Electronic structure of established and potential oxidizing agents in biological systems

Abstract

The electronic structure of 19 established and potential biological oxidants has been studied by semiempirical all-valence-electron quantum-chemical methods. Electronic ground and excited states of O 2, HO 2, HO, H 2O 2, H 3O, H 4O 2 and their (radical) ions have been investigated in order to get information on the geometry, vertical ionization potentials, vertical electron affinities and low-lying electronic excited states. The actual aim has been (i) to arrange the studied species according to their oxidizing power as given by gas-phase electron affinity. 9·HO·OH 2O ∗ 2>( 1ϵ + g).·OH>O ∗ 2( 1δ + g) >HO ∗ 2( 2A′)>O ∗ 2( 2A′)>O 2( 3ϵ - g>HO· 2) and (ii) to contribute to the thermodynamics of early changes of the O 2 molecule O 2+e→O ⊖ 2·;O ⊖ 2·+H +→HO· 2 . Moreover, it has been found theoretically that the hydrated form of the hydroxyl radical (·HO.OH 2) should be a relatively stable species with very high electron affinity (2·4 eV, INDO method). This circumstance and the theoretically predicted, extraordinarily low-lying, excited doublet state of the peroxyl radical (about 6000 cm −1) could be of biological significance.