Abstract
The n-π conjugation within the oximino group was estimated on the basis of dipole moments and of bond lengths. The dipole moments were measured on oximes I and O-methyloximes II in benzene solution, the C=N and N-O bond lengths were retrieved from the Cambridge Structural Database for all available derivatives. The results differ somewhat in the quantitative sense but can be always interpreted in terms of the classical mesomeric formulae IV ↔ V. This formalism postulates – in agreement with experiments – a lowered C=N bond order, some double bond character of the N-O bond, planarity of the C=NOR group, and an excess dipole moment (μm) oriented in the sense of a charge transfer from O toward C. Quite different results were obtained for O-acetyloximes III. While their bond lengths could agree with a weakened conjugation, the excess dipole moment is oriented from C towards O and cannot be expressed by any simple formula. Hence the mesomeric formulae may represent an acceptable description of the actual charge distribution in many cases but not in all.
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