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In this study, first-row transition metal-doped calcium oxide materials (Mn, Ni, Cr, Co., and Zn) were synthesized, characterized, and tested for the OCM reaction. Doped carbonate precursors were prepared by a co-precipitation method. The synthesis parameters were optimized to yield materials with a pure calcite phase, which was verified by XRD. EPR measurements on the doped CaO materials indicate a successful substitution of Ca

Using pulsed electron paramagnetic resonance (EPR) techniques, the low-temperature magnetic properties of the NO radical being confined in two different modified open C60-derived cages are determined. It is found that the smallest principal g value g3, being assigned to the axis of the radical, deviates strongly from the free electron value. This behaviour results from partial compensation of the spin and orbital contributions to the g3 value. The measured g3 values in the range of 0.7 yield information about the deviation of the locking potential for the encaged NO from axial symmetry. The estimated 17 meV asymmetry is quite small compared to the situation found for the same radical in polycrystalline or amorphous matrices ranging from 300 to 500 meV. The analysis of the temperature dependence of spin relaxation times resulted in an activation temperature of about 3 K, assigned to temperature-activated motion of the NO within the modified open C60-derived cages with coupled rotational and translational degrees of freedom in a complicated three-dimensional locking potential.

A series of Na 2 WO 4 /Mn/SiO 2 catalysts supported on mesoporous silica were prepared and tested for the oxidative coupling of methane in chemical looping experiments. The parameter variations were done for specific surface area, Mn-loading and support material. Chemical looping experiments reveal that highest catalytic activity is observed at 2 wt% Mn and 5 wt% Na 2 WO 4 on mesoporous silica. Repetitive methane pulse experiments allowed us to determine the oxygen uptake of the Na 2 WO 4 /Mn/SiO 2 catalyst in chemical looping experiments. We found a correlation between oxygen storage capacity and manganese loading, which is in line with the reversible change of Mn(III) concentration observed in oxidation and reduction cycles of the material. Furthermore, we observed that ethylene formation is independent of the catalyst material in chemical looping conditions. No indication for the presence of W(V) was found.

Doping of Li/MgO with additional metal oxides is suggested leading to an improved system with respect to the catalytic performance and stability when used for oxidative coupling of methane. We used Gd and Fe as dopants and characterized the resulting materials, showing that Fe seems to be completely and Gd partly incorporated into the MgO lattice. The catalytic performance is improved in most cases, but all materials still suffer from severe deactivation. A loss of Li is observed when being used under reaction conditions, but this Li loss is retarded for Fe-Li/MgO as compared to undoped Li/MgO.

Our recent studies revealed that the role of Li in Li/MgO as a catalyst for oxidative coupling of methane (OCM) is to promote MgO surface morphology change rather than serve as a constituent of catalytically active sites. Indeed, while Li/MgO is unstable at realistic conditions with respect to the loss of Li, the resulting samples show enhanced selectivity towards C2 hydrocarbons versus CO2, although the activity is low and close to pristine MgO. We explore how co-doping with alio-valent metal ions affects the catalytic performance of Li/MgO. To analyze the structure and the stability of the samples, catalysts with well-defined stoichiometry were prepared. Gd and Fe were used as dopants. These dopants are anticipated to stabilize Li in the catalyst at the reaction conditions, which allows to further clarify the role of Li. In the case of Gd/Li co-doping, changes observed in EPR and 7Li-NMR spectra indicate the formation of correlated, next-neighbor Li+.. Gd3+ pairs, co-existing with "isolated" Gd3+ ions at octahedral Mg lattice sites. For Li/Fe co-doping, no significant change in the EPR pattern is observed in the presence of Li+ ions, indicating a larger distance between the Li+ and Fe3+ cations in the MgO lattice. Hybrid DFT calculations explain the dfference between Fe and Gd co-doping by a less efficient screening of the Coulomb repulsion between Gd3+ and neighboring cations in Gd-doped samples, leading to the stabilization of LiMg near GdMg.

