Hydrogen-bond Basicity Research Articles

System Maps for Retention of Small Neutral Compounds on a Superficially Porous Ethyl-Bridged, Octadecylsiloxane-Bonded Silica Stationary Phase in Reversed-Phase Liquid Chromatography

The system constants of the solvation parameter model are used to prepare system maps for the retention of small neutral compounds on an ethyl-bridged, ocatadecylsiloxane-bonded superficially porous silica stationary phase (Kinetex EVO C18) for aqueous mobile phases containing 10–70% (v/v) methanol or acetonitrile. Electrostatic interactions (cation-exchange) are important for the retention of weak bases with acetonitrile–water but not methanol–water mobile phase compositions. Compared with a superficially porous octadecylsiloxane-bonded silica stationary phase (Kinetex C18) with a similar morphology but different topology statistically significant differences in selectivity at the 95% confidence level are observed for neutral compounds that vary by size and hydrogen-bond basicity with other intermolecular interactions roughly similar. These selectivity differences are dampened with acetonitrile–water mobile phases, but are significant for methanol–water mobile phase compositions containing <30% (v/v) methanol. A comparison of a totally porous ethyl-bridged, octadecylsiloxane-bonded silica stationary phase (XBridge C18) with Kinetex EVO C18 indicated that they are effectively selectivity equivalent.

Nitroxide stable radicals interacting as Lewis bases in hydrogen bonds: A search in the Cambridge structural data base for intermolecular contacts

1125 X-ray structures of nitroxide free radicals presenting intermolecular hydrogen bonds have been reported in the Cambridge Structural Database. We will report in this paper a qualitative and quantitative analysis of these bonds. The observation in some plots of an excluded region was statistically analyzed using convex hull and kernel smooting methodologies. A theoretical study at the MP2 level with different basis has been carried out indicating that the nitronyl nitroxide radicals (five electrons) lie just in between nitroso compounds (four electrons) and amine N-oxides (six electrons) as far as hydrogen-bond basicity is concerned.

Synthesis and Properties of Non-Aromatic Ionic Liquids and their Role in Cellulose Dissolution

The dissolution of cellulose is an important pretreatment method required for some of its catalytic conversion processes. Morpholinium-based ionic liquids (ILs) are challenging solvent choices available for “greener” and “cheaper” dissolution of cellulose. In this study, morpholinium-based ILs were prepared, and their influencing factors were experimentally investigated. The unique bipolar chemical structure derived from oxygen (electron-rich center) and nitrogen (electron-poor center) considerably enhanced the Hammett acidity function (H0) of task-ILs. N-Methyl-N-allyl-morpholinium acetate IL showed the highest H0 (2.324) and polarizability power (1.096). The anion of ILs determined the hydrogen bond basicity (β). The acetate anion contributed to β (0.88) values. As to fluid properties, the morpholinium-based ILs exhibited much lower viscosity. The properties of ILs improved the dissolution efficiency. The cellulose was directly dissolved in morpholinium-based Ils, and no other derivatives were formed. The cations and the anions of ILs studied reacted with oxygen and hydrogen atoms on the hydroxyl groups of cellulose, respectively.

A system map for the ionic liquid stationary phase 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide trifluoromethanesulfonate for gas chromatography

The solvation parameter model is used to prepare a system map for the retention of volatile organic compounds on the ionic liquid stationary phase 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide trifluoromethanesulfonate (SLB-IL61) over the temperature range 80–260°C. Retention is governed by dispersion, dipole-type and hydrogen-bonding interactions each with its own temperature dependence. The exchange of a bis(trifluoromethylsulfonyl)imide anion in SLB-IL60 for a trifluromethanesulfonate anion (SLB-IL61) results in a change in selectivity indicated by an increase in the hydrogen-bond basicity and a decrease in hydrogen-bond acidity of the stationary phase without change in either the cohesion or dipolarity/polarizability of the stationary phases. At high temperatures there are small differences in electron lone pair interactions but these are relatively unimportant in terms of selectivity differences. Since the disclosed chemical structures for SLB-IL60 and SLB-IL61 does not contain obvious hydrogen-bond acid functional groups the modest hydrogen-bond acidity of these stationary phases was unexpected but does not appear to be obviously connected to adsorption sites at the column wall. The polarity number is shown to be a poor indicator of column retention properties for SLB-IL61. Principal component analysis with the system constants as variables indicates that the retention properties of SLB-IL61 are not duplicated by any of the common poly(siloxane) and poly(ethylene glycol) stationary phase chemistries in current use for column preparation. The SLB-IL61 column is closest in separation properties to poly(cyanopropylphenyldimethylsiloxane) and poly(cyanopropylmethyldimethysiloxane) stationary phases with a high percentage of cyanopropyl-containing monomer but the two stationary phase types are not selectivity equivalent.

