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

Abstract

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.

Computational study on intramolecular electron transfer in 1,3-dintrobenzene radical anion

“…Both the C–N and N–O bonds in the charged nitro group were longer than those obtained in the vacuum, whereas the bond distances in the uncharged nitro group were less altered. These tendencies are consistent with the previous results . The distribution of bond distances is broader in the MeCN solution than in the vacuum, which reflects the fluctuation of the solute–solvent interactions.…”

“…(b). Similar observations of unsymmetrical solvation have been previously reported for the same system . As shown in Fig.…”

“…The magnitude of electronic coupling, H ab , was estimated to be 0.20 eV by the SA‐CASSCF and 0.175 eV by the MR‐CIS, which was evaluated as half of the energy gap at the midpoint between the reactant and the product. The reported H ab values range from 0.07 to 0.2 eV for DNB − in MeCN at 298 K . H ab in the vacuum is larger than in a polar medium for DNB − because the dipole moment of the upper state (4.36 D) is larger than that of the lower state (2.21 D) in the state‐crossing region.…”

“…The atomic charges for DNB − were set to the ESP charges obtained by UB3LYP/TZVP calculations with the CPCM treatment for MeCN. Although the equilibrium geometry and the effective charges of DNB − were different between the MeCN and MeOH solutions, the same parameters were used here for both the solutions for simplicity. The sums of the atomic charges in the charged and uncharged nitro groups were −0.878 and −0.235, respectively, which were similar to those reported for MeCN solution in (−0.87 and −0.25).…”

“…Yoshida et al . applied the reference interaction site model (RISM)‐multi‐configurational self‐consistent field (SCF) theory to the calculation of the one‐dimensional free energy profiles of DNB − in MeCN and MeOH solutions, whereas Renz et al . used the direct‐COSMO‐RS (conductor‐like screening model for real solvents) model in combination with the time‐dependent density functional theory method .…”