The effective spin of paramagnetic centers generated during the reaction of a mixture of CH4 and O2 on activated MgO was determined with transient nutation EPR (TN-EPR). For the first time it could be shown that apart from the generation of surface-trapped superoxide radicals O2–, a significant number of paramagnetic centers are formed with an effective spin larger than 1/2. Results from HYSCORE and other pulsed EPR experiments further give evidence that these centers are localized at the surface of the MgO catalyst. These centers are proposed to be exchange coupled clusters of holes and electrons, respectively, some of them formed at specific exposed sites of the highly structured MgO surface. Judging from the relative high concentration of the centers ascribed to hole clusters, it is tempting to assume that they are involved in the catalytic reaction network of the oxidative coupling of methane.

The magnetic properties of Mn–Na2WO4/SiO2, a promising catalyst for the oxidative coupling of methane (OCM), were investigated in two states: reduced with CH4 until reactivity ceased and reoxidized with O2 to probe for state-specific magnetic species and their involvement in the oxygen storage of the catalyst. Employing temperature- and frequency-dependent continuous-wave (cw) and pulsed electron paramagnetic resonance (EPR) spectroscopy combined with SQUID (Superconducting Quantum Interference Device) magnetization measurements allowed us to identify a variety of Mn species in different oxidation states and their role in the oxygen storage capability of the catalyst. For the reoxidized catalyst, the formation of magnetically ordered Mn(II)- and Mn(III)-containing Mn3O4 as well as Mn2O3 or MnMn6SiO12 phases were detected. The reduced catalyst exhibits almost ideal paramagnetic behavior and a strong, broad cw EPR signal consistent with the formation of short-range ordered nanosized Mn(II) oxide, demonstrating the involvement of these Mn species in the oxygen storage capability of the catalyst. In contrast, rather isolated, highly oxidized Mn(IV) as well as different Mn(III) species were observed by pulsed EPR, which are not affected by the oxidation state of the Mn–Na2WO4/SiO2 catalyst, suggesting an inaccessible, buried location, presumably in the SiO2 support. Furthermore, paramagnetic sites with an effective S = 1/2 spin are detected, whose intensity depends on the oxidation state of the sample and are thus involved in the oxygen storage capacity of the catalyst.

Using pulsed EPR and ENDOR, the full set of g matrix and vanadium hyperfine parameters of the persistent paramagnetic V4+ center in “as prepared” and H2 reduced SBA-15 supported VOx catalysts has been measured. The determination of relative signs of the vanadium hyperfine tensor elements by ENDOR using orientation selection allowed an unambiguous extraction of the isotropic part of this interaction. This allowed for identification of the persistent V4+ center as a surface exposed deprotonated vanadium site. The same site topology was found for oxidized and H2 reduced catalysts, thus indicating that the identified sites represent catalytically active centers. Hyperfine interaction with distant protons indicates formation of an oligomeric structure even for samples with vanadium loadings of less than 2 wt %. This conclusion is confirmed by applying 2D EPR for measuring the hyperfine interaction with neighboring vanadium atoms, covalently linked to reduced V4+ sites. Hence, application of 2D EPR enabled us t...

The time-window for processing electron spin information (spintronics) in solid-state quantum electronic devices is determined by the spin–lattice and spin–spin relaxation times of electrons. Minimizing the effects of spin–orbit coupling and the local magnetic contributions of neighbouring atoms on spin–lattice and spin–spin relaxation times at room temperature remain substantial challenges to practical spintronics. Here we report conduction electron spin–lattice and spin–spin relaxation times of 175 ns at 300 K in 37±7 nm carbon spheres, which is remarkably long for any conducting solid-state material of comparable size. Following the observation of spin polarization by electron spin resonance, we control the quantum state of the electron spin by applying short bursts of an oscillating magnetic field and observe coherent oscillations of the spin state. These results demonstrate the feasibility of operating electron spins in conducting carbon nanospheres as quantum bits at room temperature., Electronic decoherence due to spin-orbit and magnetic interactions limits the application of spintronic nanosystems in quantum information processing. Here, the authors report notably long spin-lattice and spin-spin relaxation times of 175 ns at room temperature in carbon nanospheres.

