Motions of diaquahydrogen ions and phase transitions in dodecatungstophosphoric acid hexahydrate studied by H-1 nuclear magnetic resonance technique

The motions of the planar diaquahydrogen ion H5O2+ in H3PW12O40. 6H(2)O are studied by H-1 nuclear magnetic resonance second moment (M-2) and spin-lat tice relaxation time (T-1) measurements. The phase transitions were observe d at 144 and 313 K by differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Comparison between the experimental and calcula ted M-2 values suggests that the pseudo-C-4 reorientation around the centra l H+ occurs at temperatures higher than 140 K and that H3O+-H2O behavior ap pears at temperatures higher than 315 K. Since the longitudinal magnetizati on relaxed nonexponentially, T-1 was also estimated theoretically by applyi ng the hindered rotation treatment to magnetic relaxation for a five-intera cting proton system. The enthalpy change (DeltaH) of 260 +/- 30 Jmol(-1) an d the difference between the experimental and calculated T-1 minimum values show that at the order-disorder phase transition point of 144 K the potent ial energy well for the H5O2+ ion changes from a single potential well to u nequal potential wells whose energy difference is 4-5 kJ mol(-1). The room temperature phase is dynamically disordered with the pseudo-C-4 reorientati on of the H5O2+ ion. (C) 2000 American Institute of Physics. [S0021-9606(00 )50941-6].