Keto-enol tautomerism in micro-hydrated acetylacetone: an atoms-in-molecules study

We theoretically investigate the keto-enol tautomerism of micro-hydrated acetylacetone molecule and compare it to the isolated acetylacetone. The minimum energy path was computed through the Climbing-Image Nudged Elastic Band method (CINEB). The density functional theory was applied in the Projector Augmented Wavefunction (PAW) formalism using the generalized gradient approximation within Perdew-Wang (PW91) parametrization. Moreover, a topological and energy partition analysis was done in the framework of the Quantum Theory of Atoms In Molecules (QTAIM). We show that the activation barrier energy decreases from the gas phase to clusters with up to three water molecules surrounding the acetylacetone and increases when four water molecules are considered. Our analysis indicates that the hydrogen bond lattice of the water molecules strongly stabilizes the transition state, leading to the decrease of the activation energy. Furthermore, the size of the proton relay formed by the solvent molecules influences the stretch of the C-alpha-H bond which explains the increase of the activation energy for the largest cluster.