Trisubstituted heteropolytungstates as soluble metal oxide analogs. III. Synthesis, characterization, phosphorus-31, silicon-29, vanadium-51, and 1- and 2-D tungsten-183 NMR, deprotonation, and proton mobility studies of organic solvent solute forms of HxSiW9V3O40x-7 and HxP2W15V3O62x-9