Magnetic and Electronic Structural Properties of the S3 State of Nature's Water Oxidizing Complex: A Combined Study in ELDOR- Detected Nuclear Magnetic Resonance Spectral Simulation and Broken-Symmetry Density Functional Theory

ELDOR-detected nuclear magnetic resonance (EDNMR) spectral simulations combined with broken-symmetry density functional theory (BS-DFT) calculations are used to obtain and to assign the 55Mn hyperfine coupling constants (hfcs) for modified forms of the water oxidizing complex in the penultimate S3 state of the water oxidation cycle. The study shows that an open cubane form of the core Mn4CaO6 cluster explains the magnetic properties of the dominant S = 3 species in all cases studied experimentally with no need to invoke a closed cubane intermediate possessing a distorted pentacoordinate Mn4 ion as recently suggested. EDNMR simulations found that both the experimental bandwidth and multinuclear transitions may alter relative EDNMR peak intensities, potentially leading to incorrect assignment of hfcs. The implications of these findings for the water oxidation mechanism are discussed.