Protonation equilibria and stepwise hydrolysis behavior of a series of thiomonophosphate anions.

The stepwise protonation constants of a series of thiomonophosphate anions, i.e., monothiomonophosphate, dithiomonophosphate, trithiomonophosphate, and tetrathiomonophosphate anions, were determined by (31)P NMR chemical shift measurements in aqueous solution. Despite the remarkably fast hydrolysis rates of these anions, the protonation processes of all thiomonophosphate anions may be evaluated accurately without any previous purification, because the NMR signals corresponding to thiomonophosphate anions and hydrolyzed residues are well resolved. The stepwise protonation constants decrease with an increase in the number of sulfur atoms bound to the central phosphorus atom. It was revealed that the logarithms of the stabilities of the proton complexes of the series of thiomonophosphate anions decrease "linearly" with an increase in the number of sulfur atoms in the anions. The intrinsic (31)P NMR chemical shifts due to orthophosphate and tetrathiomonophosphate anions show upfield shifts upon successive protonations of the anions, whereas the shifts of mono-, di-, and trithiomonophosphate anions move downfield relative to the anions upon protonation. Furthermore, more asymmetric molecular structures experience greater changes in their X-P-Y bond angles upon protonation or complex formation, leading to drastic changes in the nuclear screening. The symmetry of the molecular structure is related to the direction of the (31)P NMR chemical shift change upon successive protonation of thiomonophosphate anions.