T. Ueda et al., Structure and properties of acidic protons in anhydrous dodecatungstophosphoric acid, H3PW12O40, as studied by solid-state H-1, H-2 NMR, and H-1-P-31SEDOR NMR, J PHYS CH B, 105(23), 2001, pp. 5391-5396
The binding position and the electronic properties of acidic proton in anhy
drous dodecatungstophosphoric acid, H3PW12O40, were studied by means of H-1
, H-2 NMR, and H-1-P-31 spin-echo double resonance (SEDOR) NMR techniques.
H-1 broad line NMR spectrum showed a bell type resonance line shape with 4.
7 kHz line width and MAS NMR spectrum has a single resonance peak, suggesti
ng that the hetero-polyanion PW12O403- provides a single site to the acidic
proton. 2H NMR spectrum leads to the quadrupole coupling constant (QCC) of
210 kHz and the asymmetry parameter of the electric-field-gradient tenser
(eta) Of 0.15. The value of QCC suggests that the deuterons are bonded to o
xygen atoms in the hetero-polyanion very rigidly, and that any motion of th
e hetero-polyanion does not take place. A remarkable decay of the H-1-P-31
SEDOR signal was observed, due to the dipolar interaction between the centr
al P-31 and three protons strongly bonded to the hetero-polyanion. The P-H
distance was evaluated by simulating the decay behavior of the echo to be 0
.50 +/- 0.02 nm. Comparing this P-H distance with model structures in which
protons are bonded to corner-shared, edge-shared, or terminal oxygen atom
of WO6 unit in PW12O403-, the edge-shared oxygen atom seems to be the most
probable binding position for the acidic proton.