Inverse Kinetic Isotope Effect Research Articles

Compound-specific, intra-molecular, and clumped 13C fractionations in the thermal generation and decomposition of ethane and propane: A DFT and kinetic investigation

The distribution of stable isotopes in organic compounds within geochemical systems reflects the reactions they undergo, which are often kinetically governed and can be elucidated through quantification based on first principles: employing quantum chemical simulations to derive energy barriers and kinetic isotope effects (KIEs), followed by kinetic calculations using these parameters. In this paper, the distribution of 13C in hydrocarbon gases during the thermal generation and decomposition of ethane and propane has been addressed. The work is based on abundant and systematic isotopic data, including the compound- and position-specific 13C compositions, recent improvements in kinetic networks in hydrocarbon generation, and state-of-the-art quantum chemical methods that can process organic geochemical systems involving solid phases. Density Functional Theory computations validate the carbonium path of hydrocarbon generation and the free-radical path of their decomposition under geological conditions. The first step of each path (alkyl sidechain protonation for generation and hydrogen abstraction for decomposition) is the rate-limiting step and determines the 13C fractionations of gaseous hydrocarbons, with the reaction rate constrained by the concentration or accessibility of the active species (hydrated protons and surface free radicals in the two paths, respectively). Alkyl protonation has normal 13C KIEs, but hydrogen abstraction on the surfaces of solid organic matter has inverse ones (substituting 12C by 13C accelerates the reaction). These inverse KIEs explain three isotopic phenomena in gaseous hydrocarbons in their late thermal evolution: 1) the depletion of 13C in ethane and propane accompanied by their decreased fraction in hydrocarbon gases (isotope “rollover”); 2) the reversed 13C sequence with carbon number (δ13Cmethane > δ13Cethane > δ13Cpropane); and 3) the depletion of 13C in the center carbon atoms of residual propane. Isotopic fractionation between gaseous hydrocarbons upon generation is evaluated using the Successive Random Chain-Scission model. The covariation of δ13C in alkanes shows a depletion in CH4 compared with the values expected from the reciprocal of the carbon numbers, indicating that this deviation may be intrinsic instead of mixing with microbial methane. The comparison of intramolecular 13C fractionation of propane between computational and observed data suggests the contribution of isoprenoids at the beginning of the generation, in addition to isotopic homogenization between the center and end carbon atoms through the ring closure of intermediate cations. The 13C–13C clumping exhibits a weak KIE in both the generation and decomposition of ethane; the clumping is essentially stochastically governed. This work demonstrates how first-principle calculations can provide significant insights into the details of geochemical reactions governing the isotopic distributions.

Inverse kinetic isotope effect in singlet-to-triplet intersystem crossing contributes to greater robustness of deuterated organocatalyst

Abstract Precisely defined deuterium labeling of simplified Maruoka Catalyst® markedly increases its robustness under basic conditions, but the reason for this has remained unclear. Based on multicomponent density functional theory (MC_DFT) calculations, we find that an inverse kinetic isotope effect in singlet-to-triplet intersystem crossing contributes to the greater robustness of the deuterated catalyst.

Remotely Bonded Bridging Dioxygen Ligands Enhance Hydrogen Transfer in a Silica-Supported Tetrairidium Cluster Catalyst.

