The temperature dependence of concerted proton-electron transfer (CPET) reactions of two anthracene-phenol-pyridine (An-PhOH-py) triads is investigated in toluene. Light excitation forms an anthracene local excited state (1*An), which undergoes CPET to form a charge separated state (CSS, An•--PhO•-pyH+), which in turn undergoes CPET charge recombination (CR). In toluene, compared with polar solvents, the CSS is energetically destabilized. First, this makes another reaction competitive with CPET, which we propose is proton-coupled energy transfer (PCEnT) from 1*An to form the short-lived excited state keto tautomer of the phenol-pyridine subunit (*[PhO═pyH]). Second, it puts CR deep into the Marcus inverted region, and CSS lifetimes therefore reach several nanoseconds at room temperature. The slow kinetics makes CR to the anthracene triplet state (3*An) competitive, as well as another reaction that is strongly activated and dominates CSS deactivation at T ≥ 240 K for one of the triads. The latter is proposed to be CR via initial formation of the same [*PhO═PyH] state as above by an unusual electron transfer (ET) from An•- to pyH+, instead of CR with the juxtaposed PhO•. The two different pathways to form *[PhO═pyH] lead to CSS yields and lifetimes that vary significantly with temperature, and in markedly different ways between the triads. This is rationalized by the differences in the energies of the states involved. The results broaden the scope and understanding of the still rare phenomena of inverted CPET and PCEnT and may aid toward their use in solar fuels and photoredox catalysis.

Redox transformations at metal oxide (MOx)/solution interfaces are broadly important, and oxygen atom transfer (OAT) is one of the simplest and most fundamental examples of such reactivity. OAT is a two-electron transfer process, well-known in gas/solid reactions and catalysis. However, OAT is rarely directly observed at oxide/water interfaces, whose redox reactions are typically proposed to occur in one-electron steps. Reported here are stoichiometric OAT reactions of organic molecules with aqueous colloidal titanium dioxide and iridium oxide nanoparticles (TiO2 and IrOx NPs). Me2SO (DMSO) oxidizes reduced TiO2 NPs with the formation of Me2S, and IrOx NPs transfer O atoms to a water-soluble phosphine and a thioether. The reaction stoichiometries were established and the chemical mechanisms were probed using typical solution spectroscopic techniques, exploiting the high surface areas and transparency of the colloids. These OAT reactions, including a catalytic example, utilize the ability of the individual NPs to accumulate many electrons and/or holes. Observing OAT reactions of two different materials, in opposite directions, is a step toward harnessing oxide nanoparticles for valuable multi-electron and multi-hole transformations.

Supported iron carbide particles have long served as catalysts for CO hydrogenation (the Fischer-Tropsch synthesis, FTS) and continue to be attractive. Despite this, little is known about their chemistry. Reported here is a colloidal Fe x C nanoparticle (NP) model system that allows direct observation of surface hydrogen and CO, as well as quantification of the surface H. Dodecylamine-capped Fe x C NPs (DDA-Fe x C NPs) were synthesized through solution-phase carburization of Fe NPs and form stable colloids in low-polarity organic solvents. Treatment of these colloids with H2 or D2 produced highly hydrogenated materials, and Fourier transform infrared (FTIR) spectra of DDA-Fe x C-D n showed that most of the D binds to carbides, with at least four distinct nu(C-D) modes. The surface C-H(D) bonds were reactive, transferring hydrogen to alkenes and other reagents in solution. Titration and ICP measurements showed a 0.17:1 ratio of added H:total Fe, or roughly 40 H per 1.8 nm DDA-Fe x C NP. Conversely, CO was preferentially bound to surface Fe sites, with FTIR spectra showing a single broad nu(CO) that shifted with CO coverage or coadsorption of H2. The DDA-Fe x C NPs were active catalysts for both olefin hydrogenation and the FTS, under mild conditions and without catalyst pretreatment. The CO hydrogenation reactions yielded a broad distribution of long-chain linear paraffins and olefins. Though quantitative comparisons with typical FTS results are not possible because of our use of sealed batch reactors and other factors, the observations of high catalytic reactivity demonstrate the relevance of this model system to iron-carbide catalysis. Density functional theory (DFT) calculations on model slab surfaces with varying iron carbide stoichiometries revealed that the thermodynamically preferred surface adsorption sites are C for Hads and Fe for COads. A variety of binding sites and binding energies were found for each adsorbate. We are unaware of previous studies indicating that a diverse array of C-H bonds is the primary source of reactive H on iron carbides. Experimentally, the diversity of *C-H sites was evident in reactions with H atom donors and abstractors of different strengths, from both the reaction stoichiometries and IR spectra. The different surface-H binding energies correlate with the nu(C-D) stretching frequencies. These insights into complex iron carbide surfaces and catalysis could assist catalyst design, and they showcase the importance of stoichiometric studies of reaction intermediates.

