N. Weiden

Electron paramagnetic resonance study of atomic phosphorus encapsulated in [60]fullerene

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.

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

Abstract 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.

EPR investigation of atoms in chemical traps

Abstract 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.

Pulsed EPR and ENDOR investigation of hydrogen atoms in silsesquioxane cages

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.

Preparation and EPR characterization of N@C60 and N@C70 based peapods

Using the quartet spin of encased nitrogen atoms as an electron paramagnetic resonance (EPR) probe, it is possible to examine the fullerene/nanotube interactions in a peapod. A purification method is developed which allows low temperature filling of nanotubes with endohedral fullerenes. The paramagnetic impurities of undoped single wall carbon nanotubes (SWNT) are characterized via EPR resulting in a broad superparamagnetic signal of the remaining catalyst particles and a rather narrow signal of carbonaceous material. Comparison of EPR spectra of several nitrogen endohedral doped peapods with their analogues obtained in a solid fullerene matrix shows a significant broadening of N@C60 and N@C70 EPR signals. This broadening is related to a non‐vanishing zero‐field splitting caused by deformation of the fullerene cage upon encapsulation.

Light-induced release of nitrogen from fullerene cages

The release of atomic nitrogen from fullerene cages under irradiation with ultraviolet laserlight has been studied. The experiments show that there is a more favorable decay pathway for N@C60 than for N@C70, indicating that the lifetime of the metastable triplet state is not controlling the escape.

EPR investigation of N@C70 in polycrystalline C70 and single wall carbon nanotubes

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.

Squeezing and shielding of atoms in fullerene traps

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

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

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

EPR studies of N@C60 and its adducts

Atomic Nitrogen in its quartet ground state can be encapsulated in C60 and its derivatives of reduced symmetry. Three endohedral fullerenes were studied in solid matrices for a determination of the permanent zero-field-splitting tensors. In solution the lowering of the site symmetry by collision-induced deformations of the fullerene cage apparently leads to a time dependent zero-field-splitting of the quartet spin state. The variance of the zero-field-splitting and the correlation time of this interaction are estimated from the absolute values of T1 and T2.