No abstract

The temperature dependence of the 7 Li NMR shift was measured for LiMn 2 O 4 , LiMn 2-y Ni y O 4 (y ) 0.1, 0.25, 0.33), LiMn 2-y Co y O 4 (y ) 0.25, 0.5, 1.0), Li[Mn 2-y Li y ]O 4 (y ) 0.1, 0.33), and λ-MnO 2 spinel oxides. The 7 Li NMR shift can be separated into temperature-independent and -dependent components. The temperaturedependent shift follows the Curie-Weiss behavior of the bulk magnetic susceptibility. The temperatureindependent shift is attributed to contributions from van Vleck and diamagnetic susceptibilities. Pauli susceptibility may also contribute to the temperature-independent shift in the nickel-and cobalt-substituted spinels. Supertransferred hyperfine (STH) coupling constants were derived from the 7 Li NMR shifts and bulk magnetic susceptibility data. The progressive increase in average nominal manganese oxidation state from +3.5 to +4 results in an increase in the supertransferred hyperfine field at the 7 Li nucleus in the lithiumsubstituted samples. Replacement of manganese by either cobalt or nickel also results in a larger STH field at the 7 Li nuclei. The increase in STH field for the lithium-, nickel-, and cobalt-substituted spinel oxides may arise from a greater covalence in these materials relative to the parent LiMn 2 O 4 spinel oxide.

The nature of the Mn oxidation states involved in photosynthetic oxygen evolution has remained controversial, despite intense study by X-ray absorption and electron paramagnetic resonance spectroscopy. As an alternative approach, high-resolution Kβ X-ray fluorescence spectra have been recorded on the dark-adapted S1 state and the hydroquinone-reduced state of the oxygen-evolving complex in photosystem II. By comparison of the Kβ chemical shifts with those of appropriate model compounds, the S1 state of photosystem II is found to contain equal amounts of Mn(III) and Mn(IV). In the hydroquinone-reduced sample, a significant fraction of the Mn is reduced to Mn(II). The results are compatible with models involving conversion of Mn(III)2Mn(IV)2 to Mn(II)2Mn(IV)2 clusters.