  The O-O coupling process is considered on terminal monocoordinated oxo
centers in the gamma FeOOH hydroxide modeled by iron tetramer cubane cluster
with the Fe4O4 core. The density functional theory predicts that reactive
HO-Fe(IV)-O$\bullet$ group formed from hydroxide by second withdrawal of
proton-electron pair is capable to couple the OH moiety of water molecule with
a low barrier. This process is far more effective than direct coupling of oxo
centers on neighboring metal sites and is comparable with the coupling between
terminal oxo center and three-coordinated lattice oxo center. The competing
process of hydroxylation of oxyl oxygen to form the hydroxo group is equally
probable having similar barrier.
