Charge density analysis of some processes involving intramolecular hydrogen transfer

The variations in topology of the electron density along the reaction paths of the keto–enol tautomerism of acetaldehyde, the pinacol rearrangement of protonated 1,2-ethanediol, and the unimolecular decomposition of methanediol, were studied as examples of hydrogen transfer between, respectively, C⋯O, C⋯C, and O⋯O atoms. The evolution of atomic charges, determined using two different atomic partitionings (AIM and Hirshfeld), indicates that the main electronic charge transfer in keto–enol tautomerism takes place between the migrating hydrogen and the carbon of the carbonyl group. The topology of the electron density demonstrates that a hydrogen-bridge structure is never formed along the reaction path of the pinacol rearrangement. The methanediol decomposition follows a concerted mechanism with very small variations in the atomic charges.