Thermodynamic, reactivity and spectroscopic properties of curcumin: solvent effect

The M06/6-31+G(d) method has been used to study the reactivity of the two forms of curcumin (enol and keto). The energies needed for the three thermodynamic mechanisms: HAT, SET-PT and SPLET, have been calculated, in different solvents, to determine the most probable hydrogen atom transfer mechanism. The solvent effect is evaluated using an implicit solvation model (IEF-PCM). The results show the existence of an intramolecular hydrogen bond strength, in the enol form, which prevents the dissociation of hydrogen atom. In nonpolar solvent, the value of BDFE is lower than PA and IP; this means that HAT is the most favorable mechanism of the two forms of curcumin, while the SPLET mechanism is thermodynamically preferred in polar solvent. The UV/Vis spectra have been determined by the time-dependent density functional theory (TD-DFT) to show the maximum absorption wavelength value of curcumin and the nature of the excited states.