Unraveling the Bonding Nature Along the Photochemically Activated Paterno-Büchi Reaction Mechanism.

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 (C-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 C-O bond is formed in the ground-state surface (via C-O attack) returning from the S1 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 C-C bond formation after the TS. For the carbon-carbon attack (C-C), both reactants (formaldehyde and ethylene) are activated, leading to C-C bond formation in the S1 excited state before reaching the conical intersection region. Finally, the C-O formation occurs in the ground-state surface, resulting from the pair density flowing primarily from the C to O atom.