13C NMR contact shifts δ induced by the 2,2,6,6‐tetramethylpiperidine nitroxide radical (TMPN) were measured for N,N‐dimethylacetamide (DMA) and N,N‐dimethylformamide (DMF) in several solvents and for 5 different N,N‐dimethylamidines in acetone‐d6. The shifts δ were straight linear functions of the TMPN concentration CR and were dependent on the electron acceptor properties of the solvents, thus proving the electron donor‐acceptor nature of the interactions between the radical and the other components of a solution. It was found that the radical induced contact shift enables one to detect differences in the electron acceptor properties of methyl groups within a diamagnetic molecule. In particular the different linear relationships between ln(dδ/dCR) and the solvent acceptor number AN was obtained for each methyl group of DMA. In all the compounds studied the N‐methyl group in the conformation antiperiplanar to the double bond (C = O in amides, C = N in amidines) appeared to be a better electron acceptor site than the N‐methyl group in the synperiplanar conformation. On the basis of dδ/dCR values some conjugation between the aromatic ring and C = N bond π‐electrons in arylacetamidines and in arylformamidines is suggested.
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