Component Analysis of the Visible Absorption Spectra of I2 and Br2 in Inert Solvents: A Critique of Band Decomposition by Least-Squares Fitting

Abstract

The absorption spectra of I2 and Br2 dissolved in n-heptane and CCl4 are recorded with high precision in the visible-near-IR spectral range, at temperatures between 15° and 50 °C. The spectra are decomposed into the three contributing transitions (1Πu ← X, B0+u3Π ← X, and A 1u3Π ← X) through simultaneous least-squares fitting of the T-dependent data. The main results of the analysis are as follows. (1) In I2 the weakest A ← X band is virtually identical in shape and intensity in these solvents and in the gas phase, but with small blue shifts in solution. (2) The ∼20% increased absorption of I2 in solution appears to be largely attributable to a doubling in the intensity of the weaker of the two main transitions, 1Πu ← X, which is red-shifted in solution. (3) All three transitions in I2 shift to the red with increasing T, with the strongest effect observed for 1Πu ← X. These differences are not explained by the refraction-index-based relations commonly used to relate gas- and condensed-phase spectra. In the course of this work the method of decomposition analysis by fitting to assumed band shapes has been tested extensively, with the following observations. (1) The much-used 3-parameter Gaussian and log-normal functions do not have enough flexibility to fit single “pure” bands within the experimental precision obtainable from commercial spectrophotometers. (2) The results of such analyses can vary widely with choice of band function. (3) When comparing two such analyses, lower variance is no guarantee of a “truer” resolution. Concerning the labeling of the halogen electronic states involved in the absorption, it is recommended that for consistency the 1Πu state be designated as the C state in all the halogens.