Natural abundance O17 and S33 nuclear magnetic resonance spectroscopy in solids achieved through extended coherence lifetimes

We have found that the NMR coherence lifetime ${T}₂$ of the symmetric central $\left(\ifmmode\pm\else\textpm\fi{}\frac{1}{2}\ensuremath{\rightarrow}\ensuremath{\mp}\frac{1}{2}\right)$ transition for $¹⁷\mathrm{O}$ nuclei through a $\ensuremath{\pi}$ pulse train can be extended by over two orders of magnitude in a lattice dilute with NMR active nuclei through the use of highly selective (low-power) radio-frequency pulses. Crucial to this lifetime extension is the avoidance of coherence transfer to short-lived nonsymmetric transitions. For $¹⁷\mathrm{O}$ in $\ensuremath{\alpha}$-quartz, we obtain ${T}₂=262\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.28em}{0ex}}\text{s}$. This translates into enormous sensitivity gains for echo train acquisition schemes such as Carr-Purcell-Meiboom-Gill (CPMG). By combining satellite population transfer schemes with a low-power (2.73 kHz) CPMG on $¹⁷\mathrm{O}$ in quartz, we obtain over a 1000-fold sensitivity enhancement compared to a spectrum from a free induction decay acquired at a more typical rf field strength of $32.5\phantom{\rule{4.pt}{0ex}}\text{kHz}$. For $³³\mathrm{S}$ in ${\mathrm{K}}₂{\mathrm{SO}}₄$ the same approach yields ${T}₂=8.8\ifmmode\pm\else\textpm\fi{}0.4\phantom{\rule{4pt}{0ex}}\mathrm{s}$ and a sensitivity enhancement of 63. In both examples, these enhancements enable the acquisition of NMR spectra at 9.4 T, despite their low natural abundance and spin-lattice relaxation times of $\ensuremath{\sim}900$ and $25\phantom{\rule{4.pt}{0ex}}\text{s}$, respectively, with signal-to-noise ratios of $\ensuremath{\sim}30$ in 1 h.