Studies of the solvent effects on the internal reorganization energy for electron transfer of uracil and its anion with ONIOM

In this paper, the aqueous adiabatic electron affinity (AEA) of Uracil (U) and internal reorganization energy lambda(i) of the self-exchange electron transfer (ET) reaction between Uracil and Uracil anion radical (U-.) in aqueous solution were studied. The effect of the solvation was studied with the recently developed hybrid quantum molecular chemical method, ONIOM. In all calculations, the geometrical optimization for U and U-. was performed at B3LYP/6-31++G(d) level. As for the solvent surroundings, the seven water molecules as the first hydration shell were adopted and treated with B3LYP, PM3 and AMBER methods, namely, ONIOM (B3LYP:B3LYP), ONIOM (B3LYP:PM3) and ONIOM (B3LYP:Amber) methods, respectively. The values of AEA for Uracil, predicted by the above three methods, are small positive ones. The geometrical differences between neutral and anion radical molecules of U originate mainly from those of dihedral angles. According to the corresponding dipole moment values, the excess electron in U-. should be trapped dominantly by dipole-bound way. The calculated lambda(i) values by ONIOM (B3LYP:B3LYP) and ONIOM (B3LYP:PM3) are close to each other within 0.89%. The lambda(i) value from ONIOM (B3LYP:Amber) is in agreement with the one from SCRF-CPCM very well. Finally, the calculation results of the detailed geometries and molecular interaction mode-effect of U and U-. and related water molecules in the hydration shell were discussed. (C) 2003 Elsevier Science B.V. All rights reserved.