Do one‐step mechanisms always involve simultaneous evolution of electron density? QTAIM/IQA analysis of the Curtius rearrangement

Abstract

The Curtius rearrangement reaction is studied by using quantum theory of atoms in molecules (QTAIM) analysis of the electron density and the interacting quantum atoms (IQA) formalism. Although the rearrangements take place in one stage, two phases are distinguished when the rearranged atom is H: the first one corresponds to the separation of N2, and the second one to the N‐H/C‐H bond rearrangement. The transition state (TS) for the reaction does not represent an intermediate between reagent and product for the migration but for the isolation of the N2 molecule. When the migration is undergone by a fluorine atom, no electronic phases can be distinguished and the process is really concerted. As the migration happens closer to the TS, the TS is more similar to the product. The IQA analysis reveals different electron density evolutions for H and F migrations, and the scarce relevance (in terms of energy) of the point where BCPs appear or disappear.

The performance of exchange—correlation functionals in describing electron density parameters of saddle point structures along chemical reactions

“…Furthermore, there is a recent effort in analyzing reactions using the Interacting Quantum Atom (IQA) tools, 16,17 as represented by the studies from Milovanović et al, 18 focused on hydrogen transfer processes, from Viegas and Jensen, 19 who considered tropospheric reactions between the OH radical and α,ω‐dihydrofluoropolyethers, and also from López‐Fernández and collaborators, 20 investigating the Curtius rearrangement. Another example is illustrated by the work of Alkorta et al, 21 which uses the IQA partition for SN2 reactions with halogens.…”