The relative position of La and Lb ππ* electronic states in purine
nucleobases is a much debated topic, since it can strongly affect our understanding of
their photoexcited dynamics. To assess this point, we calculated the absorption and
magnetic circular dichroism (MCD) spectra of adenine, guanine, and their nucleosides
in gas-phase and aqueous solution, exploiting recent developments in MCD
computational technology within time-dependent density functional theory. MCD
spectroscopy allows us to resolve the intense S0→ La transition from the weak S0→ Lb
transition. The spectra obtained in water solution, by using B3LYP and CAM-B3LYP
functionals and describing solvent effect by cluster models and by the polarizable
continuum model (PCM), are in very good agreement with the experimental
counterparts, thus providing direct and unambiguous evidence that the energy ordering
predicted by TD-DFT, La < Lb, is the correct one.
