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EPR investigation of atoms in chemical traps
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Magnetic properties of atoms and molecular clusters encapsulated in fullerenes and other chemical traps are studied using various EPR and ENDOR techniques. From these results exceptional topological and electronic properties of these new compounds can be derived. Making use of the high symmetry at the site of the encased paramagnetic atom, details of the deformation of the spin density distribution resulting from the confinement are deduced. Dissolving N@C60 and N@C70 in a liquid crystal, the order parameter of the solute can be studied by invoking the fine structure interaction of confined nitrogen in its 4S3/2 ground state. Information about the interaction potential locating the neutral atoms at the cage centre is obtained from an analysis of the temperature dependence of the hyperfine coupling constant. Values deduced are in good agreement with vibrational frequencies calculated in harmonic approximation.
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... [21] However, so far, only N@C 60 and N 2 @C 60 complexes have been synthesized. [22,23] Moreover, it was found that the fullerene-based endohedral complexes N n @C 60 conserve their structure at room temperature for n 12. [21] Restricted inner space of the fullerene cage and charge transfer from the guest compound to the cage provide the feasible conditions for stabilization of the nitrogen systems. [21] Highly curved concave inner surface of the fullerene is less reactive than the convex outer surface, [24] and, therefore, it does not react with the nitrogen clusters even during the heating. ...
Using tight-binding molecular dynamics simulations, we study kinetic stability of the nitrogen cubane inside the fullerene carbon cage. The main identified mechanism of N8 decomposition is the N–N bond breaking with the further rapid “splitting” into the N2 molecules that can form the strong covalent bonds with the inner surface of the fullerene cage. It is found that the fullerene cage significantly increases the stability of the N8 cube, but its lifetime at room temperature is insufficient to observe the endohedral complex N8@C60 without the use of extreme temperature conditions. It can be synthesized and investigated only at cryogenic temperatures.
... When trapped in solids, H atom reveals wave function of reduced symmetry adapting to the electrostatic potential of the cage formed by the matrix first couple of coordination spheres. As a consequence, excluding cubic lattice, the hf interaction develops appreciable anisotropy [28][29][30][31]. The excellent linear fit in Figure 3 may be accidental. ...
Motional quantum effects of tunneling methyl radical isolated in solid gases as they appear on experimental electron paramagnetic resonance (EPR) spectra are examined. Obtained analytical expressions of the tunneling frequency for methyl rotor/torsional-oscillator utilizing localized Hermite polynomials are compared to full numerical computations and tested against experimental EPR lineshape simulations. In particular, the X-band of methyl radical was displaying partial anisotropy averaging even at lowest temperatures. EPR lineshape simulations involving rotational dynamics were applied for the accurate determination of the potential barrier and the tunneling frequency. Tunneling frequency, as the splitting between the A and E torsional levels by the presence of a periodic C3 model potential with periodic boundary conditions, was computed and related to the EPR-lineshape alteration. The corresponding C2 rotary tunneling about the in-plane axes of methyl was also studied while both the C2 and C3 rotations were compared with the rotation of deuteriated methyl radical.
In the present work, the results of molecular dynamics simulation of the central scattering of nitrogen atoms by C60 fullerenes with different impact parameters is presented. Three temperatures are considered - 0 K, 200 K, and 300 K to analyze the temperature effect on the adsorption or scattering process. The interaction of fullerenes with nitrogen atoms and behavior of the whole system in the thermal equilibrium are studied. It is possible to estimate important characteristic times of transient processes. It is found that the temperature does not play a key role in the scattering process, and for all the considered temperatures similar dependencies are obtained. The energy of scattered nitrogen atoms is equal to 5 eV which leads to an adsorption of the most of nitrogen atoms by fullerene molecules.
This review highlights the year 2005 in fullerene chemistry and includes the elegant multi-step organic synthesis of H2@C60 from an opened cage aza-thia fullerene derivative. Carbon nanotube research has continued to advance rapidly. Highlights include the impressive use of single-walled carbon nanotubes (SWNT) as delivery agents for short interfering RNA (siRNA) which resulted in gene silencing in mammalian cells. Similarly, effective, specific delivery of boron to, and retention in, tumour cells using substituted carborane-appended SWNT has been achieved which highlights the potential for the future development of new carbon nanotube-based targeted anti-cancer therapies.