Li-doped MgO is a potential catalyst for the oxidative coupling of methane, whereby surface Li + O - centers are suggested to be the chemically active species. To elucidate the role of Li in the MgO matrix, two model systems are prepared and their morphological, optical and magnetic properties as a function of Li doping are investigated. The first is an MgO film deposited on Mo(001) and doped with various amounts of Li, whereas the second is a powder sample fabricated by calcination of Li and Mg precursors in an oxygen atmosphere. Scanning tunneling and transmission electron microscopy are performed to characterize the morphology of both samples. At temperatures above 700 K, Li starts segregating towards the surface and forms irregular Li-rich oxide patches. Above 1050 K, Li desorbs from the MgO surface, leaving behind a characteristic defect pattern. Traces of Li also dissolve into the MgO, as concluded from a distinct optical signature that is absent in the pristine oxide. No electron paramagnetic resonance signal that would be compatible with Li + O - centers is detected in the two Li/ MgO samples. Density-functional theory calculations are used to determine the thermodynamic stability of various Li-induced defects in the MgO. The calculations clarify the driving forces for Li segregation towards the MgO surface, but also rationalize the absence of Li + O - centers. From the combination of experimental and theoretical results, a detailed picture arises on the role of Li for the MgO properties, which can be used as a starting point to analyze the chemical behavior of the doped oxide in future.

Sealed, deoxygenated single-wall carbon nanotubes show two characteristic electron paramagnetic resonance (EPR) signals at g = 2.07 and g = 2.00 in the temperature range from 300 to 50 K. Reversible interconversion between both components was observed. The large g-shift and the temperature dependence of the EPR susceptibility of the g = 2.07 signal indicate that this signal can be attributed to itinerant spins. At low temperatures only the g = 2.00 signal remained, which could be further characterized using microwave frequencies up to 320 GHz. The direct current conductivity of a partially aligned sample was also measured. The room temperature value was estimated as 0.7 (Ωcm)−1. The observed temperature dependence can be described by assuming temperature-activated hopping in a small-gap semiconductor with an activation energy of 3.5 meV, similar to the characteristics of the previously measured 9.4 GHz microwave conductivity.

Using a resonator-free setup, pulsed high-frequency (240 GHz) electron paramagnetic resonance (EPR) experiments on gadolinium-doped PbTiO3 and PbZrO3 samples have been performed. It could be demonstrated that echo-detected EPR spectra can be recorded routinely from these materials. These compounds are highly absorptive at microwave frequencies, thus preventing the use of microwave resonators at very high frequencies. As echo-detected EPR allows us to record the EPR absorption directly, the effect of relative suppression of broad unstructured spectral components in conventional field-derivative EPR is avoided. The analysis of the high-frequency EPR spectra indicated that Gd3+ ions are additionally also positioned at highly distorted sites. This might indicate that charge compensation leads to the formation of closely correlated Gd3+-V Pb′′-Gd3+ defects under high doping conditions in addition to Gd3+ inserted substitutionally at Pb2+ sites with undistorted oxygen and lead neighboring shells. For the orthorhombic low-temperature phase of PbZrO3 two crystallographically inequivalent Pb2+ sites of equal abundance are present. The contribution of Gd3+ inserted substitutionally at these sites could be confirmed.

Supported vanadium oxide catalysts were studied by high-frequency EPR for determination of the paramagnetic vanadium oxidation states vanadium(IV) and vanadium(III). TiO2-, γ-Al2O3-, and SiO2 (SBA-...

