ORTHO-SUBSTITUENT EFFECTS IN N-ARYLACETAMIDES - NMR AND MOLECULAR MECHANICS INVESTIGATION

H-1, C-13, N-15 and O-17 NMR spectra of N-phenylacetamide (acetanilide ) and 21 ortho-substituted acetanilides were measured and assigned, Th e observed NMR parameters are related to the Hammett substituent param eters and conformational characteristics of the acetamido moiety estim ated by molecular mechanics calculations, Significant relationships we re found for the C-13 NMR chemical shifts of C-5 (para to substituent) and the direct spin-spin coupling constant, (1)J(C, H), of C-3 (ortho to substituent) with Hammett substituent parameters, For N-15 NMR che mical shifts of the amido nitrogen, no general correlation with the Ha mmett substituent parameters was found, The interactions between funct ionalities contiguous to the carbonyl group and the amino nitrogen in acetanilides are effectively hampered owing to the increased twist ang le between the planes containing the phenyl ring and the H-N-C fragmen t in derivatives bearing bulky ortho substituents, Especially in ortho -halogen-substituted derivatives the N-15 NMR chemical shift of the am ino nitrogen is very clearly related to the twist angle between the ph enyl ring and H-N-C fragment, For O-17 NMR chemical shifts of the acet amido carbonyl, the observed variations can be related to the steric i nhibition of resonance between the benzene ring and the acetamido grou p.