Experimental determination of infrared intensities

The theoretical models for interpretation of infrared intensities presented in the subsequent chapters have been largely applied in analyzing gas-phase experimental data. Gas-phase intensities provide an unique opportunity to study in a unifonn approach the interrelations between molecular structure and intensity parameters. This is due to the fact that, in contrast to vibrational frequencies, the absorption coefficients depend strongly on the phase state and on solvent effects. Intensities of different modes of the same molecule are not influenced in a systematic way by the solvent. The variations of absorption coefficients may reach tens and hundreds percent. Accurately determined gas-phase intensities are, therefore, of fundamental importance as a source of experimoital information on intramolecular properties. 

Past difficulties in experimental measurements of integrated infrared intensities have been associated mostly with the low resolving power of spectrometers, poor accuracy on the ordinate and absence of computer facilities for band integration, deconvolution and curve fitting in overlap parts of the spectra. It is clear that presently we have far better experimental means for accurate determination of die integrated intensities of individual absorption bands. Still, however, careful considerations of a number of possible sources of errors are needed in order to obtain sufficiendy accurate intensity data. Some of these problems will be discussed later on. 

It is interesting that the methods developed for experimental determination of vibrational intensities in the gas-phase were aimed at resolving problems arising mosdy from the low resolution power of the available spectrometers at the time. It may appear that nowdays, when the researchers have access to instruments with resolution of the order of a few himdredths or even few thousandths of a wavenumber, these techniques may be of lesser importance. Although this is partly true, the current experimental approaches for experimental determination of vibrational intensities fully rely on the original developments. This is determined by the fact that these methods not only compensate for the effect of low resolution on intensities but also provide criteria for the accuracy of measurements and the influence of such phenomena as adsorption of sample gas or slow diffiision process. Thus, the extrapolation method of Wilson and Wells [11], further developed by Penner and Weber [12], is the standard approach for experimental intensity studies. 

Early attempts for experimental measurements of infrared intensities [13,14] resulted in greatly divergent values for the same molecule. It was soon realized that most of the difficulties were associated with the low resolving power of the spectrometers used [IS]. The problems arise from the fact that the incident infrared beam emerging from the monochromator is not strictly monochromatic, but contains a band of frequencies around die frequency v determined by the slit function g(v,v ). There is, thus, a perfectly good chance for the intensity of the transmitted radiation I by a cell 

The quantity of principal interest in intensity measurements is the integrated absorption coefficient A as defined by Eqs. (1.43), (1.44) and (1.47). For gas samples the respective expression is 

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