Anne-Sophie Grell

Benzene-solvated C60:1H nuclear spin-lattice magnetic relaxation

Abstract Crystalline C 60 .4C 6 H 6 was studied by NMR. Using a saturation-recovery method, the spin-lattice relaxation time, T 1 , of the protons of benzene was measured as a function of temperature between 4 K and 300 K. The 1 H data were fitted by a model where four correlation times are present. They correspond to thermally activated processes which follow an Arrhenius law: the activation energies are, respectively, 60 K, 279 K, 1365 K and 2945 K. All constituents of our system are extremely mobile and no structural phase transitions are observed near 260 K and 90 K as in the case of pure C 60 crystals. We have also studied the temperature dependence of the 1 H NMR linewidth which narrows with increasing temperature.

Structural and motional features of benzene-solvated C60 as revealed by high-resolution solid state NMR

Abstract Structural and motional features of C 60 · 4C 6 H 6 solvate has been investigated by high-resolution 13 C solid-state NMR spectroscopy. 13 C line shape analysis of the cross-polarization spectrum for the static sample at 293 K shows an unusual motional restriction of benzene molecules in this solvate. The existence of highly anisotropic motion of benzene fraction is visualized also by the detection of important homo and heteronuclear dipolar interactions. 13 C magic-angle spinning spectra reveal a complex character of the C 60 NMR signal composed of three components with slightly different isotropic chemical shifts indicating the existence of magnetically nonequivalent positions or sites.

NMR study of C60⋅C7H8 by cross polarization and polarization inversion of rare-spin magnetization

Solid state NMR spectroscopy has been used to study the molecular dynamics of the toluene-solvated C60 compound, C60⋅C7H8. From the 1H→13C cross-polarization experiments (CP), we have obtained the cross polarization time, TIS=12.6 ms for C60 and 1H spin-lattice relaxation time in the rotating frame, T1p=170 ms. By using the cross polarization inversion method (ICP), we have deduced that all toluene molecules rotate (as seen by NMR) around the axis perpendicular to the cycle and are strongly oriented at an angle of 65° between the rotation axis of the toluene molecules and the direction of the principal magnetic field.

Molecular orientational dynamics in C70S48: Investigation by 13C MAS NMR

At room temperature the MAS 13C NMR spectrum of C70S48 is identical to that of pure C70 above 323 K, except that the 13C line is shifted by 1.7 ppm compared to that of pure C70. From these results, we deduce that our system is mainly of the van der Waals type. A simulation of the low speed MAS spectrum shows that C70 molecules in C70S48 undergo a uniaxial rotation as in pure C70. This new result contradicts what had been previously published. The chemical shift of the 13C line does not vary with temperature, however the rotation of C70 slows down as the temperature is decreased and stops at ca. 150 K. Moreover the 13C spin-lattice relaxation time, T1, can be described by a single correlation time that follows an Arrhenius law with a 900 K activation energy. By NMR no phase transition is observed at 245 K contrary to dielectric relaxation measurements.