Orbital States
Molecular orbital theory predicts two low-lying excited singlet states O2(a¹Δg) and O2(b¹Σg+) (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 πg-orbitals (see degenerate energy level). The O2(b¹Σg+)-state is very short lived and relaxes quickly to the lowest lying excited state, O2(a¹Δg). Thus, the O2(a¹Δg)-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 (Te (a¹Δg <- X³Σg-) = 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(X³Σg-). 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.
Read more about this topic: Singlet Oxygen
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