Reversible Intramolecular Proton Transfer in Curcumin Crystals and Nonlinear Size Correlation

Abstract

Intramolecular proton transfer is more difficult for curcumin in the solid state than in the solution. In this study, a reversible transformation of keto-enol to the β-diketone form of curcumin was achieved by nonchemical methods. The obstacle to intramolecular proton transfer was shown to be the intermolecular hydrogen bond at the keto-enol linkage. When the intermolecular hydrogen bond in the keto-enol site of the curcumin molecule is broken, reversible proton transfer will occur more easily. The energy fluctuations and vibrational shifts in the functional group spectra were captured during the proton transfer and conformational rearrangement with increasing temperature (from 170 to 183 °C) after desolvation. The maximum energy change in the proton transfer process of curcumin occurs in the 200 μm crystal (extreme value point), influenced by the combination of the specific surface area of the crystal itself and the degree of thermal homogeneity. This paper presents an environmentally friendly and efficient method to regulate the physicochemical properties of solid organic materials.