Infrared Spectra with Molecular Structure

Correlating Infrared Spectra with Molecular Structure 

Water has been analyzed in glycerol, hydrazine. Freon, organic films, acetone, and fuming nitric acid. Absorption bands at 2.76, 1.90, and 1.40 pm are used, depending on the concentration of the test substanee. When interferences from other absorption bands are severe or very low coneentrations of water are being studied, water can be extracted with glycerol or ethylene glycol. 

Near-infrared spectrometry is a valuable tool for analyzing mixtures of aromatic amines. Primary aromatic amines are characterized by two relatively intense absorption bands near 1.97 and 1.49 pm. The band at 1.97 pm is a combination ofN—H bending and stretching modes, and the one at 1.49 pm is the first overtone of the symmetric N—H stretching vibration. Secondary amines exhibit an overtone band, but they do not absorb appreciably in the combination region. Secondary amines exhibit an overtone band, but they do 

The region between 667 and 10 cm (15-1000 pm) contains the bending vibrations of carbon, nitrogen, oxygen, fluoride with atoms heavier than mass 19, and additional bending motions in cyclic or unsaturated systems. The low-frequency molecular vibrations found in the far infrared are particularly 

The presence of an unsaturated C=C linkage introduces the stretching frequency at 1650 cm (6.07 pm), which may be weak or nonexistent if 

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Correlating Infrared Spectra with Molecular Structure... 

In a surprising recent development, Kratschmer et al. have shown that certain all-carbon molecules are produced in large quantities in the evaporation of graphite and can be isolated as soluble, well-defined solids. The major species was identiHed as molecular C ) through mass spectrometry and by comparison of the infrared spectrum with theoretical predictions for the celebrated truncated-icosahedron structure, which had earlier been proposed to account for cluster beam observations. The solid material, described as a new form of elemental carbon in a nearly pure state, has a disordered hep lattice of packed quasi-spherical molecules, but determination of the precise molecular structure awaits diffraction from well-ordered crystals. 

Infrared and Raman spectroscopy are complementary in structural determinations because some molecular vibrations that are inactive in the infrared (that is, do not result in a change in dipole moment and therefore do not cause an absorption band) do have a strong Raman line. The reverse is also true. Some bands that are weak or forbidden in the Raman spectrum are strong in the infrared spectrum. With the combined use of these techniques, the vibrational energies of a molecule can be fully described. 

We measure infrared spectra to answer questions about samples. One question we commonly try to answer is, What molecules are present in this sample , otherwise called unknown analysis. The peak positions in an infrared spectrum correlate with molecular structure, which is part of why infrared spectroscopy is useful. Over the last 100-plus years a great number of infrared spectra have been measured, and the peak positions of known molecules derived from these spectra can be used to identify the molecules in an unknown sample [1]. 

Hydroxypyridine 1-oxide is insoluble in chloroform and other suitable solvents, and, although the solid-state infrared spectrum indicates that strong intermolecular hydrogen bonding occurs, no additional structural conclusions could be reached. Jaffe has attempted to deduce the structure of 4-hydroxypyridine 1-oxide using the Hammett equation and molecular orbital calculations. This tautomeric compound reacts with diazomethane to give both the 1- and 4-methoxy derivatives, " and the relation of its structure to other chemical reactions has been discussed by Hayashi. ... 

Fig.2 shows the infrared absorption spectrum of the tin oxide film. In order to analyze the molecular structure of the deposited film, we deposited the tin oxide film on a KBr disc with thickness of 1 mm and diameter of 13 mm. Various peaks formed by surface reaction are observed including O-H stretching mode at 3400 cm, C=C stretching mode at 1648 cm, and Sn02 vibration mode at 530 cm. The formation of sp structure with graphite-like is due to ion bombardment with hydrogen ions at the surface and plasma polymerization of methyl group with sp -CHa. 

Infrared and Raman spectroscopy are in current use fo r elucidating the molecular structures of nucleic acids. The application of infrared spectroscopy to studies of the structure of nucleic acids has been reviewed,135 as well as of Raman spectroscopy.136 It was noted that the assignments are generally based on isotopic substitution, or on comparison of the spectrum of simple molecules that are considered to form a part of the polynucleotide chain to that of the nucleic acid. The vibrational spectra are generally believed to be a good complementary technique in the study of chemical reactions, as in the study76 of carbohydrate complexation with boric acid. In this study, the i.r. data demonstrated that only ribose forms a solid complex with undissociated H3B03, and that the complexes are polymeric. 

The analytical chemist can use Raman and infrared spectra in two ways. At tile purely empirical level, they provide fingerprints of the molecular structure and, as such, permit the qualitative analysis of individual compounds, either by direct comparison of the spectra of the known and unknown materials run consecutively, or by comparison of the spectrum of the unknown compound with catalogs of reference spectra. 

By comparisons among the spectra of large numbers of compounds of known structure, it lias been possible to recognize, at specific positions in the spectrum, bands which can be identified as characteristic group frequencies associated with the presence of localized units of molecular structure in the molecule, such as methyl, carbonyl, or hydroxyl groups. Many of these group frequencies differ in the Raman and infrared spectra. 

Exercise 9-11 The infrared spectra shown in Figure 9-14 are for compounds of formula C3H60 and C3H602. Use the data in Table 9-2 and the molecular formulas to deduce a structure for each of these substances from its infrared spectrum. Indicate clearly which lines in the spectra you identify with the groups in your structures. 

The study of oriented polymers with polarized infrared radiation is an equally important tool in the detailed analysis of the vibrational spectrum, since it permits us, within certain restrictions, to determine the orientation (with respect to the molecular structure) of the transition moment for a given normal mode. This makes it possible, as we shall see, not only to classify bands in the spectrum but to establish their origin. Although polarizers are available with commercial spectrometers, their use has not yet become as general as would be desirable. Some comments... 

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