1,3-Dinitrobenzene radical anion (DNB⁻), which is a typical mixed valence compound, undergoes intramolecular electron transfer (ET) in solution. It is reported that the ET rates exceed 10¹⁰ s⁻¹ in polar aprotic solvent such as acetonitrile. Formulation based on a simple one-dimensional model cannot quantitatively account for the observed ET rates, and further study has been desired for better understanding of the solvent effects on the ET. In the present study, molecular dynamics simulations were performed for DNB⁻ in the vacuum and in acetonitrile solution. In the vacuum, ET was induced by the antisymmetric C–N stretching mode on a timescale of ~100 fs, and the charge transferring between the nitro groups was much less than unity. For the acetonitrile solution, short-timescale and long-timescale simulations were performed using a droplet model of solvated DNB⁻ at 298 K. Although the mean C–N distance in the charged nitro group was longer than that in the vacuum, no ET took place in the short (~150 fs) simulations. The solvent coordinate, which was defined as the difference in the solute–solvent interaction energy between the reactant and the product, significantly fluctuated even in short-time simulations. The reorganization energies in acetonitrile were evaluated on the basis of molecular orbital (MO) calculations, and the ratio of the inner sphere and outer sphere parts, λ_i/λ_o, was estimated to be ~0.6. The results suggest that the intramolecular mode and fast solvent mode may play an important role in the present ET reaction. Copyright © 2014 John Wiley & Sons, Ltd.