Position-Specific 13C Fractionation during Liquid-Vapor Transition Correlated to the Strength of Intermolecular Interaction in the Liquid Phase.

The relationship between the strength of the intermolecular interaction in liquid and the position-specific 13C fractionation observed during distillation was investigated. A range of molecules showing different intermolecular interactions in terms of mode and intensity were incorporated in the study. Although it had previously been suggested that during evaporation the diffusive 13C isotope effect in the thin liquid layer interfaced with vapor is not position-specific, herein we show that this is not the case. In particular, the position-specific effect was demonstrated for a series of alcohols. Our hypothesis is that intermolecular interactions in the liquid phase are the source of position-specific 13C fractionation observed on the molecule. A clear trend is observed between the 13C isotope effect of the carbon bearing the heteroatom of chemical function and the relative permittivity, the solvent hydrogen bond acidity, and the solvent hydrogen bond basicity, while only a weak trend was observed when using the 13C content of the whole molecule. Furthermore, two families of products appeared when using the hydrogen bond acidity parameter for the correlation by distinguishing H-acceptor and H-donor molecules from those H-acceptors only. This strongly reinforces the hypothesis of an important role of the 13C positioned close to the interaction center.

The role and potential of morpholinium-based ionic liquids in dissolution of cellulose

The effective dissolution of cellulose prepares for its subsequently full utilization. Ionic liquids (ILs) are attractive options for “green” dissolution of cellulose. A series of morpholinium-based ILs were synthesized and applied in dissolution of cellulose. The ultrasonic assisted saturated calcium chloride solution was used to activate cellulose. The remarkable decrease (from 87% to 65%) in crystallinity of cellulose facilitated dissolution process. The type of head-corn cations significantly increased polarity (ET(30)) and hydrogen bond acidity (α) of ILs. N-Allyl-N-methylmorpholinium-based ILs showed the highest ET(30) and α (59.11kcalmol−1 and 0.51) compared to 1-Allyl-3-methylimidazolium-based ILs (57.41kcalmol−1 and 0.46) and 1-Allyl-3-methylpyridinium-based ILs (49.36kcalmol−1 and 0.33) owing to the electron-rich center of oxygen in the morpholine ring. The type of anions dominated the ability to accept hydrogen bond of ILs. Formate, acetate and bisulfate contributed more hydrogen bond basicity (β) (0.84, 0.88 and 0.91) for N-allyl-N-methylmorpholinium-based ILs in comparison with other anions. Additionally, morpholinium-based ILs exhibited appropriate viscosity. Above-mentioned performances jointly improved the dissolution of cellulose in morpholinium-based ILs. This study demonstrated the potential of using morpholinium-based ILs as a class of efficient solvents to dissolve cellulose.

Recovery of methacrylic acid from dilute aqueous solutions by ionic liquids though hydrogen bonding interaction

Abstract The recovery of carboxylic acids from dilute aqueous solution is necessary but difficult in chemical industry. Methacrylic acid (MAA) is an important chemical widely used in polymer industry. In this work, the recovery of carboxylic acid from diluted aqueous solution by liquid-liquid extraction using ionic liquids (ILs) as extractants was studied. MAA was employed as model carboxylic acid. The partition coefficients of MAA in biphasic system and the extraction efficiencies of MAA were determined for imidazolium-based ILs and quaternary ammonium salt ILs with different structure of cation and anion. The extraction conditions such as extraction time, extraction temperature, mass ratio of ILs to MAA aqueous solution, initial concentrations of MAA, and water content were evaluated. An internal mechanism of the MAA extract by ILs was revealed by combining solvatochromic study, FT-IR and quantum chemical calculations. The results show that the strong hydrogen bond basicity (β) of ILs results in the high partition coefficient of MAA, which provides guidance to molecular design of ILs for the high efficient extractive separation of MAA from diluted aqueous solution. Based on this, IL with strong hydrogen bond basicity ([N8881]Cl) was used to extract MAA from dilute aqueous solution. Compared with hexane which is conventional solvent used in MAA extractive separation, [N8881]Cl can get 49 times higher of partition coefficient value and the extraction efficiency reach 94.57% for the single-stage extraction.

