Proton triple-quantum solid-state NMR spectroscopy at slow MAS ∼10 kHz

Abstract

Solid-state NMR is a valuable tool for elucidating the structures and dynamics of materials at an atomic level. Proton multiple-quantum (MQ) /single-quantum (SQ) correlation NMR spectroscopy is widely used to probe spatial proximity among protons. In the triple-quantum (TQ)/SQ correlation experiment, the excitation of triple-quantum (TQ) coherences is traditionally achieved by a 90° pulse in conjugation with double-quantum (DQ) recoupling sequences. Nevertheless, such sequences often suffer from low TQ filtering efficiency and may lead to overlapping spinning sidebands in the indirect TQ dimension, especially at a slow MAS frequency. Herein, we design several supercycled symmetry-based RNnν γ-free TQ recoupling sequences and compare their performance via extensive numerical simulation and experiments. Experimental results further confirm that (R1816R181−6)31 pulse sequence gives the highest TQ filtering efficiency of around 20% in the slow MAS regime (∼10 kHz). The 2D TQ/SQ spectrum at slow MAS is completely free of spinning sidebands in the TQ dimension due to its γ-free nature. We establish that such a γ-free (R1816R181−6)31 pulse sequence is a superior candidate for TQ spectroscopy at slow MAS frequency.