13C → 1H Cross-Polarization NMR in Solids at Natural 13C Abundance

Abstract

We discuss the features of cross-polarization (CP) between dilute spin systems with substantial molecular motion and discuss the applicability of two different theories for the description of cross-polarization in toluene-solvated fullerene-70. The material was examined by 13C → 1H and 1H → 13C CP NMR with magic-angle spinning (MAS). 13C → 1H CP was set on perdeuterated glycine. Both solids were at the natural 13C isotopic abundance and with ca. 2% 1H content, so that 13C and 1H 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 constants. This result and the measured signal intensities are rationalized in terms of the short-correlation-time theory of CP. The toluene carbons cross-polarize only from the adjacent protons according to the classic CP theory. There are two kinds of toluene molecules with the relaxation time of 10 and 70 ms. The former molecules are single and more mobile, and the latter, less mobile, probably form aggregates. CP between the dilute spin systems is very sensitive to molecular motion.