Frequencies group

Not all vibrations are good group frequencies, and some group frequencies which maintain a fairly constant position for most 

Group frequencies are described in terms of the motions that the nuclei in a structural group in the molecule undergo during the vibration. For example, a CH3 group in a molecule can be visualized as having the following vibrations  

It will be recalled that these motions were diagrammed in Chapter 4. In describing the group frequencies of a CH3 group, one may classify them according to this scheme. However, there is some ambiguity in such a classification. 

Diagramming vibrations in this manner implies that the frequency Vi depends entirely on a stretching force constant while the bending fundamental V2 depends entirely on a bending force constant A 2. There is no contribution of A2 to the force constant that determines v, and no contribution from to the vibration V2. 

Vibrations which retain their identity can be described as group frequencies. They can be assumed to be associated with fixed groups in the molecule and they appear in fairly constant regions in a spectrum. When a vibration can be associated with a certain structural group, this is usually noted in the listing of the vibrational analysis of the molecule (for example, see Table 5-1). 

Historically, the first step in the spectroscopic analysis is an assignment in terms of group frequencies. Compilations of these are readily available for infi-ared and Raman spectra [1] however, this is not so for INS spectra. This is largely because it is only in the last decade that sufficiently high resolution INS spectra have become available that would enable the creation of such tables. Fortunately, the frequency information in the infrared and Raman tables is still valid for INS spectra, it is the intensity information that is inapplicable. 

Maleic anhydride will be used as example [2] (see Fig. 8.1 for the structure). Using the tables the spectra can be assigned as follows. The symmetric and antisynunetric C-H stretches are seen in the infrared and Raman spectra at 3132 and 3124 cm. The S5nnmetric and antisymmetric C=0 stretches are at 1857 and 1785 cm respectively. The assignment is based on the reversal of intensities between the infrared and Raman 

The analysis may seem rudimentary but for many years it formed the basis of vibrational spectroseopy. With additional information from isotopic substitution and polarisation studies it provides a successfiil means of analysis. For complex systems such as biomolecules this type of analysis is still used. The method offers a simple, useful check on the spectra recorded. It is also needed when the unexpected happens and the observed spectrum does not match expectations it provides clues for further investigation. 

It would seem from the previous discussion that there is little hope of interpreting the spectra of complex molecules. The solution of the 

The important consequence of the constancy of force constants is that, under certain conditions, the frequencies associated with the motion of particular atoms or groups of atoms will remain essentially constant from molecule to molecule. These are known as group frequencies, and they provide an important tool for the spectral characterization of a molecule. It is therefore important to inquire into the validity and limitations of this concept. 

While constancy of the electronic environment is necessary in order to insure transferable force constants, it is not sufficient to guarantee the existence of separable group frequencies. The essential characteristic of the latter is that in a particular normal vibration of the molecule as a whole the displacements may validly be said to be confined primarily to the atoms of this group. Not only this, but also such a group normal mode should remain basically unchanged from molecule to molecule. The existence of such a group frequency is intimately connected with the extent of interaction between the characteristic motions of this group and 

As an illustration of the factors which determine the separability of such group modes, we will consider the stretching vibrations of a linear triatomic molecule, XYZ. Such a molecule has 3x3 — 5 = 4 internal vibrational modes, of which two involve stretching of the X—Y and Y—Z bonds and the other two (which are degenerate, i. e., of the same frequency) bending of the XYZ angle. By the method outlined in section I. 1. we can obtain the frequencies and forms of the stretching modes when the masses and force constants are specified. The secular equation is 

The results, for several values of the masses and force constants, are given in Table 1, where we have chosen the masses to correspond roughly to H, C, and N atoms and the force constants to be approximately those of single, double, and triple bonds. In the drawings of the normal modes the scale of the displacements is not the same as that of the internuclear distances, nor is the scale of displacements the same in all modes. Only 

You should be aware, however, of certain factors which can complicate injured spectra. Phenomena such as overtone and combination l ands, Fermi resonance, and hydrogen bonding can introduce additional, and sometimes misleading, information into the spectra. It is important to be aware of these factors before tackling the interpretation of a given spectrum and so they are also discussed in this chapter. 

