The Objective Evaluation of the Positions of Infrared Absorption Maxima

The fit of polynomial series of varying degree to infrared absorption bands in the neighborhood of the maxima has been investigated, and, in the instances considered, an acceptable graduation was usually possible with a quartic equation. By differentiation of the polynomial an objective, and independently reproducible, numerical value can be obtained for the band maximum. The abscissal positions computed for the maximum converged as the degree of the polynomial was raised, provided the experimental points did not exhibit gross departures from a smooth distribution. The convergence of the roots of the quadratic, cubic, and quartic functions was employed to check the applicability of the technique in individual cases. A Lorentz curve can be fitted exactly by a parabola if the reciprocal of the absorbance is plotted as ordinate, but in practice no improvement in the fit of power series to real absorption bands was achieved by using the reciprocal ordinates.Frequency shifts in the infrared spectra of benzene, acetone, and nitromethane on solution in carbon tetrachloride have been measured by this method. The finite displacements observed contradict a recent statement in the literature that the peak frequencies of the bands in the spectra of these and other substances are uninfluenced by solution in non-polar solvents.