Determination of spectroscopic band shapes using second derivatives, part I: theory.

The molecular spectra of water, aqueous solutions, hydrogen-bonded systems, and others have massive bands that contain many overlapping components. To decipher the spectra for molecular interpretation, it is necessary to separate these. Several attempts to do this have been made without clear success. To surmount some of the difficulties, we present a novel method, which consists of quantitatively evaluating the spectral band second-derivative profiles. This aids in the determination of the original band profiles: Gaussian, Lorentzian (Cauchy), and Gauss-Lorentz products. Then the number of components in a massive absorption, their shapes, and their positions can be determined. We tested the usefulness of the method in the visible region using calibration standards: a light emitting diode emission spectrum and a holmium chloride (HoCl2) solution. To verify its utility in the infrared region, we used liquid propanol, liquid acetonitrile, and aqueous acetone.