ABSORPTION MAXIMA OF VISIBLE BAND OF IODINE IN DIFFERENT GROUPS OF SOLVENTS

The wavelength of the maximum of the visible absorption band of iodine was measured in a large number of different solvents. A transition energy increase with respect to the gas-phase value was observed in most of the solvents studied which ranged up to 15 kcal. Relating the band position to the ionization potential of the solvents yielded a striking division of the blue-shifted band into a series of characteristic solvent classes. It was found that the transition energy of iodine was affected most strongly by the electron donor ability of the solvent and less strongly by the ionization potential and steric structure of the solvent molecules. In a number of cases a reversal of the slope of wavelength vs. ionization potential was observed, which could be associated in part with a change in molecular shape of the solvents while keeping their donor character constant. Almost all of the observed changes could be explained with reference to two possible interactions: (1) charge-transfer interactions and (2) contact-charge-transfer interactions. Possibly, exchange repulsion of the excited state of iodine with nonbonding electron pairs is a further cause of the blue shifts. These interactions are very specific and, therefore, sensitive to steric conditions. It was found that the over-all molecular shape modifies the specific orbital overlap, thereby influencing the interaction process for various types of systems.