Vibrational spectra solid state effects

An appreciation of the crystal field effect on the vibrations of the Bravais cell which is repeated to build the crystal is extremely important when interpreting the vibrational spectra of many substances, since in the presence of a crystal field influence the number of observed bands in the spectrum cannot be directly determined from the formula unit which goes to make up the unit cell. In other words, there is almost always a larger number of bands to account for when investigating solid state samples. The solid state effects often cause degenerate bands to split in the same degree as symmetric and antisymmetric stretching modes split. 

The Stable carbonyl and thiocarbonyl halide molecules have been studied by IR as well as Raman spectroscopy. Normal coordinate analyses based on force constants transferred from other molecules (Urey-Bradley type), or from ab initio calculations, have aided in the vibrational assignments. Some of the unstable molecules which have been observed in the microwave have been characterized by infrared spectroscopy. The somewhat lower sensitivity of this method means that long path lengths of the gas may be needed. The identification of the various stable and unstable species in the microwave spectrum is simplified by the fact that the absorption lines are usually well resolved from each other. The widths of the bands in the infrared may make the transient species difficult to detect against the stronger absorptions of the stable side products. IR and Raman spectroscopies do have the advantage that they can be used on solid and liquid samples. Since the bands in a low temperature rare gas matrix have a narrower profile, the infrared spectrum is usually simplified over the room temperature gas phase spectrum. Moreover, the vibrational frequencies are only mildly perturbed by solid state effects. For example, CF Se has not been observed in the vapor phase, yet its vibrational dynamics are known from its matrix isolation spectrum. Table 9 gives the vibrational data for the carbonyl, thiocarbonyl, seleno-carbonyl and formyl halides. 

As we have seen, the expressions for the rate constant obtained for different models describing the lattice vibrations of a solid are considerably different. At the same time in a real situation the reaction rate is affected by different vibration types. In low-temperature solid-state chemical reactions one of the reactants, as a rule, differs significantly from the molecules of the medium in mass and in the value of interaction with the medium. Consequently, an active particle involved in reaction behaves as a point defect (in terms of its effect on the spectrum and vibration dynamics of a crystal lattice). Such a situation occurs, for instance, in irradiated molecular crystals where radicals (defects) are formed due to irradiation. Since a defect is one of the reactants and thus lattice regularity breakdown is within the reaction zone, the defect of a solid should be accounted for even in cases where the total number of radiation (or other) defects is small. 

The acquisition of solid-state FTIR spectra suitable for use in the characterization of polymorphic impurities is performed using either the Nujol mull technique, diffuse reflectance (DRIFT), or attenuated total reflectance (ATR). One should avoid the use of pelleting techniques to eliminate any spurious effects associated with compaction of the KBr pellet. The simplest approach is to prepare a mull of the sample in mineral oil, sandwich this between salt plates, and measure the spectrum using ordinary transmission techniques. The main drawback of the mull technique is that regions in the IR spectrum overlapping with carbon-hydrogen vibrational modes will be obliterated because of absorbance from the oil. 

From Equation 4.47 it is clear that the vibrational spectrum for an isolated molecule is not the same as that for the same molecule in a solid matrix, because of the so-called static field effect. This is because of the presence of vibrations, where the whole molecule vibrates in interaction with other neighboring ones, as well as the lattice vibrations, the so-called phonon vibrations, where several molecules are involved. Furthermore, in the case of polymorphism, the different crystal structures give very different spectra. Thus, infrared spectroscopy can be used to discriminate different crystal structures, follow phase transformations, as well as follow the progress of solid state reactions. 

The lattice vibration spectra of difierent isolated chains (Section II.4.3.1) represent most simple examples of the general theory and afford a hrst insight into the effect of the configuration and conformation on the spectrum of real polymer chains. For calculation of the heat capacity of polymers in the solid state they do not suffice, however because the intermcfiecular interaction forces (also in first order approximation) play a non-negligible part, mainly at lower temperatures. 

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