Quantitative Infrared Intensities of Neat Liquids: Their Measurement and Use

Abstract

This paper presents a summary of some of the work done in this laboratory over the past 6 years. It has become clear that well-aligned modern Fourier transform spectrometers are sufficiently photometrically accurate and sufficiently free from major differences between users that absolute infrared absorption intensities can be determined and transferred between well-aligned instruments with an error not exceeding about 2%. It is essential, however, to correct the baselines of transmission spectra, on the basis of measurements in cells with very long path-lengths. Such developments have led to the establishment of intensity standards for infrared spectroscopy of liquids. These have been accepted by the International Union of Pure and Applied Chemistry, and published. Such developments have also led to the reliable determination of relatively small intensity differences, such as those caused by isotopic substitution or by change in the intermolecular environment in binary liquid mixtures. Knowledge of correct intensities has allowed the development of an approximate procedure that greatly simplifies the computations. On the theoretical side, the availability of accuracy to within 2% has created the need to develop methods for obtaining molecular information from absorption intensities which introduce errors no larger than ~ 0.1 %, so that the experimental accuracy is not degraded by avoidable theoretical approximations. A major issue is the determination of the integrated intensities of overlapping bands and the assignment of the intensity under the baseline. The most appropriate intensity quantity to use is the imaginary molar polarizability, ${\alpha ''_m}(\tilde \nu )$. The spectrum of this quantity has been little used, but can be obtained easily with modern computerized spectrometers from transmission spectra, attenuated total reflection spectra, or specular reflection spectra. Most bands in the ${\alpha ''_m}$ spectrum of non-hydrogen-bonded liquids have essentially the Classical Damped Harmonic oscillator shape, and carefully fitting such bands to the spectrum is currently the most objective way to determine the integrated intensities. All the intensity above zero ordinate is usually explained by such fits if the ${\alpha ''_m}$ spectra were calculated from baseline-corrected experimental absorbance spectra. These points are illustrated in this paper by spectra taken from recent work in this laboratory.