Ketones lowered stretching frequency

Steric effects on both the amide and the acyloxyl side chain are similar. Tert-butyl and adamantyl groups on the amide side chain in 29v, 29x, 29c, and 29e (Table 2 entries 53 and 54, 63 and 65) result in lower stretch frequencies that, on average, are only 40 cm-1 higher than their precurser hydroxamic esters. Streck and coworkers have suggested that such changes in dialkyl ketones can be ascribed to destabilisation of resonance form II through steric hindrance to solvation which, in the case of tert-butyl counteracts the inductive stabilisation.127... 

Amides Simple amides have much lower carbonyl stretching frequencies than the other carboxylic acid derivatives, absorbing around 1640 to 1680 cm-1 (often a close doublet). This low-frequency absorption agrees with the resonance picture of the amide. The C=0 bond of the amide carbonyl is somewhat less than a full double bond. Because it is not as strong as the C=0 bond in a simple ketone or carboxylic acid, the amide C=0 has a lower stretching frequency. 

In the majority of metal-ac complexes, the ketonic CO stretching frequency is rather lower, 1650 cm i [53,201], than that formd for organic ketones ( 1725 cm i)- This be due either to some double boding of the metal with the -orbitals of the 0=0 group or there may be some direct interaction of the metal with the 0=0 group. 

Values of 1554 cm for the carbonyl stretching frequency in tlie radical-anion of benzophenone [16] and of 1558 cm for the radical-anion of di-ter/.-butyl ketone [17] suggest that in general Vco will be found at lower frequencies for the radical-anion than for the parent carbonyl compound. 

Branched iV-chlorohydroxamic esters exhibit much lower carbonyl frequencies in their IR spectra. Series of Ai-(phenylethyloxy)amides (Table 2, entries 1-7) and Af-butoxy-amides (Table 2, entries 12-16) show a clear movement to lower carbonyl stretch frequencies with branching alpha to the carbonyl, in accord with greater inductive stabilization of the polar resonance form III of the carbonyl (Figure la). Neopentyl (entry 17) is a special case. While the group should contribute much more inductive stabilization than ethyl, its carbonyl stretch frequency is higher. Similar changes have been noted in the IR spectra of branched ketones and have been ascribed to a degree of steric hindrance to solvation and therefore destabilization of the polar resonance form Dl". ... 

Conjugation lowers the carbonyl stretching frequencies of ketones and aldehydes because the partial pi bonding character of the single bond between the conjugated double bonds reduces the electron density of the carbonyl pi bond. The stretching frequency of... 

An infrared study including 2-furyl f-butyl ketone furnished evidence for changes in both ring and carbonyl stretching bands that could well signal an increasing tendency for the bulkier substituents to rotate out-of-plane. The study is of special interest because it is based not only upon vibrational frequencies, as is common, but mainly upon intensities, which is not. In the f-butyl ketone, the carbonyl stretching frequency and its intensity (1662 cm -1 A 6,500) are both lower than in the methyl ketone (1674 cm" 1 A 10,500).357... 

Properties of Sulphides.—An a-alkylthio-group increases the enol content of jS-dicarbonyl compounds,and causes a shift in carbonyl stretching frequency to lower values, in a-(alkylthio) thioesters, ketones, and amides the C—N stretching frequency in corresponding nitriles is similarly altered. ... 

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