In addition to = 2.00 signals seen frequently in EPR spectra of SWNT, signals at g = 2.07 of SWNT prepared by CVD were detected, exhibiting a Pauli susceptibility temperature dependence. This Pauli magnetism in combination with the large g shift is taken as evidence that these signals originate from itinerant electrons of metallic nanotubes. At temperatures below 150 K, a dominant narrow signal develops at 9 = 2.00. By applying multi-frequency EPR i ip to 319 GHz, its inhomogeneous nature was confirmed. This signal is assigned to defects of the carbon network of the tubes. Comparing room temperature EPR spectra of CVD arid arc-grown SWNT, we found a much lower concentration of metallic tubes in arc material. No =" 2.07 signals of itinerant spins could be observed, which might be also caused by the high amount of residing catalyst. A drastic increase in nonresonant microwave absorption is observed below lOKforbothtypesof samples, if a threshold microwave power level is passed. In the same temperature range a drop in EPR intensity is also detected. These observations are taken as evidence for a transition into a superconducting phase of part of the sample.

Microwave absorption (MA) is applied to study the electromagnetic properties of bulk samples of single-walled carbon nanotubes (SWNT). Using resonant MA (electron paramagnetic resonance) and also its non-resonant variant (cavity perturbation) it was possible to reveal different electronic phases of SWNT. Besides the existence of doped semiconducting nanotubes with an activation energy of 67 meV, also small-bandgap semiconducting tubes with a gap-energy of 7 meV could be detected. Additionally, at temperatures below 12 K an unconventional loss mechanism occurs. The properties of this non-linear absorptive state give evidence for the occurrence of superconducting domains in the samples.

Single wall carbon nanotubes (SWNT) prepared by the "super growth" method exhibit electron paramagnetic resonance (EPR) signals, which originate from temperature activated detrapping of spins. Non-resonant microwave absorption in the temperature range below 20 K is indicative for the opening of a small gap at the Fermi level for tubes of metallic character, which might indicate a transition into a superconducting (SC) state. The expected concurrent microwave field inhomogeneity has been detected by a transient nutation experiment.

Using multi frequency electron paramagnetic resonance (EPR) it was possible to determine the inherent fine structure (FS) interaction of nitrogen encapsulated in a C70 cage. Confinement of N@C70 in single wall carbon nanotubes (SWNT) of rather wide diameter does not significantly influence this value. The transition between isotropic molecular tumbling and immobilization was found to occur at about 170 K in both matrices, indicating that the hindrance potential is quite small in both cases.

A semiquantitative treatment of hyperfine sublevel correlation spectra of aS=1/2,I=1 spin system is presented. The derived closed analytical expressions allow the determination of components of the dipolar and quadrupolar coupling tensors. The method is applicable if nonsecular terms can be neglected and if canonical orientations of the hyperfine interactions can be selected via a dominantg matrix or hyperfine anisotropy.

Copper(II)-doped Pb[Zr0.54Ti0.46]O3 (PZT) ferroelectrics with a dopant concentration of 0.25 mol % were investigated at high magnetic fields, enabling an enhanced resolution of the structural distortion at the dopant site. The results obtained suggest that Cu2+ substitutes as an acceptor centre for [Zr,Ti]4+ in oxygen octahedra with tetragonal, monoclinic and rhombohedral distortion, confirming the model of mesoscopic mixing at the morphotropic phase boundary.

A comparative study of C60-based endohedral fullerenes was performed. From an analysis of spin relaxation times of the recently characterized Cu@C60 metallo-endofullerene, it could be shown that the encased ion occupies a well-defined off-centre position even at room temperature. Localization on the timescale of an EPR experiment is combined with charge transfer of approximately two electrons, resulting in the EPR signature of a 3d9 copper ion. This is in contrast to N@C60, for which a nearly complete decoupling from the cage is found. An upper limit for the variance of collision-induced modulation of the anisotropic nitrogen hfi is determined as (A(eff)2)(1/2) < 500 kHz by an analysis of cross peak intensities in a 2D EPR experiment. Pulsed 95 GHZ ENDOR experiments have been performed to confirm that deformation-induced hyperfine anisotropies are expected to be very small in N@C60.

