Solvation structure and dynamics of the Fe2+–Fe3+ ion pair in water

Abstract

Computer simulations are performed to study the structure of the coordination shells of Fe+2 and Fe+3 ions fixed at a very close reactive separation. The simulations show that it is possible for the two octahedral aquo complexes, i.e., Fe(H2O)62+ and Fe(H2O)63+, to come as close as 5 Å without disrupting their coordination shells. The reorientational dynamics within the hydration shells of these ions at this separation is examined by studying the time correlation functions (TCFs) of the unit vectors on the water molecules and along the iron–oxygen vector. The quantities related to the solvent polarization relaxation during a change in the charge, the dipole moment, and the quadrupole moment located at the solute ions are examined by studying the corresponding TCFs in the system. The TCFs exhibited a bimodal response, with a very fast initial relaxation due to inertial motions of the solvent, followed by a long tail corresponding to a diffusive component. The polarization fluctuations are also estimated via the cavity field time correlation function (CFTCF), which is useful in the theory of electron transfer processes. The memory kernel or the time dependent friction (TDF) of the solvent is also estimated from the force–force time correlation function. The cross correlations between the Coulombic and the non-Coulombic components of the forces at the ions contribute significantly to the TDF.