Figure 1. Chemical Properties of Singlet Oxygen. Whereas ground state molecular oxygen is in a triplet state, with two unpaired, spin-parallel electrons (A), singlet oxygen has two valence electrons spin-paired in a single orbital and a second orbital left empty. Consequently, singlet oxygen is more reactive than triplet oxygen, and can react with carbon double bonds in a variety of molecules through ene reactions (C) and Diels-Alder cycloadditions (D) that triplet oxygen cannot perform.