Understanding cation catalytic effects in electron transfer reactions at molecular scale

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.Ph.D

Encapsulation of Nanoparticles During Polymer Micelle Formation: A Dissipative Particle Dynamics Study

The formation of block copolymer micelles with and without hydrophobic nanoparticles is simulated using dissipative particle dynamics. We use the model developed by Spaeth et al. [Spaeth, J. R.; Kevrekidis, I. G.; Panagiotopoulos, A. Z. J. Chem. Phys. 2011, 134 (16), 164902], and drive micelle formation by adjusting the interaction parameters linearly over time to represent a rapid change from organic solvent to water. For different concentrations of added nanoparticles, we determine characteristic times for micelle formation and coagulation, and characterize micelles with respect to size, polydispersity, and nanoparticle loading. Four block copolymers with different numbers of hydrophobic and hydrophilic polymer beads, are examined. We find that increasing the number of hydrophobic beads on the polymer decreases the micelle formation time and lowers polydispersity in the final micelle distribution. Adding more nanoparticles to the simulation has a negligible effect on micelle formation and coagulation times, and monotonically increases the polydispersity of the micelles for a given polymer system. The presence of relatively stable free polymer in one system decreases the amount of polymer encapsulating the nanoparticles, and results in an increase in polydispersity and the number of nanoparticles per micelle for that system, especially at high nanoparticle concentration. Longer polymers lead to micelles with a more uniform nanoparticle loading

Cation- and pH-Dependent Hydrogen Evolution and Oxidation Reaction Kinetics

The production of molecular hydrogen by catalyzing water splitting is central to achieving the decarbonization of sustainable fuels and chemical transformations. In this work, a series of structure-making/breaking cations in the electrolyte were investigated as spectator cations in hydrogen evolution and oxidation reactions (HER/HOR) in the pH range of 1 to 14, whose kinetics was found to be altered by up to 2 orders of magnitude by these cations. The exchange current density of HER/HOR was shown to increase with greater structure-making tendency of cations in the order of Cs+ < Rb+ < K+ < Na+ < Li+, which was accompanied by decreasing reorganization energy from the Marcus-Hush-Chidsey formalism and increasing reaction entropy. Invoking the Born model of reorganization energy and reaction entropy, the static dielectric constant of the electrolyte at the electrified interface was found to be significantly lower than that of bulk, decreasing with the structure-making tendency of cations at the negatively charged Pt surface. The physical origin of cation-dependent HER/HOR kinetics can be rationalized by an increase in concentration of cations on the negatively charged Pt surface, altering the interfacial water structure and the H-bonding network, which is supported by classical molecular dynamics simulation and surface-enhanced infrared absorption spectroscopy. This work highlights immense opportunities to control the reaction rates by tuning interfacial structures of cation and solvents

Effectiveness and safety of 3 and 5 day courses of artemether–lumefantrine for the treatment of uncomplicated falciparum malaria in an area of emerging artemisinin resistance in Myanmar

Abstract Background Artemisinin resistance in Plasmodium falciparum has emerged and spread in Southeast Asia. In areas where resistance is established longer courses of artemisinin-based combination therapy have improved cure rates. Methods The standard 3-day course of artemether–lumefantrine (AL) was compared with an extended 5-day regimen for the treatment of uncomplicated falciparum malaria in Kayin state in South-East Myanmar, an area of emerging artemisinin resistance. Late parasite clearance dynamics were described by microscopy and quantitative ultra-sensitive PCR. Patients were followed up for 42 days. Results Of 154 patients recruited (105 adults and 49 children < 14 years) 78 were randomized to 3 days and 76 to 5 days AL. Mutations in the P. falciparum kelch13 propeller gene (k13) were found in 46% (70/152) of infections, with F446I the most prevalent propeller mutation (29%; 20/70). Both regimens were well-tolerated. Parasite clearance profiles were biphasic with a slower submicroscopic phase which was similar in k13 wild-type and mutant infections. The cure rates were 100% (70/70) and 97% (68/70) in the 3- and 5-day arms respectively. Genotyping of the two recurrences was unsuccessful. Conclusion Despite a high prevalence of k13 mutations, the current first-line treatment, AL, was still highly effective in this area of South-East Myanmar. The extended 5 day regimen was very well tolerated, and would be an option to prolong the useful therapeutic life of AL. Trial registration NCT02020330. Registered 24 December 2013, https://clinicaltrials.gov/NCT0202033