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

Solid-state NMR faces limitations for liquid sample analysis due to the incompatibility with the magic angle spinning (MAS) method used for averaging anisotropic interactions. In this work, we introduce a simple approach to study frozen aqueous solutions through a direct insertion into an NMR probe pre-cooled at 153K. The study of aqueous sodium bicarbonate, as a model solution, evidences the quench of the fast HCO₃^- ↔ CO₃^2- interconversion and leads to the observation of the distinct ¹³C resonances of each ion dispersed in the water frozen matrix. Moreover, pH variation allows the determination of "apparent" pKa in frozen conditions that are found similar to what is expected at room temperature, showing that the freezing process is not modifying the chemical equilibria. We also show that temperature variation from 153 to 213 K does not modify the ¹³C chemical shift anisotropy of CO₃^2- and HCO₃^- showing that the local dynamics of the ions in the frozen water environment is not impacted in this temperature range. Extension to ¹H-¹³C two-dimensional NMR evidences water molecules solvating the two ions, as well as the C-OH resonance from bicarbonate, proof of the effective quenching of the HCO₃^- ↔ CO₃^2- interconversion. This work is the first step towards the development of freeze-quench methods to study crystallization processes from aqueous solution by solid-state NMR including, transient intermediate, chemical exchange or polymorphism.