Theoretical study on Curcumin: A comparison of calculated spectroscopic properties with NMR, UV–vis and IR experimental data

The main target of this study is a high-level computational analysis of Curcumin, employing DFT approach with two different sets of basis functions (B3LYP/6-31G∗ and B3LYP/6-311G∗∗). Accurate quantum mechanical studies, both in vacuum and in methanol medium, are carried out with the aim to analyze the conformational equilibria, to find the most stable equilibrium structure and to define the nature of the molecular orbitals, fundamental to explain Curcumin binding characteristic. Our theoretical calculations, performed at B3LYP/6-31G∗ and B3LYP/6-311G∗∗ levels both in vacuum and in methanol medium, confirm that the keto-enolic forms are more stable than the di-keto one, whose extremely low population suggests that this structure should not influence Curcumin properties. Keto-enolic form C results the most stable, independently on calculation level and solvent (methanol) effect. HOMO and LUMO molecular orbitals are calculated for all the structures with the two sets of basis with very similar results. MEPs show that the negative charge is localized on the oxygen atoms, which, in the keto-enolic forms, point in the same direction enabling metal coordination.NMR, UV–vis and FT-IR experimental data are employed in the comparison with electronic and conformational properties of Curcumin resulting from theoretical calculations. The two different calculation levels (B3LYP/6-31G∗ and B3LYP/6-311G∗∗) give very similar results.Good linear correlations between the experimental 1H and 13C NMR chemical shifts (δexp), in methanol-d4 (MeOD) and DMSO-d6 (DMSO), and calculated magnetic isotropic shielding tensors (σcalc) are found (δexp = a · σcalc + b). A good prediction of UV–vis experimental maximum absorption (λmax) on the basis of conformer populations is obtained. A linear relation with a good correlation coefficient is observed plotting the FT-IR experimental wavenumbers vs. the calculated ones, allowing to predict FT-IR spectra.