Characteristic bands for organic compounds

The application of the above rules to all kinds of compounds has been made in order to exploit the use of infrared spectroscopy as a method of structural analysis. Empirically, it has been observed a correlation between position of certain band maxima and the presence within a molecule of organic functional groups or of particular structural features within the skeleton of molecules (see Table 10.1). This property comes from the fact that each organic functional group corresponds to a collective-type of several atoms. For a given bond, the force constant k (expression 10.3) does not vary significantly from one molecule to another. 

In the plane Bending Vibrations fS) Out-of-plane Bending Vibrations (y) 

Stretching vibration (sym.) Stretching vibration (asym.) Pianar rotation (rocking) 

Moreover, calculation reveals that the reduced mass of a given group of atoms tends toward a limiting value as the molecular mass increases (see expression 10.4). This can explain that absorption of each organic functional group corresponds to a particular value. 

As for carbonyl compounds, the first two harmonics of the fundamental vibration of C=0 (1700cm ) appear towards 3400 and 5100cm respectively. The combination bands result from the interaction of two or more modes of vibration for the same functional group and are superimposed upon the preceding band. The energy of the corresponding transition is approximately the sum of those which have given rise to it. 

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UV absorbance and fluorescence detection are only of moderate use as liquid chromatography detectors for organic compounds because most of these do not have very characteristic spectra and many do not even fluoresce. These indistinct spectra are marked by one or two broad bands. For a few classes, however, this is not the case. The polycycHc aromatic hydrocarbons (PAHs), for example, have spectra that contain several sharp bands in a distinct pattern for each PAH. For this class of compounds, these detectors are much more sensitive and give more information on the peak identities than any other type of detector. 

This chapter is devoted to the description of an easy and efficient method based on the application of gas phase Flow FTIR spectroscopy analysis for determination of adsorption characteristics of volatile organic compounds. As adsorbent beds are usually operated under dynamic conditions, the adopted analytical approach is based on gas phase composition monitoring at reactor outlet during adsorption/ desorption experiments carried out under dynamic regime. This method permits further simultaneous detection of new IR bands that may originate from adsorbate dissociation during adsorption or desorption. 

To use KBr discs for quantitative measurements it is best to employ an internal standard procedure in which a substance possessing a prominent isolated infrared absorption band is mixed with the potassium bromide. The substance most commonly used is potassium thiocyanate, KSCN, which is intimately mixed and ground to give a uniform concentration, usually 0.1-0.2 per cent, in the potassium bromide. A KBr/KSCN disc will give a characteristic absorption band at 2125 cm 1. Before quantitative measurements can be carried out it is necessary to prepare a calibration curve from a series of standards made using different amounts of the pure organic compound with the KBr/KSCN. A practical application of this is given in Section 19.9. 

Most organic compounds when subjected to infra-red radiation present characteristic absorption bands. The spectral data are compared to those obtained for standard reference wines used to calibrate the instrument. The concentration of the analytes is calculated using multiple linear regression the apparatus is computerised and can be linked to an automatic sampler. The analyte should exhibit strong absorption bands for the method to be exploitable, and it is thus only suitable for major wine or must constituents, essentially ethanol and sugars. The major qualities of this technique are simplicity of operation, high sample throughput and the lack of necessity for sample preparation - the only required sample re-treatment is the removal of carbon dioxide from musts in fermentation. 

The group frequency region falls approximately between 4000 to 1400 cm-1, and the absorption bands in it may be assigned to vibration of pairs of two (or sometimes three) atoms. The frequency is characteristic of the masses of the atoms involved and the nature of their bond, ignoring the rest of the molecule. Therefore, IR spectra are useful for determining the presence of functional groups in organic compounds alcohols (—OH), ketones (=CO), amines... 

Ultra-violet and visible spectrophotometry can be effectively used for the control of purification and specification of purity of compounds. If a compound is transparent in the near ultra-violet and the visible regions, the purification is continued until the absorbancy is reduced to a minimum (e < 1). Traces of impurities present in pure transparent organic compounds can be readily detected and estimated, provided the impurities themselves have fairly intense, absorption bands. Before a liquid is used as a spectroscopic solvent, it should be tested for spectrophotometric purity. For example, commercial absolute alcohol usually contains benzene as impurity. The absence of benzene in the Alcohol should be confirmed spectrophoto-metrically by using sufficiently large cells (4 or 10 cm cells), before using the alcohol as a solvent. The presence of carbon disulphide in carbon tetrachloride may be detected by the presence of the disulphide absorption tend at 318 mytt. The detection of the characteristic benzenoid absorption in the spectra of many organic compounds (e.g. diethyl ether, cyclohexene) showed that the bands attributed to these compounds earlier were only due to the contamination by benzene1. 

IR spectroscopy is used for the qualitative identihcation of surfactants and for differentiating between them and nonsurfactant compounds. Prior to IR spectroscopy, however, separation of the organic compound complex into different fractions, performed by, e.g., the use of thin-layer chromatography, is required to obtain meaningful spectra. °" ° By comparing the IR spectra of the isolated fractions with IR spectra of standard compounds with regard to characteristic bands, the qualitative determination of surfactants in environmental samples is possible. The method is equally applicable to anionic, ° nonionic, °" and cationic surfactants.The prerequisite for a clear identification of surfactants, however, is the availability of suitable standards. Moreover, considerable experience and knowledge are needed to interpret IR spectra of environmental samples. 

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