Solvatochromic parameters of deep eutectic solvents formed by ammonium-based salts and carboxylic acids

Deep eutectic solvents (DES) have been studied in a wide range of applications, and despite their potential as sustainable solvents, detailed knowledge on their solvatochromic parameters is still lacking. To overcome this problem, in this work, the Kamlet Taft (KT) solvatochromic parameters, namely the hydrogen-bond acidity, hydrogen-bond basicity and dipolarity/polarizability, of a wide range of DES composed of ammonium-based salts as hydrogen bond acceptors (HBAs), and carboxylic acids as hydrogen bond donors (HBDs), were determined aiming at better understanding the influence of the chemical structure of the DES components on their polarity. It is shown that the high acidity of the DES investigated is mainly provided by the organic acid present in the mixture, and that an increase of the alkyl side chain of both the HBA and the HBD species leads to a lower ability of the solvent to donate protons. On the other hand, the ammonium salt plays the major role on the hydrogen-bond basicity of DES. Contrarily to the hydrogen-bond acidity, an increase in the length of the aliphatic moieties of both the carboxylic acid and salt cation results in solvents with higher ability to accept protons. The dipolarity/polarizability of DES is mainly defined by the ionic species present, and tend to decrease with the increase of the aliphatic moiety of the organic acid. In general, DES composed of ammonium-based salts and carboxylic acids present a higher capacity to donate and accept protons when compared to most of the ionic liquids or organic molecular solvents.

Better Than the Best Polar Solvent: Tuning the Polarity of 1,2,4‐ Triazolium‐Based Ionic Liquids

AbstractIn search of better replacement for highly polar volatile organic solvents, ionic liquids (ILs) with high polarity and high polarizability were synthesized using N‐1 substituted 1,2,4‐triazolium‐based cation with trifluoromethanesulfonate as the counter anion. The temperature dependence polarity of the synthesized ILs has been investigated using solvatochromic dyes and expressed in terms of and Kamlet‐Taft parameters (dipolarity/polarizability (π*), hydrogen bond donor acidity (α), and hydrogen bond basicity (β) in the range of (293.15−353.15) K. The introduction of the –SO3H functional group and triazole ring on the cation has revealed high polarity ( &gt;1.2) and high polarizability (π*&gt; 3.2). Furthermore, to understand the extent of intermolecular interaction temperature dependence refractive indices have been measured in the temperature range (293.15−343.15) K and a positive correlation was observed with polarity parameter. Finally, the impact of structure variants in terms of the N‐1 functionalized 1,2,4‐triazole ring has been analyzed over the studied properties which is scrutinized to understand the solvation capability and intermolecular interaction in ILs.

Characterization of the interaction of glycyrrhizin and glycyrrhetinic acid with bovine serum albumin by spectrophotometric-gradient flow injection titration technique and molecular modeling simulations

In this research, the interactions of glycyrrhizin (GL) and glycyrrhetinic acid (GA) with bovine serum albumin (BSA) have been investigated by the novel method of spectrophotometric- gradient flow injection titration technique. The hard-modeling multivariate approach to binding was used for calculation of binding constants and estimation of concentration-spectral profiles of equilibrium species. The stoichiometric ratio of binding was estimated using eigenvalue analysis. Results showed that GL and GA bind BSA with overall binding constants of KGL-BSA=3.85 (±0.09)×104Lmol−1, KGA-BSA=3.08 (±0.08)×104Lmol−1. Ligand-BSA complexes were further analyzed by combined docking and molecular dynamics (MD) simulations. Docking simulations were performed to obtain a first guess on the binding structure of the GL/GA-BSA complex, and subsequently analyzed by 20 ns MD simulations in order to evaluate interactions of GL/GA with BSA in detail. Results of MD simulations indicated that GL-BSA complex forms mainly on the basis of hydrogen bonds, while, GA-BSA complex forms on the basis of hydrophobic interactions. Also, water molecules can bridge between the ligand and protein by hydrogen bonds, which are stable during the entire simulation and play an important role in stabilization of the GL/GA-BSA complexes.

