Singlet phenylhalocarbenes are shown to react with triplet oxygen and the apparent spin- forbidden oxygenation rates are strongly dependent on substituents, ie in the decreasing order of Br> Cl≫ F for halogen and of p-NO2> H≫ p-MeO for p-substituents. These results suggest the oxygenation of triplet halocarbene equilibrated with ground-state singlet, resulting in the first estimation of energy difference between the singlet and triplet states.
