GROUP INTENSITIES IN INFRARED SPECTROSCOPY

Abstract

In recent years we have used the reflection spectra of various solids and Liquids to obtain the real $n(\nu)$ and imaginary $k(\nu)$ pares of the complex indices $N(\nu) = n (\nu) + ik (\nu)$ of refraction of these materials by means of Kramers-Kronig phase-shift analysis of spectral reflectivity $R(\nu)$ measured at near-normal incidence. In certain $^cases^{1,2}$ we have found that the integrated absorption $\int k (\nu)d\nu$ of characteristic absorption bands have characteristic values that are directly proportional to the number-density of absorbing groups such as $SO_{4}{^{--}}$ regardless of changes in the ‘molecular environment’ of the group; for example, by measuring $\int k (\nu) d \nu$ for $NH_{4}^{+}$ and $SO_{4}{^{--}}$ bands in aqueous solution of known concentration we have been able to predict the general features of the spectrum of ammonium sulfate crystals by extrapolation to crystal densities. Both $n(\nu)$ and $k(\nu)$ for the crystal can thus be obtained in good approximation from solution spectra. This result is to be expected on the basis of Van Vleck principle of spectroscopic stability. We have recently studied the reflection spectra of certain liquid hydrocarbons to ascertain the possibility of attributing characteristic group intensities to $CH_{2}$ and $CH_{3}$ groups in hydrocarbon molecules. Although we have not been successful in establishing such characteristic group intensities, we have established certain empirical relationships, which will be discussed.