SOLUTION AND SOLID-STATE PROTON-TRANSFER FROM PHENOLS TO TRIPHENYLPHOSPHINE OXIDE STUDIED BY H-1, C-13 AND P-31 NMR-SPECTROSCOPY

Several complexes between substituted phenols and triphenylphosphine o xide (TPPO) were examined both in solution and in the solid state by N MR spectroscopy. The degree of proton transfer from the phenol to TPPO in solution was studied by the H-1 chemical shift of the phenolic OH proton and by the C-13 chemical shifts of the phenol C-O (C1) and para (C4) carbons. As the pK(a) of the acid decreases, the H-1 signal move s towards higher frequencies as a consequence of the deshielding produ ced by the proximity of the second oxygen, However, when the hydrogen is largely transferred to the oxide moiety, e.g. in the picric acid co mplex, the H-1 resonance shifts to lower frequencies again. In turn, t he C-13 chemical shift of C1 is displaced to high frequencies, whereas C4 shifts in the reverse direction. The solid phase was studied throu gh the changes in the P-31 Shielding tensor of the TPPO residue and by the C1 and C4 C-13 chemical shifts of the phenols, The proton transfe r process follows the same pattern observed in solution. The values of the principal components of the P-31 Shielding tensor (sigma(ii)) mov e towards those corresponding to symmetric tetrahedral phosphorus envi ronments as the pK(a) of the phenol decreases.