The electronic localization and delocalization in molecules has been receiving a lot of attention in the past few years, especially as it pertains to the study of aromatic compounds but also in the analysis of phenomena such as conjugation and hyperconjugation.

“Our paper summarizes the recent advances in methodologies used to analyze electronic delocalization from molecular quantum mechanics studies.”

Our paper summarizes the recent advances in methodologies used to analyze electronic delocalization from molecular quantum mechanics studies. The review focuses mainly on the two possible partitions of the space for the analysis of electron delocalization, those derived from the so-called atoms-in-molecules (AIM) theory and the electron localization function (ELF).

Aromaticity is a property of many cyclic organic and inorganic molecules that helps to understand the molecular structure, stability, reactivity, and magnetic properties of many molecules. Thus, the chemistry of cyclic molecules is quite dependent on the evolution of aromaticity during the reactions. This property is largely determined by the electronic delocalization of the most external electrons.

Our paper discusses the most important current methods used to analyze electronic delocalization in molecules, and, in particular, in aromatic molecules. Comprehension of such properties may help to design new inorganic clusters with unexpected chemical and physical properties.

We were especially interested in the quantification of aromaticity. The measure of aromaticity is usually done indirectly by calculating a series of different physical properties that characterize aromatic compounds such as energy stabilization, bond length equalization, unusual reactivity, characteristic spectroscopic features, and particular magnetic properties.

We have introduced several measures of aromaticity which are based on the electronic delocalization properties of molecules. One of the main problems in this field is that different indices of aromaticity afford different aromaticity ordering of molecules, and in many cases it is difficult to know which are the methods that are performing the best. Another problem is the lack of commercial programs that allow the computation of such quantities with correlated wavefunctions.

We have recently demonstrated that the aromaticity measures based on electronic delocalization properties are the most reliable measures of aromaticity among currently available methods. However, there is room for improving the methods we have implemented and we believe that this can be done sometime in the next few years.

Very recently a series of all-metal cluster compounds with aromatic characteristics have been synthesized. Such molecules may be useful for their technical applications as specific and very efficient catalysts, drugs, and other novel materials with as yet unimagined properties! The stability of such species is largely determined by the electron delocalization of the most external electrons. Therefore, the study of such phenomena gives clues about which clusters will be the most stable and, consequently, can help in orienting the design of new clusters with unexpected properties.