This work revisits the topological characterization of the Diels-Alder reaction between 1,3-butadiene and ethylene. In contrast to the currently accepted rationalization, we here provide strong evidence in support of a representation in terms of seven structural stability domains separated by a sequence of 10 elementary catastrophes, but all only of the fold type, i.e., C4H6 + C2H4 :[FF]F † -0 : C6H10. Such an unexpected finding provides fundamental new insights opening simplifying perspectives concerning the rationalization of the CC bond formation in pericyclic reactions in terms of the simplest Thom's elementary catastrophe, namely the one-(state) variable, one-(control) parameter function.
In this work, the 2s+2s (face-to-face) prototypical example of a photochemical reaction has been re-examined to characterize the evolution of chemical bonding. The analysis of the electron localization function (as...
1,3-Cyclohexadiene
ring opening has been studied within the bonding
evolution theory (BET) framework. We have focused on describing for
the first time the electron pair rearrangements leading to the cis-1,3,5-hexatriene (HT) product from CHD. The nature of
bonding in this process begins with the weakening of the double bonds
in the Franck–Condon region. Along the 11B surface,
the C–C sigma bond weakens. Meanwhile, its density redistributes
toward the whole CHD ring, mainly over double bonds. Breaking of this
bond occurs on the 21A surface due to the symmetrical splitting
of pair density from this region. This density redistributes toward
the reaction center once the pericyclic minimum is reached. The formation
of the double bonds that characterize HT occurs gradually in the ground
state. However, near the 21A/11A intersection,
these bonds are partially established.
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.
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