Binary Nonaqueous Solvent Systems Exhibiting Characteristics of Dielectric Continua

Abstract

The continuum as a description of medium effects upon chemical processes often fails when applied to reactions in mixed solvents. Test specifications for the cosolvent as a continuum are now stated in a more precise form and these make use of the Kirkwood-Onsager and Block-Walker functions in constant. As a result, two additional binary solvents have been identified which appear to meet the tests, namely the 1-butanol : 1-propanol and nitrobenzene : acetonitrile cosolvent systems. In modeling the influence of polar solvents upon both electrolytic conductance and reaction kinetics in nonaqueous media most contemporary theories have initially made use of the dielectric continuum assumption to represent the solvent environment (Covington and Dickinson, 1973). When stated in its simplest form (Amis, 1966), that depicts the solvent as being the fully homogeneous surroundings for all solute species including reactants, products and transition state structures involved in the given process. Likewise, it requires that the polarity of that continuum be expressed as an appropriate function in either constant (E) alone or coupled with the square of the index of refraction for the bulk solvent. For the latter, the solvent reorganization energy functions of Marcus (1963) and their extension by Lay (1986) to hydrogen bonding solvents are representative of current continuum descriptions which have been applied to electron transfer processes in solution. By-and-large, the continuum serves best as a solvent environmental description for pure solvents of the aprotic dipolar (AD) and aprotic highly dipolar (AHD) classes in the Chastrette-Purcell (1985) general classification scheme. Beyond the discrete point (single solvent) tests of that assumption, the model has been used with reactions in binary solvent mixtures, and it is in such cosolvent systems which serve as dielectrically tunable media that the continuum is often quantitatively invalid. For mixed solvents, the failure of the model may arise from several sources at the molecular level. These include a range of solvent-solvent interactions from strong