Electronic absorption spectra of iodo- and bromomethanes

A careful study has been made of the light absorption of the iodomethanes in the spectroscopic region between 1900 Å and 5000 Å. The compounds used, methyl iodide, methylene iodide, iodoform, and carbon tetraiodide, were all carefully purified and precautions were taken against photolytic decomposition prior to the spectroscopic measurements. It was possible to resolve the absorption curves for the last three iodomethanes into Gaussian components with the aid of an analogue computer designed for this purpose. Using the resolved components as a guide to counting the number of electronic transitions, a symmetry treatment yielded results in satisfactory qualitative agreement with experiment. Solvent effects were studied, the results being consistent with an assignment of all of the "low-intensity" bands to n→σ*-transitions, in agreement with the previous assignments for methyl iodide. The bromomethanes, methylene bromide, bromoform, and carbon tetrabromide were also examined in a series of solvents, with results that appeared to fit the expectations for the iodomethanes.