Micellar and Solvent Effects on the Geometrical Isomerism of Hydroxamic Acids and Their Anions

Hydroxamic acids and their anions exhibit geometrical isomerism due to amide-like resonance and an increase in the C−N bond order which can be monitored by ¹H-NMR spectroscopy. N-Phenylpropiono- and dodecanohydroxamic acids, 1a and 2a, respectively, exist as E-isomers in DMSO-d₆ and CD₃OD but 1a is an E−Z mixture in D₂O. The corresponding hydroxamate ions, 1b and 2b, also exist as E-isomers in DMSO-d₆, but Z-isomers form on addition of D₂O and Z-2b is dominant with χ_D2O ≥ 0.9. In CD₃OD E- and Z-isomers coexist, but Z-2b becomes dominant on addition of D₂O. The E:Z ratio of 1b is very similar in cetyltrimethylammonium bromide (CTABr) and in D₂O, but only Z-2b is detected in cationic CTABr micelles and in anionic, zwitterionic, and nonionic micelles. Formation of Z-2b in micelles and water-rich mixed solvents is favored by hydrophobic interactions between phenyl and n-alkyl groups, despite the proximity of anionoid oxygens. In micelles of sodium dodecyl sulfate the N-phenyl residue of 2b is in a more aqueous region of the micelle−water interface than in micelles of the other surfactants.