Improving coherence lifetimes for the NMR measurement of 1H-X distances.

The measurement of heteronuclear interatomic distances to proton nuclei is often hindered by rapid 1H decoherence under the application of a dipolar recoupling sequence. This decay is dominated by sensitivity to radiofrequency and rotor instabilities that may be improved through experiment design. We compared dipolar recoupling scheme combinations both experimentally and using spin dynamics simulations to maximize coherence lifetimes and enable the measurement of weaker dipolar interactions. From these investigations, we observed that the windowed R1253 recoupling scheme and a double-echo pulse sequence were the most robust approach for minimizing the influence from typical experimental imperfections. We applied this combination to investigate the dipolar interactions between 1H and 25Mg in Mg(OH)2, where the 1H environment features a strong homonuclear dipolar coupling network, and where 25Mg possesses a small gyromagnetic ratio. We also applied the technique to probe the surface termination of LaScO3 nanocrystals.