Lactones group frequencies

The carbonyl group frequencies in the IR spectra of pyranopyrandiones are characteristic of simple lactones (Figure 14). 

Characteristic group frequencies of lactams are shown in Figure 39. As in lactones, the size of the... 

Although the valence bond description of pyran-2-one could include a zwitterionic contribution involving the carbonyl group, thus making the ring aromatic, there is little evidence to support this. For example, the IR carbonyl frequency (1740 cm ) is typical of an unsaturated lactone, and the chemical shifts of the ring protons in the H NMR spectrum indicate that there is no ring current. 

The IR spectra of coumarin and isocoumarin show carbonyl frequencies characteristic of lactones. There is no evidence to suggest that contributions from betaines (207) and (208) are significant. Moreover, EHT MO calculations predict high ir-bond orders for the carbonyl groups in these molecules. IR data and C=0 7r-bond orders are presented in Table 10. 

The IR absorption frequency for the exocyclic carbonyl group in the sydnones in the range 1750-1770 cm-1 has suggested a similarity to -lactones (1770 cm-1) rather than to tropones (1638 cm-1). This is somewhat disturbing if the sydnone is considered to have the ionic character of structure (1). However, some A3-oxazolones absorb in the 1760 cm-1 region. IR absorption data for some recently prepared sydnones are given in Table 3 (see also 63PMH(2)229>. 

Note The frequencies given for the C=0 stretching vibrations for anhydrides, acid chlorides, esters, lactones, aldehydes, ketones, carboxylic acids and amides refer to the open chain or unstrained functional group in a nonconjugated system. If the carbonyl group is conjugated with a double bond or an aromatic ring, the frequency is 30 cm less. If it is... 

A second alkaloid without the oxazolidine ring, but with the carbino-lamine system is cycloneosamandaridine (XXVII). The IR-spectrum evidences a five-membered lactone ring (band at 1780 cm i) as well as the carbinolamine group (bands at 1050/1180 cm i). The IR-spectrum of the X-acetyl compound no longer shows these hands at 1050-1180 cm i, hut an additional carbonyl frequency at 1735 cm i for an aldehyde group. The mass spectrum confirms the molecular formula as well as structure XXVII(id). 

The most direct way of looking at the specific interactions is by using spectroscopic measurements. Infra-red spectroscopy is the technique which has been most commonly used to study mixtures involving polymers. Studies of blends of PVC with polycaprolactone showed shifts of 4-6 cm 1 in the carbonyl band of polycapro-lactone relative to the pure polymer 99 l00), but this Figure should be treated with caution as the peak probably consists of the sum of a shifted and an unshifted peak and it is difficult to say what the frequency of the shifted peak would be, or what fraction of the carbonyl groups are, or can be, involved in the interaction. Frequency shifts have also been shown to exist in blends of poly(methyl methacrylate) with poly(vinylidene fluoride)63). 

Another spectrometric method is IR, which apart from identifying functional groups is useful in revealing the conjugated lactone functionality. Coumarins are isomeric with chromones, but the two classes differ considerably in their IR spectra. The carbonyl stretching frequency in coumarins (a-pyrones) is observed in the region 1700-1750 cm- whereas in chromones (y-pyrones) it is found at 1650 cm- [8]. 

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