NMR Cross-Polarization when <i>T</i>_IS&gt;<i>T</i>₁_ρ; Examples from Silica Gel and Calcium Silicate Hydrates

Abstract

NMR cross-polarization (CP) measurements are usually analyzed assuming that the cross-polarization time TIS of magnetization transfer from the abundant I spins to the rare S spins is shorter than the relaxation time T1ρ in the rotating frame of the I spins (fast CP regime). Here, it is shown that the reverse situation (TIS > T1ρI, slow CP regime) may occur, for instance, for the 1H→29Si transfer in commonly encountered inorganic materials and that analyzing the experimental data in this case under the usual fast CP assumption will, beside leading to erroneous dynamic parameters, underestimate the number of spins by a factor ∼TIS/T1ρ. Experimental ways to distinguish between the two situations are presented, the most efficient being to resort to the TORQUE experiment. Examples are given on silica gel and calcium silicate hydrate (CSH) samples for which the proper analysis allows a deeper insight into the nature of the protonated surfaces. This slow CP regime may be expected to occur also in some organic materials, for instance in 13C NMR of polyaromatic compounds.