Intensities of infrared spectral bands

Traditionally, the absolute intensities of infrared spectral bands have been difficult to measure accurately and there is now less activity in the area than was once the case. However, with the advent of F.T. I.R. techniques, the subject seems ripe for a revival. 

The specific surface area of the fresh and used catalysts was measured by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 473 K prior to the measurements and the Dubinin equation was used to calculate the specific surface area. The acidity of investigated samples was measured by infrared spectroscopy (ATI Mattson FTIR) by using pyridine (>99.5%, a.r.) as a probe molecule for qualitative and quantitative determination of both Bronstcd and Lewis acid sites (further denoted as BAS and LAS). The amounts of BAS and LAS were calculated from the intensities of corresponding spectral bands by using the molar extinction coefficients reported by Emeis (23). Full details of the acidity measurements are provided elsewhere (22). 

While infrared and Raman spectrum both involve vibrational and rotational energy levels, they are not duplicates of each other but rather complement each other (see Fig. 1.31). This is because the intensity of the spectral band depends on how effectively the photon energy is transferred to the molecule and the mechanism for photon energy transfer differs in the two techniques. This will be shown below. 

In a practical sense the effects described in the foregoing discussion place a severe limitation on the applicability of spectral studies of adsorbed molecules to the detailed elucidation of the adsorption process and of the stereochemistry involved in surface catalysis. Since the absorption intensity may be either enhanced or decreased as a result of adsorption on a surface, and may either increase or decrease with variation in surface coverage, it becomes very difficult indeed to use spectral data as a measure of the surface concentration of adsorbed species. This is of particular importance when more than one species occupies the surface e.g., physisorbed and chemisorbed species. In this case the absolute concentration of either species on the surface cannot be measured directly nor can it be reliably inferred from a comparison of the intensity of the bands corresponding to these two species. Moreover, in the identification of an adsorbed species the relative intensities of two or more bands characteristic of that species e.g., the CH stretching and the CH deformation frequencies for adsorbed hydrocarbons, cannot be used as evidence for the structure of the adsorbed species since the absorption coefficients of the individual bands may change in opposite directions as a function of surface coverage. Thus the relative intensities of such bands cannot be compared to the relative intensities of the same bands observed in solution or in the gas phase. A similar difficulty arises when attempts are made to use the electronic spectra of adsorbed molecules to complement the infrared spectra for identification purposes. 

The time-dependent classical statistical mechanics of systems of simple molecules is reviewed. The Liouville equation is derived the relationship between the generalized susceptibility and time-correlation function of molecular variables is obtained and a derivation of the generalized Langevin equation from the Liouville equation is given. The G.L.E. is then simplified and/or approximated by introducing physical assumptions that are appropriate to the problem of rotational motion in a dense fluid. Finally, the well-known expressions for spectral intensity of infrared and Raman vibration-rotation bands are reformulated in terms of time correlation functions. As an illustration, a brief discussion of the application of these results to the analysis of spectral data for liquid benzene is presented. 

Visible and UV spectrometry are of secondary importance to other spectral methods for the identification and structural analysis of unknown compounds. This is a direct consequence of the broad bands and rather simple spectra which make differentiation between structurally related compounds difficult. As an adjunct to infrared, magnetic resonance and mass spectrometry, however, they can play a useful role. They can be particularly helpful in confirming the presence of acidic or basic groups in a molecule from the changes in band position and intensity associated with changes in pH (p. 369). 

The most recent fairly comprehensive review Of the vibrational spectra of transition metal carbonyls is contained in the book by Braterman1. This provides a literature coverage up to the end of 1971 and so the subject of the present article is the literature from 1972 through to the end of 1975. Inevitably, some considerable selectivity has been necessary. For instance, a considerable number of largely preparative papers are not included in the present article. Tables A-E provide a general view of the work reported in the period. Table A covers spectral reports and papers for which topics related purely to vibrational analysis are not the main objective. Papers with the latter more in view are covered in Table C. Evidently, the division between the two is somewhat arbitrary. Other tables are devoted to papers primarily concerned with the spectra of crystalline samples — Table B — to reports of infrared and Raman band intensities — Table D and sundry experimental techniques or observations - Table E. Papers on matrix isolated species, which are covered elsewhere in this volume, are excluded. 

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