Experimental aspects of 14N overtone RESPDOR solid-state NMR spectroscopy under MAS beyond 60 kHz

Abstract

Nitrogen-14 (14N) overtone (OT) spectroscopy under fast magic angle spinning (MAS) conditions (>60 kHz) has emerged as a powerful technique for observing correlations and distances between 14N and 1H, owing to the absence of the first-order quadrupolar broadenings. In addition, 14NOT allows selective manipulation of 14N nuclei for each site. Despite extensive theoretical and experimental studies, the spin dynamics of 14NOT remains under debate. In this study, we conducted experimental investigations to assess the spin dynamics of 14NOT using the rotational-echo saturation-pulse double-resonance (RESPDOR) sequence, which monitors population transfer induced by a14NOT pulse. The 14NOT spin dynamics is well represented by a model of a two-energy-level system. Unlike spin-1/2, the maximum excitation efficiency of 14NOT 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 14NOT spin dynamics, is nearly independent of the B0 field. Consequently, the filtering efficiency of RESPDOR experiments exhibits negligible dependence on B0 when the 14NOT pulse length is optimized. The study also identifies the optimal experimental conditions for 14NOT/1H RESPDOR correlation experiments.