Mesityl oxide carbon-13 chemical shift

Fig. 7 Mobility-shift assay for the determination of dissociation constant of the complex between anti-DNP rat monoclonal IgG21) antibody and charged ligands that contained the A-dinitrophenyl group. Mesityl oxide (MO) served as EOF marker, bovine carbonic anhydrase (CAB) and bovine a-lactalbumin (LA) as internal references. The DNP ligands with a charge of —1 (A) und —9 (B), respectively, were used as additives to the running buffer. (Reprinted with permission from Ref. 30. Copyright 1995 American Chemical Society.)...

An exception to the straightforward correspondence between C shifts in zeolites (or other catalysts) and solution values occurs when the structure of the compound is significantly perturbed on the catalyst. The most common example is protonation equilibria on acidic catalysts. Indeed, there have been a number of reports of the use of protonation shifts of amines 151,521, phosphines (151, and phosphine oxides (531 as probes of catalyst acidity. Similar effects are (x ca-sionally encountered in in situ experiments when a basic molecule is formed as an intermediate or product. An interesting case is the conversion of acetone to hydrocarbons on zeolites, which may involve the intermediacy of diacetone alcohol, mesityl oxide, phorone, and isophorone—all ketones. The chemical shifts of the carbonyl carbons of all these species in acidic zeolites were found to be up to 10 ppm downfield of the corresponding values in reference compilations. Furthermore, although the chemical shifts of the olefinic carbons a to the carbonyl were in reasonable agreement with values for CDCI solutions, the resonances of the olefinic carbons p to the carbonyl were very broad and shifted 20-30 ppm downfield 54. ... 

Acetone, for example, has a chemical shift of 205 ppm in neat solution and a shift of 244 ppm in sulfuric acid. This equilibrium also explains the olefinic carbon shifts, as illustrated for the case of mesityl oxide. 

Farcasiu has suggested that shifts of probe molecules like mesityl oxide be used in an acidity scale based on NMR measurements (57). Farcasiu and coworkers showed that whereas the a olefinic carbon shifted only 2-3 ppm going from CDCI3 to 100% H2SO4, the p olefinic carbon moved downfield by nearly 50 ppm. Thus, in the in situ study of acetone conversion, the apparent chemical shift discrepancies are accounted for by protonation equilibria. 

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