How Malonaldehyde Bonds Change during Proton Transfer

The proton transfer mechanism in malonaldehyde has been investigated from the points of view of the topological analysis of the electron localization function and of the catastrophe theory. The calculations were carried out at the B3LYP hybrid functional level with the ccp-VTZ basis set. Complementary calculations were performed at the HF, MP2, BLYP, and B3LYP levels with the STO-3G, 6-31G, and 6-31G** basis sets in order to discuss the stability of the method. The mechanism of the reaction does not involve any reorganization of the skeleton: it is localized in the hydrogen bond region and involves three chemical structures corresponding to the reactant, the transition state, and the product. It is shown that the proton detachment is in fact the covalent breaking of the OH bond, and by symmetry its attachment on the other oxygen is also a covalent process. The analysis of the catastrophe shows that the reaction is driven by two active coordinates related to the OH and OO distance and provides a method for the determination of the limiting paths.