We report the enantioselective hydrodifluoroalkylation of alkenes proceeding via an asymmetric hydrogen atom transfer (HAT) event catalyzed by thiol-containing tetrapeptides. Photocatalytic generation of a difluoroacetyl radical followed by carbon-carbon bond formation results in a prochiral carbon-centered radical that engages with the chiral catalyst. A trialkylamine reductant is proposed to turn over the catalyst in this net-reductive transformation. Notably, incorporating an (S)-β-methyl-substituted cysteine as the N-terminal residue improved selectivity relative to that of the native N-terminal cysteine (Cys) residue, and X-ray crystallographic analysis supports the conformational underpinning of this effect. A range of enantioenriched γ-substituted amides were synthesized in up to a 96:4 enantiomeric ratio, demonstrating the broad functional group tolerance of this method. Models accounting for asymmetric induction are proposed with supporting DFT calculations.

Carbon radicals are key to many biological and organic reactions, and stable triphenylmethyl (trityl) radicals are often used to model these processes. Here, we report reactions of a RuIII-pyridine-imidazolate complex (RuIII(pyIm), where pyIm = pyridine-imidazolate) with a series of para-substituted trityls (R = MeO, tBu, Ph, and CF3) that surprisingly proceed via C-C bond formation. Reactions of metal complexes with carbon radicals more commonly form C-X bonds (X = O, N, or halide), as in the ubiquitous rebound mechanism. RuIII(pyIm), however, reacts by a Minisci-type substitution to form RuII(5-trityl-imidazole) complexes. The reduced imidazole complexes can be oxidized to the corresponding RuIII(5-trityl-imidazolate) by abstraction of the imidazole NH hydrogen by an additional trityl. These reactions form triphenylmethanes and a mixture of tritylated RuII and RuIII complexes. The imidazolate is rendered electrophilic, as required for Minisci reactions, by the RuIII center that accepts 1e- in the reaction. Alternatively, the reaction can be viewed as a trityl radical coupling with the RuIII(pyIm) SOMO that has contributions from the imidazolate π-HOMO and a half-filled t2g orbital on RuIII. Because of kinetic competition between the C-C bond formation and H-abstraction steps, more electron-rich trityls form mostly RuII-trityl-imidazoles, while electron-poor trityls form RuIII-trityl-imidazolates.

Photoelectrochemical generation of a potent organic hydride donor at silicon is demonstrated. Two different oxide-coated p-type silicon photoelectrodes reduced 1,2,3,5,6-pentamethyl-1H-benzo[d]imidazol-3-ium hexafluorophosphate, [PMBI][PF6], to its corresponding imidazole hydride, PMBIH, in the presence of a proton source. Under 1 sun illumination, in acetonitrile with 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) buffer, the p-Si photoelectrodes convert PMBI+ to PMBIH with good Faradaic efficiencies (FEs): 78% FE at -2.3 V vs Fc+/0 for Si|TiO2 and 83% FE at -2.6 V vs Fc+/0 for Si|SiO2 (where Si|SiO2 represents silicon coated with an oxide layer). Generally, the Si|TiO2 catalyzed the reaction at milder potentials than Si|SiO2, but the Si|SiO2 had better selectivity for PMBIH generation over H2 evolution than Si|TiO2. In light of prior studies of these photoelectrodes, the differences in selectivity and onset potential suggest a difference in mechanism, likely an outer-sphere electron transfer (ET) mechanism at the SiO2 surface and potentially a proton-coupled ET process at the TiO2 surface. To help understand reaction efficiency and identify areas of improvement, a thermochemical model for understanding net hydride transfer from the semiconductor to an acceptor in solution was developed. We find that the reactions in the present system are quite downhill. The high overpotentials (even while maintaining selectivity over H2 evolution) emphasize the need for improved catalysts. This approach to evaluate the thermodynamics of net hydride transfer should be broadly valuable for electrochemical and photoelectrochemical processes.

