Solid-State 1H, 13C, and 17O NMR Characterization of the Two Uncommon Polymorphs of Curcumin

Curcumin is known to exist in three polymorphs (forms I, II, and III) in the solid state. The most common polymorph of curcumin (form I) is monoclinic in the space group P2/n, whereas the other two uncommon forms are both orthorhombic with the space groups of Pca21 and Pbca for forms II and III, respectively. While crystal structures are known for all three polymorphs of curcumin, their solid-state NMR signatures are incomplete in the literature. In this study, we report a complete set of solid-state 1H, 13C, and 17O NMR data for form III of curcumin. In addition, we discovered that form III of curcumin prepared by repeated drying from methanol is not stable, which undergoes slow structural transition to form II in the solid state over a period of days at room temperature. As a result, we were able to obtain new solid-state 17O NMR data for form II of curcumin, which complements the existing solid-state 1H and 13C NMR data for this polymorph in the literature. We compare experimental NMR parameters with calculated values by the GIPAW DFT and dispersion corrected DFT-D2 methods. We found that while the computed 13C chemical shifts are generally in excellent agreement with experimental values, the quality of the computed 1H and 17O NMR chemical shifts is less satisfactory. This may imply that it is necessary to include the nuclear quantum effects in future quantum chemical calculations to account for potential proton tunneling and dynamics.