¹H and ¹³C NMR studies of isomerism in hydroxamic acids

Abstract 1 H and 13 C NMR studies of N ‐methyl‐substituted hydroxamic acids, RCON(CH 3 )OH (R = CH 3 , C 2 H 5 and C 6 H 5 ), show that the series exhibits cis – trans isomerism about the CN bond. The Z / E ratio increases in the series CH 3 < C 2 H 5 < n ‐C 5 H 11 < n ‐C 6 H 13 for a given solvent, indicating that steric interaction between R and the OH group is more important than that between R and the CH 3 group. In DMSO‐ d 6 the Z isomer is preferentially stabilized by solvation, whereas in C 6 D 6 and toluene‐ d 8 the unsolvated E form is thermodynamically more stable, as predicted by ab initio molecular orbital calculations. (12) At high concentrations (in CDCl 3 , CD 2 Cl 2 , C 6 D 6 and toluene‐ d 8 ) the E isomer may also be stabilized by intermolecular association. In the O ‐methyl‐substituted series, RCONHOCH 3 (R = CH 3 , C 6 H 5 ), although the Z isomer predominates in all solvents, at low temperatures there is evidence for enolic forms.