Singlet oxygen

Molecular Orbital diagram for singlet oxygen. Quantum mechanics predicts that this configuration with the paired electrons is higher in energy than the triplet ground state.

Singlet Oxygen is the common name used for the two metastable states of molecular oxygen (O₂) with slightly higher energy than the ground state triplet oxygen.

Molecular oxygen differs from most molecules in having an open-shell triplet ground state, O₂(X³Σ_g^-). Molecular orbital theory predicts two low-lying excited singlet states O₂(a¹Δ_g) and O₂(b¹Σ_g⁺)(for nomenclature see article on Molecular term symbol). These electronic states differ only in the spin and the occupancy of oxygen's two degenerate antibonding π_g-orbitals (see Degenerate energy level). The O₂(b¹Σ_g⁺)-state is very short lived and relaxes quickly to the lowest lying excited state, O₂(a¹Δ_g). Thus, the O₂(a¹Δ_g)-state is commonly referred to as Singlet Oxygen.

The energy difference between ground state and singlet oxygen is 94.2 kJ/mol and corresponds to a transition in the near-infrared at ~1270 nm. In the isolated molecule, the transition is strictly forbidden by spin, symmetry and parity selection rules, making it one of natures most forbidden transitions. In other words, direct excitation of ground state oxygen by light to form singlet oxygen is very improbable. As a consequence, singlet oxygen in the gas phase is extremely long lived (72 minutes). Interaction with solvents, however, reduces the lifetime to microsecond or even nanoseconds.

The chemistry of Singlet Oxygen is different from that of ground state oxygen. Singlet Oxygen can participate in Diels-Alder reactions and ene reactions. It can be generated in a photosensitized process by energy transfer from dye molecules such as Rose Bengal, Methylene Blue or porphyrins, or by chemical processes such as spontaneous decomposition of hydrogen trioxide in water or the reaction of hydrogen peroxide with hypochlorite. It is the active species in photodynamic therapy.

Direct detection of singlet oxygen is possible through its extremely weak phosphorescence at 1270 nm, which is not visible to the eye. However, at high singlet oxygen concentrations, the fluorescence of the so-called singlet oxygen dimol (simultaneous emission from two singlet oxygen molecules upon collision) can be observed as a red glow at 634 nm.

References

1. Interpretation of the atmospheric oxygen bands; electronic levels of the oxygen molecule R.S. Mulliken Nature (journal) Volume 122, Page 505 1928
2. Physical Mechanisms of Generation and Deactivation of Singlet Oxygen C. Schweitzer, R. Schmidt Chemical Reviews Volume 103, Pages 1685-1757 2003