The surface immobilization of molecular catalysts is attractive because it combines the benefits of homogeneous and heterogeneous catalysis. However, determining the surface coverage and distribution of a molecular catalyst on a solid support is often challenging, inhibiting our ability to design improved catalytic systems. Here, we demonstrate that the combination of scanning transmission electron microscopy (STEM) and image analysis of the individual positions of heavy atoms in transition metal complexes via a convolutional neural network (CNN) allows statistically robust determination of the surface coverage and distribution of immobilized molecular catalysts. These observations provide information about how changes in the functionalization conditions, attachment group, and structure of the molecular catalyst affect the surface coverage and distribution, providing insight into the chemical mechanism of surface immobilization. The method could be generally valuable for correlating the surface coverage and distribution to the activity, selectivity, and stability of a catalytic system.

The mechanism of proton-coupled electron transfer at the surface of titanium-substituted polyoxovanadate-alkoxide clusters can be tuned by judicious selection of substrate.

Coatings and termination strategies for silicon photoelectrodes can protect the semiconductor from oxidation during photoelectrochemical reactions that produce chemicals and fuels. However, these modifications must not inhibit interfacial charge transfer to fuel-producing catalysts and mediators. Terminating the silicon lattice with organic moieties affords stable photoelectrodes that exhibit large photovoltages. Methyl-terminated silicon can be employed to drive the selective reduction of CO2 by molecular catalysts without the competitive hydrogen evolution observed with hydrogen-terminated electrodes. When the silicon is instead coated with a thin silicon oxide layer grown through a rapid thermal annealing strategy, stability and large photovoltages are also achievable. We show that the proton-coupled electron transfer reduction of benzimidazolium to a dihydrobenzimidazole organohydride mediator proceeds without competitive hydrogen evolution with these thermal oxide-coated silicon. Collectively, these studies provide key foundations for hybrid photoelectrodes that drive fuel production with sunlight.

The selective reduction of CO 2 to formate using molecular catalysts immobilized on high surface area porous silicon is described. Manganese complexes of the form (Rbpy)Mn(CO)3Br (bpy = 2,2'-bipyridine) were prepared with silatrane groups on the bpy ligand for attachment to oxide-coated porous silicon (SiOx-porSi). SiOx-porSi wafers were formed by heating hydrogen-terminated p-type porous silicon wafers under air and the manganese complexes were immobilized on SiOx-porSi by heating at 80 °C. The resulting Mn@SiOx-porSi photoelectrodes are photoelectrocatalysts for CO 2 reduction in acetonitrile containing 2.0 M triethylamine and 2.0 M isopropanol, yielding formate with high selectivity (>96%) and current density (~0.6 mA/cm2), excellent reproducibility, and a photovoltage of 280 mV at -1.75 V (versus ferrocenium/ferrocene) under 1 sun illumination. The applied potential is close to the equilibrium potential for CO 2 reduction to formate. This work presents rare examples of immobilized molecular catalysts for CO 2 reduction to formate, and the first on semiconducting silicon.

