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Tautomeric and E ‐ Z equilibria of the herbicide clethodim in water and organic solvents: A nuclear magnetic resonance and theoretical study
Author(s) -
Caputo Maricel,
Colasurdo Diego D.,
Allegretti Patricia E.,
Laurella Sergio L.
Publication year - 2020
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.4108
Subject(s) - tautomer , chemistry , chemical shift , resonance (particle physics) , hydrogen bond , solvent , computational chemistry , proton , molecule , stereochemistry , organic chemistry , atomic physics , physics , quantum mechanics
The herbicide clethodim can exist as different tautomeric forms and also as E ‐ Z isomers. Previous works have made some approach to the matter of tautomerization and have always stated that the E form was the only one stable. In this work, the investigation on the tautomeric equilibrium of clethodim is carried out in solution by means of proton nuclear magnetic resonance ( 1 H NMR), carbon‐13 nuclear magnetic resonance ( 13 C NMR), and correlation spectroscopy (COSY) experiments in four solvents (CDCl 3 , acetone‐ d 6 , dimethylsulfoxide (DMSO)‐ d 6 , and D 2 O). Major tautomeric species have been identified, and equilibrium constants have been calculated from the integration of hydrogens of the chloroallyl radical of clethodim. E ‐ketoenolimine is the only detectable tautomer in CDCl 3 . Mixtures of E ‐ketoenolimine and E ‐diketoenamine tautomers are observed in acetone‐ d 6 and DMSO‐ d 6 (in ratios of 8:92 and 53:47, respectively). In water, the most abundant neutral tautomers are E ‐ketoenolimine, E ‐diketoenamine, and Z ‐ketoenolimine (with a tautomeric ratio of 66:20:14). Identification of the E and Z tautomers is reported. Thermodynamic parameters (obtained by analysis of the effect of temperature) and theoretical calculations (energies and chemical shifts) support the assignations and tautomeric ratios when considering specific clethodim‐solvent hydrogen bonds. In the case of water, calculations suggest double bonded complexes that would be the cause of stability of the tautomers. These new observations on the structure of clethodim in solution open a new series of questions on its absorption mechanism, and they are also a tool for further chemical modification of this herbicide.