The electronic structure of the endohedral fullerene 14N@C60 spatially ordered in the crystal state by its supramolecular assembly with a buckycatcher C60H28 was investigated by X-band electron paramagnetic resonance (EPR) spectroscopy. The zero-field splitting (ZFS) found in the complex is significantly enhanced in comparison to the earlier reported data for the analogous systems. The large ZFS, presumably induced by the molecular order in the supramolecular complex makes it an attractive material for the future quantum control and coherence studies.
The synthesis, characterisation, chemistry and uses of a wide arrange of polyhedral silsesquioxanes of the general formula SinO3/2nRn (where R=H, alkyl, aryl, etc., and n=4, 6, 8, 10 etc.) is described. Much of the discussion is centred on the compounds Si8O12R8 which have become important building blocks for a wide range of materials useful in, for example, nanocomposites, dendrimers, and optical materials. The synthesis of POSS molecules can be carried out from monomeric Si-containing precursors or by modification of more complicated siloxanes, these various methods are described. The physical properties and spectroscopic parameters of many POSS compounds are also described together with a review of computational studies on their structures.
For La@C82 embedded into the polycarbonate film, an intensive H‐electron‐nuclear double resonance (ENDOR) has been revealed. Since La‐EMF does not contain hydrogen atoms, it evidences for the electron spin density on the matrix protons. Furthermore, for the solutions of La‐EMF in hexamethylphosphoramide (HMPA), the paramagnetic shift of the P NMR signal of bulk solvent molecules has been revealed. These findings testify the partial localization of unpaired electron outside the fullerene cage. The “transparency” of the fullerene shell to the electron spin density may serve the important factor in designing the fullerene‐based materials.
The aim of this research was to answer the question whether the area of localization of unpaired electron in a paramagnetic endohedral metallofullerene is restricted by the fullerene shell or a “spin-leakage” beyond the fullerene cage is possible. Herein, we report an ENDOR investigation of La@C82 embedded into the polycarbonate films. The intensive 1H-ENDOR signal has been revealed. Since the La-EMF does not contain hydrogen atoms, this result testifies to the contact hyperfine interaction of the unpaired electron of La-EMF with the matrix hydrogen atoms, i.e. electron spin density on the polymer protons. We also report a NMR investigation of a liquid solution of the same La-EMF in hexamethylphosphoramide (HMPA), molecules of which contain the NMR active nucleus, phosphorus-31. The paramagnetic shift of the 31P NMR signal of bulk HMPA molecules in the presence of La-EMF has been revealed. Thus, much as the charge-transfer complexes, the paramagnetic EMF molecules can form the spin-transfer complexes in which the electron spin density partially localizes beyond the fullerene cage on atoms of the matrix in which the EMF molecules are embedded.
Since endohedral fullerenes exhibit unique features there is growing demand for their production and purification in high quality and quantity. This work shows how the chromatographic enrichment process can be optimized, for the separation of N@C60 and N@C70 from their respective empty parent compounds.
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.
The idea(1) that fullerenes might be able to encapsulate atoms and molecules has been verified by the successful synthesis of a range of endohedral fullerenes, in which metallic or nan-metallic species are trapped inside the carbon cage(2-13). Metal-containing endohedral fullerenes have attracted particular interest as they might exhibit unusual material properties associated with charge transfer from the metal to the carbon shell. However, current synthesis methods have typical yields of less than 0.5%, and produce multiple endohedral fullerene isomers, which makes it difficult to perform detailed studies of their properties. Here we show that the introduction of small amounts of nitrogen into an electric-are reactor allows for the efficient production of a new family of stable endohedral fullerenes encapsulating trimetallic nitride clusters, ErxSc3-xN@C-80 (x = 0-3). This 'trimetallic nitride template' process generates milligram quantities of product containing 3-5% Sc3N@C-80, which allows us to isolate the material and determine its crystal structure, and its optical and electronic properties. We find that the Sc3N moiety is encapsulated in a highly symmetric, icosahedral C-80 cage, which is stabilized as a result of charge transfer between the nitride cluster and the fullerene cage. We expect that our method will provide access to a range of small-bandgap fullerene materials, whose electronic properties can be tuned by encapsulating nitride clusters containing different metals and metal mixtures.