Solubility and selective oxidation of 2-chloroethyl ethyl sulfide in imidazole-based ionic liquids

In this study, a novel decontamination method against mustard agent simulant 2-chloroethyl ethyl sulfide (CEES) was conducted with 30% H2O2 aqueous solution in 1-butyl-3-methylimidazolium based ionic liquids (ILs). More than 99% of the CEES was oxidized within H2O2/[BMIm]HCO3 at a concentration of 20mgmL−1 in 30min with the corresponding sulfoxide yield more than 97%. Decontamination was effective at very low temperatures (−32°C). [BMIm]HCO3 can be recycled 4 times without significant decrease in decontamination efficacy. The decontamination efficacy against CEES was mainly depended on the hydrophilic-hydrophobic property and the ability of activating H2O2. The singlet oxygen (1O2) generated chemically in H2O2/[BMIm]HCO3 was able to oxidize CEES to corresponding 2-chloroethyl ethyl sulfoxide (CEESO) and ethyl vinyl sulfoxide (EVSO). The ILs was oxidized by the reaction intermediate to 2-imidazolone, which in turn inhibited the formation of corrosive 2-chloroethyl ethyl sulfone (CEESO2). The influencing factors about imidazole-based ILs hydrogen-bond basicity (β), ILs alkyl chain length, oxidant/CEES mole ratio, reaction temperature and ILs reusability of the decontamination system were also checked.

Capillary column coated with geminal ionic liquids as stationary phases for gas chromatographic separation

Four geminal ionic liquids (GILs), namely, 1,4-bis(1,1′-butyl-3,3′- methylene- imidazolium)-benzene bis[(trifluoromethyl)sulfonyl]imide (BBMIB-NTf2), 1,4- bis(1,1′-butyl-3,3′-methylene-imidazolium)-benzene tetrafluoroborate (BBMIB-BF4), 1,4- bis(1,1′-butyl-3,3′-methylene-imidazolium)-benzene hexafluophosphate (BBMIB-PF6), and 1,4-bis(1,1′-methyl-3,3′-methylene-imidazolium)-benzene bis[(trifluoromethyl) sulfonyl] imide (BMMIB-NTf2), were synthesized. They were statically coated onto the inner walls of fused-silica capillary columns and used as stationary phases for gas chromatography. The evaluation of BBMIB-NTf2, BBMIB-BF4, BBMIB-PF6, and BMMIB-NTf2 as stationary phases is reported here for the first time. These new stationary phases exhibit efficiencies of at least 2.3 × 103 plates per meter. Abraham solvation parameter model was used to evaluate the solvation characteristics. The system constants indicated that the dipolarity/polarizability and the hydrogen-bond basicity play a major role among five molecular interactions between stationary phases and solute molecules. A fundamental understanding into the solvation characteristics of these GILs can be used as a guide to choosing the appropriate geminal ionic liquids for specific applications in various fields. The chromatographic separation performance was evaluated by a Grob test mixture, n-alkanes, alcohols, and aromatic isomers. Furthermore, the thermal stability was tested. The present results demonstrate that these geminal ionic liquids stationary phases possess excellent chromatographic separation performance and good thermal stability (at least up to 270 °C) and may be applicable as gas chromatography stationary phases for more application.

Descriptors for ferrocene and some substituted ferrocenes

We have used literature data on solubilities in 58 solvents to obtain Abraham (Absolv) descriptors for ferrocene. The required descriptor E was obtained through known measurements of molar refraction, and the V descriptor was obtained through comparison of partial molal volumes with McGowan volumes, Vx, for 58 compounds. Ferrocene is quite hydrophobic, with our calculated water-octanol partition coefficient as 3.70 (log Ps), it has no hydrogen bond acidity, only a small hydrogen bond basicity, but a significant dipolarity. Descriptors for a number of derivatives of ferrocene were also obtained from solubility data; these derivatives were acetylferrocene, diacetylferrocene, acetoacetylferrcene and diacetoacetylferocene. In addition, descriptors for alkylferrocenes were estimated by comparison. Substituent effects on ferrocene are very similar to substituent effects on purely organic compounds, and could be used to estimate descriptors for a variety of monosubstituted ferrocenes.

