Evaluation of excitation schemes for indirect detection of 14N via solid-state HMQC NMR experiments

TL;DR

This study compares four excitation schemes for indirect detection of 14N in solid-state NMR, finding that the Selective Long Pulse (SLP) scheme offers superior efficiency, robustness, and ease of setup for nitrogen analysis.

Contribution

The paper introduces a comparative analysis of four 14N excitation schemes, highlighting the advantages of SLP for practical and reliable solid-state NMR experiments.

Findings

DANTE and SLP outperform XiX and HP in excitation efficiency.

SLP is robust across various quadrupole interactions and setup conditions.

SLP is simple to implement, making it suitable for non-specialist use.

Abstract

It has previously been shown that ${}^{14}$N NMR spectra can be reliably obtained through indirect detection via HMQC experiments. This method exploits the transfer of coherence between single- (SQ) or double-quantum (DQ) ${}^{14}$N coherences, and SQ coherences of a suitable spin-1/2 'spy' nucleus, e.g., ${}^1$H. It must be noted that SQ-SQ methods require a carefully optimized setup to minimize the broadening related to the first-order quadrupole interaction (i.e., an extremely well-adjusted magic angle and a highly stable spinning speed), whereas DQ-SQ ones do not. In this work, the efficiencies of four ${}^{14}$N excitation schemes (DANTE, XiX, Hard Pulse (HP), and Selective Long Pulse (SLP)) are compared using J-HMQC based numerical simulations and either SQ-SQ or DQ-SQ ${}^1$H-{${}^{14}$N} D-HMQC experiments on L-histidine HCl and N-acetyl-L-valine at 18.8 T and 62.5 kHz MAS. The results demonstrate that both DANTE and SLP provide a more efficient 14N excitation profile than XiX and HP. Furthermore, it is shown that the SLP scheme: (i) is efficient over a large range of quadrupole interaction, (ii) is highly robust to offset and rf-pulse length and amplitude, and (iii) is very simple to set up. These factors make SLP ideally suited to widespread, non-specialist use in solid-state NMR analyses of nitrogen-containing materials.