A longstanding challenge in catalysis by noble metals has been to understand the origin of enhancements of rates of hydrogen transfer that result from the bonding of oxygen near metal sites. We investigated structurally well-defined catalysts consisting of supported tetrairidium carbonyl clusters with single-atom (apical iridium) catalytic sites for ethylene hydrogenation. Reaction of the clusters with ethylene and H2 followed by O2 led to the onset of catalytic activity as a terminal CO ligand at each apical Ir atom was removed and bridging dioxygen ligands replaced CO ligands at neighboring (basal-plane) sites. The presence of the dioxygen ligands caused a 6-fold increase in the catalytic reaction rate, which is explained by the electron-withdrawing capability induced by the bridging dioxygen ligands, consistent with the inference that reductive elimination is rate-determining. Electronic-structure calculations demonstrate an additional role of the dioxygen ligands, changing the mechanism of hydrogen transfer from one involving equatorial hydride ligands to that involving bridging hydride ligands. This mechanism is made evident by an inverse kinetic isotope effect observed in ethylene hydrogenation reactions with H2 and, alternatively, with D2 on the cluster incorporating the dioxygen ligands and is a consequence of quasi-equilibrated hydrogen transfer in this catalyst. The same mechanism accounts for rate enhancements induced by the bridging dioxygen ligands for the catalytic reaction of H2 with D2 to give HD. We posit that the mechanism involving bridging hydride ligands facilitated by oxygen ligands remote from the catalytic site may have some generality in catalysis by oxide-supported noble metals.

Inverse kinetic isotope effect of proton and deuteron permeation through pyridinic N-doped graphene

Graphene is becoming an increasingly popular platform for developing high-efficiency hydrogen isotope separation technologies operated at room temperature. However, its performance can be strongly affected by contributions from atomic defects. Understanding the influence of defects on the hydrogen isotope selectivity of graphene is thus imperative but remains elusive. In this work, we investigated the electrically driven proton/deuteron permeation through nitrogen-doped graphene membranes with varying N concentrations and configurations. The results show that N-doping significantly enhances the proton and deuteron permeability of graphene (by one order of magnitude). More importantly, deuterons are found to permeate faster through pyridinic N-containing defects than protons, demonstrating a pronounced inverse kinetic isotope effect (KIE) with kH/kD of 0.75. Conversely, pyrrolic N-containing defects exhibit a normal KIE (kH/kD = 1.56). The exclusive contribution of the pyridinic N to the inverse KIE is confirmed by the reversed KIE factor of the pyridinic N-dominated sample subjected to modification. Theoretical calculations indicate that the explicit dependence of KIE on N configuration originates from the different leading mechanisms of proton/deuteron permeation through these N-containing defects, which is governed by the size and the chemisorption imparted by defects. These findings provide fundamental insights into the correlation between defect characteristics and proton/deuteron transport in graphene, and contribute to developing economical hydrogen isotope separation technologies through the defect engineering of graphene.

Effects of pH-dependent speciation on the photolytic degradation mechanism of phosphonates

Phosphonates, commonly used as strong metal chelating agents in household and industrial products, partially persist through water treatment processes, releasing significant amounts into the environment and raise concerns about potential environmental impacts. This study investigated effects of pH-dependent speciation of phosphonates on their photolysis by a medium-pressure mercury lamp in aqueous solution under ambient air conditions. Different species of aminotrimethylphosphonic acid (ATMP), and its most important transformation products iminodimethylphosphonic acid(IDMP) and aminomethylphosphonic acid(AMPA) were analyzed by liquid chromatography in combination with isotope ratio (CSIA) and high resolution mass spectrometry. UV radiation (<320 nm) was required for the photolytic degradation of ATMP, IDMP, and AMPA, and their photolysis was influenced by pH-dependent speciation. Degradation rate constant of ATMP was affected by pH, but a direct correlation between kobs and pH was not found. In contrast, IDMP and AMPA exhibited a direct correlation between pH and kobs during photolysis. CSIA revealed an inverse carbon kinetic isotope effect (KIE) for ATMP, while a normal carbon KIE was observed for IDMP and AMPA. Quantum chemical calculations supported degradation pathway interpretation and KIE understanding. Energy barrier calculations distinguished N-C vs C-P bond cleavage, favoring N-C cleavage on average, consistent with observed transformation products. Further calculations, considering possible transition states, provide first promising hints for elucidation of present KIEs.

