Linear free energy relationship analysis of solvent effects on singlet oxygen reactions

Linear solvation energy relationship, LSER, and theoretical linear solvation energy relationship, TLSER, formalisms are applied to the analysis of solvent effects on singlet oxygen reactions. Treatments allow quantitative evaluation of solvent effects and are powerful tools for interpreting the mechanism of the process. Both formalisms are applied to several amino derivatives, where for all types of solvents there is a single pattern, implying a common reaction mechanism involving charge transfer intermediates. The relative contribution of the different descriptors depends upon the compound considered, but a common feature is a significant negative dependence on the alpha parameter, which measures solvent acidity. Correlation equations obtained from analysis of singlet oxygen reactions with 1,3-dienes also exhibit a common dependence on the rho(H) parameter, that accounts for the cohesive energy of the solvent, and reflects the negative activation volume associated with concerted or partially concerted reactions. These formalisms can be also used to determine the main reaction center in polyfunctional compounds (e.g. boldine, vincamine, reserpine, and furosemide), to detect changes in the reaction mechanism with solvent properties (furosemide), and to evaluate the relative contribution of tautomers in equilibrium to the total reaction rate (piroxicam). This treatment could be used to analyze singlet oxygen reactivity in microheterogeneous systems and to predict relative rate constants in these systems.