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

Abstract

a In this work, we applied the theory of Atoms in Molecules to the study of carbocationic transition states involved in two different reaction mechanisms, using the proposed mechanism of ethylene dimerization over Brønsted-acid site of zeolite catalyst as a study case. We report the main results of the analysis of the Laplacian of electron density distribution and Bader's atomic charges to increase our understanding of stereoelectronic aspects of these chemical reactions. At B3LYP/6-31++G(d,p)//B3LYP/6-3G(d,p) level, our results show that the envelopes of Laplacian distribution provide valuable information about the relative orientation of the species involved in the reaction, their interaction with the catalyst surface, and its stability. The stabilization and formation of these transition states depend on the availability of electrons in the environment. Such availability plays a key role to stabilize the positive charge on the carbocationic center and, thus, to stabilize the formed carbocation. The analysis of envelopes of Laplacian could be used to study different organic and inorganic chemical reactions and provide new insights to increase our understanding of these chemical reactions.

Molecular Insights on the Role of (CTA+)(SiO−) Ion Pair into the Catalytic Activity of [CTA+]–Si–MCM-41

“…MEPs based on electron density provide valuable information about the shape, volume, and electronic properties of are also observed the catalytic system, and it have been previously used in heterogeneous catalysis in order to predict interactions between the adsorbate and the catalyst [26,27]. Also, previous studies of reactions on zeolites showed that the stabilization and formation of intermediates and transition states, depend on the availability of electrons on the surface and the electronic availability plays a crucial role to stabilize the formed species [23,28,29].…”

“…Kubota et al reported that the high activity of Si-MCM-41 with a quaternary ammonium ion occluded emerges only when both parts form a composite [4]. Martins and co-workers studied the Knoevenagel condensation reaction using Si-MCM-41 molecular sieves as basic catalyst, where the pores were occluded by the SDA, and suggested from 29 Si NMR spectroscopy and O1s XPS measurements, that the active site is the ≡SiO − CTA + ionic pair, which is the basic site itself, also they suggested that the reaction should occur in the pore mouth [5]. Other work showed that different mesoporous materials synthesized with surfactants of different lengths in the carbon chain ensure greater catalytic activity when the occluded cation is more voluminous, this was attributed to the more weaker interaction with SiO − sites, thus increasing the SiO − basicity [9].…”