Theoretical study of temperature and solvent dependence of the free-energy surface of the intramolecular electron-transfer based on the RISM-SCF theory: application to the 1,3-dinitrobenzene radical anion in acetonitrile and methanol.

The free-energy surfaces along the intramolecular electron-transfer reaction path of the 1,3-dinitrobenzene radical anion in acetonitrile and methanol are investigated with the reference interaction site model self-consistent field theory. Although acetonitrile and methanol have similar values of the dielectric constant, the free-energy profiles are quite different. In the methanol solution, the charge is strongly localized on one of the nitrile substituents due to a strong hydrogen bond between 1,3-dinitrobenzene and the solvent, while the polarization is not so large in the case of acetonitrile. The temperature dependence of the reorganization energy, the coupling strength, and the activation barrier is evaluated in both acetonitrile and methanol. The reorganization energy and the activation barrier decrease with increasing temperature for both cases. The electronic coupling strength also shows a similar tendency in the temperature dependence; it increases with increasing temperature in both solvents but with different rates. The behavior is explained in terms of the strong polarization induced by the hydrogen bond between the solute and solvent in the methanol solution.