Intensity of Infrared Bands

For a vibration to give rise to the absorption of infrared radiation, it must cause a change in the dipole moment of the molecule. The larger this change, then the more intense the absorption band will be. 

A dipole moment is a vector sum, so therefore CO2 in the ground state has no dipole moment. If the two C=0 bonds are stretched symmetrically, there is still no net dipole and thus there is no inhrared activity. However, in the asymmetric stretch mode the two C=0 

This chapter introduced the ideas which are fundamental to the understanding of infrared spectroscopy. 

Initially, the electromagnetic spectrum was considered in terms of various atomic and molecular processes, with classical and quantum ideas being introduced. 

We then turned to the vibrations of molecules and how they produce an infrared spectrum. The various factors which are responsible for the position and intensity of infrared modes were also discussed. 

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For a more precise calculation of intensities of infrared bands it is necessary to take into account the variation of the dipole function with intemuclear distance,... 

Fine structure of infrared bands Intensity of infrared bands Intensity of Raman bands Polarization of infrared bands Reflection spectrum in the infrared region Resonance Raman effect... 

A powerful characteristic of RAIR spectroscopy is that the technique can be used to determine the orientation of surface species. The reason for this is as follows. When parallel polarized infrared radiation is specularly reflected off of a substrate at a large angle of incidence, the incident and reflected waves combine to form a standing wave that has its electric field vector (E) perpendicular to the substrate surface. Since the intensity of an infrared absorption band is proportional to / ( M), where M is the transition moment , it can be seen that the intensity of a band is maximum when E and M are parallel (i.e., both perpendicular to the surface). / is a minimum when M is parallel to the surface (as stated above, E is always perpendicular to the surface in RAIR spectroscopy). 

Reactant and product analyses were obtained from the intensities of infrared absorption bands by successive subtraction of absorptions by known species. Low noise reference spectra for UDMH and several reaction products were generated for this purpose in order to minimize the increase in the noise level of the residual spectrum with each stage of subtraction. 

Since the formation of NO2 can occur homogeneously, it was of interest to establish whether adsorbed NO could be oxidized. NO was adsorbed at 225 C, after which the infrared cell was purged with He and subsequently a stream of 10.1% O2 in He was allowed to flow over the catalyst. Prior to the introduction of the 02-containing stream, the only features evident were those for mono- and dinitrosyls. In the presence of O2 at 225 °C, the intensities of the bands for both mono- and dinitrosyl species attenuated and new features appeared at 1628 and 1518 cm-, corresponding to nitrate and nitrito species, respectively. A similar experiment carried out in the absence of O2, showed only a small decrease in the intensity of the nitrosyl bands due to NO desorption and the absence of bands for nitrate and nitrito species during a 30 min purge in He at 225 °C. 

Figure 10.6. In situ Fourier transform infrared spectra of decane SCR-NO in the presence and absence of hydrogen on Ag/Al203 at 200°C. Evolution of intensities of the bands characteristic for adsorbed species (monodentate nitrates 1245 cm-1, bidentate nitrates 1295 cm-1, —CN 2150 cm-1 and —NCO 2230 cm-1. 1000 ppm NO, 6vol.%02,750 ppm decane, Oor 1000 ppm H2 (reproduced with permission from Ref. [12]).

Figure 7 Hydroperoxide index (HI) determined from mid-infrared spectroscopy (ratio of the integrated intensity of the 3,552 cm 1 band to the integrated intensity of the band at 2,010 cm-1) as a function of total hydroperoxide content measured by iodiometric titration.

In the normal-incident transmission measurements of LB films deposited on transparent substrates, the electric vector of the infrared beam is parallel to the film surface (Figure 5A). Therefore, only absorption bands which have the transition moments parallel to the film surface can be detected by this method. On the other hand, in the above-mentioned RA measurements, in which the p-polarized infrared beam is incident upon the LB film prepared on Ag-evaporated substrates at a large angle of incidence, we have a strong electric field perpendicular to the film surface as shown in Figure 5B. Therefore, in this case, only absorption bands which have the transition moments perpendicular to the film surface can be detected with a large intensity enhancement. Thus, if the molecules are highly oriented in the LB films, the peak intensities of particular bands should be different between the transmission and RA spectra. 

In some cases crystalline polymers show additional absorption bands in the infrared spectrum, as in polyethylene ( crystalline band at 730 cm amorphous band at 1300 cm" ) and polystyrene (bands at 982,1318, and 1368 cm" ). By determining the intensity of these bands it is possible to follow in a simple way the changes of degree of crystallinity caused, for example, by heating or by changes in the conditions of preparation. 

The reaction can be conveniently monitored by infrared spectroscopy by observing the intensity of the band at 2150 cm. 

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