Modification of Infrared Spectra of the Herbicide Glyphosate induced by pH variation

An infrared spectroscopy study was conducted on the herbicide Glyphosate (Roundup) at increasing pH. The IR bands followed the dissociation of the Glyphosate acidic groups at the different pHs. In particular the IR bands at about 1180 and 1090 cm‐1 were attributed to the P‐O stretching of the P‐OH and P‐O groups of the phosphono radical, respectively. In fact, the band at around 1180 cm‐1 appeared progressively reduced with increasing pH, whereas the absorption at about 1090 cm‐1 was concomitantly enhanced. This attribution had not been clearly indicated up to now in the literature, where the IR bands at about 1180 and 1090 cm‐1 were instead generally attributed to the asymmetric and symmetric stretchings of the P‐O bond of all three possible forms of the phosphono radical (‐PO3H2, ‐PO3H‐and ‐PO3 2‐). An increasing broadth of the bands was related to the enhanced number of possible intermolecular hydrogen bonds when the Glyphosate acidic functions were increasingly dissociated at higher pH.