Component analysis of the visible absorption spectra of I_2 and Br_2 in inert solvents:a critique of band decomposition by least-squares fitting

摘要

The absorption spectra of I_2 and Br_2 dissolved in n-heptane and CCl_4 are recorded with high precision in the visible-near-IR spectral range,at temperatures between 15deg and 50degC.The spectra aredecomposedintothe three contributing transitions (~1II_u<-X,B0~+_u~3II<-X,and Al_u~3II<-X)through simultaneous least-squares fitting of the T-dependent data.The main results of the analysis are as follows.(1) In I_2 the weakest A<-X band is virtually identical in shape and intensity in these solvents and int he gas phase,but with small blue shifts in solution.(2) The approx 20% increased absorption of I_2 in solution appears to be largley attributable to a doubling in the intensity of the weaker of the weaker of the two main transitions.~1II_u<-X,which is red-shiftedin solution.(3) All three transitions in I_2 shift to the red with increasing T,with the strongest effect observed for ~1II_u<-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,withthe 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) results of such analyses can vary widely with choice band function.(3) When comapring 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 ~1II_u state be designated as the C state in all the halogens.