Uses of the Infrared Spectrum

Since every type of bond has a different natural frequency of vibration, and since two of the same type of bond in two different compounds are in two shghtly different environments, no two molecules of different structure have exactly the same infrared absorption pattern, or infrared spectrum. Although some of the frequencies absorbed in the two cases might be the same, in no case of two different molecules will their infrared spectra (the patterns of absorption) be identical. Thus, the infrared spectrum can be used for molecules much as a fingerprint can be used for hrrmans. By comparing the infrared spectra of two substances thought to be identical, you can establish whether they are, in fact, identical. If their infrared spectra coincide peak for peak (absorption for absorption), in most cases the two substances wdl be identical. 

FIGURE 2.2 The approximate regions where various common types of bonds absorb (stretching vibrations only bending, twisting, and other types of bond vibrations have been omitted for clarity). 

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Although some aspects of the proposed assignments will require additional confirmation in order to be more certain that they are correct, it appears at present that the basic framework of a set of assignments for isotactic polystyrene exists. These assignments enable the use of the infrared spectrum of an oriented... 

Searching for Chemically Stable Conducting Polythiophenes—Use of the Infrared Spectrum... 

Next is the use of the infrared spectrum for the identification of the conformation of the backbone (see ref.154. for more details). In a previous spectroscopic analysis on model compounds the out-of-... 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

This general behaviour is characteristic of type A, B and C bands and is further illustrated in Figure 6.34. This shows part of the infrared spectrum of fluorobenzene, a prolate asymmetric rotor. The bands at about 1156 cm, 1067 cm and 893 cm are type A, B and C bands, respectively. They show less resolved rotational stmcture than those of ethylene. The reason for this is that the molecule is much larger, resulting in far greater congestion of rotational transitions. Nevertheless, it is clear that observation of such rotational contours, and the consequent identification of the direction of the vibrational transition moment, is very useful in fhe assignmenf of vibrational modes. 

Fig. 4 Restoration by maximum entropy (solid curve) of a portion of the Q branch of the v3 band of the infrared spectrum of CH4 near 3014 7 cm-. Triangles indicate the experimental inputs /m used in the restoring scheme, some of which lie outside the plotted region and are not shown.

We therefore studied the effect of temperature and of concentration on the position of the hydroxyl peak in simple alcohols (methanol, ethanol, etc.) in the pure state, and in carbon tetrachloride or chloroform solutions. Some of the results of this work have already been reported [1]. A plot of peak position against concentration gives curves such as that in Fig. la. Interpretation of this type of curve from the N.M.R. data alone is impossible. It is clear that several different species (monomer, dimer, polymers) are contributing their effect, but because of the averaging phenomenon only a single OH peak, representing the weighted mean of all these species, is observed. We have now used infrared spectral data to clarify the situation. A careful examination of the infrared spectrum of all normal aliphatic alcohols... 

The intense absorption of water over most of the infrared spectrum restricts the regions where aqueous solutions of carbohydrates can be usefully studied. Absorbance subtraction makes it possible to eliminate water absorbance and magnify the remaining spectral features to the limit of the signal-to-noise ratio. Many other data-processing techniques, such as the ratio method,4 the least-squares refinement,5 and factor analysis,6 should be of benefit in the study of carbohydrate mixtures. 

The objectives of this study were (1) to reveal trends between selected regions of the infrared spectrum and accepted indicators of coalification (2) to determine the nature and degree of interdependency among these data using statistical analyses and (3) to evaluate the usefulness of FTIR data in measuring degree of coalification. 

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. 

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