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 1 H-NMR spectroscopy. N -Phenylpropiono- and dodecanohydroxamic acids, 1a and 2a, respectively, exist as E -isomers in DMSO- d 6 and CD 3 OD but 1a is an E − Z mixture in D 2 O. The corresponding hydroxamate ions, 1b and 2b, also exist as E -isomers in DMSO- d 6, but Z -isomers form on addition of D 2 O and Z - 2b is dominant with χ D 2 O ≥ 0.9. In CD 3 OD E - and Z -isomers coexist, but Z - 2b becomes dominant on addition of D 2 O. The E: Z ratio of 1b is very similar in cetyltrimethylammonium bromide (CTABr) and in D 2 O, 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.