Solids crystal vibrations

DIott D D 1988 Dynamics of molecular crystal vibrations Laser Spectroscopy of Solids 7/ed W Yen (Berlin Springer) pp 167-200... 

Raman spectroscopy is particularly useful for investigating the structure of noncrystalline solids. The vibrational spectra of noncrystalline solids exhibit broad bands centered at wavenumbers corresponding to the vibrational modes of the corresponding crystals (Figure 5). In silicate glasses shifts in the high-wavenumber bands... 

The molecule S12, like Se, is of Dsd symmetry but in the soHd state it occupies sites of the much lower C211 symmetry [163]. Due to the low solubihty and the thermal decomposition on melting only solid state vibrational spectra have been recorded [2,79]. However, from carbon disulfide the compound Si2-CS2 crystallizes in which the S12 molecules occupy sites of the high Sg symmetry which is close to 03a [163]. The spectroscopic investigation of this adduct has resulted in a revision [79] of the earher vibrational assignment [2] and therefore also of the earlier force constants calculation [164]. In Fig. 24 the low-temperature Raman spectra of S12 and Si2-CS2 are shown. 

Most of the four above-mentioned properties for Raman spectra can be explained by using a simple classical model. When the crystal is subjected to the oscillating electric field = fioc " of the incident electromagnetic radiation, it becomes polarized. In the linear approximation, the induced electric polarization in any specific direction is given by Pj = XjkEk, where Xjk is the susceptibility tensor. As for other physical properties of the crystal, the susceptibility becomes altered because the atoms in the solid are vibrating periodically around equilibrium positions. Thus, for a particular... 

The spectra in Figure 12.24 clearly show that the constituent ions in the liquid and in the respective solid salts vibrate rather independent of the surroundings. Therefore the liquid spectrum looks much like the sum of the solid salts. This conclusion is of course not new, but nevertheless it is still quite applicable in the evaluation of many IL Raman (and IR) spectra. However, the presence of conformational equilibria for both of the IL ions makes a closer study worth while. We therefore recommend the interested reader to study the work by Umebayashi et al. [114] in which subtle spectral band shape details, for example, around 930-880 cm are evaluated to show information on the eq-envelope trans-TT and ax-envelope trans-TT interconversion of the [C4QIm]+ ion in the liquid. Also note that the crystal structure of the [CjC4pyr][Tf2N] salt was recently solved it contained the eq-envelope trans-TT conformer of the cation [115]. Also conformers of symmetry Cj and C2 of the [Tf2N] ion show their presence hurried in the band at 400-440 cm-i [109]. 

Dlott DD. Dynamics of molecular crystal vibrations. In Yen W, ed. Laser Spectroscopy of Solids II. Berlin Springer-Verlag, 1988 167-200. 

The particles in a solid are very close together and have an orderly, fixed arrangement. They are held in place by the attractive forces that are between all particles. Because solid particles can vibrate only in place and do not break away from their fixed positions, solids have fixed volumes and shapes. That is, no matter what container you put a solid in, the solid takes up the same amount of space. Solids usually exist in crystalline form. Solid crystals can be very hard and brittle, like salt, or they can be very soft, like lead. Another example of a solid is ice, the solid state of water. 

These improvements were made by Debye, and independently by Born and Karman (1912). They rest on the following considerations. Up to this point we have dealt with the individual atoms of the solid (crystal), as if they performed undisturbed harmonic vibrations independently of one another. This, however, is by no means the case. 

The quantitative side of the matter is less definite as far as solid crystals are concerned, because the vibrations of the solid are in reality very complex and can only be described in rough approximation by a single frequency. In fact a complicated spectrum of frequencies must be invoked to do justice to the finer details of behaviour. Nevertheless, the operation of the first of the quantum rules is clearly shown by what has been described. 

With anisodimensional molecules states have indeed been observed which lie between the crystalline and the amorphous (mesophases, liquid crystals). Mark recognizes the following transitions between solid crystals and amorphous liquids 1°. Three-dimensional crystal the centres of gravity of the units are fixed (apart from vibration), rotations are not possible. Examples hexamethylene tetramine, urea. 2°, Crystal with rotating molecules the centres of the particles are fixed rotation about one or more axes is possible. Examples NH4CI, sodium stearate at higher temperatures. 

For example, Levich and co-workers put forward a polaronic energy transfer mechanism (which is applicable firstly in solid crystals) for the activation of ions in solution, the object being to explain the continuity in the current-potential relation. This is because in their view, the vibrational-rotational levels of ions in solution remain separated by the same amounts as in the gas phase. However, this objection is not cogent. Thus, the results of Moore et show that there are enough translator frequencies in water to justify a model in which liquid water contains a sufficient number of free... 

For example, in a recent tutorial, Kettle and co-authors described the solid-state vibrational spectroscopy of bis(dicarbonyl-77-cyclopentadienyliron) [4]. Although it is well established, solid-state spectroscopy is given cursory treatment in standard physical chemistry textbooks. The iron compound makes an interesting case study because its cis and trans isomers crystallize in the same space group. Raman and infrared spectroscopy are given equal consideration in the discussion. 

---