Experimental Aspects of 14N Overtone RESPDOR Solid-State NMR Spectroscopy under MAS Beyond 60 Khz

Nitrogen-14 (N-14) overtone (OT) spectroscopy under fast magic angle spinning (MAS) conditions (>60 kHz) has emerged as a powerful technique for observing correlations and distances between N-14 and H-1, owing to the absence of the first-order quadrupolar broadenings. In addition, N-14(OT) allows selective manipulation of N-14 nuclei for each site. Despite extensive theoretical and experimental studies, the spin dynamics of N-14(OT) remains under debate. In this study, we conducted experimental investigations to assess the spin dynamics of N-14(OT) using the rotational-echo saturation-pulse double-resonance (RESPDOR) sequence, which monitors population transfer induced by a(14)N(OT) pulse. The N-14(OT) spin dynamics is well represented by a model of a two-energy-level system. Unlike spin-1/2, the maximum excitation efficiency of N-14(OT) coherences of powdered solids, denoted by p, depends on the radiofrequency field (rf-field) strength due to orientation dependence of effective nutation fields even when pulse lengths are optimized. It is also found that the p factor, contributing to the N-14(OT) spin dynamics, is nearly independent of the B-0 field. Consequently, the filtering efficiency of RESPDOR experiments exhibits negligible dependence on B-0 when the N-14(OT) pulse length is optimized. The study also identifies the optimal experimental conditions for N-14(OT)/H-1 RESPDOR correlation experiments. (c) 2024 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).