ISSN:
0030-4921
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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 (ΔG298≠) 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 G298≠ 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.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1270110106
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