat bath theory (FHBT), the binding energy for C2 emission from C70(+) and the activation energy for N elimination from N@C70(+) were deduced from KERDs in the light of a recent finding that fragmentation of fullerene cations proceeds via a very loose transition state. The activation energy measured for N extrusion from N@C70(+) was found to be lower than that for C2 evaporation, higher than the value from its neutral molecule N@C70 obtained on the basis of thermal stability measurements, and coincident with the theoretical value. The results provide confirmation that the proposed extrusion mechanism in which the N atom escapes from the cage via formation of an aza-bridged intermediate is correct.
The endohedral fullerene epoxide N@C60O was synthesised, isolated by High Performance Liquid Chromatography (HPLC), and characterised by Electron Spin Resonance (ESR). This nitrogen radical displays predominantly axial symmetry characteristics as expected for a monoadduct, evidenced by a zero-field splitting D parameter of 6.6 MHz and an E parameter of 0.5 MHz in powder at 77 K. Photo- and thermally-activated silencing of the nitrogen radical were observed, the latter showing the evolution of a new spin signal during heating at 100 degrees C. We suggest that loss of nitrogen spin is due to coupling with a radical formed by opening of the epoxide ring. This implies that the reaction of C60O with C60 in the solid state proceeds via a radical, rather than ionic, intermediate.
Using density functional theory within the generalized gradient approximation, I analyzed the electronic structure of a C(60)-ferrocene hybrid [= C(60) (*) FeCp] around HOMO in comparison with that of ferrocene, where C(60) (*) and Cp denote C(60)(CH(3))(5) and a cyclopentadienyl ring. HOMO-LUMO gap is significantly smaller than that of ferrocene because of the intervention of pi(C(60) (*)) states below LUMO. In addition, geometrical and electronic structures of N@C(60) (*) FeCp are also investigated. I find that there are two isomers with the energy difference of 0.13 eV. In one of the two, the encased nitrogen atom is located at the center of the fullerene cage. The Fe atom is eta(5)-coordinated to both Cp and R*, where R* is a five-membered ring of C(60) (*) cage. On the other hand, the atom is coordinated to R* with eta(4)-hapticity, and the nitrogen atom is bonded to a carbon atom of the R* ring in the other isomer. Upon the isomerization between the two isomers, there occurs a partial transfer of spin density between the nitrogen and Fe atoms as well as the creation and breaking of a C-N bond.
By applying a new technique for dynamic nuclear polarization involving simultaneous excitation of electronic and nuclear transitions, we have enhanced the nuclear polarization of the nitrogen nuclei in 15N@C60 by a factor of 10(3) at a fixed temperature of 3 K and a magnetic field of 8.6 T, more than twice the maximum enhancement reported to date. This methodology will allow the initialization of the nuclear qubit in schemes exploiting N@C60 molecules as components of a quantum information processing device.
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 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 automata is frustrated due to the degeneracy of the electronic transitions. However, we can construct a quantum celluar 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 automata operation and obtain a rough figure of merit for the 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 towards meeting the fifth criteria: qubit readout. Comment: 16 pages, Latex, 5 figures, See http://planck.thphys.may.ie/QIPDDF/ submitted to Phys. Rev. A
Dynamic nuclear polarization transfers spin polarization from electrons to nuclei. We have achieved this by a new method, simultaneously exciting transitions of electronic and nuclear spins. The efficiency of this technique improves with increasing magnetic field. Experimental results are shown for N@C60 with continuous-wave microwaves, which can be expected to produce even higher polarization than the corresponding pulsed techniques for electron spins greater than 1/2. The degree of nuclear polarization in this case can be easily monitored through the intensities of the well resolved hyperfine components in the EPR spectrum. The nuclear spin-lattice relaxation time is orders of magnitude longer than that of the electrons.
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.
The stability of geodesic structures was realized and exploited by the architect R. Buckminster Fuller. It was not suspected that such structures are realized on a molecular scale until 1985, when Kroto et al. observed a striking prominence of 60-atom carbon clusters in certain molecular beam experiments.¹ They suggested that C60 possessed the truncated icosahedral geometry of a soccer ball. The C70 mass peak was also prominent, and a series of larger C2n clusters was observed to extend out to the limit of the spectrometer. These observations gave rise to the elegant picture of a family of closed-shell carbon molecules, the fullerenes, consisting of 12 pentagonal and two or more six-membered rings.2, 3 The discovery of new methods to produce and purify macroscopic quantities of fullerenes⁴⁻⁸ has allowed spectacular confirmation of this picture.
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.
We report the first isolation of strontium- and barium-based metallofullerenes and their characterisation by UV-vis-NIR absorption spectroscopy. The absorption spectra of the present samples (Sr/Ba@C82 and Sr/Ba@C84) show some similarity to their calcium-based analogues and are very different from those of yttrium- and lanthanum-based metallofullerenes. Assuming the barium atom to be in the 2+ state, which has an ionic radius of 1.34Å, the Ba-containing species represent the largest ionic radii-containing metallofullerenes so far isolated.
Is host-guest chemistry possible with fullerenes? Yes, but not as easily as once thought in the euphoric first weeks of preparative fullerene chemistry. Although the occurrence of metal-ion inclusion is still controversial, indisputable evidence for inclusion of He-3 and He-4 within C60.+ and C70.+ radical ions has now been obtained. A reaction was initiated by collision of C60.+ and C70.+ ions with H2, D2, Ar, SF6, and He gases, but only He could be included, which agrees with the results of the initial calculations.
The magnetic resonance spectrum of the 4S ground state of atomic nitrogen-14 has been observed using the microwave absorption method. The magnetic-dipole hyperfine interaction constant is A=10.45+/-0.02 Mc/. No evidence was obtained for electric quadrupole interaction.
The results of a variety of experiments used to characterize fullerenes, metallofullerenes and alkali-metal intercalated C60 are reported. It is shown that differential scanning calorimetry and NMR characterization of the orientational phase transition in C60 provide sensitive means to assess the purity and crystallinity of fullerene samples. The results of a mass-spectrometric investigation of metallofullerene samples produced by co-vaporization of carbon and metals are described. An account is given of some electron paramagnetic resonance results obtained for LaC82 and solid-state NMR results obtained on an alkali-intercalated fullerite, Rb3C60, which show for the first time the presence of magnetically inequivalent carbons in underivatized C60. Together these experiments yield a great deal of information about the phase purity, molecular dynamics and structure of a variety of fullerene materials.
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.
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 4He and 3He. The amount corresponds to one 4He for every 880,000 fullerene molecules. The 3He/4He 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 3He and neon by heating fullerene in their presence.
Encapsulating atoms or molecules inside fullerene cages could give rise to a myriad of novel molecules and materials. The existence of such species is now strongly supported by a growing body of experimental evidence. Fullerene& ndash;metal complexes generally thought to be endohedral are being produced and purified in milligram quantities, and their structure and properties are beginning to be explored
Laser vaporization of graphite in a high-pressure supersonic nozzle produced a remarkable stable C60 molecule in high yield with striking and exciting new properties. Specifically it was found that the stability of the new species arises from its unique ability to close into a spheroidal, aromatic molecule in the form of a truncated icosahedron. This should be an exceptionally strong binding site for a wide range of atoms. Evidence is presented for clusters of carbon atoms which form very stable complexes with lanthanum atoms, particularly C60 La. Such aromatic egg-shell complexes of metal atoms may be stable enough to survive in normal condensed-phase chemical environments.
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.
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.
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.
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.