Deprotonation of Glyphosate studied with X-ray Photoelectron Spectroscopy

LUIS MIGUEL RODRÍGUEZ; PEDANO, MARÍA LAURA; ALBORNOZ, M.; MARTIARENA, M.L.; ZAMPIERI, G.

S. C. de Bariloche

Congreso; SLAFES XXIII; 2018

Centro Atómico Bariloche

The three chemical groups of glyphosate undergo deprotonation as the pH of the solution is increased. There is a generally accepted sequence for the deprotonation that has been studied mainly by infrared spectroscopy (IR) and nuclear magnetic resonance (NMR), but controversy has arisen recently about the order in which the carboxylic and phosphonic groups deprotonate. X-ray Photoelectron spectroscopy (XPS) is also well suited for these studies because the protonated and deprotonated moieties of the molecule can in principle be distinguished through changes of the core-level binding energies. With XPS, however, only dry droplets of the solution deposited on a substrate can be analysed, what opens the question of the possible influence of the surface on what is being investigated.In this report we analyse the deprotonation of the amino and phosphonic groups in solutions with increasing pH in the range 1 < pH < 12 through the N1s and P2s photoelectron spectra measured in droplets deposited on Si. O1s and C1s spectra were also measured, but the use of these spectra is hindered by the presence ofspurious components ascribed to contamination. We have found that the N1s core level undergoes a large shift upon deprotonation (∼ 2 eV), in good agreement with previous reports. The deprotonation, however, occurs at a smaller pH than reported in NMR studies, what is attributed to the influence of OH groups on the Si surface. On the contrary, we have found only a minor change of the P2s core level in the whole series. We present also results of calculations performed with density-functional theory which are in good agreement with the experimental findings.