On the existence of a spin bipolaron. Evidence from the 183W NMR of mixed valence polyoxotungstate anions

The 183W NMR chemical shifts of polyoxotungstates reduced by two electrons regarded as a bipolaron have been reconsidered. It is assumed that the bipolaron formed by two W induces a spin bipolaron in nearby tungsten atoms manifested in NMR spectra by their substantial shielding. A quantitative estimation of the chemical shifts resulting from the delocalization process is given.