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Аннотация Di-μ-oxo MnIII−MnIV dimers are models for coupled, mixed-valence manganese in the oxygen-evolving centers of green plants. Using a recently reported method of exchanging water oxygen into the di-μ-oxo cross-bridges , we have incorporated 17O into the μ-oxo cross bridges of the MnIII−MnIV bipyridyl dimer for study of oxygen electron-spin hyperfine couplings by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). The ENDOR evidence was for a di-μ-oxo 17O hyperfine coupling of 12.8 ± 1.0 MHz. Narrow and highly resolved EPR features from dimers exchanged with H216O (I = 0) became broadened when the dimer was prepared by exchanging the cross bridging oxygens with H217O (I = 5/2). The EPR broadening due to 17O was quantitatively reproduced by a model where the dimer has two equivalent di-μ-oxo cross-bridging 17Ο, and the 17O hyperfine coupling was highly consistent with that determined by ENDOR. This work explicitly points out evidence for covalent spin transfer to the cross-bridging di-μ-oxo oxygens which provide chemical bonds and antiferromagentic coupling between the mixed-valence manganese centers.