Curly arrows, electron flow, and reaction mechanisms from the perspective of the bonding evolution theory

Abstract

Despite the usefulness of curly arrows in chemistry, their relationship with real electron density flows is still imprecise, and even their direct connection to quantum chemistry is still controversial. The paradigmatic description - from first principles - of the mechanistic aspects of a given chemical process is based mainly on the relative energies and geometrical changes at the stationary points of the potential energy surface along the reaction pathway; however, it is not sufficient to describe chemical systems in terms of bonding aspects. Probing the electron density distribution during a chemical reaction can provide important insights, enabling us to understand and control chemical reactions. This aim has required an extension of the relationships between the concepts of traditional chemistry and those of quantum mechanics. Bonding evolution theory (BET), which combines the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT), provides a powerful method that offers insight into the molecular mechanism of chemical rearrangements. In agreement with the laws of physical and aspects of quantum theory, BET can be considered an appropriate tool to tackle chemical reactivity with a wide range of possible applications. In this work, BET is applied to address a long-standing problem: the ability to monitor the flow of electron density. BET analysis shows a connection between quantum mechanics and bond making/forming processes. Likewise, the present approach retrieves the classical curly arrows used to describe the rearrangements of chemical bonds and provides detailed physical grounds for this type of representation. We demonstrate this procedure using the test set of prototypical examples of thermal ring apertures, and the degenerated Cope rearrangement of semibullvalene.

Deciphering the Curly Arrow Representation and Electron Flow for the 1,3-Dipolar Rearrangement between Acetonitrile Oxide and (1S,2R,4S)-2-Cyano-7-oxabicyclo[2.2.1]hept-5-en-2-yl Acetate Derivatives

“…31 BET allows us to perceive electronic rearrangement and bonding changes taking place along a reaction pathway. 32 − 42 During the BET analysis, only three types of bifurcation catastrophes have been found in chemical reactivity and two of them have been identified in the present study: the fold catastrophe, corresponding to the creation or annihilation of two critical points of different parity, and the cusp catastrophe, which transforms one critical point into three (and vice versa) such as in the formation or the breaking of a covalent bond.…”

Topological investigation of the reaction mechanism of glycerol carbonate decomposition by bond evolution theory

“…22,23 For more than twenty years, the topological analysis of the ELFs has been adapted to the study of different types of chemical bonds [24][25][26] and reaction mechanisms. [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] Indeed, such an approach enables to divide the space into different regions corresponding to the chemical objects developed in Lewis valence theory 19 or in the Valence Shell Electron Pair Repulsion (VSEPR) theory, [42][43][44] i.e. the bonds, free pairs, p systems, etc.…”

Real-Space Approach to the Reaction Force: Understanding the Origin of Synchronicity/Nonsynchronicity in Multibond Chemical Reactions

“…In this last region the covalent interactions become dominant, thus driving the process towards the final products. [22][23][24] In this sense, the scope of these interactions in determining the initial orientation mode of the reactants can be responsible of the mechanistic issues that have been exclusively ascribed to the TS such as the stereo-and regioselectivity, as well as the synchronicity/asynchronicity of multibond chemical reactions. 17,25 Therefore, a deeper understanding of the reaction mechanisms can be achieved by combining the classical IRC-based analyses with the specific role of non-covalent interactions (the so-called molecular recognition).…”