Vibration spectra, interpretation

Theoretical interpretation of molecular vibration spectra is not a simple task. It requires knowledge of symmetry and mathematical group theory to assign all the vibration bands in a spectrum precisely. For applications of vibrational spectroscopy to materials characterization, we can still interpret the vibrational spectra with relatively simple methods without extensive theoretical background knowledge. Here, we introduce some simple methods of vibrational spectrum interpretations. 

The computational prediction of vibrational spectra is among the important areas of application for modem quantum chemical methods because it allows the interpretation of experimental spectra and can be very instrumental for the identification of unknown species. A vibrational spectrum consists of two characteristics, the frequency of the incident light at which the absorption occurs and how much of the radiation is absorbed. The first quantity can be obtained computationally by calculating the harmonic vibrational frequencies of a molecule. As outlined in Chapter 8 density functional methods do a rather good job in that area. To complete the picture, one must also consider the second quantity, i. e., accurate computational predictions of the corresponding intensities have to be provided. 

As the interpretation of the vibrational spectrum of sohd state samples can provide information concerning the cation-anion interactions, it is of special interest to the chemist. In some cases information about the crystal structure can be obtained from the interpretation of spectra of the solid state when the substance possesses an anion of high symmetry. 

Polyethylene has been studied spectroscopically in greater detail than any other polymer. This is primarily a result of its (supposedly) simple structure and the hope that its simple spectrum could be understood in detail. Yet as simple as this structure and spectrum are, a satisfactory analysis had not been made until relatively recently, and even then significant problems of interpretation still remained. The main reason for this is that this polymer in fact generally contains structures other than the simple planar zig-zag implied by (CH2CH2) there are not only impurities of various kinds that differ chemically from the above, but the polymer always contains some amorphous material. In the latter portion of the material the chain no longer assumes an extended planar zig-zag conformation, and as we have noted earlier, such ro-tationally isomeric forms of a molecule usually have different spectra. Furthermore, the molecule has a center of symmetry, which as we have seen implies that some modes will be infrared inactive but Raman active, so that until Raman spectra became available recently it was difficult to be certain of the interpretation of some aspects of the spectrum. As a result of this work, and of detailed studies on the spectra of n-paraffins, it now seems possible to present a quite detailed assignment of bands in the vibrational spectrum of polyethylene. 

The identification of species adsorbed on surfaces has preoccupied chemists and physicists for many years. Of all the techniques used to determine the structure of molecules, interpretation of the vibrational spectrum probably occupies first place. This is also true for adsorbed molecules, and identification of the vibrational modes of chemisorbed and physisorbed species has contributed greatly to our understanding of both the underlying surface and the adsorbed molecules. The most common method for determining the vibrational modes of a molecule is by direct observation of adsorptions in the infrared region of the spectrum. Surface spectroscopy is no exception and by far the largest number of publications in the literature refer to the infrared spectroscopy of adsorbed molecules. Up to this time, the main approach has been the use of conventional transmission IR and work in this area up to 1967 has been summarized in three books. The first chapter in this volume, by Hair, presents a necessarily brief overview of this work with emphasis upon some of the developments that have occurred since 1967. 

The vibrational spectrum of H+ is even harder to interpret. Absorption increases at all frequencies in the infrared and the already broad water bands get broader, but not symmetrically. The additions to the water bands have been interpreted as the new bands of the HsO+ unit in H+ (Falk and Gigufere, 1957). The suggested frequencies are shown in Table 9. On the other hand it has been suggested that the rapid proton shifts from one oxygen to another precludes a band spectrum for that unit in water (Ackermann, 1961) and its absorption has been... 

When interpreting time-resolved mid-IR spectra, it is beneficial to consider the influence of rotational dynamics on the vibrational spectrum of a heteronuclear diatomic. It was shown more than 30 years ago that the vibrational absorption spectrum of a diatomic is related to its transition dipole correlation function (/z(0) /r(t)> through a Fourier transform (10) ... 

This experiment is concerned with the rotational fine structure of the infrared vibrational spectrum of a linear molecule such as HCI. From an interpretation of the details of this spectrum, it is possible to obtain the moment of inertia of the molecule and thus the intemuclear separation. In addition the pure vibrational frequency determines a force constant that is a measure of the bond strength. By a study of DCI also, the isotope effect can be observed. 

A natural development of this work was to study the effects of structural perturbations like conformational changes on the valence bands of stereoisomers. We have considered the four conformations relevant to the interpretation of the vibrational spectrum of polyethylene (J ). The theoretical calculations on those four conformers of polyethylene T, TG, G and TGTG revealed so important differences in the shape of densities of states that... 

Spectrum comparison is the simplest method to interpret a vibration spectrum. We do not need to have much theoretical knowledge of spectrum interpretation for doing so. A sample can be identified if its spectrum matches a reference in both band positions and intensities... 

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