Study of the interactions between Brønsted acids and triethylphosphine oxide in solution by 31P NMR: evidences for 2:1 species
The variation of the 31P chemical shift of triethylphosphine oxide (TEPO) in CDCl3 solution with a series of Brønsted acids at different molar ratios allows to determine the value for the 1:1 species (δ1:1), which is much lower than the reported value at infinite dilution. This value correlates with the pKa of the acid in two zones, for acids stronger and weaker than TEPO-H+. The acid strength also controls the exchange rate in solution. The evolution of the chemical shift at high acid/TEPO molar ratios indicates the existence of a second TEPO-acid interaction, which is also dependent on the acid strength. This interaction is much more favorable in the case of a diacid, which shows chemical shifts higher than expected for its pKa1 value.