Measurement of NMR cross-polarization (CP) rate constants in the slow CP regime: Relevance to structure determinations of zeolite-sorbate and other complexes by CP magic-angle spinning NMR

When analyzing I -> S variable contact time cross-polarization (CP) curves, the spin dynamics are usually assumed to be describable in the "fast CP regime" in which the growth of the S spin magnetization is governed by the rate of cross polarization while its decay is governed by the rate of I spin Tip relaxation. However, in the investigation of the structures of zeolite-sorbate and other complexes by polarization transfer this will not necessarily be the case. We discuss the measurement of I -> S CP rate constants under the "slow CP regime" in which the rate of Tip relaxation is fast compared to the rate of cross polarization, leading to a reversal of the usual assumptions such that the rate or growth is governed by the rate of I spin Tip relaxation while the decay is governed by the rate of cross polarization (and the S spin Tip relaxation). It is very important to recognize when a system is in the slow CP regime, as an analysis assuming the normal fast CP will lead to erroneous data. However, even when the slow CP regime is recognized, it is difficult to obtain absolute values for the CP rate constants from fits to standard CP curves, since the CP rate constant is correlated to the scaling factor, the contribution from Si-29 T-1 rho relaxation is ignored, and it is difficult to obtain reliable data at very long contact times. The use of a S-29{H-1} CP "drain" or "depolarization" experiment, which measures absolute values of the CP rate constants, is therefore proposed as being most appropriate for theses situations. To illustrate the importance of these observations, measurements of the H-1-Si-29 CP rate constants in the p-dichlorobenzene/ZSM-5 sorbate-zeolite complex by Si-29{H-1} CP and CP drain magic-angle spinning (MAS) NMR experiments are presented and compared and used to determine the location of the guest sorbate molecules in the cavities of the host zeolite framework.