Solvent effect on protonation of tpps in water-DMF mixtures

The protonation of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin was investigated in aqueous solutions of N,N-dimethyformamide at 25 °C and 0.1 mol.dm-3 sodium perchlorate. The solvent effect on value of protonation constant was examined by using the linear solvation energy relationship concept. The value of logK1,logK2 and logKt was correlated with the macroscopic (dielectric constant) and microscopic Kamlet-Taft parameters (a, b and p*) of binary mixtures. The solvent effects were analyzed in the terms of Kamlet, Abboud and Taft model (KAT). Multiple linear regression were used to find the contribution of the microscopic parameters containing a (hydrogen-bond acidity), p* (dipolarity/polarizability) and b (hydrogen-bond basicity). It was found that a and b were the most predominant descriptors. Also, relationship with reciprocal of dielectric constant was obtained based on Born’s model, showing the significance of specific solute-solvent interactions. Therefore the hydrogen bonding interactions between solute and solvent components are mainly responsible for the change in protonation constants of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin in water- N,N-dimethyformamid binary mixtures. KEY WORDS: Protonation, TPPS, Solvent effects, Aqueous mixture, DMF Bull. Chem. Soc. Ethiop. 2016, 30(3), 457-464DOI: http://dx.doi.org/10.4314/bcse.v30i3.14

The influence of hydrogen bonding on partition coefficients.

This Perspective explores how consideration of hydrogen bonding can be used to both predict and better understand partition coefficients. It is shown how polarity of both compounds and substructures can be estimated from measured alkane/water partition coefficients. When polarity is defined in this manner, hydrogen bond donors are typically less polar than hydrogen bond acceptors. Analysis of alkane/water partition coefficients in conjunction with molecular electrostatic potential calculations suggests that aromatic chloro substituents may be less lipophilic than is generally believed and that some of the effect of chloro-substitution stems from making the aromatic π-cloud less available to hydrogen bond donors. Relationships between polarity and calculated hydrogen bond basicity are derived for aromatic nitrogen and carbonyl oxygen. Aligned hydrogen bond acceptors appear to present special challenges for prediction of alkane/water partition coefficients and this may reflect 'frustration' of solvation resulting from overlapping hydration spheres. It is also shown how calculated hydrogen bond basicity can be used to model the effect of aromatic aza-substitution on octanol/water partition coefficients.

Production of Levoglucosenone and Dihydrolevoglucosenone by Catalytic Reforming of Volatiles from Cellulose Pyrolysis Using Supported Ionic Liquid Phase

This paper presents a novel method for continuous production of a biomass-derived platform chemical, levoglucosenone (LGO), from cellulose without its pretreatment or use of solvent. First, cellulose is pyrolyzed, and then the volatiles are reformed over a catalyst consisting of a type of ionic liquid supported over porous char. The ionic liquid, having a moderate hydrogen-bond basicity, performs well in the dehydrative conversion of levoglucosan (LGA) and anhydrosugar oligomers in the volatiles to LGO at 275 °C. The catalytic reforming to LGO is highly selective, and consequently, the yield of LGO is determined mainly by the pyrolysis conditions that produce the LGO precursors. The highest LGO yield we obtained was 31.6% on a cellulose carbon basis (24.6 wt %) with fast pyrolysis that produced more precursors than the slow one. Furthermore, the reaction system is applicable to the production of dihydrolevoglucosenone (DLGO), a promising biobased alternative to dipolar aprotic solvents. Addition of hydrog...

Electromembrane extraction with alkylated phosphites and phosphates as supported liquid membranes

A range of alkylated phosphates and phosphites were for the first time investigated as potential supported liquid membranes (SLMs) for electromembrane extraction (EME) of basic drugs from human plasma samples. Six polar basic drugs were used as model analytes for initial testing of the different SLMs. From this initial testing, bis(2-ethylhexyl) hydrogen phosphite (DEHPi), tris(2-butoxyethyl) phosphate (TBoEP), and tributyl phosphate (TBP) were found to be successful SLMs. These solvents have very high hydrogen-bond basicity, high polarity-polarizability, and no hydrogen-bond acidity. Based on this observation, hydrogen-bonding interactions were hypothesized to play a dominant role in the partition of the charged polar model analytes into the SLM. The successful solvents also contained alkyl moieties equivalent to 12–18 carbon atoms. Interestingly, similar phosphates/phosphites with aromatic character were unsuccessful. Alkylated phosphates/phosphites with less carbon atoms resulted in high current flowing in the system, whereas similar structures with >18 carbon atoms provided very low recoveries and extremely low system-current. Based on this knowledge, an SLM of TBP mixed with 40% NPOE (2-nitrophenyl octyl ether) was proposed as a new and successful SLM for EME of both polar and non-polar basic drugs. An assay based on EME-HPLC/UV with a TBP/NPOE SLM was evaluated from two-fold diluted human plasma under physiological pH conditions for both polar and non-polar basic drugs. The evaluation data were found to be acceptable for all analytes.

