Carbon-13 chemical shifts of carbonyl group

Table 7.55 Carbon-13 Chemical Shifts of Carbonyl Group 7.106...

Carbon-13 chemical shifts of representative aldehydes [284] and ketones [285-288] are collected in Tables 4.27 and 4.28. Inspection of the data shows that a, / , and y effects are up to 7, 2, and — 1 ppm, respectively. These increments are significantly smaller compared with those reported for alkyl carbons. Obviously, the electron releasing effect of alkyl groups (( +(-/-effect) slightly attenuates positive polarization of the carbonyl carbons. 

Carbon-13 chemical shifts of transition metal carbonyls [471, 473] decrease as one proceeds down a given group of the periodic table, as demonstrated for the triad Cr(CO)6, Mo(CO)e, and W(CO)e in Table 4.71. Substitution of CO by cyclopentadienide and other organic ligands deshields the carbonyl carbon nuclei. This is exemplified by iron pentacarbonyl (211 ppm) in comparison to cyclopentadienylirondicarbonyl-dimer, which displays one avaraged carbonyl signal ( 240 ppm) at room temperature [488] due to rapid cis-trans isomerization and intramolecular bridged-terminal carbonyl interconversion ... 

The carbon-13 chemical shifts contained in this section display the chemical shifts and spectrum patterns produced by the carboxylic acid ester functional group (R-C(=0)- O-R ). The carbonyl side of the bond exerts a weak to intermediate deshielding effect on the adjacent (C1) carbons of both aliphatic and aromatic compounds. The oxygen side of the bond, on the other hand, has a strongly deshielding effect on these carbons. 

The introduction of fluorine or fluorinated groups into dianhydrides, on the other hand, increases the reactivity. The NMR chemical shift of carbonyl carbons (8 ) of P6FDA (13), the only existing perfiuorinated dianhydride, resonated at about 4 ppm upfield from those of conventional unfluorinated dianhydrides (18). Although 8q are not as closely correlated with rate constants (18), this upfield shift suggests considarable increase of reactivity. P6FDA is therefore expected to compensate for the low reactivity of perfiuorinated diamines. 

The carbon-13 NMR spectrum of acetone oxime has three resonances, the deriva-tized carbonyl carbon at 154.5 ppm and the two nonequivalent CH3 groups at 21.5 and 14.7 ppm. The difference between the chemical shifts of the two methyl groups, 6.8 ppm, is primarily a steric compression shift. This is clearly indicated in the carbon NMR spectrum of methyl ethyl ketoximes (Scheme 1), where the two oxime substituents are not sterically identical and thus the two isomers are not present in equal amounts. In methyl... 

C chemical shift anisotropies of the unsaturated carbons. The chemical shift parameters used in the Ar-Al-PA for the unprotonated and proto-nated aromatic carbons, as well as for the carbonyl groups, are shown in Fig. 80. 

Carbon-13 NMR spectra of tungsten derivatives (CO)sWC(CH3)(SC6H4Y) were obtained by Ward et al. (127) (Table XXXVII). Only small variations of cis and trans carbonyl and carbene carbon chemical shift were detected as the nature of the para group, Y, was varied. The authors concluded that the lack of any conjugative effect indicated that the phenyl ring was probably perpendicular to the carbene plane. 

Heptan-2-one is the bee pheromone mentioned on p. 48. It has no symmetry so all its seven carbon atoms are different. The carbonyl group is easy to identify (208.8 ppm) but the rest are more difficult. The two carbon atoms next to the carbonyl group come at lowest field, while C7 is at highest field (13.9 ppm). It is important that there is the right number of signals at about the right chemical shift. If that is so, we are not worried if we cannot assign each frequency to a precise carbon atom (such as atoms 4, 5, and 6, for example). As we said before, don t be concerned with the intensities of the peaks. 

C-13 NMR of copolymers provides information regarding composition of a copolymer, monomer sequence distribution and end chain propagation mechanism (22-24). In the present studies, TCA-ST copolymers were studied at different mole compositions in order to see the effect of mole composition on the splitting pattern. An alkyl ester group of a copolymer displayed certain influence on the chemical shift of the carbonyl and quaternary carbon resonances. Quarternary carbons in ST-ST-ST triad always resonate at higher field than that of TC-TC-TC triad. 

Carbon-13 NMR spectra are diagnostic of both aldehydes and ketones because of the characteristic chemical shift of the carbonyl carbon. Partly because of the electronegativity of the directly bound oxygen, the carbonyl carbons in aldehydes and ketones appear at even lower field (—200 ppm) than do the 5p -hybridized carbon atoms of alkenes (Section 11-4). The carbons next to the carbonyl group also are deshielded relative to those located farther away. The NMR spectrum of cyclohexanone is shown in Figure 17-4. 

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