Electron Paramagnetic Resonance (EPR) Volume 17 highlights major developments in this area reported up to the end of 1999, with results being set into the context of earlier work and presented as a set of critical yet coherent overviews. The topics covered describe contrasting types of application, ranging from biological areas such as EPR and ENDOR studies of metalloproteins and evidence of free-radical reactions in biology and medically-related systems, to experimental developments and applications involving EPR imaging, the use of very high fields, and time-resolved methods. Critical reviews of applications involving bacterial photosynthesis, spin-labelling and spin-probes studies of self-assembled systems, and organometallic chemistry are also included. As EPR continues to find new applications in virtually all areas of modern science, including physics, chemistry, biology and materials science, this series caters not only for experts in the field, but also those wishing to gain a general overview of EPR applications in a given area. Specialist Periodical Reports provide systematic and detailed review coverage in major areas of chemical research. Compiled by teams of leading authorities in the relevant subject areas, the series creates a unique service for the active research chemist, with regular, in-depth accounts of progress in particular fields of chemistry. Subject coverage within different volumes of a given title is similar and publication is on an annual or biennial basis.
The 21 Ne, 83 Kr and 131 Xe NMR spectra of these noble gases dissolved in various thermotropic liquid crystals were recorded in the nematic phases at varying temperatures. The spectra display splittings due to the interaction between the nuclear quadrupole moment and the electric field gradient (EFG) at the nuclear site and, consequently, allow one to investigate electric field gradients. The temperature dependence of the EFG is found to be such that the field gradient arising from the electric multipoles of the liquid crystal molecules is not sufficient alone to explain the observations, but one additional contribution has to be taken into account. The quadrupole couplings were analysed in terms of a model based on the contributions from the liquid crystal molecules and from the deformation of the electron cloud due to the collisions of the atoms with liquid crystal molecules. The model provides fairly detailed information on the EFG contributions as well as on the liquid crystal order.
ESR measurements of aqueous solutions of Fe3+ containing various electrolytes were made at X- and Q-band frequencies. It was found that the line shape and integrated intensity depend on the pH and on the type and concentrations of counteranions in the solution. Addition of LiCl and NH4NCS result in reduction in the integrated intensity and subsequent disappearance of the ESR signal. This is interpreted in terms of the formation of monosubstituted complexes which give signals too broad to observe. From the results, formation constants have been calculated. Successive addition of NH4F results in the formation of complexes of the type [Fe(H2O)6−nFn]3−n having different ESR linewidth ranging from 10 to 1000 G. There is a strong reduction in the linewidth on going from X- band to Q-band frequencies. The results are explained in terms of electronic relaxation via the modulation of the zero field splitting (zfs) parameter. From the analysis of the data it was possible to derive values for the zfs interactions and the associated correlation times. The results indicate that the modulation of the zfs interaction is due to collisions between complex and solvent molecules rather than diffusional tumbling.
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.
The Fermi contact hyperfine interaction of hydrogen isotopes in liquid and solid water is below the vacuum value, shows a mass dependence, and has a negative temperature coefficient in the liquid. Furthermore, it shows a pronounced solvent isotope effect in H2O/D2O mixtures. This behavior is analyzed in terms of Dalgarno–Lewis perturbation theory for the atom in a spherical parabolic solvent potential and with a phenomenological model for spin delocalization onto solvent molecules. The results support previous suggestions that hydrogen atoms induce clathrate‐like cages in liquid water. These may resemble the static structures of noble gas clathrates except that they are of transient dynamic nature in liquid water. © 1995 American Institute of Physics.
An E.S.R. line-shape model is developed for fast tumbling three-spin systems in nematic liquid crystals. The line positions are calculated from a spin hamiltonian, which considers Zeeman, exchange, dipolar and hyperfine interactions of the three unpaired electrons. The dominant spin relaxation process, determining the line-widths, is assumed to result from the anisotropy of the zero-field splitting coupled to the molecular motion. The predictions of the theory are tested by comparison with the temperature-dependent E.S.R. spectra of trisverdazyl radicals in 4,4′-azoxydianisole. Good agreement is found between experimental and simulated spectra. A detailed analysis provides values for the solute order parameters [Pbar] 2 and [Pbar] 4. They correspond surprisingly well to predictions of the molecular-field theories of nematic liquid crystals.