Molecular mechanism of hydrogen release reactions: Topological analysis using the electron localization function G. Gopakumar a , Vinh Son Nguyen a,b , Minh Tho Nguyen a, * a Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium b Faculty of Chemistry, University of Education, Hanoi, Viet Nam Received 28 November 2006; accepted 15 January 2007 Available online 1 February 2007 Abstract There is significant interest in the development of new materials for chemical hydrogen storage for the transportation sector. Elec- tronic structure calculations using various methods, reveal that the borane molecule (BH 3 ) could act as an efficient bifunctional acid– base catalyst in the H 2 -elimination reactions of XH n YH n systems (X, Y = B, C, N). Such a catalyst is needed as the elimination of H 2 from isoelectronic ethane and borane amine compounds, proceeds with an energy barrier much higher than that of the A–B bond energy. The catalytic effect of BH 3 has been probed by an analysis of the electronic densities of the transition structures using the atoms-in-mol- ecules (AIM) and electron localization function (ELF) approaches. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Electron localization function; Atoms-in-molecules; Hydrogen generation; BH 3 catalyst; Electronic structure; ELF; Density functional theory; Topology of electron density; Molecular graph; Mechanism; Electron reorganization 1. Introduction In 1916, Lewis published his seminal paper entitled ‘‘The Atom and the Molecule’’ [1] in which a theory on the cubic atom was described. The essence of this theory, which was based on a set of six coherent postulates, is that a molecu- lar system is composed of different interacting subsystems. The electronic subsystems are regrouped in pairs, which are situated either around a nucleus or between two nuclei. Since then, the Lewis concept of electron pairs became pre- dominant in the interpretation of electronic structure of molecules, and deeply rooted in the thinking system of chemists, in such a way that any new chemical concept, in order to be widely accepted, needs to have a clear corre- spondence with the Lewis picture. From a quantum chemical point of view, the Lewis model of electron pairs corresponds to a non-symmetrized total wavefunction, in which different electrons are associated with different densities. However, the molecular anti-symmetrized determinantal wavefunction obtained according to a self-consistent field (SCF) procedure corre- sponds, in general, to an electron density delocalized over the whole molecular skeleton, and therefore is not always in line with the popular views of chemical structure. As a consequence, from the early times of quantum chemistry, there have been numerous attempts to partition the elec- tron density in terms of electron pairs. Daudel introduced in 1953 the loge concept [2], and the loge theory was subsequently developed [3,4]. Using criteria such as the probability of occurrence of electronic events, the missing information function, or the fluctuation of the number of electrons, this theory led to a partition of the molecular space into loges, that are regions associ- ated with cores, bonds and lone pairs. In general, the best partition into loges corresponds to a minimal fluctuation of the number of electrons concentrated in the loges (the n-electrons loges). It is also able to handle the organization of the spins of electrons [5]. Overall, this loge partition 0166-1280/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2007.01.028 * Corresponding author. E-mail address: minh.nguyen@chem.kuleuven.be (M.T. Nguyen). www.elsevier.com/locate/theochem Journal of Molecular Structure: THEOCHEM 811 (2007) 77–89