Nitrogen is an important and ubiquitous element in many areas of science, but NMR observation of the 99.6% natural-abundance 14N isotope (spin = 1) is extremely challenging in the solid state. This is primarily due to the large first-order quadrupolar interaction that broadens both of the single-quantum transitions over frequency ranges of many MHz. Overtone NMR spectroscopy, in which the much narrower Δm = 2 (overtone) transition is directly excited and observed, is a potential solution to this problem. The concept of 14N overtone NMR has been around for decades but has failed to catch on due to various associated difficulties. Recently, however, this approach has been successfully combined with magic angle spinning (MAS) for the first time, resulting in a unique method for directly acquiring 14N NMR spectra in which multiple sites are well-resolved.1
The latest developments in solid-state 14N overtone NMR will be presented, including 1H-14N correlation experiments under ultra-fast MAS2 and signal enhancement opportunities using dynamic nuclear polarisation (DNP). Some key applications of these techniques are also highlighted.