Partition Coefficients of Organics between Water and Carbon Dioxide Revisited: Correlation with Solute Molecular Descriptors and Solvent Cohesive Properties.

High-pressure phase behavior of systems containing water, carbon dioxide and organics has been important in several environment- and energy-related fields including carbon capture and storage, CO2 sequestration and CO2-assisted enhanced oil recovery. Here, partition coefficients (K-factors) of organic solutes between water and supercritical carbon dioxide have been correlated with extended linear solvation energy relationships (LSERs). In addition to the Abraham molecular descriptors of the solutes, the explanatory variables also include the logarithm of solute vapor pressure, the solubility parameters of carbon dioxide and water, and the internal pressure of water. This is the first attempt to include also the properties of water as explanatory variables in LSER correlations of K-factor data in CO2-water-organic systems. Increasing values of the solute hydrogen bond acidity, the solute hydrogen bond basicity, the solute dipolarity/polarizability, the internal pressure of water and the solubility parameter of water all tend to reduce the K-factor, that is, to favor the solute partitioning to the water-rich phase. On the contrary, increasing values of the solute characteristic volume, the solute vapor pressure and the solubility parameter of CO2 tend to raise the K-factor, that is, to favor the solute partitioning to the CO2-rich phase.

Modulating hydrogen-bond basicity within the context of protein-ligand binding: A case study with thrombin inhibitors that reveals a dominating role for desolvation

Understanding subtle aspects of hydrogen bonding is a challenging but crucial task to improve our ability to design ligands with high affinity for protein hosts. To gain a deeper understanding of these aspects, we investigated a series of thrombin inhibitors in which the basicity of the ligand's group that accepts an H-bond from Gly216 was modulated via bioisosterism; e.g., a C=O acceptor was made electron deficient or rich via bioisosteric replacements of the adjacent moiety. Although the ligand's binding affinity was anticipated to improve when the H-bond basicity is increased (due to stronger H-bonding with the protein), we herein present data that unexpectedly revealed an opposite trend. This trend was attributed to a dominating role played by desolvation in determining the relative binding affinity. For example, a decrease in the H-bond basicity reduces the desolvation penalty and, as experimentally observed, improves the binding affinity, given that the reduction in the desolvation penalty dominates the change in the net contribution of the ligand's interactions with the protein. The current study, therefore, reveals that desolvation can be a major underlying cause for the apparently counterintuitive structure-activity relationship (SAR) outcomes, and indicates that understanding this factor can improve our ability to predict binding affinity and to design more potent ligands.

Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore-Imidazole-Phenol Complex: Stepwise Oxidation and Concerted Reduction.

Proton-coupled electron transfer (PCET) reactions were studied in acetonitrile for a Photosystem II (PSII)-inspired [Ru(bpy)2(phen-imidazole-Ph(OH)((t)Bu)2)](2+), in which Ru(III) generated by a flash-quench sequence oxidizes the appended phenol and the proton is transferred to the hydrogen-bonded imidazole base. In contrast to related systems, the donor and acceptor are strongly coupled, as indicated by the shift in the Ru(III/II) couple upon phenol oxidation, and intramolecular oxidation of the phenol by Ru(III) is energetically favorable by both stepwise and concerted pathways. The phenol oxidation occurs via a stepwise ET-PT mechanism with kET = 2.7 × 10(7) s(-1) and a kinetic isotope effect (KIE) of 0.99 ± 0.03. The electron transfer reaction was characterized as adiabatic with λDA = 1.16 eV and 280 < HDA < 540 cm(-1) consistent with strong electronic coupling and slow solvent dynamics. Reduction of the phenoxyl radical by the quencher radical was examined as the analogue of the redox reaction between the PSII tyrosyl radical and the oxygen-evolving complex. In our PSII-inspired complex, the recombination reaction activation energy is <2 kcal mol(-1). The reaction is nonadiabatic (VPCET ≈ 22 cm(-1) (H) and 49 cm(-1) (D)) and concerted, and it exhibits an unexpected inverse KIE = 0.55 that is attributed to greater overlap of the reactant vibronic ground state with the OD vibronic states of the proton acceptor due to the smaller quantum spacing of the deuterium vibrational levels.