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

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 (σ_p) 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 σ_p+ (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.