Deprotonation of the amine group of Glyphosate studied by XPS and DFT

Highlights: • N1s spectra show that the last deprotonation of glyphosate occurs at the amine group. • P2p spectra exhibit only minor changes in the range 2 < pH < 12. • DFT calculations also predict that the last deprotonation occurs at the amine group. • DFT core-level binding energies in good agreement with XPS measurements. Glyphosate (N-(phosphonomethyl)glycine, C₃H₈NO₅P) in aqueous solutions undergoes three deprotonation reactions generally assigned to the carboxylate oxygen (pK_a1 ~ 2.2), the phosphonate oxygen (pK_a2 ~ 5.5), and the amine group (pK_a3 ~ 10.2). A couple of recent studies (J. Phys. Chem. A 2015, 119, 5241–5249 and J. Phys. Chem. B 2016, 120, 2132–2137), however, have put forward a new sequence in which the amine group deprotonates before the phosphonate oxygen. In this study we present experimental and theoretical data with the aim of resolving the controversy over the order of the last two deprotonations. On the experimental side, we analysed the evolution of the N1s photoemission spectrum with the pH of the aqueous solution, finding conclusive evidence that the last deprotonation occurs at the amine group. On the theoretical side, we compared the stabilities of several configurations for each glyphosate moiety, finding again that the last group that undergoes deprotonation is the amine group.