Proton-coupled electron transfer reactivities of electronically divergent heme superoxide intermediates: a kinetic, thermodynamic, and theoretical study.

From Europe PMC via Jisc Publications RouterHistory: epub 2021-05-27, ppub 2021-07-01Publication status: PublishedFunder: Biotechnology and Biological Sciences Research Council; Grant(s): BB/J014478/1Heme superoxides are one of the most versatile metallo-intermediates in biology, and they mediate a vast variety of oxidation and oxygenation reactions involving O2(g). Overall proton-coupled electron transfer (PCET) processes they facilitate may proceed via several different mechanistic pathways, attributes of which are not yet fully understood. Herein we present a detailed investigation into concerted PCET events of a series of geometrically similar, but electronically disparate synthetic heme superoxide mimics, where unprecedented, PCET feasibility-determining electronic effects of the heme center have been identified. These electronic factors firmly modulate both thermodynamic and kinetic parameters that are central to PCET, as supported by our experimental and theoretical observations. Consistently, the most electron-deficient superoxide adduct shows the strongest driving force for PCET, whereas the most electron-rich system remains unreactive. The pivotal role of these findings in understanding significant heme systems in biology, as well as in alternative energy applications is also discussed

Antireflection coatings from analogy between electron scattering and spin precession

We use the analogy between scattering of a wave from a potential, and the precession of a spin-half particle in a magnetic field, to gain insight into the design of an antireflection coating for electrons in a semiconductor superlattice. It is shown that the classic recipes derived for optics are generally not applicable due to the different dispersion law for electrons. Using the stability conditions we show that a Poisson distribution of impedance steps is a better approximation than is a Gaussian distribution. Examples are given of filters with average transmissivity exceeding 95% over an allowed band

Lighting the Shadows of the Canon: A Modern and Contextual Reexamination of Victor Herbert’s Cello Concerto No. 2 Op. 30

In this dissertation, I explore the unique musical interests of Victor Herbert’s Cello Concerto No. 2, Op. 30 (1894) with the intent to promote the piece from its ‘fringe’ repertory status. The research presented synthesizes historical information, traditional analysis, and material surveys with a physical exploration from the solo cellist’s point of view to show effective methods for maximizing drama without sacrificing clarity, promoting a singing, vocal quality of the solo cello, and creating a unique duality of tone combining romantic gravitas with the lightheartedness of operetta. In addition, I track the concerto’s reception and repertory status from its premiere in 1894 to 2021 using a survey of existing reviews, articles, public discourse, and recordings. This research aims to encourage more concert programming of the Op. 30, and in the process, makes the argument that canonization and the related false dichotomy between ‘light’ and ‘serious’ music styles have been detrimental forces in the consolidation of the cello concerto repertoire

Introduction to St. Thomas Aquinas : The Essence of the Summa Theologica and the summa Contra Gentiles

New Yorkxxx, 690 p.; 18 c

Using Thermodynamics and Mechanism to Understand and Improve Dioxygen Reduction Electro- and Photoelectrocatalysts

The dioxygen reduction reaction (ORR) is a key process for renewable energy technologies such as batteries and fuel cells. Reliance on dioxygen as the terminal oxidant in fuel cell technologies requires the development of electrocatalysts that proceed at low overpotentials with fast rates, while maintaining adequate selectivity for H2O production over thousands of hours of operation. While many advances have been made, the discovery of efficient and inexpensive ORR electrocatalytic materials remains a holy-grail of energy science. The study of soluble, molecular electrocatalysts allow for more detailed structure : activity analyses to be made than for electrocatalytic materials, providing an atomic level understanding of catalytic barriers and opportunities for improvement. Chapters 1-5 of this thesis develop a rational approach for analyzing, comparing, and improving homogeneous and molecular ORR electrocatalysts in non-aqueous solvents, and use this approach to study the reduction of dioxygen to water using iron porphyrin electrocatalysts. Quantification of the equilibrium potential for O2/H2O under the conditions of study allowed, for the first time, accurate estimations of the reaction overpotential in non-aqueous solvents. Knowledge of the reaction overpotential proved critical for comparing electrocatalysts under the somewhat diverse conditions encountered for homogeneous catalysts with varying solubilities in organic solvents. A detailed kinetic and mechanistic study was then conducted on iron tetraphenyl porphyrin, Fe(TPP), which revealed that the turnover limiting step is protonation of the iron(III) superoxo adduct, formed via pre-equilibrium dioxygen binding to FeII(TPP). The protonation step was found to have a large kinetic barrier, suggesting that targeting proton delivery to the active site may improve the ORR activity of iron porphyrin electrocatalysts. Studies of eleven substituted iron porphyrin ORR electrocatalysts all showed high selectivity for the 4H+/4e - reduction to water. The turnover frequencies (TOFs) were found to correlate with the reduction potential required to initiate electrocatalysis, in two log(TOF):overpotential linear free energy relationships (LFERs). The iron porphyrin electrocatalysts with well-positioned proton donors above the active site fell upon the same LFER as those without such proton relays, suggesting that the second coordination sphere does not directly participate in the rate-limiting proton transfer. These results contradict the general sense that well-positioned proton relays should decrease kinetic barriers. However, some iron porphyrin catalysts in the series can break the LFER, leading to more efficient catalysis. Computational studies suggest that, rather than directly participating in an intramolecular proton transfer, the second coordination sphere of some iron porphyrins can hydrogen bond with the O2 adduct to influence the thermochemistry for proton transfer. Importantly, the presence of these LFERs was shown to stem from the electrocatalyst E1/2 influencing the thermodynamics for O2 binding and proton transfer. Analogies are drawn between these linear free energy relationships and the scaling relationship analyses used for electrocatalytic materials for the ORR. Using the mechanism, rate law, and thermochemistry, the log(TOF) : overpotential correlations were then derived for ORR catalyzed by iron porphyrins. Given that the TOF is a function of the catalytic rate law (TOF = k cat[O2][HA]) and the overpotential is a function of the reaction conditions, the predicted correlation between log(TOF) and effective overpotential is independently. derived for changes in the reaction conditions or for changes to the catalyst E1/2. For each parameter varied, a unique correlation coefficient was identified and shown to agree with experimental data. The very shallow dependence between log(TOF) and the pK a of the acid used was used to enable Fe(TPP) catalyzed ORR to break the prior LFERs by 104 s-1 in TOF. These scaling relations highlight how decoupling the ET, PT and substrate binding events can lead to diverse scaling relationships, providing opportunities for improving the activity of a catalytic system by targeting the medium, as opposed to the catalyst. In chapter 6, an exploratory research project on driving the ORR using sunlight to produce hydrogen peroxide (H2O2) is discussed. H2O2 is a commodity chemical with diverse applications in water purification, as an oxidant, and as a liquid fuel. Preparation of nickel(II) oxide photocathodes sensitized with simple dyes revealed that these photocathodes are surprisingly active for H2O2 production, proceeding to produce H2O2 with unity faradaic efficiency at low overpotentials (<20 mV). The reaction is found to proceed via outer sphere electron transfer from reduced dyes to O2, forming superoxide, which disproportionates in solution, forming H2O2. Remarkably, these unoptimized systems are among the most active photocathodes for H 2O2 production. These results are promising for developing the delocalized production of H2O2 using dye-sensitized photoelectrosynthesis cells