Simulation of the UV/Vis spectra of flavonoids

The UV/Vis properties of 14 naturally occurring flavonoids, including flavones, flavonols, and isoflavones, were investigated. The TDDFT approach in combination with the B3LYP method and 6-311+G(d, p) basis set was applied. To avoid the Kohn-Sham orbitals, whose shapes are misleading when used to interpret and describe electronic transitions, the natural bond orbital analysis was applied. Taking into account the energy gap, spatial separation, and character of the π bonding, lone pair, and π* antibonding natural localized molecular orbitals (NLMOs), the "NLMO clusters" were constructed. NLMO cluster is a molecular moiety characterized with distinguished electron density. Our investigation showed that the TDDFT and NBO approaches are complementary, implying that the results from the two theories can be combined to better understand the redistribution of electron density upon excitation. Agreement between the predictions of the TDDFT approach and those based on the NLMO clusters is excellent in the case of major electronic transitions and small excitation energies.