The intrinsic iron(III) impurity center in polycrystalline lead titanate was investigated by means of high-frequency electron paramagnetic resonance (EPR) spectroscopy in order to determine the local-environment sensitive fine structure parameter D. At a spectrometer frequency of 190 GHz, spectral analysis of a powder sample was unambiguously possible. The observed mean value D = +35.28 GHz can be rationalized if Fe3+ ions substitute for Ti4+ at the B-site of the perovskite ABO3 lattice forming a directly coordinated iron - oxygen vacancy defect associate. A consistent fit of the multi-frequency data necessitated use of a distribution of D values with a variance of about 1 GHz. This statistical distribution of values is probably related to more distant defects and vacancies., 6 pages, 3 figures, 1 table, to appear in J. App. Phys, 96 (2004)

The analysis of dipolar and quadrupolar lanthanum hyperfine data measured with electron paramagnetic resonance (EPR) and electron nuclear double resonance reveals that at low temperatures no significant change occurs at the internal binding site of the endohedral complex. We interpret this result as indicative of freezing of the large-scale motion of the encased ion, which is observed at room temperature. Averaging of hyperfine interactions is fast on the time scale of the EPR experiment, preventing drastic changes of dipolar and quadrupolar hyperfine interaction (hfi), providing that the equilibrium position is unchanged. The detection of hfi in disordered samples was possible by invoking orientation selection in the 94-GHz EPR spectrum. Quadrupolar hfi could be directly measured for the first time in a metallo-endohedral fullerene complex.

Spin density distributions and isotropic hyperfine coupling constants of N@C 60 , P@C 60 , N@C 70 , N@C 60 (CH 2 ) 6 , and N@C 60 (SiH 2 ) 6 are theoretically investigated by testing B3LYP and MP2 methods for the free N and P atoms. When N is encapsulated inside C 60 , the calculated spin density at nucleus N increases. In P@C 60 , the corresponding increase is much larger. N@C 60 has a slightly larger spin density at N than N@C 70 . These agree well with the observed isotropic hyperfine coupling constants. Upon addition of CH 2 groups on the N@C 60 cage, the spin density at N is slightly decreased. However, addition of SiH 2 groups increases significantly the spin density at the nitrogen nucleus.

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.

High sensitivity and spectral resolution provided by pulsed electron paramagnetic resonance and electron nuclear double resonance techniques at high Larmor frequencies open the way for a study of atoms in chemical traps. As an example we studied deuteron atoms encased in silsesquioxane cages to probe the cage symmetry as function of temperature. An analysis of the temperature dependence showed that the system undergoes a structural phase transition near 100 K. At this temperature the character of distortion of the ideal cubic symmetry changes from oblate to prolate (or vice versa). With quantum chemical methods, a model for cage escape of the encased atom could be derived. The calculated escape barrier of 0.9 eV is close to the experimental value derived by thermal release experiments. Although the encased deuterium atom exhibits an isotropic hyperfine coupling constant nearly identical with the free atom value, a spin population analysis revealed that approximately 10% of the spin density is transferred to the cage. We therefore conclude that confinement of the hydrogen atom leads to a compression of its wave function compensating the decrease of spin density. In this respect the system falls somewhat short of the properties of an ideal cage, being defined by well decoupled atomic and molecular wave functions.

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.

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.

By performing high-resolution EPR and ENDOR experiments on nitrogen atoms encapsulated in C 60 , 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 C 60 . 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 13 C 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.

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 sp...

The synthesis and EPR spectroscopic investigations of a family of six endohedral fullerenes, namely, N@C60 (1), N@C61(COOC2H5)2 (2), N@C66(COOC2H5)12 (3), N@C66(COOC2D5)12 (4), N@C61(COOC2D5)2 (5), and N@C70 (6), containing atomic nitrogen in the 4S3/2 ground state is described. The parent systems N@C60 (1) and N@C70 (6) were synthesized by nitrogen ion implantation. The syntheses of the C2v-symmetric monoadducts 2 and 5 and the Th 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 C60. 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 monito...

