On the mechanism of dioxygen formation from a di-mu-oxo-bridged manganese dinuclear complex.
Density functional theory (DFT) calculations, using the Becke-Perdew gradient-corrected functional with a triple-zeta-plus-polarization basis set, have been used to characterize the [(H(2)O)(H(3)N)(3)Mn(mu-O)(2)Mn(NH(3))(3)(OH(2))](q)(+) (q = 2-5) complexes. This structure has been proposed as a possible model for the oxygen-releasing site of the photosystem II (PSII) reaction center. We have performed full optimizations to locate stationary points in various spin states for each of the +2 to +5 charge states. Our calculations indicate that O(2) release from the vacuum-phase +5 charge state complex is barrier inhibited, in contrast to the results of a recent DFT study. We report several new di-mu-oxo-bridged stationary points with spin multiplicities of S = (1)/(2), (3)/(2), and (5)/(2) and effective metal oxidation states of Mn(IV)Mn(V) for the +5 charge state. Finally, calculations employing the 'conductorlike screening model' (COSMO), to address the inclusion of solvent effects, indicate that dissociative O(2) release from the +5 charge state model complex is inhibited by a major barrier and is therefore apparently highly disfavored.
Department of Chemistry, The Faculties, The Australian National University, Canberra ACT 0200, Australia.
Simon
S
Petrie
Robert
R
Stranger