Amide stretching frequencies

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. 

Spectral Characteristics. The iafrared stretching frequency of the penicillin P-lactam carbonyl group normally occurs at relatively high frequencies (1770 1815 cm ) as compared to the absorptions for the secondary amide (1504-1695 cm ) and ester (1720-1780 cm ) carbonyl groups. 

The role of IR spectroscopy in the early penicillin structure studies has been described (B-49MI51103) and the results of more recent work have been summarized (B-72MI51101). The most noteworthy aspect of a penicillin IR spectrum is the stretching frequency of the /3-lactam carbonyl, which comes at approximately 1780 cm" This is in contrast to a linear tertiary amide which absorbs at approximately 1650 cm and a /3-lactam which is not fused to another ring (e.g. benzyldethiopenicillin), which absorbs at approximately 1740 cm (the exact absorption frequency will, of course, depend upon the specific compound and technique of spectrum determination). The /3-lactam carbonyl absorptions of penicillin sulfoxides and sulfones occur at approximately 1805 and 1810 cm respectively. The high absorption frequency of the penicillin /3-lactam carbonyl is interpreted in terms of the increased double bond character of that bond as a consequence of decreased amide resonance, as discussed in the X-ray crystallographic section. Other aspects of the penicillin IR spectrum, e.g. the side chain amide absorptions at approximately 1680 and 1510 cm and the carboxylate absorption at approximately 1610 cm are as expected. 

A criterion for the position of the extent of the mesomerism of type 9 is given by the bond order of the CO bond, a first approximation to W hich can be obtained from the infrared spectrum (v C=0). Unfortunately, relatively little is known of the infrared spectra of amide anions. How-ever, it can be assumed that the mesomeric relationships in the anions 9 can also be deduced from the infrared spectra of the free amides (4), although, of course, the absolute participation of the canonical forms a and b in structures 4 and 9 is different. If Table I is considered from this point of view, the intimate relationship betw-een the position of the amide band 1 (v C=0) and the orientation (0 or N) of methylation of lactams by diazomethane is unmistakeable. Thus the behavior of a lactam tow ard diazomethane can be deduced from the acidity (velocity of reaction) and the C=0 stretching frequency (orientation of methylation). Three major regions can be differentiated (1) 1620-1680 cm h 0-methylation (2) 1680-1720 cm i, O- and A -methylation, w ith kinetic dependence and (3) 1730-1800 em , A -methylation, The factual material in Table I is... 

Carbonyl stretch frequencies, carbonyl 13C and amide 15N chemical shifts for a wide range of A-acyloxy-A-alkoxyamides are listed in Table 2 together with those of the precursor hydroxamic esters. Spectroscopically, mutagens can be categorised into six types ... 

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... 

Similar to the TGA experiments, in situ Infrared Spectroscopy has been used to follow the amide bond stretching frequencies while heating under various atmospheres. ° These experiments (see Fig. 7.2) suggest that dendrimer removal requires relatively forcing conditions to maximize CO adsorption on supported Pt catalysts prepared from A variety of activation conditions have been... 

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". ... 

The amide carbonyl vibrational frequencies of A-acyloxy-Af-alkoxyamides are similar to that observed for the twisted l-aza-2-adamantanone (98, 1731 cm ) . It is apparent from the extensive data available for both A-chlorohydroxamic esters (Table 2, Section in.B.2) and Af-acyloxy-A-alkoxyamides that when an amide nitrogen lone pair loses conjugation with the carbonyl (either through twisting/pyramidalization or, in the case of anomeric amides, pyramidalization alone), the configuration is analogous to an ester rather than a ketone. As with esters, acid halides and anhydrides or diacyl peroxides , the carbonyl stretch frequency is higher than that of ketones and aldehydes... 

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 ". 

The carbonyl stretch frequencies (in the condensed phase) for hydrazines bearing branched amide side chains (Table 14, entries 13, 14 and 17) exhibit the usual reduction... 

Piperazine-2,5-diones possess two cis amide bonds. In the infrared, the CO stretching bands occur at 1670-1690 cm 1 and the NH stretching frequency at 3180-3195 cm-1. More detailed analysis of IR and Raman spectra have been carried out [84SA(A)481, 84SA(A)503]. 

N—H Stretching Vibrations In dilute solution in nonpolar solvents, primary amides show two moderately intense NH stretching frequencies corresponding to the asymmetrical and symmetrical NH stretching vibrations. These bands occur near 3520 and 3400 cm-1, respectively. In the spectra of solid samples, these bands are observed near 3350 and 3180 cm-1 because of hydrogen bonding. 

Box 3.3 Effect of Ring Strain on the Carbonyl Stretching Frequencies of Lactones (Cyclic Esters) and Lactams (Cyclic Amides)... 

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