Hydrogen atom transfer (HAT) reactions and their kinetic barriers ΔGHAT‡ are important in organic and inorganic chemistry. This study examines factors that influence ΔGHAT‡, reporting the kinetics and thermodynamics of HAT from various ruthenium bis(acetylacetonate) pyridine-imidazole complexes to nitroxyl radicals. Across these 36 reactions, the ΔGPT° and ΔGHAT° can be independently varied, with different sets of Ru complexes primarily tuning either their pKas or their E°s. The ΔΔGHAT‡ are analyzed using multiple linear free energy relationships (LFERs), the first largely experimental study of its kind. The barriers vary most strongly with the overall driving force, ΔΔGHAT‡ = 0.28 × ΔΔGHAT°, but are also affected by HAT intrinsic barriers (λ), sterics, and the thermochemical e-/H+ imbalance of the reactions, |ΔGPT° - ΔGET°|. The latter is a small but significant effect, revealed only by comparing LFERs. The imbalance analysis is closely related to traditional explanations of polar effects, but it is quantitative: ΔGHAT‡ shifts by ∼4% with changes in |ΔGPT° - ΔGET°|. This is the same dependence as was observed for purely organic HAT from toluenes─a remarkable result because traditional explanations of organic polar effects, e.g., using X-H bond polarities, do not apply to the Ru complexes in which the e- and H+ are spatially separated. This work demonstrates the strong similarities between different kinds of HAT reactions when viewed through the lens of H+/e- (PCET) free energies. This lens also shows that ΔGHAT‡ are ∼10-fold more sensitive to changes in ΔGHAT° and λ than to the e-/H+ free-energy imbalance.

Colloidal gold nanoparticles (AuNPs) have myriad scientific and technological applications, but their fundamental redox chemistry is underexplored. Reported here are titration studies of oxidation and reduction reactions of aqueous AuNP colloids, which show that the AuNPs bind substantial hydrogen (electrons + protons) under mild conditions. The 5 nm AuNPs are reduced to a similar extent with reductants from borohydrides to H-2 and are reoxidized back essentially to their original state by oxidants, including O-2. The reactions were monitored via surface plasmon resonance (SPR) optical absorption, which was shown to be much more sensitive to surface H than to changes in solution conditions. Reductions with H-2 occurred without pH changes, demonstrating that hydrogenation forms surface H rather than releasing H+. Computational studies suggested that an SPR blueshift was expected for H atom addition, while just electron addition likely would have caused a redshift. Titrations consistently showed a maximum redox change of the 5 nm NPs, independent of the reagent, corresponding to 9% of the total gold or similar to 30% hydrogen surface coverage (similar to 370 H per AuNP). Larger AuNPs showed smaller maximum fractional surface coverages. We conclude that H binds to the edge, corner, and defect sites of the AuNPs, which explains the stoichiometric limitation and the size effect. The finding of substantial and stable hydrogen on the AuNP surface under mild reducing conditions has potential implications for various applications of AuNPs in reducing environments, from catalysis to biomedicine. This finding contrasts with the behavior of bulk gold and with the typical electron-focused perspective in this field.

TiO2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule-semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO2 reduction, N-2 reduction, and H-2 evolution. On both n-type Si and irradiated p-type Si, TiO2 reduction is proton-coupled with a 1e(-):1H(+) stoichiometry, as demonstrated by the Nernstian dependence of the Ti4+/3+E1/2 on the buffer pK(a). Experiments were conducted with and without illumination, and a photovoltage of similar to 0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO2/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO2 films after reduction reveals restructuring with the formation of islands, rendering TiO2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on "protective" metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.

A high-surface-area p-type porous Si photocathode containing a covalently immobilized molecular Re catalyst is highly selective for the photoelectrochemical conversion of CO2 to CO. It gives Faradaic efficiencies of up to 90% for CO at potentials of -1.7 V (versus ferrocenium/ferrocene) under 1 sun illumination in an acetonitrile solution containing phenol. The photovoltage is approximately 300 mV based on comparisons with similar n-type porous Si cathodes in the dark. Using an estimate of the equilibrium potential for CO2 reduction to CO under optimized reaction conditions, photoelectrolysis was performed at a small overpotential, and the onset of electrocatalysis in cyclic voltammograms occurred at a modest underpotential. The porous Si photoelectrode is more stable and selective for CO production than the photoelectrode generated by attaching the same Re catalyst to a planar Si wafer. Further, facile characterization of the porous Si-based photoelectrodes using transmission mode FTIR spectroscopy leads to highly reproducible catalytic performance.

