O Stretching Vibrations in Esters

C—O stretching vibrations of esters actually consist of two asymmetrical coupled vibrations C—C(=0)—O and O—C—C, the former being more important. These bands occur in the region of 1300-1000 cm"1. The corresponding symmetric vibrations are of little importance. The C—O stretch correlations are less reliable than the C=0 stretch correlations. 

The C-O-C stretching vibration in carboxylic esters is characterized by an absorption band between 1100 cm-1 and 1280 cm-1 [70]. In the case of pure methyl formate one observes a broad band of strong intensity between 1150 and 1220 cm-1. Considering the shift in aqueous solution the band at 1230 cm-1 in Fig. 2.12 could be attributed to the C-O-C stretching vibration of methylformate. 

C=O Stretching Vibrations The C=O absorption band of saturated aliphatic esters (except formates) is in the 1750-1735 cm-1 region. The C=0 absorption bands of formates, a,/3-unsaturated, and benzoate esters are in the region of 1730-1715 cm-1. Further conjugation has little or no additional effect upon the frequency of the carbonyl absorption. 

C—O stretching band in the region where a weaker band occurs for ketones. There is overlapping in the C=0 frequency of esters or lactones and acids, but the OH stretching and bending vibrations and the possibility of salt formation distinguish the acids. 

The band at 1730 cm is most likely due to ester formation, the band at 1670 cm is less easily assigned but given that the phenoxy radicals are present in the early stages of photo-degradation (based on ESR work mentioned above) then quinone type species are likely candidates or some other, unsamrated carbonyl moiety. The broad band in the region 1000-1300 cm is likely to be due to various C-O stretching vibrations and is typically observed for acid peroxides [Davies, 1993]. 

The two most polar bonds in esters (containing the -CO-O-C-unit) are the C=0 and C-O bonds and these bonds produce the strongest bands in the spectrum of any ester (summarized in Table 4.3). Aromatic and aliphatic esters may be differentiated, as both C=0 stretching and C-O stretching vibrations produce bands in different ranges aliphatic esters produce C=0 and C-O bands at 1750-1730 and 1300-1100 cm respectively, while aromatic esters produce C=0 and C-O bands at 1730-1705 and 1310-1250 cm respectively. 

The reactions of NaOH with cellulose in natural fiber yielded cellulose-ONa compound and removed impurities from the fiber surface [11,12]. This is confirmed by the FTIR spectroscopic analysis as shown in Figure 14.1. The FTIR spectrum of the raw wood clearly shows the absorption band in the region of3407 cm, 2917 cm and 1736 cm due to O-H, C-H and C = O stretching vibration respectively. These absorption bands are due to hydroxyl groups in cellulose, carbonyl groups of acetyl ester in hemi-cellulose and carbonyl aldehyde in lignin. 

A definitive statement about the reaction step following the P=S bond scission, i.e. the cleavage of the P-O bond to form aryloxy groups or the scission of the C-O bond to give aryl group, cannot be done on the basis of the FT-IR spectra since the superposition of the C-O stretching vibration band of the ester compounds (at ca. 1175 cm ) produced by the oxidation of the base oil with the vC-O-(P) band results in DI values which are not representative of the decrease in absorbance of the latter. 

The IR and Raman spectra of polyvinyl acetate are provided in Reference Spectrum 10. Key features of the IR spectrum of PVAc are the C=0 stretching vibration at 1740 cm , the C—O stretching vibration of the ester group at 1230 and 1020 cm , an absorption due to methyl CH bending at 1375 cm , and CH2 bending (scissors) vibration of the methylene groups at 1440 cm . An important copolymer of vinyl acetate is ethylene-vinyl acetate copolymer, EVA. For this copolymer, an increase in the number of —CH2— groups, due to the insertion of ethylene molecules into the polymer... 

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