High-dispersed CeOx species on mesopores silica to accelerate Ni-catalysed CO2 methanation at low temperatures

CO2 methanation is a key step in the power-to-gas technology for storing and distributing renewable energy as energy vectors. Due to thermodynamic constraints, highly active catalysts below about 300 °C are necessary. We demonstrate that their preparation is possible by supporting Ni nanoparticles over highly dispersed CeOx species decorating mesopores silicate (mSiO2) obtained by a template-free method. The CeOx decorated mSiO2 exhibits a 3-fold increase in activity and enhanced CH4 selectivity (98.7 % at 300 °C) compared to the Ni/mSiO2 catalyst. The performances are superior compared to the literature data. A systematic characterisation by physisorption, XPS, XAFS, UV–Vis and HR-TEM shows a uniform dispersion of highly dispersed CeOx within the mesoporous silica framework. The CO2 hydrogenation properties were investigated in a fixed-bed flow reactor and complemented by CO2-TPD, CO2-TPSR, in-situ FTIR, and D2 kinetic isotope experiments. The results show that highly dispersed CeOx exhibits excellent CO2 adsorption and dissociation abilities. The inverse kinetic isotope effect indicates that CO2 activation is the rate-determining step of the reaction. CO2 is strongly adsorbed on CeOx to form bidentate carbonate, which accelerates the generation of CO while avoiding the adsorption of CO2 on Ni nanoparticles.

Acid‐Promoted Electron Transfer with Inverse Kinetic Isotope Effect for Disproportionation of Nitroxyl Radicals in CH3CN

AbstractThough electron transfer (ET) and proton transfer (PT) are extensively involved in organic reactions, they remain unclear in acid‐enabled disproportionation of nitroxyl radicals, especially in organic solvents. Here we unveil the acid‐promoted electron transfer (APET) mechanism with inverse kinetic isotope effect (KIE) for the disproportionation of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) in CH3CN. Kinetic experiments relying on on‐line detection of electron paramagnetic resonance (EPR) was conducted together with theoretical calculations, giving consistent insights on the mechanisms. Moreover, the significant effect of water as cosolvent was revealed. The findings here will encourage further mechanistic understandings and method developments for nitroxyl‐participated organic reactions.

Engineering C-C Bond Cleavage Activity into a P450 Monooxygenase Enzyme.

The cytochrome P450 (CYP) superfamily of heme monooxygenases has demonstrated ability to facilitate hydroxylation, desaturation, sulfoxidation, epoxidation, heteroatom dealkylation, and carbon-carbon bond formation and cleavage (lyase) reactions. Seeking to study the carbon-carbon cleavage reaction of α-hydroxy ketones in mechanistic detail using a microbial P450, we synthesized α-hydroxy ketone probes based on the physiological substrate for a well-characterized benzoic acid metabolizing P450, CYP199A4. After observing low activity with wild-type CYP199A4, subsequent assays with an F182L mutant demonstrated enzyme-dependent C-C bond cleavage toward one of the α-hydroxy ketones. This C-C cleavage reaction was subject to an inverse kinetic solvent isotope effect analogous to that observed in the lyase activity of the human P450 CYP17A1, suggesting the involvement of a species earlier than Compound I in the catalytic cycle. Co-crystallization of F182L-CYP199A4 with this α-hydroxy ketone showed that the substrate bound in the active site with a preference for the (S)-enantiomer in a position which could mimic the topology of the lyase reaction in CYP17A1. Molecular dynamics simulations with an oxy-ferrous model of CYP199A4 revealed a displacement of the substrate to allow for oxygen binding and the formation of the lyase transition state proposed for CYP17A1. This demonstration that a correctly positioned α-hydroxy ketone substrate can realize lyase activity with an unusual inverse solvent isotope effect in an engineered microbial system opens the door for further detailed biophysical and structural characterization of CYP catalytic intermediates.

Deuterium Solvent Kinetic Isotope Effect on Enzymatic Methyl Transfer Catalyzed by Catechol O-methyltransferase.

