ENDOR spectra of the intercalated water molecules in vanadyl(IV, V) phosphate pentahydrate

The ENDOR spectra of the metastable expanded layer solid VOPO4·5H2O and its congener VOPO4·2H2O [both containing very small amounts of vanadium (IV)] have been measured at 4 K. The latter gave no ENDOR, whereas the former provided a spectrum in which three different proton signals ascribable to two interlayer water molecules were detected. From a reconstruction of the interlayer, taking into account the layer expansion of 3.1 Å due to the intercalated water molecules, it is suggested that VOPO4·5H2O is an example of a 'pillared' material.