Amides 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. 

Hydrogen bonding to a carbonyl group causes a shift to lower frequency of 40 to 60 cm k Acids, amides, enolized /3-keto carbonyl systems, and o-hydroxyphenol and o-aminophenyl carbonyl compounds show this effect. All carbonyl compounds tend to give slightly lower values for the carbonyl stretching frequency in the solid state compared with the value for dilute solutions. 

Amides Primary Secondary 3540-3500 (w) 3400-3380 (w) 1310-1250 (s) 1150-1095(m) 3491-3404 (m-s) 1190-1130 (m) 931-865 (m-s) 430-395 (w-m) Both bands lowered ca 150 cnr1 in solid state and H bonding Interaction of NH bending and CN stretching lowered 50 cnr1 in nonbonded state Rocking of NH2 Two bands lowered in frequency on H bonding and in solid state... 

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". ... 

Following the trend towards lower carbonyl IR stretch frequencies, branching alpha to the amide carbonyl (Table 5, entries 53, 54, 62, 63 and 65) affects the shifts for mutagens and hydroxamic esters similarly and causes a marked downfield shift of up to 6 ppm relative to the acetamide substrate (Table 5, entry 60). These effects, as well as the smaller than expected downfield shift with ferf-butyl and neopentyl side chains are, as with the Ai-chlorohydroxamic esters, due to the combined influence of a stabilizing alkyl inductive effect together with destabilizing desolvation of the polar form of the amide carbonyl ". 

A strong carbonyl absorption is evident in the spectra of all amides, although the frequency of absorption varies somewhat with the structure of the amide. Thus primary amides generally absorb near 1680 cm 1, whereas secondary and tertiary amides absorb at slightly lower frequencies. The N—H stretching frequencies of amides are closely similar to those of amines and show shifts of 100 cm-1 to 200 cm 1 to lower frequencies as the result of hydrogen bonding. Primary amides have two N—H bands of medium intensity near 3500 cm 1 and 3400 cm 1, whereas secondary amides, to a first approximation, have only one N—H band near 3440 cm 1. However, a closer look reveals that the number, position, and intensity of the N—H bands of mono-substituted amides depend on the conformation of the amide, which can be either cis or trans ... 

Here again the effect of dilution has been studied by Richards and Thompson (1947), by Mizushima and others (1950), and by Darmon and Sutherland (unpublished). The effects found were similar to those observed on the C=0 frequency in carboxylic acids (Davies and Sutherland, 1938). Thus in concentrated solutions, in pure liquids, or in the solid state where hydrogen bonding is bound to be prevalent, this band lies at a markedly lower frequency than for dilute solutions the difference in this CO frequency being about 40 cm. i (Fig. 1). It should be noticed that this is less than the change in the NH stretching frequency, which was several hundred cm. Typical values for this frequency in a simple monosubstituted amide are ... 

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