Understanding cation catalytic effects in electron transfer reactions at molecular scale

Abstract

Cation catalytic effects from spectator counter ions on rate of electron transfer (ET) reactions have been well-documented in both homogeneous and heterogeneous electron transfer reactions. People have found that the cation independent reaction path way is 300 times slower than cation (potassium) facilitated reaction rate. This cation specificity is usually observed in electron transfer involving high-valent, anionic redox couples, and for cationic redox centers, the change in reaction rate is relatively insignificant. Moreover, the observed cation specificity seemed to be independent of the shape or geometry of the redox centers, and also the shape or identity of the electrode.

Despite the ubiquity of cation catalytic effects in aqueous ET reactions, the mech- anism behind this effect is not yet fully understood. In known literature, people have proposed two different mechanisms concerning the cation specific effects: (1) indirect and (2) direct pathways. In indirect pathway, cations modify the extended hydrogen bonding structure of water in the solution; thereby, modifying the reorganization en- ergy associated with the electron transfer. In direct pathway, cations pair up with the redox centers to modulate the local solvation characteristics, which in turn changes not only the reorganization energy but aslo the coupling between the redox centers.

In Ch. 2, we quantified the indirect effects from the cations using various statis- tical tools. Moreover, we developed basic intuitions for the likely causes of cation specificity using model redox centers. Our analyses in this chapter reveal that cations exert no significant change in water’s hydrogen bond geometry outside the their first few solvation shells. More importantly, in Marcus picture, collective electrostatics fluctuations drive ET, and we found that the cations have no effect on electrostatics fluctuation of the bulk solvent. These findings indicate that cations exert no signif- icant indirect effect on these ET reaction, and the direct effects are the likely cause of the observed change in ET rate. We then investigated the role of ion pairing in cation specific effects, and found that more highly charged anions tend to pair more strongly with cations and this ion pairing significantly affects the local electrostatics fluctuations around the anionic redox centers, likely causing the observed changes in experimental rates.

In Ch. 3, we applied the intuitions obtained in Ch. 2 to a real, ferri/ferrocyanide redox couple as a test case. Contrary to our expectation, ferri/ferrocyanide display no cation specific trend in outer-sphere reorganization energy for both homogeneous ET case and heterogeneous ET case. Further investigation into the effects of redox charge distribution reveals that charge distribution has significant effect on outer-sphere reor- ganization energy trend. We found that as we transition from a very concentrated to a more scattered charge distribution, the trend in outer-sphere reorganization energy slowly disappears. This implies that for some redox centers, the cation specificity is likely caused by changes in both reorganization energy and cation induced coupling values. We can use this as a general guideline for designing better redox centers in the future. Preliminary coupling calculations on representative MD configurations indicate that the coupling values are highly sensitive to orientation and number of explicit solvent molecules included in the calculations. This means an intuitive and qualitative explanation for cation specific coupling trend is out of reach for ferri/fer- rocyanide redox couple, and a quantitative explanation for observed experimental ET rate shift can be obtained by calculating ensemble averaged coupling values for each cation.