Activation entropy of electron transfer reactions

Abstract

We report microscopic calculations of free energies and entropies for intramolecular electron transfer reactions. The calculation algorithm combines the atomistic geometry and charge distribution of a molecular solute obtained from quantum calculations with the microscopic polarization response of a polar solvent expressed in terms of its polarization structure factors. The procedure is tested on a donor-acceptor complex in which ruthenium donor and cobalt acceptor sites are linked by a four-proline polypeptide. The reorganization energies and reaction energy gaps are calculated as a function of temperature by using structure factors obtained from our analytical procedure and from computer simulations. Good agreement between two procedures and with direct computer simulations of the reorganization energy is achieved. The microscopic algorithm is compared to the dielectric continuum calculations. We found that the strong dependence of the reorganization energy on the solvent refractive index predicted by continuum models is not supported by the microscopic theory. Also, the reorganization and overall solvation entropies are substantially larger in the microscopic theory compared to continuum models.

Dynamical Arrest of Electron Transfer in Liquid Crystalline Solvents

“…Because, for large solutes with the effective radius R 0 . σ, the range of k values affecting solvation is k < π/R 0 , the approximation S L -(k) = S L (0), resulting in continuum solvation, is reasonably accurate (deviations from this approximation are responsible for observable effects on the solvation entropy 54,55 ). The perpendicular component S xx (k) in liquid crystals is governed by repulsions of long molecular rods at the distance of shortaxis diameter = σ and is, therefore, qualitatively similar to S L -(k) in isotropic solvents.…”

“…The slope of the continuum free energy is, however, incorrect, as is often the case with continuum calculations of the solvation entropy. [54][55][56]58 …”

Equilibrium solvation in quadrupolar solvents

“…Data in columns 2 and 10 in Table VI have been calculated by using S m (k) from MD simulations (12-site benzene). Equations (35) and (38) provide an alternative analytical route to S m (k) through the effective linear quadrupole moment with its magnitude given by Eq. (22).…”

“…The fit covers the range of reduced densities ρ * = 0.5 − 1.0. (35)- (38) are also used to define the structure factors of the 12-site model of benzene (Fig. 2) approximated by the axial quadrupole with the magnitude given by Eq.…”

“…Table VIII lists the calculated thermodynamic parameters and rates of charge separation in complex 1 in acetonitrile in the experimental temperature range between 255 and 335 K. 67 The calculations of the free energy gap and the reorganization energy are done by using the recently developed microscopic theory of dipolar solvation. 37,38 The dispersion part of the ET driving force in acetonitrile is calculated according to Eqs. (71) and (72) with the solvent parameters listed in Table V.…”