Dynamics of calcium oxalate monohydrate in high and low temperature phases using ¹⁷ O solid-state NMR

Calcium oxalates are naturally occurring minerals, with the monohydrate form CaC ₂ O ₄ ·H ₂ O (COM) being the most stable. COM contains two crystallographically different water sites. We investigate the details of water internal dynamics in the high and low temperature phases of COM using ¹⁷ O central transition solid-state NMR line shapes, as well as laboratory and rotating frame relaxation rates. The measurements were performed either under static or magic angle spinning conditions and in a wide temperature range from 343 to 180 K. The combination of all measurements allows for precise constraints on motional mechanisms, rate constants, and amplitudes of motions. The high temperature phase is dominated by large-angle fluctuations with an amplitude of about 100 ° , identical in both sites. During the phase transition between 323 to 300 K, these large-angle jumps freeze out in one of the water sites, while remaining active in the other. In the low temperature phase from 280 to 180 K, small-angle fluctuations of 2-8 ° in amplitude dominate the relaxation. Transverse relaxation rates also point to the existence of a very slow collective “rocking” motion down to about 220-200 K.