H-1 AND C-13 NMR-STUDIES OF ISOMERISM IN HYDROXAMIC ACIDS

H-1 and C-13 NMR studies of N-methyl-substituted hydroxamic acids, RCON(CH3)OH (R = CH3, C2H5 and C6H5), show that series exhibits cis-trans isomerism about the C-N bond. The Z/E ratio increases in the series CH3 < C2H5 < n-C5H11 < n-C6H13 for a given solvent, indicating that steric interaction between R and the OH group is more important than that between R and the CH3 group. In DMSO-d6 the Z isomer is preferentially stablilized by solvation, whereas in C6D6 and toluene-d(s) the unsolvated E form is thermodynamically more stable, as predicted by ab inito molecular orbital calculations. (12) At high concentrations (in CDCl3, CD2Cl2, C6D6 and toluene-d(s) the E isomer may also be stabilized by intermolecular association. In the O-methyl-substituted series, RCONHOCH3 (R = CH3, C6H5), although the Z isomer predominates in all solvents, at low temperatures there is evidence for enolic forms.