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 electronic structure of endohedral Sc@C82 with a C82 cage of C2v symmetry has been studied by ab initio Hartree-Fock (HF) calculations. The optimized position of Sc in the configuration of minimum energy is predicted to be on the two-fold axis of the fullerene cage. In the corresponding configuration Sc is above the center of a hexagon of site symmetry C2 . This structure of C2v symmetry is nearly degenerate with Cs and Ct structures with Sc slightly displaced from the center of the coordinated hexagon. The binding energy of the endohedral complex is larger than 3.55 eV. The ab initio HF data of the C2v topoisomer of the fullerene unit are compared with new experimental findings and HF results derived for the 3(C2) topoisomer of the C82 cage.

The electronic structure of endohedral Sc@C 82 has been studied by different computational techniques. To optimize the geometry of Sc@C 82 and the bare C 82 cage, ab initio Hartree-Fock (HF) calculations in the minimal STO 3G basis have been performed. The split-valence 3–21G ∗ basis set has been used to derive the Sc binding energy and atomic net charges. The optimized position of Sc is strongly off-center. The coordination number of the endohedral atom is six. Sc is located above the 66 bond of a pyracylene unit in the pocket region of C 82 a geometrical arrangement which minimizes the ScC contacts to the carbon atoms of the second coordination sphere. The spatial reorganization process of the carbon atoms in the transition from C 82 to Sc@C 82 are compared with spatial reorganization processes encountered in the transition from pyracylene C 14 H 8 to the corresponding dianion C 14 H 8 2− and from icosahedral C 60 to the dodeca anion C 60 12− . The excess electrons in the corresponding systems lead to a suppression of the bond-length alternation in the pentagon fragments. The π electronic structure of the different systems has been described by simple ‘resonance’ structures. Furthermore, we have discussed the 13 C and 45 Sc hyperfine interaction (hfs) in Sc@C 82 . The 13 C hfs data have been simulated by local spin density and INDO calculations. The present findings are compared with experimental observations and the results of other computational studies.

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 250 K, a thermally activated process, which is tentatively attributed to a hopping 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 a high 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 a fluctuating zero-field splitting, sensed by the quartet spin state of the central encapsulated nitrogen atom.

An analysis of the temperature dependence of the line width variation of topoisomers (I) and (II) of La@C82 revealed that both molecules exhibit practically identical rotational correlation times from room temperature down to the freezing point of CS2. Although this result indicates confinement of the metal ion, from the increased line width of13C satellites in isomer (II) we conclude that in this molecule there might be increased mobility within a confine volume. Three stable topoisomers of La@C90 were identified for the first time. One of these topoisomers was separated by HPLC technique. We were unable, however, to detect any nuclear spin dependence of the EPR line width even at low temperatures. This might indicate that in this case the ion is no longer confined to a specific binding site, although a more trivial explanation assuming small anisotropies of all hyperfine interactions cannot yet be ruled out.

Using pulsed EPR techniques, the electron spin relaxation properties of nitrogen atoms encapsulated in C 60 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@C 60 are the dominant spin relaxation mechanism.

Photochemically induced electron transfer in homogeneous systems (using triethylamine donor) and heterogeneous systems (using photoexcited TiO2 suspension) was applied in in situ reduction of [60]fullerene. The anion radicals generated were characterized by means of EPR and VIS/near-IR spectroscopy. Narrow EPR lines were found. Radical A with gA=2.0000 and peak-to-peak width, ppA=0.09mT was observed as the primary product; followed by its consecutive product B with gB=2.0006, ppB=0.04mT, and in some cases product C with gC=2.0009 and ppc

C60 is photochemically reduced in the presence of tetraphenylborate anion to C60 −. The primary products A and B are characterized with narrow EPR lines (< 0.1 mT), and then, consecutively, radical C with a 3 mT wide EPR line is observed, which correlates with the formation of a sediment.

