Infrared active bond vibrations

The infrared spectrum therefore consists of a number of absorption bands arising from infrared active fundamental vibrations however, even a cursory inspection of an i.r. spectrum reveals a greater number of absorptions than can be accounted for on this basis. This is because of the presence of combination bands, overtone bands and difference bands. The first arises when absorption by a molecule results in the excitation of two vibrations simultaneously, say vl5 and v2, and the combination band appears at a frequency of -I- v2 an overtone band corresponds to a multiple (2v, 3v, etc.) of the frequency of a particular absorption band. A difference band arises when absorption of radiation converts a first excited state into a second excited state. These bands are frequently of lower intensity than the fundamental absorption bands but their presence, particularly the overtone bands, can be of diagnostic value for confirming the presence of a particular bonding system. 

Symmetry concepts can be extremely useful in chemistry. By analyzing the symmetry of molecules, we can predict infrared spectra, describe the types of orbitals used in bonding, predict optical activity, interpret electronic spectra, and study a number of additional molecular properties. In this chapter, we first define symmetry very specifically in terms of five fundamental symmetry operations. We then describe how molecules can be classified on the basis of the types of symmetry they possess. We conclude with examples of how symmetry can be used to predict optical activity of molecules and to determine the number and types of infrared-active stretching vibrations. 

The frequency of vibration for an infrared-active bond can be estimated using Hooke s law for the vibration of a simple harmonic oscillator such as a vibrating spring. Hooke s law predicts that the frequency of vibration increases when (1) the bond strength increases and (2) the reduced mass of the vibrating system decreases. 

It can be seen that for C H systems with m > 4 regardless of the size of the ring four infrared-active normal vibrations and seven Raman-active ones are to be expected. Since the members of the series with m = 3 and 4 are known only with phenyl substituents or in complexes, it is impossible to observe their n.v. unaffected by conjugation or complex bonding effects. However, for the higher members, it suffices for establishing structures to find spectra with only a few bands, namely the four IR- and seven Raman-active ones. In addition. Table II shows that the rule of mutual exclusion is strictly valid, although Ds and do not have a center of inversion. 

The frequencies of these vibrations generally decrease in the order v > 8 > y > x. Not all vibrations can be observed absorption of an IR photon occurs only if a dipole moment changes during the vibration. The intensity of the IR band is proportional to the change in dipole moment. Thus species with polar bonds (e.g. CO, NO and OH) exhibit strong IR bands, whereas molecules such as H2 and N2 are not infrared active at all. 

In C70, because of its lower DSh symmetry, there are five kinds of non-equivalent atomic sites and eight kinds of non-equivalent bonds. This means that the number of normal vibrations increases for C70 in comparison to C60. Although there are now 204 vibrational degrees of freedom for the 70-atom molecule, the symmetry of C70 gives rise to a number of degenerate modes so that only 122 modes are expected. Of these 31 are infrared-active and 53 are Raman-active. 

Since only two vibrations were observed, they were assigned to the parallel and perpendicular vibrations of hydrogen atoms in bridging sites, although it was realized that this was superficially incompatible with the observation that these two vibrations were both infrared-active. Calculations by Muscat32 have shown that the most stable sites on the Pd surface are hollow sites in which the adsorbed H atoms are multiply bonded to atoms in and below the surface. In these sites both parallel and perpendicular excitations would be infrared-active, as suggested previously.30... 

In the infrared spectrum of polyethylene (Fig. 4.1-2A) this band is split into a doublet at 720 and 731 cm This factor group splitting (Fig. 2.6-1 and Sec. 2.7.6.4) is a result of the interaction between the molecules in crystalline lattice areas. It may be used to investigate the crystallinity of polymers (Drushel and Iddings, 1963 Luongo, 1964). Polyethylene has a unit cell of the factor group Dih (compare Secs. 2.7.5 and 2.7.6.3) which contains a -CH2-CH2- section of two neighboring chains. Each of these sections has a center of inversion in the middle of the C-C bond (Fig. 4.1-3). Therefore the rule of mutual exclusion (Sec. 2.7.3.4) becomes effective The vibrations of the C-C bonds cannot be infrared active and further there are no coincidences of vibrational frequencies in the infrared and Raman spectrum of linear polyethylene. 

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