Infrared active bond frequencies

Figure II-9 illustrates the four normal modes of vibraiion of a tetrahedral XY4 molecule. All four vibrations are Raman active, whereas only P3 and are infrared active. Fundamental frequencies of XH4-type molecules are listed in Table II-60. The trends p, and v > V4 hold for ihe majority of the compounds. The XH stretching frequencies may be lowered whenever the XH4 ions form hydrogen bonds with counterions. In the same family of the periodic table, the XH stretching frequency decreases as the mass of the X atom increases. Shirk and Shrivef noted, however, that the p, frequency and the corresponding force constant show an unusual trend in Group HI A ...

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. 

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. 

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. 

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. 

Table II-2n lists the vibrational frequencies of XY2-type metal halides. Most of these data were obtained in inert gas matrices. Although the structures of these halides are classified as either linear or bent, it should be noted that the bond angles in the latler type range from 95° to 170°. Thus, some bent molecules are almost linear. These two types can be distinguished by the infrared activity of the f, mode it is active for bent but not active for linear molecules. However, this simple criterion has led to conflicting results in some cases. The bond angle of the YXY-type molecule can be determined by the metal (X atom) isotope frequencies of the modes observed in inert gas matrices. If a pair of P3 frequencies is determined, the bond angle (2a) can be calculated from the equation ...

Figure 11-12 shows the seven normal modes of vibration of square-planar XY4 molecules. Vibrations P3, nd p are infrared active, whereas Pi, P2 and P4 are Raman active. Table II-6j lists the vibrational frequencies of some ions belonging to this group XeF, (Sec. I-10) is an unusual example of a neutral molecule which takes a square-planar structure. Bos worth and Clark measured the relative intensities of Raman-active fundamentals of some of these ions and calculated their bond polarizability derivatives and bond anisotropies. They also measured the resonance Raman spectra of several [AuBr4] salts in the solid state, and observed progressions such as np,... 

The XY,-type molecules are veiy rare. Both IF, and ReF, arc known to be pentagonal-bipy rami dal (D ,), and their vibrational spectra have been assigned completely, as shown in Table II-9. The A, ", and , vibrations are Raman active, and the A 2 and , vibrations are infrared active. According to Eysel and Seppelt, IF, undergoes minor dynamic distortions from 0 symmetry which cause violation of the Djj, selection rules for combination bands but not for the fundamentals. Normal coordinate analysis shows that the axial bonds are definitely stronger and shorter t ian the equatorial ones. Table 11-9 also lists the frequencies and assignments of the [U02Fs] ion of Dj, symmetry. 

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