The rcdd-infrared spectrum can be divided into four regions, and the nature of a group frequency may generally be determined by the region in which it is located. 

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These characteristic absorption regions called group frequencies allow the analyst to detect the different elemental patterns and from them to reconstruct the molecule either by dej duct ion or by comparison with library reference spectra. The libraries contaih severaY hundred thousand spectra. 

Socrates G 1994 Infrared Characteristic Group Frequencies Tables and Charts 2nd edn (Chichester Wiley)... 

Bellamy, L. J. (1980) The Infrared Spectra of Complex Molecules, Volume 2 Advances in Infrared Group Frequencies, Chapman and Hall, London. 

Uv—vis Spectra do not offer the unique group frequencies and fingerprinting abiUty of the it, but different chtomophotes exhibit absorptions at snecific wavelengths, A, and have characteristic intensities. These ate tabulated in handbooks as X and S, where S is a molar decadic absorption... 

Some of these group frequencies are listed in Table 19.1, and amore complete correlation table is provided in Appendix 11. 

Infrared spectra absorption bands, (T) 839 group frequencies, 742 modes of vibration, 742... 

G. Socrates, Infrared and Raman characteristic group frequencies, John Wiley Sons LTD, Baffins Lane, Chichester, West Susses P019 lUD, England, 2001, p. 1-347. 

The interactions of photons with molecules are described by molecular cross-sections. For IR spectroscopy the cross-section is some two orders of magnitude smaller with respect to UV or fluorescence spectroscopy but about 10 orders of magnitude bigger than for Raman scattering. The peaks in IR spectra represent the excitation of vibrational modes of the molecules in the sample and thus are associated with the various chemical bonds and functional groups present in the molecules. The frequencies of the characteristic absorption bands lie within a relatively narrow range, almost independent of the composition of the rest of the molecule. The relative constancy of these group frequencies allows determination of the characteristic... 

Cytogenetic research showed that the average group frequency of cells with chromosomal aberrations in lymphocytes is many times higher in personnel producing the fungicide zineb (5.53%, with fluctuations from 4.00-8.50%) than in the control group (0.95%) [A97]. 

Infrared spectroscopy is an excellent tool in iminoborane chemistry, which readily permits, to distinguish between iminoboranes and nitrile-borane adducts and to identify monomeric and dimeric forms of iminoboranes. This event is due to the fact that the i>CN of CN multiple bonds absorbs outside the fingerprint region and can be considered to be a valuable group frequency even when mixed with other vibrational modes. In some cases other vibrations like NH, BH, B-halogen or B-S stretching modes are helpful for determining the structure of iminoboranes. 

Diatomic Molecules. Polyatomic Molecules. Characteristic Vibration Frequencies. Factors Affecting Group Frequencies. 

The occurrence of characteristic vibration or group frequencies can be explained in terms of relative masses and of force constants using the classical analogy of weights and springs as depicted in Figure 9.14. 

The value of infrared spectrometry as a means of identification of unknown compounds and to investigate structural features is immense. Spectra are used in an empirical manner by comparison of samples with known materials and by reference to charts of group frequencies. A simplified correlation chart is shown in Table 9.8. The interpretation of infrared spectra is best considered by discussing the prominent features of a representative series of compounds. 

The presence of group frequencies or "finger print" regions in infrared spectra make vibrational spectroscopy a key analytical method in identifying classes of molecules. 

A dipole-dipole model reproduces factor group frequencies of i>(CO) in a variety of dinuclear metal carbonyls... 

C. Pratt Brock and L. L. Duncan. Anomalous space group frequencies for monoalcohols C HmOH. Chem. Mater. 6, 1307 1312 (1994). 

Table 22.1 Stretching Frequencies found in Group Frequency Region...

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