Aliphatic stretching region

The results of a components analysis for the aliphatic stretching region are shown in Table V. The loadings of a variable to the components are similar in nature to correlation coefficients, therefore, the critical value used for significance at the 95% probability level is 0.404 (10). Only the significant loadings are given in Table V. Component 1 contains five variables and... 

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... 

Inspect the C—H stretching region and identify the bands as either of aliphatic or aromatic/olefinic origin. 

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. 

Table II. Band Assignments for the Aliphatic Stretching and Aromatic Out-of-plane Bending Regions...

Before the FTIR data were analyzed together with coalification parameters in a components analysis, it was first necessary to select which variables to use. In general, the number of variables should not exceed one-third of the number of samples (in this study the maximum number is 8). Components analyses were performed on data for the aliphatic stretching and aromatic out-of-plane bending regions of the infrared spectrum in order to eliminate those variables that did not provide new information. 

The spectra of nitriles (R—C=N) are characterized by weak to medium absorption in the triple-bond stretching region of the spectrum. Aliphatic nitriles absorb near 2260-2240 cm"1. Electron-attracting atoms, such as oxygen or chlorine, attached to the carbon atom a to the C=N group reduce the intensity of absorption. Conjugation, such as occurs in aromatic nitriles, reduces the frequency of absorption to 2240-2222 cm"1 and enhances the intensity. The spectrum of a typical nitrile is shown in Figure 2.31. 

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. 

The most intense band is the C=0 stretch, at 1730 cm indicating that the aldehyde carbonyl is part of the aliphatic portion of the molecule. Looking at the C—H stretching region, the sharp aldehydic C—H bands appear at 2725 and 2825 cm . The doublet... 

Figure 15. Scale expanded aliphatic C—H stretching region of the spectrum of a vitrinite concentrate (bottom) and second derivative of the spectrum (top).

Figure 16. The resolution of the aliphatic C—H stretching region into five bands.

The most intense band is the C=0 stretch, at 1730 cm", indicating that the aldehyde carbonyl is part of the aliphatic portion of the molecule. Looking at the C-H stretching region, the sharp aldehydic C H bands appear at 2725 and 2825 cm". The doublet appears due to resonance with an overtone of the aldehydic C-H bend at 1390 cm". This is called Fermi resonance and is always seen in aldehydes. The sharp peaks above 3000 cm" are the aromatic C-H stretching peaks and the peaks between the aromatic and aldehydic peaks are due to the aliphatic C-H stretches. The sharp... 

Tertiary aliphatic amines are difficult to characterize in the 1300-1000 cm region. If there is no absorption in the NH stretch region and a tertiary amine is suspected, a spot check may be made by mixing 2 drops of the suspected amine and 1 drop of a mixture. Absorption near 2600 cm amine hydrochloride. Tertiary amines may also be detected by changes in the aliphatic groups next to the nitrogen. 

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