Very High Resolution ¹⁷O NMR Evidence for Displacive Behavior in Hydrogen-Bonded Solids: Squaric Acid

Abstract

Magic angle sample spinning (MAS)⁷O NMR studies of the paraelectric-antiferroelectric phase transition (at T_C = 373 K) of the model hydrogen-bonded compound squaric acid (H₂C₄O₄ ) reveal a significant displacive component in the microscopic mechanism of the transition. The high resolution (400% enhancement over conventional MAS) was obtained by utilizing single crystals. All four oxygens were clearly shown to be chemically different at T < T_C . The peak assignment was supported by quantum theoretical calculations of the ¹⁷O isotropic chemical shifts using a pentamer model of the crystal structure. There was a clear break in the isotropic part of the chemical shift on lowering the temperature through T_C , implying that the phase transition involves a distortion of the whole H₂C₄O₄ framework, and not just the order-disorder rearrangement of the H's i.e., future models of the transition should include a displacive component, in addition to an order-disorder part. The observation of the double at T > T_C implies that the two O─H⋅⋅⋅O chains retain their difference in the paraelectric phase as well. This is consistent with the one-dimensional Ising chain model, in contrast to the more prevalent, two-dimensional, C₄O₄ square-lattice model.

Keywords:

• NMR
• hydrogen bonded solids

Acknowledgments

This research was supported in part by a grant from the National Science Foundation and Florida State University. We acknowledge Centro de Computación Científica of Universidad Autónoma de Madrid for the computer time.

Notes

^a Experimental-theoretical linear correlation: ¹⁷O δ_iso = 209–0.64 ¹⁷O σ_iso (Sd = 25 ppm, R = 0.983).