What is singlet oxygen?
Oxygen makes up about 20% percent of the air and is essential to life and various chemical reaction.
The oxygen molecule is a diradical, as its lowest electronic state is a triplet (3Σg-) state in which two unpaired electrons are distributed in the two highest occupied degenerate orbitals.
Oxygen in the triplet state (3O2) is not very reactive. However, excitation of the molecule will result in the rearrangement of the electron spins and the orbital occupancy to form two possible singlet electronic states, 1Δg and 1Σg+, who are highly reactive.
Singlet oxygen (1O2) is increasingly used for medical applications such as respiratory, skin and cancer treatment, and industrial applications such as water treatment and inducing chemical reactions.
The 1Σg+ is very reactive and has a relatively short lifetime as it tends to quickly relax to the lower energy 1Δg state.
Therefore, the 1Δg singlet state, that is only 23 kcal above that of the ground state, is the state involved in most oxygen reactions that do not involve radicals and is the state that is referred to when discussing oxygen singlet state, 1O2.