Theoretical calculations on the transition energies of the UV-visible spectra of curcurnin pigment in turmeric

Abstract

Theoretical calculations based on the Pariser-ParrPople (PPP) hamiltonian have been performed on two forms of a model compound resembling the curcumin pigment in turmeric. The transition energies for various possible <i>n→π</i>* and <i>π→π</i>* transitions have been computed for the singlet-singlet transitions. The most consistent transition energies for the neutral curcumin are calculated as λ_max(<i>n→π</i>*) =430 nm and λ_max (<i>π →π</i>*) = 261.5 nm compared to the corresponding experimental values of 420-430 nm and 265 nm, respectively. The curcumin anion obtained upon addition of OH^- to curcumin exhibits considerable red shift. The theoretical <i>n→π</i>* and <i>π→π</i>* λ_max values are calculated to be 483 and 343 nm, respectively. The charge densities and bond orders derived from the PPP hamiltonian reveal considerable delocalization of <i>π</i>-electrons in the curcumin compound and a structure close to the quinonoid form of the curcumin anion.