A Computational Study (TDDFT and RICC2) of the Electronic Spectra of Pyranoanthocyanins in the Gas Phase and Solution.

The conformational structures and UV-vis absorption electronic spectra of a class of derived anthocyanin molecules (pyranoanthocyanins) have been investigated mainly by means of density functional (DFT) and time-dependent DFT methods. Pyranoanthocyanins are natural pigments present in aged wines and absorb at shorter wavelengths (around 500 nm) than the parent anthocyanin compounds, giving an orange-brown colored solution. The investigated molecules are derived from the reaction of glycosylated malvidin, peonidin, and petunidin with enolizable molecules (acetaldehyde and pyruvic acid) and vinyl derivatives. During wine storage, the concentration of pyranoanthocyanins increases with time, and analytical measurements (e.g., UV-vis spectroscopy) can characterize aged wines by color analysis. The prediction of absorption electronic spectra from TDDFT results, with the inclusion of water bulk solvation effects through the conductor-like polarizable continuum model, gives an absolute mean deviation from experimental absorption maxima of 0.1 eV and a good reproduction of the spectra line shape over the visible range of the spectrum. TDDFT calculated excitation energies agree with those obtained from ab initio multireference coupled cluster with the resolution of identity approximation (RICC2) methods, calculated at DFT gas-phase geometries.