Unraveling reaction mechanisms by means of Quantum Chemical Topology Analysis

Abstract

A chemical reaction can be understood in terms of geometrical changes of the molecular structures and reordering of the electronic densities involved in the process; therefore, identifying structural and electronic density changes taking place along the reaction coordinate renders valuable information on reaction mechanism. Understanding the atomic rearrangements that occur during chemical reactions is of great importance, and this perspective aims to highlight the major developments in quantum chemical topology analysis, based on the combination of electron localization function and catastrophe theory as useful tools in elucidating the bonding and reactivity patterns of molecules. It reveals all the expected, but still ambiguous, elements of electronic structure extensively used by chemists. The chemical bonds determine chemical reactivity, and this technique offers the possibility of their visualization, allowing chemists to understand how atoms bond, how and where bonds are broken/ formed along a given reaction pathway at a most fundamental level, and so, better following and understanding the changes in the bond pattern. Their results clearly herald a new era, in which the atomic imaging of chemical bonds will constitute a new method for examining chemical structures and reaction mechanisms. The important feature of this procedure is that in practice the scope of its values is systemindependent. In addition, from a practical point of view, it is cheap to calculate and implement because wave functions are the required input, which are easily available from standard calculations. To capture these results two reaction mechanisms: isomerization of C(BH) 2 carbene and the thermal cycloheptatriene-norcaradiene isomerizations have been selected, indicating both the generality and utility of this type of analysis.

Curly arrows meet electron density transfers in chemical reaction mechanisms: from electron localization function (ELF) analysis to valence-shell electron-pair repulsion (VSEPR) inspired interpretation

“…The potential applications of the method cover the determination of the mechanisms of reactions belonging to organic as well as inorganic chemistry as done in recent studies. 79,81,82,122,136,143,146,[167][168][169][170][171][172][173] We believe that the approach proposed here can thus be useful not only in a variety of circumstances to provide quantitative insights into the nature of chemical reactivity and for the modelling of reaction mechanism, based on the electron density transfers but also it should become a powerful tool in the chemical education of undergraduate students because our theoretical findings can serve as a general guideline for the study and analysis of the chemical structure and reaction mechanisms. Our understanding of the chemical structure and reactivity is usually built up from and dependent upon such intuitive concepts as atom in molecule, chemical bond, lone pair, Lewis structure, etc.…”

“…In particular, BET retrieves the classical curly arrows used to describe the rearrangements of chemical bonds for a given reaction mechanism, providing detailed physical grounds for this type of representation. [79][80][81][82][83] A successive detection of the electron density changes along a chemical reaction, in which a continuous redistribution of r(r) proceeds, can provide valuable information on the interconnection of the structure of the charge density distribution and the nuclear geometry. This paper, as a tribute to the centenary of the publication of G. N. Lewis' ground breaking paper on The Atom and the Molecule, 5 presents a short review of determination of reaction mechanisms by the BET procedure with the goal to reconcile how curly arrows meet electron density transfers in chemical reaction mechanisms.…”

Understanding the mechanism of the decomposition reaction of nitroethyl benzoate through the Molecular Electron Density Theory

“…To this end, the Bonding Evolution Theory (BET) [21], which combines the topological analysis of the electron localisation function (ELF) [22] with the mathematical Catastrophe Theory (CT), was proposed [23][24][25]. BET has proved to be a useful quantum chemical tool for the study of reaction mechanisms [26][27][28][29].…”

Chemical structure and reactivity by means of quantum chemical topology analysis

“…Nizovtsev [169] has also studied the activation of C-H bond in CH 4 by Pd atom as well as electronic rearrangements during the Inversion of Lead Phthalocyanine [170] from a BET perspective to establish electron density redistribution in the course of structural rearrangements. Different examples where BET has been successfully applied to rationalize chemical reactivity have been reviewed comprehensively [155,158,171].…”