Bonds and lone electron pairs can be made “visible” when the electron density distribution is used to calculate the electron localization function (ELF). This paper presents a computer‐graphics image of ELF in colors which represent the extent of the localization (at the right a black‐and‐white picture of N2).

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.

Connecting the accurate Quantum Mechanics to the chemical view is the first of foremost purposes of interpretative methods in general, and topological analysis in particular. In this field of methods, the Maximum Probability Domains (MPD) analysis, is conceptually appealing but has not been extensively applied yet. In this study we provide the general vision coming out from MPD on the two main family of bonds: polar-covalent and ionic bonds. An interesting picture arises concerning the MPD solution associated to covalent bonds, displaying a prolate shape that extends preferentially in the orthogonal direction to the bond axis, and not along it. The polarity of the bond only affects marginally the domain shape, though further probability analysis seems to allow quantifying it. Concerning the ionic bond, a resonating picture emerges, which is compatible, and refines, the usual electrostatic vision of two oppositely-charged atoms in interaction.