  The magnetism of the polyoxometalate cluster {Mo75V20}, containing a sawtooth
ring of 10 corner-sharing triangles located on the equator of the barrel-shaped
molecule, has remained debatable since it is masked by contributions from
impurities as well as temperature-independent paramagnetism. In this article we
demonstrate the usefulness of ESR measurements since the temperature dependence
of the ESR intensity can discriminate between impurity and molecular
contributions. We determine the exchange parameters and therefore also the
low-lying spectrum of {Mo75V20}, especially the low-lying singlet states which
so far have been probed solely by specific heat measurements.
