Homonuclear decoupled INADEQUATE NMR methods with improved sensitivity and resolution in solid-state NMR

Abstract

The two-dimensional (2D) refocused INADEQUATE NMR experiment, which correlates double-quantum (DQ) and single-quantum (SQ) coherences, is widely used to probe the chemical connectivities in solids. Nevertheless, the multiplets along the F₂ dimension reduce the resolution and sensitivity of this experiment. The Composite-Refocusing (CR) technique with two excitation pulses has been proposed to suppress these multiplets in 2D INADEQUATE spectra of liquids. Recently (Kolyagin et al., J. Phys. Chem. Lett., 13 (2022) 10793), we showed that this technique can also be applied to suppress doublets in 2D ²⁹Si INADEQUATE spectra of ²⁹Si-enriched zeolites, which resulted in improved sensitivity and resolution. We investigate here how this INADEQUATE-CR scheme can also be applied for two other spin-1/2 isotopes: ¹³C and ³¹P. We also demonstrate the possibility to accelerate the acquisition of these 2D INADEQUATE-CR spectra with a very simple bi-exponential non-uniform sampling (NUS). For instance, in the case of ³¹P nuclei in SnP₂O₇, the use of the INADEQUATE-CR method with NUS yields a 7.5-fold reduction in experimental time with a simultaneous 1.4–1.5 gain in resolution with respect to a conventional INADEQUATE acquisition. Furthermore, we analyze the origins of the possible artifacts in these 2D spectra, including mismatching between J-coupling constants and refocusing delays, differences in relaxation times, coupling with a proton bath, and spin systems containing multiple identical nuclei. Based on this analysis, we introduce a new z-filtered INADEQUATE-CR version, which produces artifact-free 2D spectra, even in the presence of several distinct J-couplings and relaxation times or for multi-spin systems, and notably samples with high proton density.