Structure Distortion of Fullerenes

We report an electron spin resonance (ESR) study on a C60 anion and a metal (M) encapsulated in fullerene (C ) (a metallofullerene M C ). The anisotropy components of the g-factor of Cg0 were determined accurately from the analysis of angular-dependent ESR spectra of single crystal Cg0 salt. The evaluation of the g-factor was performed according to the classification of symmetry of the C60 geometry. It was found out from the evaluation that the molecular structure of Cg0 should he distorted to lower symmetry, C2h or C,. The variety of ESR spectra of metallofullerenes of La C s was obtained in terms of a g-factor, a hyperfine coupling constant, and a line width. In the case of the isomer I of La C80 and the isomer II of La C84, an abnormally large line width was measured. The molecular structure with high symmetry would reflect on the specific spin dynamics. 

Some fullerenes have a molecular structure with high symmetry [1], It is well known that the lh structure of C60 was experimentally proved by 13C-NMR measurements [2]. Spectroscopic study on fullerenes offers a likely playground for the discussion of molecular symmetry in 3D fashion [3-10]. Two examples of the study on the cage structure distortion of fullerenes are given in this report. 

ADVANCES IN QUANTUM CHEMISTRY, VOLUME 44 ISSN 0065-3276 DOI 10.1016/S0065-3276(03)44020-3 

Our other example is the temperature-dependent ESR study of the series of La C isomers in CS2 solution [9,10], which revealed the influence of the cage structure on the electronic structure of endohedral La ions. The anisotropic ESR parameters, such as A g, A a, and the quadrupole interaction, were determined. The quantitative discussion of these parameters indicated that the electronic structure of all La C could be described as La3+ C. The various values of g close to ge reflected the relative position of the low-lying orbital to that of the tt orbital of the cage. Interesting features such as extra spin dynamics other than those induced by molecular tumbling in solution were suggested for La Cg0-I and La C84-II. The line width at maximum slope u ms of the ESR spectrum was fitted to an expression of the form [15]. 

Here the parameters ge, /rB, coz, /, Mh and rr were the g-factor of the free electron, the Bohr magneton, the microwave frequency of measurement, the nuclear spin quantum number, its projection, and the molecular rotational correlation time in solution. The anisotropic parameters, A g, A a and eQVj 1(21 — 1) were estimated from line width coefficients, K, K2 and K4, respectively. Thus the analysis of the temperature dependences of coefficients K, K2 and K4 gave the anisotropic parameters, A g, A a and eQV/1(21 — 1) for all molecules. On the other hand, 

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It was discovered, however, that the spherical aromaticity of the icosahedral fullerenes C20, Cjq and CgQ depends on the filling of the Jt-sheUs with electrons [107]. As pointed out in Section 14.3.1 no distortion of the cage structure is expected in these fullerenes if their shells are fully filled. Closed-shell situations are realized if the fullerene contains 2(N -1-1) Jt electrons. This is closely related to the stable noble-gas configuration of atoms or atomic ions [108]. In this case the electron distribution is spherical and all angular momenta are symmetrically distributed. Correlation of the aromatic character determined by the magnetic properties is shown in Table 14.3. 

Molecule C2o is the smallest one from all the fullerenes [9] and has the form of dodecahedra (point symmetry group Yh). We consider here only polymerized structures (clusters) which are formed by the pairs of bridge like bonds directed along molecules second order axes. The clusters formation is accompanied by the distortion of the geometry of molecules that leads as sequence to decreasing the symmetry both molecule and cluster (for example the symmetry group of cluster (C2o)s is only D2h). 

Based on the results of calculations of the vibrational spectrum it is possible to estimate the influence of fullerene defects on the properties of the superconductive phase. As was mentioned above, Tc is proportional to the logarithmic average of normal vibrational modes. Because considered defects only locally distort the structure of the Ceo shell, in the first approximation it is possible to consider coupling constants unchanged and therefore the exp(-l/A,) component as a constant. 

Let us note in addition that the layered sulfides M0S2 and WS2 have been found to form nanotubes and other fullerene-type structures, on account of their highly folded and distorted nature that favors the formation of rag and tubular structures. Such materials have been synthesized by a variety of methods [78] and exhibit morphologies, which were described as inorganic fiillerenes (IF), single sheets, folded sheets, nanocrystals, and nested IFs (also known as onion crystals or Russian dolls ). 

One of the clearest cases is obtained for a salt of bis(triphenylphosphine)-iminium ion (PPN+) and Cf(0. It was found [10] that the distorted fullerene molecule exhibits, as can be seen in Fig. 4, an axial elongation with a rhombic squash, in good agreement with the type of JTD described previously. While in the ideal icosahedral structure the distances of the carbon atoms from the center of the molecule are exactly equal, in the distorted structure there is a spread of 0.04 A around the mean value of 3.542 A. 

A very similar irridium complex is of particular interest because it has been attached both to Ceo and to C70, and x-ray structures published.[Ba91a, Ba91b] C70 has four distinct C-C bonds at the fusion of two six-membered rings, in contrast to the case of Ceo, where aU 6 6 bonds are symmetry-equivalent. The Ir complex attaches only to one of these bonds, apparently the one which can be deformed away from the fullerene with minimal added distortion. 

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