 The 1Σ singlet is 13 120.9 cm−1 (157.0 kJ/mol or 1.6268 eV) above the ground state.Radiative transitions between the three low-lying electronic states of oxygen are formally forbidden as electric dipole processes.The two singlet-triplet transitions are forbidden both because of the spin selection rule ΔS = 0 and because of the parity rule that g-g transitions are forbidden.