Theoretical study of Z isomers of A-type dimeric proanthocyanidins substituted with R=H, OH and OCH₃: stability and reactivity properties.

The stereochemistry of A-type dimeric proanthocyanidins was studied, focusing on the factors that determine it, and the changes that occur with R = OCH₃, R' = H, and R = OH, R' = H as substituents, starting with the study of the conformational space of each species. Using molecular dynamics at a semiempirical level, and complementing with functional density calculations, two conformers of lowest energy were characterized for R = H, eight conformers for R = OH, and three conformers for R = OCH₃. Electronic distributions were analyzed at a higher calculation level, thus improving the basis set. Intramolecular interactions were examined and characterized by the theory of atoms in molecules (AIM). Detailed natural bond orbitals (NBO) analysis allowed the description of subtle stereoelectronic aspects of fundamental importance for understanding the stabilization and antioxidant function of these structures. The study was enriched by a deep analysis of maps of molecular electrostatic potential (MEP). The coordinated analysis of MEP, together with the NBO and AIM results, allowed us to rationalize novel distribution aspects of the potential created in the space around a molecule.