Configuration and conformation of acyl derivatives of hydroxylamine. Part 22. Hydroxamoyl chlorides. A revision

The analysis of the i.r. spectra of hydroxamoyl chlorides (Ia–d) under different conditions points to the Z-configuration in solution, i.e. the same configuration as established previously in the crystalline state by X-ray measurement. The previous arguments favouring the E-configuration are thus rebutted. This conclusion is supported by empirical correlations of i.r. frequencies, 1H and 13C n.m.r. chemical shifts, and by a revised analysis of dipole moments taking into account an aliphatic derivative and referring to the O-benzoyl derivatives (II) as model compounds. The original erroneous assignment was caused by the use of less efficient model compounds and by the low sensitivity of the dipole moments of (I) to the configuration.