[60]-Fullerene and its four ortho-quinodimethane adducts, one of them in oligomeric form, were reduced by three different methods, (a) by cathodic reduction, (b) in photoexcited TiO2 suspension, and (c) quenching photoexcited fullerene by triethylamine donor. The radicals obtained were mainly investigated by EPR and also characterized by vis—near-IR measurements. All three methods of generation gave two identical radical products: the primary radical A, which is a fullerene mono-anion, with EPR spectral parameters gA = 2.0000 and peak-to-peak width ppA = 0.09 mT for [60]-fullerene, ppA = 0.105 for its adducts and ppA = 0.19 mT for the fullerene oligomer. The near-IR band of A at 1077 nm for [60]-fullerene shiftd into the region of 1010 nm for its ortho-quinodimethane adducts. As a consecutive product of A, radical B was found with gB = 2.0006 and ppB = 0.07 mT for all [60]-fullerene adducts. The EPR signal of B converts to A after stopping photogeneration, resulting in a total increase of the spin susceptibility in the form of fullerene mono-anion A. The formation of fullerene di-anion or fullerene associates is considered as an alternative.

By comparing nuclear spin relaxation rates detected via EPR linewidth variations for La@C82 and Sc@C82, the nuclear quadrupole interaction of the scandium endohedral fullerene was determined as eQq/(I(21−1)h)=0.5(2) MHz. This small value could indicate that the oxidation state of the central ion is equal for both molecules in contrast to theoretical predictions.

Parts of the potential energy surface of endohedral Y@C 82 have been studied in a three-dimensional configuration space confined to the spatial degrees of freedom of Y. The optimized position of the endohedral atom is strongly off-center and corresponds to a pyracylene topology with Y located above a 6-6 bond connecting two pentagons. The calculated minimum is explained by the pocket structure of the C 82 cage and the high acceptor capability of fullerence pentagons. We have employed a Hartree-Fock ab initio approach defined by a split-valence 3-21G double zeta basis to search for the optimized position of Y embedded in a C 82 shell of C 2 symmetry. The present computational results are compared with experimental data and findings of other quantum chemical calculations.

The phosphorescence spectrum of [70]-fullerene in a glassy toluene matrix has been investigated at 77, 4.2 and 1.2 K. At 77 K, only one decay time (47.0 ms) has been observed while at 4.2 and 1.2 K two decay times (46.7, 5.6 ms and 46.6, 24.2 ms, respectively) were recorded. The zero field splitting parameters |D|=0.00489(25) cm−1 and |E|=0.00135(7) cm−1 were measured at 1.2 K using a pulsed ODMR technique.

Paramagnetic centers generated by swift heavy ion irradiation of LiF crystals could be identified as electrons trapped at regular anion vacancy sites (F centers). Well-resolved electron-nuclear double resonance (ENDOR) spectra resulting from the hyperfine interaction with 7Li and 19F nuclei located in six different shells could be recorded. In order to preserve the millimeter- sized crystals, a cavity-free setup was used for the ENDOR experiments at an electronic Larmor frequency of 240 GHz. Apparently even under conditions of extremely high local energy loss in the ion track, the local density of persistent F centers is still sufficiently low to prevent distortions of the ionic crystal. The spread of hyperfine coupling constants was less than 5 %. Neither in electron paramagnetic resonance (EPR) nor in ENDOR spectra there was evidence for different types of paramagnetic centers. When performing ENDOR by applying the radiofrequency pulse directly after the 3-pulse Mims- type microwave sequence, an anomalous ENDOR effect was observed. The observed “positive” and “negative” ENDOR response can be attributed to efficient hole and anti-hole formation in the inhomogeneously broadened EPR spectrum and can be used to determine the sign of hyperfine coupling constants.

The analysis of a two-dimensional EPR spectrum gives direct evidence for a dominant nuclear quadrupole interaction at the site of the encapsulated La3+ ion. From the observed nuclear spin transition rates we deduce eQVzz/[I(2I − 1)h] ≈ 0.5(3) MHz, assuming that the correlation time of the electric field gradient axes is determined by molecular reorientation of the molecule in solution. The non-vanishing value of the quadrupole interaction is further proof for an off-center position of the metal ion.