C¹³ CHEMICAL SHIFTS IN ORGANIC CARBONYL GROUPS

Abstract

The results of a study of the resonance position of the carbonyl carbon nucleus in an extensive series of carbonyl derivatives are reported. A discussion of the major factors affecting the position of these carbon resonances is presented and some of the potential applications of these data to chemical problems are indicated.

13C N.M.R. STUDIES: PART VI. CARBON-13 SPECTRA OF α,β-UNSATURATED CARBONYL COMPOUNDS

“…All of the data in Tables 11-V were obtained using the neat liquids, but to examine the effect of hydrogen bonding a t the carbonyl oxygen on the shieldings of these nuclei some compounds were examined in methanol solution as well (Table VII). (1). Certain consistent trends can be seen for the effects of further alkyl substitution on the alp-unsaturated carbonyl system by comparing the rest of the data.…”

“…While the overall consistency of the effects of inethyl substitution tends t o support the present assignments, a potential method of confirming our conclusions seemed worth investigating. I t has been established that hydrogen-bonding solvents tend to reduce the shielding of carbonyl carbon nuclei relative t o that measured for the pure liquid or in nonpolar solvents (1,4,5,35). This reduction, although not particularly large unless protonation occurs (35), is significant and, furthermore, in aromatic systenls the effect appears t o be reflected a t the terminal carbon nucleus as well [i.e., C-4 in acetophenone (I)].…”

“…Similarly, inethyl substitution a t C-5 in the 2-cyclohexenone derivatives is without effect a t the carbonyl carbon. Since it is well established that conjugation tends to shield carbonyl carbons (1) and that the carbonyl shieldings reflect the extent of conjugati7on (4,5,28), it is not surprising that /3-methyl substitution tends to shift the carbonyl absorption toward higher field. The introduction of a second p-methyl group produces an additional shielding effect as illustrated by the values for mesityl oxide and 3-methyl-2-cyclohexenone relative to 3-pentenone and 2-cyclohexenone, respectively.…”

Direct evidence by carbon‐13 NMR spectroscopy for the erythro configuration of the sphingoid moiety in Gaucher cerebroside and other natural sphingolipids

“…This observation indicates that in the preferred conformation of ceramide and glucosylceramide the steric hindrance between the allylic hydroxy group and the olefinic proton H-4 or between the allylic hydroxy group and the carbonyl oxygen is greater in the threo than in the erythro configuration. In addition, the downfield shift observed for the resonance of the carbonyl carbon of threo-ceramide and glucosyl-threo-ceramide suggests an intramolecular hydrogen bonding between the carbonyl oxygen and the hydroxyl group [19]. This hydrogen bonding should lessen the polarity of these threo compounds with respect to the erythro compounds.…”

Nuclear magnetic resonance studies. XVI. Oxygen-17 shieldings of some substituted acetophenones

“…for the same change in substituent. An interpretation of the 13C results was advanced (1) in terms of the resonance forms contributing to the structures of these compounds in which it was suggested that the location of a nucleus within the aryl system could be an important factor governing substituent effects on its shielding. In the fluorobenzenes the fluorine nucleus is at the end of the resonance system while the carbonyl carbon in the aryl ketones is one atom from the end.…”