Infrared spectrum molecules

Nevertheless, a molecule possesses sufficient vibrations so that all its frequencies taken together can be used to characterize it. In this sense the infrared spectrum is generally considered to be a molecule s fingerprint."... 

XI-1C) as well as alongside it. The infrared spectrum of CO2 adsorbed on 7-alumina suggests the presence of both physically and chemically adsorbed molecules [3]. 

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

Until 1962 the infrared and Raman spectra of thiazole in the liquid state were described by some authors (173, pp. 194-200) with only fragmentary assignments. At that date Chouteau et al. (201) published the first tentative interpretation of the whole infrared spectrum between 4000 and 650 cm for thiazole and some alkyl and haloderivatlves. They proposed a complete assignment of the normal modes of vibration of the molecule. 

The study of the infrared spectrum of thiazole under various physical states (solid, liquid, vapor, in solution) by Sbrana et al. (202) and a similar study, extended to isotopically labeled molecules, by Davidovics et al. (203, 204), gave the symmetry properties of the main vibrations of the thiazole molecule. More recently, the calculation of the normal modes of vibration of the molecule defined a force field for it and confirmed quantitatively the preceeding assignments (205, 206). 

Fingerprint region (Section 13 20) The region 1400-625 cm of an infrared spectrum This region is less character istic of functional groups than others but varies so much from one molecule to another that it can be used to deter mine whether two substances are identical or not Fischer esterification (Sections 15 8 and 19 14) Acid cat alyzed ester formation between an alcohol and a carboxylic acid... 

Figure 6.7 Rotational transitions accompanying a vibrational transition in (a) an infrared spectrum and (b) a Raman spectrum of a diatomic molecule...

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. 

The simplicity of the infrared spectrum of solid Cgo (see Fig. 9), which shows four prominent lines at 527, 576, 1183, 1428 cm each with Ti symmetry [4], provides a convenient method for characterizing Cqq samples [4, 88]. The IR spectrum of solid Cqq remains almost unchanged relative to the isolated Ceo molecule, with the most prominent addition being the weak feature at... 

More complicated molecules, with two or more chemical bonds, have more complicated absorption spectra. However, each molecule has such a characteristic spectrum that the spectrum can be used to detect the presence of that particular molecular substance. Figure 14-17, for example, shows the absorptions shown by liquid carbon tetrachloride, CCfi, and by liquid carbon disulfide, CS2. The bottom spectrum is that displayed by liquid CC14 containing a small amount of C. The absorptions of CS2 are evident in the spectrum of the mixture, so the infrared spectrum can be used to detect the impurity and to measure its concentration. 

For molecules in which at least one internal rotating part has a dipole moment component perpendicular to the axis of rotation, there should appear directly in the infrared spectrum transitions between the torsional states. For most cases these would be quite far in the infrared region and therefore more difficult to observe. Nevertheless a few molecules of this type have been studied in this region, and such transitions have been reported.6... 

When one of the cartesian coordinates (i.e. x, y, or z) of a centrosymmetric molecule is inverted through the center of symmetry it is transformed into the negative of itself. On the other hand, a binary product of coordinates (i.e. xx, yy, zz, xz, yz, zx) does not change sign on inversion since each coordinate changes sign separately. Hence for a centrosymmetric molecule every vibration which is infrared active has different symmetry properties with respect to the center of symmetry than does any Raman active mode. Therefore, for a centrosymmetric molecule no single vibration can be active in both the Raman and infrared spectrum. 

Except in simple cases, it is very difficult to predict the infrared absorption spectrum of a polyatomic molecule, because each of the modes has its characteristic absorption frequency rather than just the single frequency of a diatomic molecule. However, certain groups, such as a benzene ring or a carbonyl group, have characteristic frequencies, and their presence can often be detected in a spectrum. Thus, an infrared spectrum can be used to identify the species present in a sample by looking for the characteristic absorption bands associated with various groups. An example and its analysis is shown in Fig. 3. 

Infrared frequencies are characteristic for certain bonds in molecules and they can often be used to identify chemisorbed species on surfaces. The infrared spectrum of CO or NO can sometimes also be used to recognize sites on the surface of a catalyst, as the following example shows. 

The specific requirement for a vibration to give rise to an absorption in the infrared spectrum is that there should be a change in the dipole moment as that vibration occurs. In practice, this means that vibrations which are not centrosymmetric are the ones of interest, and since the symmetry properties of a molecule in the solid state may be different from those of the same molecule in solution, the presence of bands may depend on the physical state of the specimen. This may be an important phenomenon in applying infrared spectroscopy to the study of AB cements. 

Deposition of iron atoms into a thin film of 2, with Mw = 3100 (Table I), resulted in a product with an infrared spectrum consisting of bands at 2085 and 1840 cm-l. The former corresponds to an isocyanide ligand possessing a linear structure (B) while the latter is associated with bent isocyanide ligands (B ). Comparison with results for small-molecule analogues shows that the spectrum is consistent with formation of Fe(CN-[P])5 (17). 

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