Hydrocarbons, group frequencies

Thus, hydrocarbon group frequencies are the only recognizable absorption bands in the Nujol spectrum. These group frequencies, of course, also are found in the n-hexane spectrum. The remaining bands in the hexane spectrum become merged into background absorption in the mixture of alkanes in the mineral oil and a very simple spectrum emerges. 

The discussion thus far has emphasized sensitivity of the frequency of C02 s v3 mode to local stress, sensitivity of its absorption intensity to IR polarization, and sensitivity of both properties to resonant coupling of dimers. For the type of crystals under consideration, which consist mostly of simple hydrocarbon groups, these factors probably dominate in determining the IR spectral characteristics. Other factors can be involved, however, and although they can make simple interpretation of the spectra more problematic, they can also provide additional information about the environment of the C02 probe molecule. The following examples illustrate how one can track the motion of C02 over distances of 1-15 A by observing its proximity to free radical centers or to halogen or deuterium substituents in the crystal lattice. This information complements the previously discussed structural studies, which related to structure within the dimer rather than to the location of the C02 in the crystal matrix. 

Even when the spectrum of an adsorbed species proves to be that of only one of those anticipated, difficulties of recognition can arise because of the perturbing effects of the bonded metal atoms on the well known characteristic frequencies (wavenumbers) of hydrocarbon groups 8-10). 

One of the great advantages of vibrational spectroscopy is that many hydrocarbon groupings, such as CH3, CH2, C=C, C=C, etc., have characteristic vibration frequencies/wavenumbers, many of which fall in the wavenumber ranges transmitted by oxide supports. Furthermore, for use in connection with... 

Group-Characteristic Frequencies/Wavenumhers (cm l of Ci Hydrocarbon Groups Directly Bonded to Metal Atoms in Comparison with Those of Analogous Unsubstituted Hydrocarbons... 

More recently Tibbitt, Bell and Shen (56,57) carried out quantitative determinations of the structure of plasma polymerized hydrocarbons by infrared spectroscopy. Typical spectra of films deposited on sodium chloride substrates are shown in Figure 15. Absorptions peaks were identified to be associated with various group frequencies, corroborated with the findings from NMR spectra of dilute solutins of the oily products. From the knowledge of the extinction coefficients, concentrations of the functional groups in the plasma polymerized materials were determined. 

We saw that the hydrocarbon groups in triglycerides can differ in the length of the carbon chain and in the frequency of double bonds between their carbon atoms. The liquid triglycerides in vegetable oils have more carbon-carbon double bonds than the solid triglycerides in animal fats. The more carbon-carbon double bonds a triglyceride molecule has, the more likely it is to be liquid at room temperature. 

Aryl Halides (Chlorobenzene). The final system to be considered in this section is the aryl hakde, chlorobenzene. Based on the above assignments the group frequencies of the complete hydrocarbon portion and the heteroatom function group can be assigned as in Table 8.25 (also see Chapter 8W, IR section. Fig. W8.39). 

Spectral analysis based on group frequencies and the spectra-structure correlation method is well established in IR and Raman spectroscopy. These methods are also becoming popular in NIR spectroscopy, and recently Weyer and Lo (30) wrote a valuable review article on the relationship between NIR bands and chemical structure. They compared NIR spectra of selected aliphatic hydrocarbons, those of selected aromatics, those of selected alcohols, and so on and discussed detailed spectra-structure correlations in the NIR region. Tables for group frequencies in the NIR region are also available in some NIR books (3,4). 

If one atom has a mass very much different from the masses of the atoms in the remainder of the molecule, then the structural groups with this atom will have characteristic group frequencies. An example would be a unit such as —C— As—, i.e., the case where isolated arsenic atoms are found in a hydrocarbon. The C— As stretching frequency is fairly distinct since the vibration is not likely to couple with another. Other examples are C—Cl, C— Br, C—metal, etc. A C—group would be expected to give less satisfactory group frequencies since the masses of C and N are similar and both are involved in other chemical bonds. 

TABLE 5-IV. Characteristic Group Frequencies for Normal Paraifin Hydrocarbons (CCI4 and CS2 Solutions) (Similar Group Frequencies Are Found for Liquid and Solid State Spectra)... 

Branched-chain hydrocarbons may contain tertiary CH groups. The group frequencies for this CH unit are not as well defined as those of CH3 and CH2 groups and are generally weak since the number of tertiary CH groups is usually much less than that of CH3... 

TABLE 5-V. Characteristic Group Frequencies for Branched-Chain Hydrocarbons... 

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