Reversible Intramolecular Proton Transfer in Curcumin Crystals and Nonlinear Size Correlation

Intramolecular proton transfer is more difficult forcurcumin inthe solid state than in the solution. In this study, a reversibletransformation of keto-enol to the & beta;-diketone form of curcuminwas achieved by nonchemical methods. The obstacle to intramolecularproton transfer was shown to be the intermolecular hydrogen bond atthe keto-enol linkage. When the intermolecular hydrogen bond in theketo-enol site of the curcumin molecule is broken, reversible protontransfer will occur more easily. The energy fluctuations and vibrationalshifts in the functional group spectra were captured during the protontransfer and conformational rearrangement with increasing temperature(from 170 to 183 & DEG;C) after desolvation. The maximum energy changein the proton transfer process of curcumin occurs in the 200 & mu;mcrystal (extreme value point), influenced by the combination of thespecific surface area of the crystal itself and the degree of thermalhomogeneity. This paper presents an environmentally friendly and efficientmethod to regulate the physicochemical properties of solid organicmaterials. A crystal engineering-basedapproach has achieved reversibleintramolecular proton transfer in the solid state of curcumin, withthe highest transfer rates occurring in the intermediatesize crystal samples. The methods of solvation and desolvation areconsidered effective methods to regulate the molecular spacing andmolecular packing density in the solid state.

Automated summary

In this study, a reversible transformation of curcumin from keto-enol to β-diketone form was achieved by nonchemical methods. The obstacle to intramolecular proton transfer was found to be the intermolecular hydrogen bond at the keto-enol linkage. Energy fluctuations and vibrational shifts were observed during the proton transfer process with increasing temperature, and the maximum energy change occurred in the 200 μm crystal. This study presents an environmentally friendly and efficient method to regulate the physicochemical properties of solid organic materials.