Laplacian of the electron density: a hole-lump interaction as a tool to study the stereoelectronic control of chemical reactions

ZALAZAR, M. FERNANDA; PERUCHENA, NÉLIDA MARIA

Conferencia; 12th Latin American Conference on Physical Organic Chemistry; 2013

An important feature of the reactions in the field of heterogeneous catalysis is the concept of concerted mechanism with reaction coordinates that can be very complicated. This causes reaction pathways with low activation energies due to the complete compensation of the energy necessary for bond breaking in the reactant compounds, with the energy released in bond making in the final products. According to Bader, a chemical reaction corresponds to the combination of a charge concentration in the valence shell charge concentration of the base with a charge depletion in the valence shell charge concentration of the acid.1 The geometry of approach of the acid and base molecules is predicted through the alignment of the corresponding "lumps" and "holes" in their Laplacian distributions. That is, the reactivity is related to the magnitude of the charge concentration and the depth of the charge depletion. Theoretical studies about reaction mechanisms are usually limited to the determination of the energetic paths that connect reactants, transition states, and products. Additionally in this work, we propose the topological analysis of the Laplacian distribution, in conjunction with the atomic charges integrated over the atomic basin, as a tool to investigate the stereoelectronic control of chemical reactions. In this context, the Atoms in Molecules1 theory was applied for the study of carbocationic transition states (TS) involved in two different reaction mechanisms proposed in a selected olefins reaction over Brønsted acid site, in order to gain a deeper understanding of stereoelectronic aspects of these chemical reactions. Electron densities were obtained at B3LYP levels using a 6-31++G(d,p) basis set, and the Gaussian 03 program. The Laplacian and atomic properties were calculated using the Aim2000 program. Additionally, envelopes of Laplacian of the electron density of transition states of both mechanisms were computed and compared. The three-dimensional distribution of the Laplacian showed that the hole in the Laplacian of the carbon atom in carbocationic specie fit the lumps on the atoms in the adjacent molecules (catalyst and other olefin reactant), both species being linked by the intermolecular bond paths (the bonds formed can be regarded as lump-hole interactions), in addition the atomic charge on the carbon atoms is relatively small. Then, our results show that the Laplacian distribution provide valuable information about the relative orientation of the species involved in the reaction, the interaction with the catalyst surface and the stability. That is, the stabilization and formation of a transition state (and then the chemical reaction) depends on the electron availability of the framework of catalyst, or in other words of the availability of electrons in the environment which plays a key role in stabilizing the carbocation formed. 1 Bader, R.F.W. Atoms in Molecules. A Quantum Theory; Oxford Science Publications: London, 1990.