IR-active vibrational modes

Molecules generally have a large number of bonds and each bond may have several IR-active vibrational modes. IR spectra are complex and have many overlapping absorption bands. IR spectra are sufficiently complex that the spectmm for each compound is unique and this makes IR spectra very useful for identifying compounds by direct comparison with spectra from authentic samples "fingerprinting"). 

The IR transmission spectra (Fig. 3) of the fullerene precipitated from the solution saturated with air show four predominant absorption peaks at 526.7, 576.2, 1182.7 and 1429.7 cm 1 that could be attributed to the IR active vibration modes (Flu) of the C o molecule with high symmetry (//,). The IR spectra of 1,2 dichlorobenzene and isopropyl alcohol are also presented on Fig. 3 for comparison. The appearance of the IR bands with the peaks at 746.2, 1034.4, 1126.1, 1249.6 and 1454.2 cm 1 confirmed the presence of 1,2 dichlorobenzene in the fullerene (see spectra 1 and 2, Fig. 3). At the same time no visible bands corresponding to the isopropyl alcohol were observed in the IR transmission spectra (see spectra 2 and 3, Fig. 3). There is a peak at 1537 cm"1 that is still difficult for interpretation. In fact, precipitation leads to the introduction of mainly solvent molecules but not the molecules of precipitator in the structure of fullerene. 

Because the two commonest forms of coordinated N03 have the same effective symmetry, hence the same number of ir-active vibrational modes, criteria for distinguishing between them must be based on the positions of the bands rather than their number. In practice, the situation is quite complex and there are no entirely straightforward criteria. This is because the array of frequencies depends on both the geometry and strength of coordination. 

Figure 4.8-26 Absorption of IR-active vibrational modes in La2 vSr,vCu04 at different doping concentrations Sr doping (a) and Cu/Zn substitution (b).

In Section 4-4-2, a method was described for using molecular symmetry to determine the number of IR-active stretching vibrations. The basis for this method is that vibrational modes, to be IR active, must result in a change in the dipole moment of the molecule. In symmetry terms, the equivalent statement is that IR-active vibrational modes must have irreducible representations of the same symmetry as the Cartesian coordinates A, y, or z (or a linear combination of these coordinates). The procedure developed in Chapter 4 is used in the following examples. It is suggested as an exercise that the reader verify some of these results using the method described in Chapter 4. 

How many IR active vibrational modes does CS2 possess, and why [Hint CS2 is isostructural with CO2.]... 

In the development of the technique, experiments on SFg have been very important. The conditions are much more favourable for SFg than for UFg. The isotope shift is 17 cm between and in the IR active vibrational mode that involves asymmetric stretching of two S-F bonds. The spectrum has a typical P, Q and R branch structure and the whole region of absorption for the rotational level population distribution that is obtained at room temperature is 15 cm". Thus, the isotopic molecules are spectroscopically totally separated. Furthermore, the vibrational transition in SFg well matches the emission of a free-running pulsed CO2 laser. 

Check the character table in Appendix 3. In the rightmost column, the T2 irreducible representation has the x, y, and z labels. Therefore, only the T2-labeled vibrations will be IR-active, and the conclusion is that CCI4 will have only two IR-active vibrational modes. These modes, being triply degenerate, represent six of the nine normal vibrations of CCI4. 

Methane, CH4, has only two IR-active vibrational modes. Comment on the expected number of IR-active vibrational modes of CH3D, where one hydrogen atom is replaced by a deuterium. 

Meanwhile, the charge excitations couple with the lattice vibrations and allow some symmetrical vibrational modes (Raman-active modes) to become infrared active by breaking the local symmetry [75]. This was first recognized in the doping and photoexcitation studies of rran -polyacetylene [62, 76, 77]. Moreover, the amplitude mode formalism developed by Horovitz [78] has been successful in explaining the one-to-one correspondence between the photoinduced and the doping-induced IR-active vibrational modes and their relationship to the Raman modes of the pristine polymer. 

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