Singlet Oxygen - Orbital States

Orbital States

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 the lowest energy of O₂ in the singlet state and the lowest energy in the triplet state is about 11340 kelvin (T_e (a¹Δ_g <- X³Σ_g-) = 7882 cm−1, 94.3 kJ/mol, 0.98 eV) Molecular oxygen differs from most molecules in having an open-shell triplet ground state, O₂(X³Σ_g-). Although the three lowest energy states of oxygen can be described by the simple scheme in the figure below, this is a simplification. The excited states of oxygen are made up of combinations of electronic states. The electrons paired in the same orbital, while the first excited state involves states with the electrons in separate degenerate orbitals, as might be expected from Hund's rule.