| Abstract
| - When analyzing I → S variable contact time cross-polarization (CP) curves, the spin dynamics are usuallyassumed to be describable in the “fast CP regime” in which the growth of the S spin magnetization is governedby the rate of cross polarization while its decay is governed by the rate of I spin T1ρ relaxation. However, inthe investigation of the structures of zeolite−sorbate and other complexes by polarization transfer this willnot necessarily be the case. We discuss the measurement of I → S CP rate constants under the “slow CPregime” in which the rate of T1ρ relaxation is fast compared to the rate of cross polarization, leading to areversal of the usual assumptions such that the rate or growth is governed by the rate of I spin T1ρ relaxationwhile the decay is governed by the rate of cross polarization (and the S spin T1ρ relaxation). It is very importantto recognize when a system is in the slow CP regime, as an analysis assuming the normal fast CP will leadto erroneous data. However, even when the slow CP regime is recognized, it is difficult to obtain absolutevalues for the CP rate constants from fits to standard CP curves, since the CP rate constant is correlated tothe scaling factor, the contribution from 29Si T1ρ relaxation is ignored, and it is difficult to obtain reliable dataat very long contact times. The use of a 29Si{H} CP “drain” or “depolarization” experiment, which measuresabsolute 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 1H−29Si CP rate constants in thep-dichlorobenzene/ZSM-5 sorbate−zeolite complex by 29Si{1H} CP and CP drain magic-angle spinning (MAS)NMR experiments are presented and compared and used to determine the location of the guest sorbate moleculesin the cavities of the host zeolite framework.
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