Molecular Dynamics Simulations of Bimolecular Electron Transfer: Testing the Coulomb Term in the Weller Equation

Abstract

Reliable estimation of the driving force of photoinduced electron transfer between neutral reactants is of utmost importance for most practical applications of these reactions. The driving force is usually calculated from the Weller equation which contains a Coulomb term, C, whose magnitude in polar solvents is debated. We have performed umbrella sampling molecular dynamics simulations in order to determine C from the potentials of mean force between neutral and ionic donor/acceptor pairs of different sizes in solvents of varying polarity. According to the simulations, C in polar solvents is a factor of 2 more negative than typically calculated according to the Weller equation.Use of the π-stack contact distance in the Weller equation instead of the van der Waals radius recovers the correct value of C, but this is mostly fortuitous due to the compensating effects of overestimating the dielectric screening at contact and neglecting both charge dilution and desolvation.

Unusual Behavior of Ketoximes: Reagentless Photochemical Pathway to Alkynyl Sulfides

“…72−74 Solvent molecules play a major role in stabilizing these intermediates. 75 In nonpolar solvents, complexes often rapidly decay by emission of light. In polar solvents, dissociation of the complex into solvent separated ions or free ions is more favorable.…”

“…The molar extinction coefficient (ε) of 3a is 7.24 × 10 3 M –1 cm –1 at 352 nm, exhibits no deterioration of its molecular complexity on absorption of light, and can behave as a good photosensitizer (Figure d). − Since the whole system works perfectly when it was irradiated with 370 nm UV LED, and no observation of charge-transfer bands was found, a strong indication was going toward the photochemical event which emphasized the electron transfer due to the fact that an organic molecule or photosensitizer ( 3a here) after being excited at a particular wavelength to its excited triplet state ( 3 3a *) with a quencher ( 2a ) results in the formation of a triplet ion pair, which has a longer lifetime than a singlet ion pair. Return electron transfer for a triplet ion pair to give the ground-state molecules is also spin-forbidden; therefore, the ion pair has a better chance of undergoing further chemical reactions before return electron transfer can occur. − Solvent molecules play a major role in stabilizing these intermediates . In nonpolar solvents, complexes often rapidly decay by emission of light.…”

Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad

“…This assumption should hold for the second oxidation wave describing oxidation of the imidazole group as the phenyl spacer should prevent strong interaction with the electrical charge on the Mn. The potential of the Ru unit, which in the cyclic voltammogram is oxidized in presence of oxidized imidazole, could be more susceptible to be affected by electrostatic interactions as the extent of shielding by polar solvents at such short distances has been questioned [31]. This may lead to a possible error for determination of the Ru III/II potential which therefore should be considered with more caution.…”

Molecular Dynamics Simulations of Bimolecular Electron Transfer: the Distance-Dependent Electronic Coupling

“…Unfortunately, Δ G ( r ), λ­( r ), and H RP are not experimental observables and can only be studied directly using computational methods. In a recent publication, we employed umbrella sampling molecular dynamics (MD) simulations to demonstrate that point-charge/dielectric continuum models do a poor job of describing the molecular reactant pairs, and this can have a significant impact on the estimates of Δ G ( r ). In the present article, we will use similar simulations to study the effect of reactant pair separation and orientation on H RP for intermolecular ET reactions.…”

“…Using our earlier simulations in ref as a starting point, our present goal is to systematically study the distance dependence of the electronic coupling for intermolecular ET and test the prediction of eq when the distribution D/A pair structures is taken into account. To this end, we have performed umbrella sampling classical MD simulations of D/A pairs where r COM , the center-of-mass distance between D and A, is the biased collective variable.…”