Measurement of Radical Cation UV-Visible Spectrum in a Polycrystalline Freon Matrix at Liquid Nitrogen Temperature by Diffuse Reflectance Spectroscopy

Abstract

Recently, the chemistry of radical cations in the condensed phase has aroused wide interest. These intermediate species are involved in a variety of fundamental processes including redox, electrochemical, and photochemical reactions as well as many catalytic chemical processes. Also, their intrinsic chemical properties have found direct applications in syntheses. It is therefore important to understand the structure and chemistry of these species. Structural information in the condensed phase has recently been obtained through ESR and optical studies of isolated radical cations generated by radiolytic oxidations in various Freon matrices at low temperature. Briefly, high-energy irradiation at 77 K of a frozen Freon matrix (commonly CFCl3) containing a dispersed solute causes ionization of the matrix molecules followed by positive-hole migration through the matrix to the solute molecules having a lower ionization potential. The most popular Freon matrices adopted for ESR studies are CFCl3, CF3CCl3, and CF2ClCFCl2. Unfortunately, optical transmission studies of radical cations in these matrices are severely hindered due to the polycrystallinity of these matrices at low temperature. Instead, a Freon mixture made of equivolume amounts of CFCl3 and CF2BrCF2Br is usually used for optical studies since it can form a transparent glass upon being frozen. Consequently, a rigorous comparison between ESR and optical results may be obscured due to the use of different matrices in the two spectroscopic methods. Clearly, it would be advantageous if both measurements could be made on the same matrix system.