Direct Detection of a Chemical Equilibrium between a Localized Singlet Diradical and Its σ‐Bonded Species by Time‐Resolved UV/Vis and IR Spectroscopy

Localized singlet diradicals are key intermediates in bond homolyses. The singlet diradicals are energetically much less stable than the σ-bonded species. In general, only one-way reactions from diradicals to σ-bonded species are observed. In this study, a thermal equilibrium between a singlet 1,2-diazacyclopentane-3,5-diyl diradical and the corresponding σ-bonded species was directly observed. The singlet diradical was more stable than the σ-bonded species. The solvent effect clarified key features, such as the zwitterionic character of the singlet diradical. The effect of the nitrogen atoms is discussed in detail.