CHARACTERIZATION OF INFRARED-ABSORPTION BAND SHAPES .2. EXPERIMENTAL STUDIES

The profiles of twenty-five well defined i.r. absorption bands have been measured, to test various methods for characterizing their shapes in the context of a realistic approach to the mathematical separation of overlapping peaks. Inverse polynomials of the type y = a(1 + bx2 + cx3 + dx4)-1 prove to be very convenient and, unlike truncated moments and log/log plots of band shapes, do not require the location of an accurate experimental baseline. The second derivative approach could not be tested as it proved impossible to make measurements without incurring noise or some distortion of the profiles. Most of the bands examined are a close approximation to the Lorentzian shape, with slight asymmetry. Curve fitting of synthetic spectra consisting of Lorentzian peaks distorted by the use of an inverse polynomial with a term in x4, and of mixtures of three of the compounds used for the profile study was undertaken in terms of the commonly assumed Lorentzian shape. The results show that modest differences between the assumed and the actual shapes, which are manifest in a worsening of the overall fit, can be tolerated. Problems posed by lack of knowledge of the number of component bands and their half widths are more serious. A few of the profiles studied deviate appreciably from Lorentzian; these are explicable in terms of association effects and solvent/solute interactions. © 1978.