Absorption spectra and formation constants of steroid-iodine complexes

Studies have been made of the ultraviolet and visible absorption spectra of steroid-iodine charge transfer complexes. Iodine complexes with keto-steroids in Freon solution have absorption maxima at 250-270 mμ, and the complexes involving carbon double bonds as donors have maxima at 320-360 mμ. Equilibrium constants range from 0.4 to 12 liters/mole. At higher concentrations, dimer complex formation was observed for 5α-cholestan-3-one with a formation constant of 350 liters/mole. Broad peaks at 280 and 390 mμ characterize the dimer spectrum. It is suggested the dimer rearranges to an "inner" complex involving an I₃^-. The enthalpies of dimers formation are surprisingly high, about 15-20 kcal/mole. Conditions for the precipitation and crystallization of solid complexes were investigated. The spectra of several solid complexes are somewhat similar to the dimer spectra.