Experimental and computational ¹⁷O solid-state NMR investigation of Na- and K-(bi)carbonate salts

The importance of (bi)carbonate salts cannot be understated. They are vital to the Earth's geology and ecosystems and are used as precursors by chemists for the synthesis of functional materials. Naturally, solid-state NMR (ssNMR) appears as the spectroscopic tool of choice to probe the atomic-level structure and dynamics of (bi)carbonate salts. Of the possible nuclei available as spectroscopic probes in carbonate and bicarbonate ions (i.e., ¹H, ¹³C, and ¹⁷O), oxygen-17 is highly attractive. Yet, it is seldom employed, largely due to its low natural abundance (0.04 %) and lack of practical enrichment protocols. Recently, we reported an effective ¹⁷O-labeling strategy involving mechanochemistry of Na₂CO₃·H₂O, Na₂CO₃, NaHCO₃, K₂CO₃·1.5H₂O, and KHCO₃, and recorded their ¹⁷O NMR spectral fingerprints near room temperature. In this work, ultra-low temperature (i.e., 100 K) ¹⁷O ssNMR spectra of these phases are acquired at two magnetic fields, 14.1 and 18.8 T, to extract the ¹⁷O NMR parameters δ_iso, C_Q, and η_Q for the different oxygen sites, and to further study the influence of dynamics on the spectra. We compare the experimental ¹⁷O NMR parameters to those computed with GIPAW-DFT calculations both on static models, and after averaging by molecular dynamics (MD) simulations. This approach was taken to aid in analyzing the structure-spectra relationships and shed light on the dynamics. Lastly, we report the static GIPAW-DFT calculations of ¹⁷O NMR parameters for a series of other carbonate salts of interest, further expanding upon current experimental ¹⁷O ssNMR results.