n → σ* Absorption spectra of saturated organic compounds containing bromine and iodine

Ultra-violet absorption spectra of a total of twenty-two saturated organic iodides and bromides were extensively measured in several polar and nonpolar solvents, in order to clarify the nature of long wavelength bands characteristic of these compounds having lone pair electrons. The results show that the absorption bands appear at about 260 and 200 mμ for alkyl iodides and alkyl bromides, respectively. The oscillator strengths were calculated with LCAO MO and AO approximations for the n → σ* transitions as well as for the first Rydberg transitions of the various alkyl bromides and iodides. Comparison of the calculated and experimental results indicated that the longest wavelength absorption bands for the alkyl halides are not due to the Rydberg transition but due to the n → σ* transition, as suggested by Mulliken. The f-values calculated by the aid of AO approximation were in better agreement with the corresponding experimental values than those by the LCAO method. A sharp strong band around 1940 Å was assigned to the first Rydberg transition from the comparison of experimental and calculated oscillator strengths for ethyl iodide chosen as an example. Solvent effect, as well as, alkylation effect on the ultra-violet absorption bands was investigated for the compounds. It was found that the n → σ* bands show a blue shift with the increase of solvent polarity and show a red shift in the order of the primary, secondary and tertiary alkylations of the carbon atom attached to iodine. Absorption spectra of methylene iodide and iodoform, which may be resolved to give four individual bands, were successfully interpreted in terms of the appropriate combination of upper and lower orbitals splitted by mutual interactions between the iodine atoms in these compounds.