Steady-State Free Precession and Solid-State NMR: How, When, and Why

Abstract

Nuclear magnetic resonance (NMR) is a well-established technique, to study crystalline and amorphous solids, that provides rich structural and dynamical insights. Many solids NMR experiments, however, are challenged by sensitivity considerations, which demand the use of optimized data acquisition protocols. When inhomogeneous broadenings dominate the lineshapes, as is often the case when dealing with quadrupolar nuclei, the application of multiple echoes by means of Carr-Purcell Meiboom-Gill (CPMG) sequence variants can considerably facilitate the acquisition of solid-state NMR data. This study explores the use of steady-state free precession (SSFP) experiments, another proposal derived from Carr’s early work. SSFP sequences are commonly utilized in magnetic resonance imaging acquisitions, which are also dominated by (gradient-imposed) inhomogeneous broadenings, where they are well-known to provide the highest sensitivity per unit time. SSFP sequences are also common in nuclear quadrupole resonance—yet to our knowledge they have been less explored within the realm of high-field solids NMR on static and spinning samples. The present study examines the application of SSFP to these scenarios, in particular, in what concerns its ability to enhance the sensitivity of wide-line spectra. The regimes when SSFP could be advantageous compared to CPMG acquisitions are examined, and sensitivity enhancements of between 1 and 2 orders of magnitude per unit time are theoretically predicted and experimentally demonstrated in certain instances. Aspects in need of improvement to enable a wider use of SSFP-based approaches, particularly to wide-line quadrupolar studies, are also examined.