NMR study of the prototropic tautomerism of tris(tetrabutylammonium)hydrogen pyrophosphate salt in solution and in the solid-state

The structure of tris(tetrabutylammonium)hydrogen pyrophosphate (HPP) has been studied in solution and in the solid-state by ³¹P NMR. GIAO/DFT calculations of the isolated molecule in its open and closed conformations, of their water complexes and the solvent effects (continuum model) have been useful to discuss the experimental NMR data. Contrary to literature reports, the ³¹P NMR spectra of HPP in solution have a single band, that broadened considerably when the temperature is lowered till 193 K without reaching the coalescence. In the solid-state, two very close signals are observed that the calculations do not reproduce adequately.