Experimental and Computational 17O Solid-State NMR Investigation of Na- and K-(Bi)carbonate Salts

Abstract

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 δiso, CQ, and ηQ 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.