Carbon‐13 NMR spectra of a series of para‐substituted N,N‐dimethylbenzamides. Comparisons of linear free energy relationships for carbon‐13 and nitrogen‐15 chemical shifts and rotation barriers

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Abstract
All carbon‐13 chemical shifts for 11 para‐substituted N,N‐dimethylbenzamides in 1 mole % chloroform solution are reported, with assignments based upon double resonance experiments, analogy to chemical shifts of benzamide, and self‐consistency between experimental and calculated values using recognized substituent parameters. In contrast to earlier reports, the aryl carbon chemical shift assignments for N,N‐dimethylbenzamide are C‐2, 127.0; C‐3, 128.7; C‐4, 129.4, and for p‐chloro‐N,N‐dimethylbenzamide are C‐1, 134.6; C‐4, 135.5 ppm, relative to internal TMS. Good Hammett correlations (σp) are reported for 13C chemical shifts of C‐1 (σ = 11.9 ppm) and even for the carbonyl group (σ = −2.3 ppm) but are markedly improved if correlated with σp+ (σ = 9.5 ppm) and Dewar's F (σ = −1.9 ppm), respectively. Excellent Swain–Lupton F and R correlations were found for some of the 13C chemical shifts and yielded values for percent resonance contributions to transmission of substituent effects as follows, C‐1, 75 ± 4%; C‐2, 51 ± 3%; CO, 31±2%. These are compared to similar values calculated from the CO of benzoic acids of 34±10%, and from the nitrogen‐15 chemical shifts of benzamides of 56±2%. Correlations of these 13C δ values and 15N δ values with rotation barriers (ΔG) for N,N‐dimethylbenzamides were examined, and it was found that while CO δ values correlated only poorly the C‐1 δ values correlated very well, but the best correlation was for 15N δ values of benzamides. It is suggested that Δ G and δ 15N are intrinsically related due to their numerical correlation, and the close similarity in percent resonance contribution of substituent influence on these parameters.

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