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

Abstract

Understanding the distance dependence of the parameters underpinning Marcus theory is imperative when interpreting the results of experiments on electron transfer (ET). Unfortunately, most of these parameters are difficult or impossible to access directly with experiments, necessitating the use of computer simulations to model them. In this work, we use molecular dynamics simulations in conjunction with constrained density functional theory calculations to study the distance dependence of the electronic coupling matrix element, |H RP|, for bimolecular ET. Contrary to what is typically assumed for such intermolecular reactions, we find that the magnitude of |H RP| does not decay exponentially with the center-of-mass separation of the reactants, r COM. The addition of other simple measures of donor/acceptor (D/A) orientation did not improve the correlation of |H RP| with r COM. Using the minimum distance separation, r min, of the reactants as the structural descriptor allowed the system to be partitioned into high-coupling/close-contact and low-coupling/non-contact regimes, but large fluctuations of |H RP| were still found for the close-contact reactant pairs. Despite the persistent large fluctuations of |H RP|, its mean value was found to decay piecewise exponentially with increasing r min, which was attributed to significant changes in the average D/A pair structure. The results herein advise one to use caution when interpreting the experimental results derived from spherical reactant models of bimolecular ET.

Resolving Halide Ion Stabilization through Kinetically Competitive Electron Transfers

“…Hence, electron transfer from the stabilized halide occurs by tunneling through the halide-sequestering tmam ligand. While direct contact between the donor and acceptor promotes electronic coupling, 71 the tmam ligand is formally reduced in the excited state and therefore does not provide strong coupling to the acceptor orbital predominantly localized on the metal and cyclometallating ligands. 72 , 73 Indeed, prior reductive quenching studies reported no measurable kinetic difference when an electron donor was covalently tethered proximal or distal to the ligand formally reduced in the excited state.…”

Elucidating the Mechanism of Bimolecular Photoinduced Electron Transfer Reactions

“…Bottom: examples of Pe .– /DMA .+ conformations in acetonitrile with the same r COM = 0.85 nm but with small (left) and large (right) V values. Adapted with permission from ref . Copyright 2021 American Chemical Society.…”

“…Simulations of the quenching dynamics using MD is certainly a promising approach. After early works in the 90s, − MD simulations were recently applied again to bimolecular ET processes to obtain information that are not easily accessible experimentally. ,, They concerned the estimation of ΔG CS as well as the distance dependence of V mentioned above. , As the reorganization energy can also be obtained from MD simulations, the ET probability can in principle be calculated from Marcus theory. However, the latter is only valid for nonadiabatic ET, i.e.…”

Sub-millisecond Translational and Orientational Dynamics of a Freely Moving Single Nanoprobe

“…All of these problems are in their native environments and three-dimensional in nature and thus would benefit from the application of the technique described here. In addition, this technique’s unprecedented coupling of 3D spatial and orientational information at high time resolution points to future experiments to probe intramolecular dynamics, such as electron transfer reactions, − in heterogeneous systems. This may, in the future, give unprecedented insight into orientation-dependent chemical dynamics in systems that may themselves be spatially and dynamically complex.…”

“…Orientation is a quantity that can have an intimate impact on reactivity . To give but one example, the electronic coupling parameter which modulates an electron transfer reaction is orientation dependent. − Orientation can also function as a sensitive reporter on the nanoscale environment, with previous work in bulk and at the single-particle level probing such a quantity. This is relevant in complex systems such as cells, where the local environment can vary strongly with time and positionalso potentially affecting chemical dynamics.…”