Carbon comparison with proton shifts

C NMR studies, especially in the solid state (83H(20)1713), are of value in studies of tautomerism (83H(20)1713, 86ZC378). Solid state studies on imidazole (and pyrazole) show there are three distinct signals for the annular carbon atoms (imidazole C-2, 136.3 C-4, 126.8 C-5, 115.3 ppm). Proton exchange does not occur in the solid, hence the compounds resemble their crystal structures. Comparison with the corresponding chemical shifts for 1-methylimidazole (137.6, 129.3, 119.7 ppm) implies that the tautomerism has been frozen in the solid state (81CC1207). Solid-state examination of 2,2 -bis-lH-imidazole also reveals frozen tautomerism. 

Rapidly equilibrating (degenerate) carbenium ions are characterized by averaged carbon-13 shifts and carbon-proton coupling constants. 2,3-Dimethylbutyl in comparison with 2-propyl cation is an example [497]. Only two instead of four signals are observed at — 80 °C due to a rapid 1,2-hydride shift. 

The 13C spectrum of cularine (34) (Fig. 6, Table VI), has been reported by Wenkert et al. (6). The chemical shifts of the aliphatic carbon atoms were assigned by comparison with those of laudanosine (28) and follow also from those of the simple isoquinolines (Section II). In the aromatic region the resonances of the pair of protonated carbon atoms at C-5 and C-6 were differentiated from those at C-2 and C-5 by the observation of... 

The optimized geometries (MPW1PW91/6-31G) and chemical shifts were determined for both diastereomers. Comparison of 15 distances between protons determined by the ROE experiment and by computation led to a mean absolute error of 7.8 percent for 26a and 3.0 percent for 26b, suggesting that the latter is the correct structure. Similar comparison was then made between the experimental chemical shifts of 12 of the carbon atoms with the computed values of the two isomers. The mean absolute error in the chemical shifts of 26a is 3.7 ppm, but only 0.8 ppm for 26b. Both methods give the same conclusion conicasterol F has structure 26b. 

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