Computational study of the dimerization of glyphosate: mechanism and effect of solvent

A computational study on the structure and stability of different series of glyphosate (Glyph) dimers comprising nonionized (N) and zwitterionic structures (Z) for neutral monomers, followed by an analysis of energetics of Glyph dimerization process have been performed by means of quantum chemical calculations in different media. Optimized geometries for energy minima, as well as relative potential and free energies of the possible various conformers of each series of Glyph dimer were computed as a function of the medium at B3LYP-D3/6-311++G(2d,2p) level. The solvation model based on density (SMD) is employed for all solution phase computations. Non-ionized dimers (DN), anion-cation (AC) and either zwitterion-zwitterion (DZP and DZC) or non-ionized-phosphonate zwitterion (NZP) ionized neutral forms of Glyph dimer are predicted to exist in the gas phase and in solution in large contrast to Glyph monomers. The DZC dimer form exhibiting a centrosymmetric arrangement of two carboxylate zwitterion units was found to be the most stable dimer structure in all media. In aqueous solution, the DZP and AC dimer type structures are significantly stabilized by hydration. The tautomerisms between DZC, DZP and AC dimer type structures have been investigated in the gas phase and in solution. The DZC type structures are more prone to experience proton transfer in water than in the gas phase and in cyclohexane. The mechanism for the tautomerization process in neutral ionized Glyph dimers proceeds via two direct proton transfer paths: DZP ⇋ AC ⇋ DZC. Results show that solvents play a key role in modulating the energetics of the dimerization process of Glyph. Solvation in cyclohexane, favors the dimerization process however, hydration opposed it. In aqueous solution, the mechanism of the dimerization of Glyph from its phosphonate zwitterionic monomer form (ZP) could be described by a set of equilibria including direct proton transfer paths as follows: 2ZP ⇋ DZP ⇋ AC ⇋ DZC. According to our results, in aqueous solution, DZC Glyph dimers and their corresponding DZP and AC tautomers should be present in higher concentration than phosphonate zwitterionic Glyph monomers for high Glyph concentration, a fact that seems controversial in the literature.