Magic angle spinning NMR on single crystals as a new aid in characterizing phase transitions: application to squaric acid

Accurate measurement of δ_ISO, the isotropic part of the NMR chemical shift tensor, is usually difficult for solids. This study suggests that the technique of magic angle sample spinning (MAS), employing single crystals, might be suitable for this purpose. In our ¹³C CP(cross polarization)/MAS measurements on squaric acid (H₂C₄O₄), single crystals yielded about five-fold narrower line widths than powder samples. The resolution enhancement thus obtained enabled us to measure δ_ISO for all four carbons of squaric acid, and to show that the average O-H distance can serve as an order parameter for the phase transition at 373 K. Additionally, the ¹³C δ_ISO data suggest that the transition mechanism involves a significant displacive component, in addition to the known order-disorder behavior, thus demonstrating the high potential of the single crystal MAS-NMR methodology for studying the microscopic mechanism of phase transitions.