Intensity of Infrared Absorption

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. 

The calculation of the electro-optical parameters describing Raman intensities is not yet very advanced, because of the paucity of data. Nevertheless, some success was achieved in calculations of the intensity of infrared absorption. The results on trans and gauche bond-rotation in ethylene glycol146 could be taken as a model for carbohydrates. Indeed, similar electro-optical parameters (/aCH, /aOH, /aCC, and /aCO) were calculated. This leads to the expectation that calculations of the intensity of the vibrational spectra of carbohydrates may be accomplished in the near future. In addition, the delicate problem of accounting for molecular interactions in calculating infrared intensities could be approached as it was for v(CCC) and i CO) vibrations in acetone.149 This will allow interpretation of weak, as well as strong, i.r. bands, in order to determine the structural properties of molecules. 

The dipolar MSSR applies strictly to a flat metal surface. However, the consideration by Pearce and Sheppard (93) that adsorbed layers are typically a few angstroms (tenths of nanometers) thick in relation to the diameters of larger metal particles in catalysts (up to tens of nanometers) led to the consideration that the MSSR could have substantial effects on the intensities of infrared absorptions from adsorbed species on metal catalysts with large particles. It has been estimated that parallel modes of vibration will have their infrared absorption bands substantially attenuated at metal particle diameters of greater than 2 nm (94). This is proving to be a very important consideration in the interpretation of the infrared spectra from adsorbed hydrocarbon species on metal catalysts (20, 95, 96) and has recently become widely accepted as valid (52, 54, 57, 62, 97). 

In all cases cited the depolymerized coal product contained phenol. Ouchi showed that as depolymerization progressed the intensity of infrared absorption attributed to phenolic hydroxyl increased regularly. 

The problems of peak assignment in infrared spectroscopy are probably greater than those faced in analogous NMR experiments. Furthermore, infrared transition probabilities are highly variable owing to symmetry considerations and other molecular properties. In the absence of calibration, relative intensities of infrared absorption bands do not accurately reflect relative concentrations. By characterizing the infrared properties of appropriate model compounds and... 

From the above discussion when dpjdq, or more rigorously, VqPy is zero or has a slope discontinuity, there are likely to be slope discontinuities in the combined density of states, as revealed by infrared and Raman spectra of two-phonon processes. Points in the Brillouin zone where each of the components of VqP = 0 are known as critical points. The intensity of infrared absorption or Raman scattering depends upon quantum mechanical matrix elements which are, in general, not simple to evaluate. However, by using symmetry considerations and group theoretical methods, the various modes can be assigned as infrared or Raman active. 

Many other properties measured by spectroscopic methods can be computed as derivatives of the energy. The force constants (second-derivative w.r.t. the nuclear coordinates, r) allow one to calculate harmonic vibrational frequencies and the corresponding normal modes. The derivatives of the dipole moment and of the polarizability w.r.t. the normal modes are proportional to the intensity of infrared absorptions and of Raman bands, respectively. 

Few papers have appeared which are concerned with the intensities of infrared absorption by lattice modes in molecular solids. For polar molecules, it is clear that librational motions will cause the absorption of light by a mechanism analogous to that which occurs for rotational spectra in the gas phase. Calculations have been made of such infrared absorption intensities by Schnepp (1967) and by Friedrich (1970). Friedrich used a theory which takes into account the polarizability of the medium in a specific way. For this case, the intensity is proportional to the square of the molecular dipole moment. Section IVB.2 is concerned with this topic. 

Some Aspects of the Quantum-Chemical Interpretation of Integrated Intensities of Infrared Absorption Bands. - S. R McGlynn,... 

This paper will present some basic theory on the intensities of infrared absorption bands. Also, the interdependence of the instrumental paramenters, slit width, scan time, and resolution in absolute quantitative spectroscopy will be discussed. 

G. Amat, Method of Measuring the Intensity of Infrared Absorption Bands, Compt, rend, 233, 372-374, 1951. 

R. G. Gillis, The Intensity of Infrared Absorption Bands—A Bibliography, Australian, Commonwealth, Dept., Supply Defense Standards Lab. Tech. Memo 2, 48 pp., 1958. 134 references. 

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