HETEROGENEITY OF CROSS RELAXATION IN SOLID-STATE NMR

The most commonly referenced theories on Hartmann-Hahn cross polarization have been challenged by some new experimental observations, especially under mismatched Hartmann-Hahn conditions. This paper shows that the key to settling this conflict lies in abandoning the basic assumption of homogeneous spin temperature for the rare spins. A series of special cross-relaxation experiments has been designed and performed on (13CH2O)n, 96% 13C-enriched, and ferrocene. From these measurements it can be shown that under mismatch the rare S spins are actually divided into several categories, each participating in the cross relaxation with a different rate. This means that the cross relaxation is heterogeneous; i.e., the S spins do not behave as a reservoir with a homogeneous spin temperature during the cross-relaxation process. The heterogeneity is due to the fact that for the flip-flop of an Sl pair to be energy conserving, the rotating-frame Zeeman-energy imbalance must be compensated for by II dipolar interactions. Because the local field changes from site to site, there may be a portion of the SIn groups which have such dipolar couplings. This results in the polarization transfer in some SIn groups taking place on the tens of microseconds time scale, while others polarize much more slowly. © 1991.