Unraveling the Bonding Nature Along the Photochemically Activated Paterno‐Büchi Reaction Mechanism

Abstract

The photochemically activated Paterno-Büchi reaction mechanism following the singlet excited-state reaction path was analyzed based on a bonding evolution framework. The electronic rearrangements, which describe the mechanism of oxetane formation via carbon-oxygen attack (CÀ O), comprises of the electronic activation of formaldehyde and accumulation of pairing density on the O once the reaction system is approaching the conical intersection point. Our theoretical evidence based on the ELF topology shows that the CÀ O bond is formed in the ground-state surface (via CÀ O attack) returning from the S 1 surface accompanied by 1,4-singlet diradical formation. Subsequently, the reaction center is fully activated near the transition state (TS), and the ring-closure (yielding oxetane) involves the CÀ C bond formation after the TS. For the carbon-carbon attack (CÀ C), both reactants (formaldehyde and ethylene) are activated, leading to CÀ C bond formation in the S 1 excited state before reaching the conical intersection region. Finally, the CÀ O formation occurs in the ground-state surface, resulting from the pair density flowing primarily from the C to O atom.

Photochemically Induced 1,3‐Butadiene Ring‐Closure from the Topological Analysis of the Electron Localization Function Viewpoint

“…Following such interests devoted to unraveling key bonding events occurring in photo-induced pericyclic reactions, we have concentrated recent interests on the usefulness of the so-called bonding evolution theory (BET). [22][23][24][25] BET [26] is based on the topology analysis of the electron localization function (ELF� h r ð Þ) [27,28] performed along a chosen reaction coordinate. [26,[29][30][31] ELF constitutes a measure of pair electron localization according to the Pauli-principle; in particular, the ELF gradient field, rh r ð Þ, provides a representation in terms of Lewis-like structures from its critical points (CPs).…”

“…Following ongoing interests [22][23][24][25] to further enhance our current knowledge of photochemical processes by exploiting the electron-pair approach, this work focuses on describing the reaction mechanism of one of the simplest photochemical pericyclic processes, namely the conversion of cis-1,3-butadiene (cis-BD) [44] into cyclobutene (CB), see scheme 1.…”

“…We aim to contribute to a deeper understanding of chemical bonding events in the electronically excited states. [22][23][24][25]…”

“…Following such interests devoted to unraveling key bonding events occurring in photo‐induced pericyclic reactions, we have concentrated recent interests on the usefulness of the so‐called bonding evolution theory (BET) [22–25] . BET [26] is based on the topology analysis of the electron localization function (ELF≡ ) [27,28] performed along a chosen reaction coordinate [26,29–31] .…”

“…We propose to study these details within the topological‐based framework offered by the BET. We aim to contribute to a deeper understanding of chemical bonding events in the electronically excited states [22–25] …”