We report a series of isomeric, dicationic Re(bpy(2+))(CO)(3)I complexes with bpy (2,2 '-bipyridine) modified by two phenyl-CH2-(NMe3)(+) pendants with cations located at variable distances from the active site for electrocatalytic CO2 reduction in CH3CN/2.8 M H2O. The position of the cationic groups dramatically increases the rate of catalysis by similar to 800-fold, from 1.2 to 950 s(-1), with only a minor increase in overpotential. Acceleration is due to stabilization of the initial CO2 adduct and lowering of Delta G double dagger for C-OH bond cleavage by Coulombic stabilization of anionic charges. Performance may be enhanced by accumulation in the electrochemical double layer. Transition state stabilization in the optimized isomer unlocks the low overpotential "protonation-first" pathway, highlighting the sizable effects of subtle structural optimization.

Added electrons and holes in semiconducting (nano)materials typically occupy "trap states," which often determine their photophysical properties and chemical reactivity. However, trap states are usually ill-defined, with few insights into their stoichiometry or structure. Our laboratory previously reported that aqueous colloidal TiO2 nanoparticles prepared from TiCl4 + H2O have two classes of electron trap states, termed Blue and Red. Herein, we show that the formation of Red from oxidized TiO2 requires 1e - + 1H+, while Blue requires 1e - + 2H+. The two states are in a protic equilibrium, Blue reversible arrow Red + H+, with K eq = 2.65 mM. The Blue states in the TiO2 NPs behave just like a soluble molecular acid with this K eq as their K a, as supported by solvent isotope studies. Because the trap states have different compositions, their population and depopulation occur with the making and breaking of chemical bonds and not (as commonly assumed) just by the movement of electrons. In addition, the direct observation of a 2H+/1e - trap state contradicts the emerging H atom transfer (1H+/1e -) paradigm for oxide/solution interfaces. Finally, this work emphasizes the importance of chemical stoichiometries, not just electronic energies, in understanding and directing the reactivity at solid/solution interfaces.

Most redox processes that break/form bonds involve net 2e− changes, and many are coupled to protons. Yet most proton‐coupled electron transfer (PCET) studies focus on 1e–/1H+ reactions. Reported here is a family of molecular models that undergo tunable 2e–/2H+ redox changes. Complexes [(X2bpy)RuII(en*)2](PF6)2 and [(X2bpy)RuIV(en*‐H)2](PF6)2 have been synthesized with bpy = 2,2’‐bipyridine with 4,4’‐subtitutions X = ‐NMe2, ‐OMe, ‐Me, ‐H, ‐CF3; and en* = 2,3‐dimethyl‐2,3‐butanediamine. They have been characterized by IR, UV‐vis, and NMR spectroscopies, XRD, electrochemistry, mass spectrometry, DFT and (TD)DFT computations. The introduction of electron‐withdrawing and donating groups at the 4,4’‐position of the bpy ligand affects the complexes’ redox potentials, pKa’s, and Bond Dissociation Free Energies (BDFEs) of the N‐H bonds in the en* ligands. The average BDFEs for the overall 2e−/2H+ PCET span over 5 kcal/mol. Notably, these complexes all show marked potential inversion over an extended range, ΔpKa > 25 units and ΔE0 > 1.4 V. Potential inversion remains despite the electronic influence of bpy’s substitutions which regulate N‐H properties several bonds apart by trans‐effect over dπ‐molecular orbitals at Ru‐center. The experimental and computational results presented in this work support the presence of strong coupling between electrons and protons, for modelling insights of 2e−/2H+ transfer reactivity.

Most redox processes that break/form bonds involve net 2e(-) changes, and many are coupled to protons. Yet most proton-coupled electron transfer (PCET) studies focus on 1e(-)/1H(+) reactions. Reported here is a family of molecular models that undergo tunable 2e(-)/2H(+) redox changes. Complexes [(X(2)bpy)Ru-II(en*)(2)](PF6)(2) and [(X(2)bpy)Ru-IV(en*-H)(2)](PF6)(2) have been synthesized with bpy=2,2'-bipyridine with 4,4'-subtitutions X=-NMe2, -OMe, -Me, -H, -CF3; and en*=2,3-dimethyl-2,3-butanediamine. They have been characterized by IR, UV-vis, and NMR spectroscopies, XRD, electrochemistry, mass spectrometry, DFT and (TD)DFT computations. The introduction of electron-withdrawing and donating groups at the 4,4'-position of the bpy ligand affects the complexes' redox potentials, pK(a)'s, and Bond Dissociation Free Energies (BDFEs) of the N-H bonds in the en* ligands. The average BDFEs for the overall 2e(-)/2H(+) PCET span over 5 kcal/mol. Notably, these complexes all show marked potential inversion over an extended range, Delta pK(a)>25 units and Delta E-0>1.4 V. Potential inversion remains despite the electronic influence of bpy's substitutions which regulate N-H properties several bonds apart by trans-effect over d pi-molecular orbitals at the Ru center. The experimental and computational results presented in this work support the presence of strong coupling between electrons and protons, for modelling insights of 2e(-)/2H(+) transfer reactivity.

