NMR cross-polarization when T-IS > T-1 rho; Examples from silica gel and calcium silicate hydrates

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