Amides carbonyl absorption

Scheme 30) (6). Infrared spectra of the products possess a normal amide carbonyl absorption, indicating that the products are not present on the dipolar form (30) but rather as the neutral A -thiazoline tautomer (31 or 32) (6). 

Another IR spectrophotometric method for the determination of salicylamide among other medicinally used amides was reported (1 5). The amide carbonyl absorption band at 1732 cm l was used as the basis of quantitative assay. 

The partial IR spectrum of acid Subfraction 1 shows IR absorption at 3460 cm because of the pyrrolic nitrogen N-H absorption of carba-zole-like compounds. Amide carbonyl absorption appears at 1685 cm" The partial IR spectrum of acid Subfraction 2 shows the same two IR bands and additional bands at 3585 cm and 1650 cm owing to phenols and a second amide type. The partial IR spectrum of acid Subfraction 3 shows phenol absorption at 3585 cm S pyrrolic nitrogen absorption at 3460 cm S and strong carbonyl absorption at 1695 cm and 1725 cm characteristic of carboxylic acid dimers and monomers. In addition, absorption of hydrogen-bonded carboxylic acid and phenolic hydroxyl groups can be seen in the region of 3500-2300 cm" ... 

The partial IR spectrum of base Subfraction 1 shows N-H absorption at 3460 cm S amide absorption at 1690 cm S and aromatic absorption at 1600 cm. Base Subfraction 2 shows increased amounts of aromatic absorption at 1600 cm and an additional band at 1720 cm S which is thought to be an amide carbonyl absorption. Subfraction 3 shows N-H absorption at 3460 cm S amide absorption at 1685 cm S and large amounts of aromatic absorption at 1600 cm that shows asymmetry typical of pyridine benzologs. Strong bases such as pyridine benzologs appear to be the predominant basic compound type in Subfraction 3. 

Similar acylated isomunchnones were obtained with benzoyl isocyanate, phenyl isothiocyanate, and benzoyl isothiocyanate, as summarized for 516 517 (Fig. 4.155). " The authors " suggested that the NMR and IR spectra are more consistent with acylated isomunchnones than with bicyclic adducts 511. Thus the presence of an exchangeable proton at 9-11 ppm in the H-NMR spectrum and the amide carbonyl absorption in the IR spectrum at 1670 cm are not consistent with bicyclic lactam 511 but rather with isomunchnone amide 513. Likewise, the NMe absorption at 3.72 ppm in the H-NMR spectra of these adducts is at too low field to be expected for 511. When the authors follow the reaction in an NMR tube, they observe peaks for the 1 1 adduct 511 after 10 s, at 5.02 ppm (bridgehead methine) and 2.97 ppm (//-Me), which disappear after 30 min to give the NMR spectrum of 513. 

In addition, the substitution of other functional groups in the immediate vicinity of the amide link often gives rise to marked spectral changes. The spectra of compounds containing the CO—NH—CO structure, for example, are sufficiently different from those of normal amides to enable them to be differentiated. Urethanes and anilides correspond more closely with normal amides, but in the substituted ureas the interpretation of the 1600 cm" region becomes extremely difficult due to the complexity of the NH absorptions. Insofar as it can be recognised, however, the amide carbonyl absorption appears at a normal frequency. Amido-acids have been considered elsewhere (Chapter 13) on account of the affinities they show to amino-acids. 

The higher frequencies of the /3-lactam carbonyl absorption in fused systems has been attributed to increased inhibition of amide resonance as the /3-lactam ring becomes less planar (b-72mI50900 p. 303). For the 3-cephems (61) there is also the possibility of enamine resonance which could further reduce the ability of the /3-lactam nitrogen to contribute to amide resonance. 

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. 

This section also discusses l,5-bcnzodiazepin-2-ones, the corresponding thiones and benzo-diazepinediones. The benzodiazepinones exist in the 1,3-dihydro form 4 A rather than as the 1,5-dihydro tautomers 4B, as shown by 1HNMR spectroscopy.251,252 1Z/-1,5-Benzodiazepine-2,4(3//,5Z/)-diones 5 show NH absorptions at ca. 3180 and 3190, and amide carbonyl bands at ca. 1710 and 1660cm 1.253... 

The polyamides and polyureas exhibited broad, intense N-H stretches around 3300 cm- , a very strong carbonyl stretching vibration was present at 1630 cm- . The amide II band was evident near 1540 cm- . jn addition, sp C-H stretches occurred around 3100 cm- an(j asymmetric and symmetric sp3 c-H stretches at 2950 and 2860 cm- , respectively. The polyurethane showed the carbonyl absorption near 1700 cm-1 and C-0 stretches in the vicinity of... 

A -Acyloxy-A -alkoxyamides are characterised by two high frequency double bond absorptions corresponding to the stretching modes of the ester and amide carbonyls. 

IR spectroscopy is useful for the identification of some of the functional groups in an organic molecule. The technique also provides a fingerprint of the molecule and its comparison with authentic specimen often confirms the structure of that molecule. The IR spectra of AHLs show characteristic absorption peaks at 1780,1710,1650 cm-1 arising from the lactone ring, 3-oxo (when present), and amide carbonyl, respectively [15,16]. 

Albertamine, (-)-leontalbamine, and (+)-leontismidine were isolated from Leontice albertii and L. Smirnovii (228,229). They have the same composition, C,5H24N202. The IR spectra of these alkaloids are characterized by the absorption bands giving evidence for the presence of hydroxyl and amide carbonyl groups. There is also absorption (except in the albertamine spectrum) in the region of 2700-2800 cm attributed to trans-quinolizidine systems. The UV spectra show absorption maxima at 220 nm. 

The elucidation of the hydroxypyrazine-pyrazinone tautomerism has been made using spectral methods. An IR spectral analysis focuses on the carbonyl absorption of the amide group in the keto tautomer. A more useful method is UV spectroscopy, that is, the objective structure in solution is easily estimated by comparison with the UV spectra of bond-fixed compounds related to the two tautomers, namely O-methylated and N-methylated derivatives 9 and 10, which are prepared by methylation of the hydroxypyrazines or pyrazinones with diazomethane (Scheme 1). The above two investigations were achieved by this methodology. 

All amides show a carbonyl absorption band known as the amide I band. Its position depends on the degree of hydrogen bonding and, thus, on the physical state of the compound. 

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