Abstract
We describe a two-dimensional high-resolution solid-state NMR methodology for correlating the dynamics of molecular rearrangements around the critical points of equilibrium phase transitions in molecular solids. It combines the techniques of temperature-jump and two-dimensional (2D) NMR spectroscopy. The two spectral dimensions are the isotropic chemical shifts at different sample temperatures. The technique is illustrated by elucidating the dynamic rearrangement of the C4O4 units of squaric acid (H2C4O4) in relationship to the mechanism of its antiferroelectric phase transition at TN ∼ 373 K. These results clarify the apparent discrepancy between the conclusions derived earlier from NMR, X-ray, and Raman and neutron scattering studies. They were a direct consequence of the significant enhancement in the NMR spectral resolution through this technique, which thus might prove to be a significant new aid in understanding the mechanisms of phase transitions in molecular solids.
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