plot. The behavior of the TX(T) data near the structural phase transition temperature T01 ~ 261 K (see §7.1.3) provides a quantitative measure of 701 for highly crystalline samples, as well as sensitive characterization information about the crystalline microstructure of C60 samples [16.5, 18, 24, 28].

The spin-lattice relaxation time 7, is described by the relation rr^ïSe (,61)

where a>0 is the nuclear Larmor frequency, yn is the nuclear gyromagnetic ratio, (h2m) is the time average of the squared local field seen by the nucleus, and r is the correlation time of the precessing nucleus which follows a thermal activation model t = t0exp(Ea/kBT) (16.2)

where Ea denotes an activation energy for this process. Because of the functional form of Tx given in Eq. (16.1), it is easy to determine the value of t at the temperature where Tx is a minimum. For example, measurement of Tx at a frequency of 75 MHz yields a minimum in Tx at 233 K where r ~ l/«0 ~ 10"9 s [16.5]. By varying the resonant frequency «„ (through variation of the externally applied field), the correlation time for the molecular motion can be determined as a function of temperature. The quantity (h2m) is particularly sensitive to the axis of rotation of the molecule during the Tx measurement, since h2 is given by the relation h2 = ^H2A2(i + -n2") (16.3)

where H0 is the resonant magnetic field, A depends of the principal values cr,i of the chemical shift tensor through A = 2[a33 - (trn + a-22)/2]/3, and i] — (<ru — cr22)/A. In fact, the NMR lineshape shown in Fig. 16.4 for C70 is strongly dependent on the effects of the anisotropy of the chemical shift, which is presented schematically in Fig. 16.6 for the cases of a C70 molecule in isotropic rotation (high temperature), uniaxial rotation (intermediate temperature), and static orientation (low-temperature).

The changes in the local magnetic field due to the structural phase transition near Tm (see §7.1.3) are sensitively measured by the temperature dependence of as shown in Fig. 16.5. However, the molecular motion is too fast to provide information about this phase transition from the NMR linewidth itself. Detailed measurements of the temperature dependence of the decay of a saturation pulse below and above Tm have been made for C60 [16.18], yielding Ea ~ 690 K (59.5 meV) for temperatures T > Tm and 2980 K (257 meV) for T < Tm, where Tm is the temperature of the phase transition near 261 K. The observed dependence of 1/71, on the external magnetic field H goes as 1/r, ~ tH2 for r < w^1 and T{ ~ r

C70 molecule NMR Spectrum

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