NMR Cross-Polarization when T_IS>T_1ρ; Examples from Silica Gel and Calcium Silicate Hydrates

Abstract

NMR cross-polarization (CP) measurements are usually analyzed assuming that the cross-polarization time T IS of magnetization transfer from the abundant I spins to the rare S spins is shorter than the relaxation time T 1F in the rotating frame of the I spins (fast CP regime). Here, it is shown that the reverse situation (T IS > T 1F I , slow CP regime) may occur, for instance, for the 1 Hf 29 Si 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 ∼T IS /T 1F . 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 13 C NMR of polyaromatic compounds.

Determination of T1ρH Relaxation Rates in Charred and Uncharred Wood and Consequences for NMR Quantitation

“…The indirect methods (VCT and VSL) have the disadvantage of low sensitivity for rapidly relaxing components. A limited number of studies have used a combination of two or three of the above methods to give a more complete picture of T 1r H relaxation processes [6,9,26,43,47,51,53,55,56].…”

Influence of glass composition and alteration solution on leached silicate glass structure: A solid-state NMR investigation

“…The fact that these peaks decrease significantly in the SL spectra (Fig. 2c) suggests that they are characterized by higher mobility than other species (leading to a shorter T 1 q time (Klur et al, 2000) but to a slower decrease in HE experiments as a result of the motional averaging of the dipolar couplings). The two broad peaks observed at about 4 and 6.5 ppm can be assigned accordingly to basic-leached samples.…”

Silicon-29 cross-polarization/magic-angle-spinning NMR study of inorganic–organic hybrids: homogeneity of sol–gel derived hybrid gels

“…It is noteworthy that Eqs. (1) and (2) are valid not only in the situation T SiH < T H 1q but also in the case of T SiH > T H 1q [32], value is fixed to the average among those of T 2 , T 3 , and Q 3 shown in Table 1 for each hybrid gel. The same T th values are assumed for TEOS-RTES gels of the same type.…”