Bicarbonate/carbonate speciation in NaHCO3 frozen solutions studied by low-temperature 13C solid-state NMR

Solid-state nuclear magnetic resonance (NMR) has been shown to be a powerful technique to investigate structure, composition, and dynamics of a wide class of materials. In this work we introduce a new method in order to study by low temperature solid state NMR frozen liquid samples after direct insertion into the cold NMR probe at 153K. We demonstrate that this approach allows the magic angle spinning of frozen aqueous solutions and an efficient 1H -> 13C cross polarization process. In particular, the application to 13C-enriched sodium bicarbonate (NaH13CO3) solutions, leads to the quenching of the fast carbonate (CO32-) <-> bicarbonate (HCO3-) chemical exchange and the observation of the two distinct 13C resonances corresponding to both species possibly dispersed in an amorphous water matrix. Moreover, pH variation allows the study of the carbonate speciation and the construction of speciation diagram in frozen conditions. The “apparent pKa” determined at 153K is dependent on the concentration in the 25 mM-400mM range similarly to what is observed in solution at room temperature evidencing that the frozen process is not modifying the chemical equilibria. We also observed that the CO32- and HCO3- molecular dynamics, related to the 13C chemical shift anisotropy, is significantly stable between 153 to 213 K, after insertion at 153K, showing the stability of the glassy water matrix in this temperature range. Extension to two-dimensional NMR spectra, through the recording of 2D 1H-13C HETCOR evidences the water molecules solvating the ions, as well as the bicarbonate protons, proof of effective quench of the CO32- <-> HCO3- interconversion. This work is the first step towards the study of reaction intermediates involved in the crystallization of calcium carbonates, through the freeze quenching of reactive solutions at specific timepoints of the reaction.