Absorption bands aliphatic stretching region

Table V. Rotated Matrix of Factor Loadings for Components Analysis of FTIR Absorption Bands in the Aliphatic Stretching Region...

Aliphatic stretching region in vitrinite, rotated matrix of factor loadings for components analysis of FTIR absorption bands, 1l4t... 

The conversion of a carboxylic acid to a salt can serve as confirmation of the acid structure. This is conveniently done by the addition of a tertiary aliphatic amine, such as triethylamine, to a solution of the carboxylic acid in chloroform (no reaction occurs in CC14). The carboxylate ion thus formed shows the two characteristic carbonyl absorption bands in addition to an ammonium band in the 2700-2200 cm-1 region. The O—H stretching band, of course, disappears. The spectrum of ammonium benzoate, Figure 3.24, demonstrates most of these features. 

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. 

Naturally, the bands in this region may well represent a blend of the (v = 1) —(v = 2) and (n = 2) — (n = 3) aromatic CH stretching transitions with overtones and combinations involving aromatic CC stretches as well as aliphatic CH stretches. Many PAHs which do not have aliphatic side groups show weak absorptions near these frequencies. For example, Fig. 6 shows that chrysene, pyrene and coronene all show substructure on a broad component. Chrysene and coronene show a peak at about 2910 and 2845 cm-1 while pyrene has a broad (weak) plateau from 2950-2880 cm-1, which is similar to the emission plateau observed from the astronomical object BD + 30°3639 [44]. In the laboratory spectra these are due to overtone and combination bands which have been perturbed sufficiently by solid state effects to absorb weakly [35, 36, 37, 38, 39]. The perturbations within the PAH clusters that are suspended in salt pellets induce IR activity and broaden the individual bands causing them to overlap. In free vibrationally excited PAHs, perhaps Fermi resonances between the overtones and combinations of C-C stretching vibrations with the highly excited C-H modes can sufficiently enhance the intensities of these presumably weak bands to produce the observed intensites. 

Absorption bands in the typical C—H stretching region at 3000-2870 cm-1 usually showing two maxima at 2925 and 2855 cm-1 related to CH2 and CH3 aliphatic groups. These bands are well defined in the spectra obtained from microbial biomass of soils and there can be assigned to cell wall and capsular polysaccharides (Filip, 1978) 27). But they are also common in spectra of other biological materials. 

Since several bands may appear in the C—H stretch region, it is probably a good idea to decide only whether the absorptions are acetylenic (3300 cm ), vinylic or aromatic (>3000 cm ), aliphatic (<3000 cm ), or aldehydic (2850 and 2750 cm ). Further interpretation of C—H stretching vibrations may not be worth extended effort. The C—H bending vibrations are often more useful for determining whether methyl or methylene groups are present in a molecule. 

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