Practical considerations on the use of low RF-fields and cosine modulation in high-resolution NMR of I = 3/2 spin quadrupolar nuclei in solids

On the applicability of cosine-modulated pulses for high-resolution solid-state NMR of quadrupolar nuclei with spin > 3/2

“…We first briefly remind of a few essential practical points behind these WURST-lp/coslp-MQ-based experiments established in the previous studies of I = 3/2 spin systems. [25][26][27]31 (i) For MQ excitation/conversion, the two R-pulses need to be identical and symmetrically applied to produce a coherent evolution of the two inner STs, 25,26 to serve as inversion pulses between the 1Q-CT and 3Q coherences. (ii) In lp-MQMAS, the R-pulses are applied far off-resonance from the CT signals with a large offset (irr) on the ST manifold, 25 whereas in coslp-MQMAS, the R-pulses are applied near to the CT signals (irr), ideally on the center-band position of the inner STs, and are cosine-modulated (cos).…”

“…S2 for {I}-1 H MQ/ST-HETCOR experiments. It must be noted that even if the number and type (hp, lp, coslp or CT-selective) of rf-pulses in the sequences do not depend on the spin value, the timings and selected coherence levels used in the pulse programs are different between those used with I = 3/2, 31 or I > 3/2 (here). Examples of TopSpin pulse programs for WURST-coslp-MQMAS and WURST-coslp-MQ-HETCOR experiments are provided in the SI of this article for I > 3/2 and in that of our previous study for I = 3/2.…”

“…Examples of TopSpin pulse programs for WURST-coslp-MQMAS and WURST-coslp-MQ-HETCOR experiments are provided in the SI of this article for I > 3/2 and in that of our previous study for I = 3/2. 31 Prior to any STMAS acquisition, an accurate adjustment of the spinning axis to the magic angle was performed using a DQF version of the split-t1 shifted-echo pulse sequence on the sample of interest itself. 42 Chemical shift scales, shown in ppm, were referenced using the sample of interest itself as a secondary reference, and all 2D spectra were referenced according to the unified representation.…”

“…These two approaches stem from the lp-STMAS scheme, 27 previously proposed to manipulate the innermost STs with low RF requirement. In this article and in the previous one, 31 we use a notation of the sequences that slightly differs from the original ones. 25,26 We call them lp-MQMAS and coslp-MQMAS, instead of lpMQMAS and cos-lpMQMAS, to emphasize the fact that they are MQMAS methods, and we add WURST at the beginning to emphasize the fact that this sweeping is a very important part of these sequences.…”

“…Our study concluded that WURST-coslp-MQbased experiments are highly recommended for (i) low-gamma nuclei, (ii) large rotor diameters, and (iii) samples with large CQ values. 31 For I > 3/2 spin systems, at the time of writing, only 17 O WURST-coslp-MQMAS results have been reported in the acquisition of 3D 1 H/ 13 C/ 17 O, 32 or 2D 1 H/ 17 O, 33 isotropic HETCOR spectra at B0 = 18.8 T.…”