As already described in section 2.3 (e), dioxygen, O2, in the ground state has two unpaired spins in its molecular orbitals, shows paramagnetism and is called triplet
dioxygen. In the excited state, the spins are paired and dioxygen becomes diamagnetic,
which is called singlet dioxygen. Singlet dioxygen is important in synthetic chemistry,because it has characteristic oxidation reactivity. Singlet dioxygen is generated in a
solution by an energy transfer reaction from a photo-activated complex or by the
pyrolysis of ozonides (O3 compounds).