Infrared and Raman Activity

Among the total number of normal vibration modes in a molecule, only some can be detected by infrared spectroscopy. Such vibration modes are referred to as infrared active. Similarly, the 

Type Stretching Bending vibrations Torsion vibrations 

To be infrared active, a vibration mode must cause alternation of dipole moment in a molecule. A molecule has a center of positive charge and a center of negative charge. If these two centers 

Mathematically, the requirement of infrared activity is expressed that the derivative of dipole moment with respective to the vibration at the equilibrium position is not zero. 

To be Raman active, a vibration mode must cause polarizability changes in a molecule. When a molecule is placed in an electric field, it generates an induced dipole because its positively charged nuclei are attracted toward the negative pole of the field and its electrons are attracted toward the positive pole of the field. Polarizability (a) is a measure of the capability of inducing a dipole moment (/x) by an electric field. It is defined in the following equation. 

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The thiosulfate ion has tetrahedral symmetry and the six fundamental modes are both infrared and Raman active. The calculated frequencies (3) are in good agreement with experimental values (4). 

Raman and infrared spectroscopy provide sensitive methods for distinguishing Ceo from higher molecular weight fullerenes with lower symmetry (eg., C70 has >5/1 symmetry). Since most of the higher molecular weight fullerenes have lower symmetry as well as more degrees of freedom, they have many more infrared- and Raman-active modes. 

The free Sy molecule is of Cs symmetry but in its various solid allotropes it occupies sites of Ci symmetry [154]. In any case, in these point groups all fundamental modes are infrared and Raman active and no degeneracies occur. Four allotropes of Sy (a, p, y, S) have been identified by Raman spec-... 

Figure 2. Selected infrared and Raman active vibrational modes of C12H14.

For gaseous ethylene bands labeled I and R are infrared and Raman active, respectively. See Stoicheff (43) for liquid ethylene. 

The high sensitivity of tunneling spectroscopy and absence of strong selection rules allows infrared and Raman active modes to be observed for a monolayer or less of adsorbed molecules on metal supported alumina. Because tunneling spectroscopy includes problems with the top metal electrode, cryogenic temperatures and low intensity of some vibrations, model catalysts of evaporated metals have been studied with CO and acetylene as the reactive small molecules. Reactions of these molecules on rhodium and palladium have been studied and illustrate the potential of tunneling spectroscopy for modeling reactions on catalyst surfaces,... 

In a molecule with a centre of inversion, the irreducible representations in r are of u-type and those in P are of y-type and since cannot coincide with both a u and a (7-type irreducible representation, no fundamental frequency for this type of molecule can be both infrared and Raman active. 

The number of vibrational modes of a molecule composed of N atoms is 3N — 6 (or 3N — 5 if linear). We may find which of these are infrared and Raman active by the application of a few simple symmetry arguments. First, infrared energy is absorbed for certain changes in the vibrational energy levels of a molecule. For a vibration to be infrared active, there must be a change in the dipole moment vector... 

As compared to eq. (7.3.5), the only difference is that the silent mode Z 2u in that expression is converted to Z iu here, and now V2 has Z 2g symmetry and V4 has Big symmetry. All deductions concerning infrared and Raman activities remain unchanged. 

The spectra of infrared and Raman active normal vibrations (see Sec. 2.7) depend on whether the sample is in the liquid, gaseous or crystalline state. However, the main features which are needed to identify a molecule or to elucidate its structure are clearly visible in spectra of any state. 

Spectra of molecules in the crystalline state, i.e., of molecular crystals, are obtained from molecules which are at fixed positions (sites) in the lattice (Fig. 2.6-1C). Normal (first-order) infrared and Raman spectra can be seen as spectra of hyper molecules , the unit cells (Schneider 1974, Schneider et al., 197.5). As a consequence, any molecular vibration is split into as many components as there are molecules present in the unit cell. Their infrared and Raman activity is determined by the symmetry of the unit cell. In addition, the translational and rotational degrees of freedom of molecules at their sites are frozen to give rise to lattice vibrations translational vibrations of the molecules at their sites and rotational vibrations about their main inertial axes, so-called librations. 

Figure 2.7-4 Examples of tetra-atomic molecules belonging to different point groups. They can be distinguished by the number of polarized and depolarized Raman active vibrations, the infrared active vibrations and the coincidences of the frequencies of infrared and Raman active bands.

The chalcogenocyanate ions are linear triatomic species belonging to the point group The three normal modes of vibration shown in Fig. 1 are both infrared and Raman active. The vibrations are commonly described as though group frequencies existed unmixed in these ions,... 

Chamritski I, Bums G (2005) Infrared- and Raman-active phonons of magnetite, mghemite, and hematite, a computer simulation and spectroscopic study. J Phys Chem B 109 4965-4968... 

All modes are infrared and Raman active unless specified otherwise. Within a point group the vibrational modes are numbered from the highest symmetry species and from the highest frequency within any symmetry species. 

Referring to the character tables ofC2v, C v and 2 oo/ identify the symmetry species of infrared and Raman active normal modes of H2O, NH3 and CO2. 

Referring to the character table in Exercise 13.7, determine the symmetry species of normal modes in benzene CsHe which can be infrared and Raman active. 

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