Dynamics of calcium oxalate monohydrate in high and low temperature phases using 17O solid-state NMR
Liliya Vugmeyster1, Kusum Yadav1, Sean T Holmes2
1Department of Chemistry, University of Colorado at Denver, Denver, CO, 80204, USA.
Abstract:
Calcium oxalates are naturally occurring minerals, with the monohydrate form CaC2O4·H2O (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 17O 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 and 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.
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