| Abstract
| - The zwitterionic nature of N-(phosphonomethyl)glycine (glyphosate, PMG) and the associated intramolecular hydrogen bonding is investigated with IR spectroscopy and ab initio frequency calculations. New evidence is presented for the structures of Cu(PMG)-, CuH(PMG), and Cu(PMG)24- in aqueous solution, based on the combined findings of IR and EXAFS spectroscopies.
- The varying degrees of protonation of N-(phosphonomethyl)glycine (PMG, glyphosate) were investigated withinfrared (IR) spectroscopy and ab initio frequency calculations. The zwitterionic nature of PMG in solution wasconfirmed, and intramolecular hydrogen bonding was identified. Successive protonation of the PMG moleculefollows the order amine, phosphonate, carboxylate. Intramolecular hydrogen bonding is indicated to exist at allstages of protonation: between both RCO2- and RNH2+ and RPO32- and RNH2+ in HL2- (where L representsthe ligand PMG); between RCO2- and RNH2+ in H2L-; predominantly between RPO32- and RNH2+ in H3L.There are strong indications that the zwitterion is intact throughout the pH range investigated. Results from IRand extended X-ray absorption fine structure (EXAFS) spectroscopies provide new evidence for structures ofN-(phosphonomethyl)glycinecopper(II) complexes. The structures of 1:1 complexes, CuL- and CuHL, areessentially the same, differing only in protonation of the phosphonate group. Copper(II) lies at the center of aJahn−Teller distorted octahedron with all three donor groups (amine, carboxylate, phosphonate) of PMG chelatingwith copper(II) to form two five-membered chelate rings oriented in the equatorial plane. EXAFS indicates thatoxygen (most likely a water molecule) is a fourth ligand, which would thus occupy the fourth corner in theequatorial plane of the elongated octahedron. CuL24- most probably forms an isomeric mixture in solution, andthere are indications that this mixture is dominated by complexes where two PMG ligands are bound to copper(II) via equatorial and axial positions, with both phosphonate and carboxylate donor groups responsible for chelationat axial positions.
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