Computational characterization of the herbicide metolachlor and its mono-hydroxylated photodegradation products

Abstract

Electronic structure calculations have been performed for the characterization of the different conformers of the herbicide metolachlor (MC), (2-chloro-N-(2-methyl-6-ethylphenyl)-N-(2-methoxy-1-methylethyl) acetamide), the radical structures obtained through the photo chemical breaking of the C–Cl bond and the most important, experimentally identified mono-hydroxylated photo-fragmentation products. Only the S-metolachlor enantiomer that shows herbicide activity was studied. The goal of the study has been twofold. The first target has been the conformational analysis of the neutral molecule and the corresponding radical. Ten different conformers were identified for the S-metolachlor enantiomer, all of them exhibiting a near perpendicular disposition of the amidyl and the phenyl planes. They were classified into two families of minima depending on the orientation of the carbonyl group (either cis or trans with respect to the phenyl ring). All the minima in the S-cis family were found to lie higher than those in the S-trans family. The free energy of isomerization between the lowest minima of each family was found to be $$ \Delta G_{298}^{0} $$ = 6.2 kcal/mol, while the isomerization barrier around the amide C–N bond causing atropisomerism was calculated to be 34.9 kcal/mol (in good agreement with the experimental energy of activation, 36.9 ± 3 kcal/mol) [45]. The second part of the work is devoted to the computational characterization of the most important isomeric forms of the metolachlor radical and the major photodegradation mono-hydroxylated metabolites formed through the coupling of the metolachlor radical with OH. Reaction enthalpies and free energies for the various hydroxylation pathways have been calculated. Based on their order, the relative significance of the various hydroxylation channels is justified and the preference in the phenyl ring hydroxylation is established. The lowest exothermicity value determined, − 5.8 kcal/mol at the M06/6-31 + G(d,p) level, corresponds to phenyl group hydroxylation at the p-position with regard to C–N bond, in excellent agreement with the experimental findings.