Learning the science of heterogeneous catalysis and electrocatalysis always starts with the simple case of a flat, uniform surface with an ideal adsorbate. It has of course been recognized for a century that real catalysts are more complicated. For the increasingly complex catalysts of the 21st century, this Perspective argues that surface heterogeneity and non-ideal binding isotherms are central features, and their implications need to be incorporated in current thinking. A variety of systems are described herein where catalyst complexity leads to broad, non-Langmuirian surface isotherms for the binding of hydrogen atoms - and this occurs even for ideal, flat Pt(111) surfaces. Modern catalysis employs nanoscale materials whose surfaces have substantial step, edge, corner, impurity, and other defect sites, and they increasingly have both metallic and non-metallic elements MnXm, including metal oxides, chalcogenides, pnictides, carbides, doped carbons, etc. The surfaces of such catalysts are often not crystal facets of the bulk phase underneath, and they typically have a variety of potential active sites. Catalytic surfaces in operando are often non-stoichiometric, amorphous, dynamic, and impure, and often vary from one part of the surface to another. Understanding of the issues that arise at such nanoscale, multi-element catalysts is just beginning to emerge. Yet these catalysts are widely discussed using Bronsted/Bell-Evans-Polanyi (BEP) relations, volcano plots, Tafel slopes, the Butler-Volmer equation, and other linear free energy relations (LFERs), which all depend on the implicit assumption that the active sites are "similar" and that surface adsorption is close to ideal. These assumptions underly the ubiquitous intuition based on the Sabatier Principle, that the fastest catalysis will occur when key intermediates have free energies of adsorption that are not too strong nor too weak. Current catalysis research often aims to minimize the complexity of non-ideal isotherms through experimental and computational design (e.g., the use of single crystal surfaces), and these studies are the foundation of the field. In contrast, this Perspective argues that the heterogeneity of binding sites and binding energies is an inherent strength of these catalysts. This diversity makes many nanoscale catalysts inherently a high-throughput screen wrapped in a tiny package. Only by making the heterogeneity part of the foundation of catalysis models, sorting the types of active sites and dissecting non-ideal binding isotherms, will modern catalysis learn to harness the inherent diversity of real catalysts. Controlling and exploiting diversity rather than avoiding it will help to optimize complex modern catalysts and catalytic conditions.

Rate constants for hydrogen atom transfer (HAT) reactions of substituted toluenes with tert-butyl, tert-butoxy, and tert-butylperoxyl radicals are reanalyzed here using the free energies of related proton transfer (PT) and electron transfer (ET) reactions, calculated from an extensive set of compiled or estimated pKa and E° values. The Eyring activation energies ΔGHAT‡ do not correlate with the relatively constant ΔG°HAT, but do correlate close-to-linearly with ΔG°PT and ΔG°ET. The slopes of correlations are similar for the three radicals except that the tBu• barriers shift in the opposite direction from the oxyl radical barriers─a clear example of the qualitative "polar effect" in HAT reactions. When cast quantitatively in free energy terms (ΔGHAT‡ vs ΔG°PT/ET), this effect is very small, only 5-10% of the typical Bell-Evans-Polanyi (BEP) effect of changing ΔG°HAT. This analysis also highlights connections between polar effects and the concepts of "asynchronous" or "imbalanced" HAT reactions in which the PT and ET components of ΔG°HAT contribute differently to the barrier. Finally, these observations are discussed in light of the traditional explanations of polar effects and the potential for a rubric that could predict the extent to which contra-thermodynamic selectivity may be achieved in HAT reactions.