C-13-]H-1 CROSS-POLARIZATION NMR IN SOLIDS AT NATURAL C-13 ABUNDANCE

We discuss the features of cross-polarization (CP) between dilute spin systems with substantial molecular motion and discuss the applicabili ty of two different theories for the description of cross-polarization in toluene-solvated fullerene-70. The material was examined by C-13 - -> H-1 and H-1 --> C-13 CP NMR with magic-angle spinning (MAS). C-13 - -> H-1 CP was set on perdeuterated glycine. Both solids were at the na tural C-13 isotopic abundance and with ca. 2% H-1 content, so that C-1 3 and H-1 spin species were both dilute. We have found that CP between the fullerene carbons and the toluene protons in either direction is not site-specific and its kinetics are governed by the same time const ants. This result and the measured signal intensities are rationalized in terms of the short-correlation-time theory of CP. The toluene carb ons cross-polarize only from the adjacent protons according to the cla ssic CP theory. There are two kinds of toluene molecules with the rela xation time T-1 rho(H) of 10 and 70 ms. The former molecules are singl e and more mobile, and the latter, less mobile, probably form aggregat es. CP between the dilute spin systems is very sensitive to molecular motion.