Catechol o-methyltransferase plays a key role in the metabolism of catecholamine neurotransmitters. At present, its catalytic mechanism, overall structure, and kinetic characteristics have been basically clarified, but few people have paid attention to the function of solvents on enzymatic methyl transfer reactions. The influence of solvents on enzymatic reactions has always been a fuzzy hot topic. In addition, as a well-studied typical methyltransferase, COMT is a good test bed for exploring the source of the solvent isotope effect, which is a powerful tool in enzymatic mechanism research. We have measured the kinetic parameters of methyl transfer catalyzed by COMT in both normal water (H2O) and heavy water (D2O) by high-performance liquid chromatography (HPLC) in the range of pL 6 ~ 11. The kinetic characteristics of COMT in H2O and D2O were significantly different under different pH/pD conditions. Significant solvent kinetic isotope effects (SKIE) were obtained, especially inverse solvent kinetic isotope effects (SKIE < 1) were observed in this methyl transfer reaction for the first time. Traditional factors which could interpret the solvent isotope effect were ruled out. It's suggested that the solvent might affect the overall conformation as well as the flexibility of protein through non-covalent forces, thus altering the catalytic activity of COMT and leading to the solvent isotope effect.

Solvent isotope effects in the catalytic cycle of P450 CYP17A1: Computational modeling of the hydroxylation and lyase reactions

The catalytic cycle of the cytochromes P450 (CYP) requires two electrons from a protein redox partner and two protons from water to generate the main catalytic intermediate, a ferryl-oxo complex with π-cation on the heme porphyrin ring, termed Compound 1. The protonation steps are at least partially rate-limiting, therefore the steady-state rates of P450 catalysis are usually slower in deuterated solvent (D2O) by a factor of 1.5–3. However, in several P450 systems a pronounced inverse kinetic solvent isotope effect (KSIE ∼0.4–0.7) is observed, where the reaction is faster in D2O. This raises an important mechanistic question: Is this inverse solvent isotope effect compatible with Compound 1 catalyzed reactions, or is it indicative of another catalytic intermediate being involved? In this communication we use exhaustive numerical modeling of the P450 steady-state kinetics to demonstrate that a significant inverse KSIE cannot be obtained for a pure Compound 1 driven catalytic cycle of P450. Rather, an alternative, protonation independent, catalytic intermediate needs to be introduced. This result is applicable to the broad spectrum of P450s in nature, but as an example we use the extensively documented inverse isotope effect in the human steroid biosynthetic P450 CYP17A1 where the involvement of a heme peroxo anion intermediate has been characterized. Based on this analysis, we show that the observation of an inverse KSIE can be used as a general mechanistic probe for reaction cycle intermediates in the cytochromes P450.

Spectating the proton migration on catalyst with noninnocent ligand in aqueous electrochemical CO2 reduction

It is of great significance to investigate aqueous CO2 reduction on catalysts with redox-noninnocent ligands as they effectively facilitate CO2 reduction but circumvent the H2 evolution. By developing the methodology of rapid-scan FT-IR in external reflection mode, the pathway of CO2 reduction on iron porphyrin was in-situ monitored. It’s uncovered that coupled with first-electron reduction of [FeIITAPP]0, the porphyrin nitrogen is protonated as the observation of NH+ ammonium band in IR spectra, which avoids the formation of Fe–H structure for proton reduction. Moreover, an inverse kinetic isotope effect (KIE = 0.59) in formation of *COOH indicates the protonated porphyrin-N would also act as a proton relay for the efficient inner-sphere proton transfer to the activated CO2 molecules. This work experimentally identified the proton migration pathway via the reduced porphyrin ligand as proton shuttle for the first time and should benefit the design of the catalyst of high CO2 reduction selectivity.

Characterization of paramagnetic states in an organometallic nickel hydrogen evolution electrocatalyst

Significant progress has been made in the bioinorganic modeling of the paramagnetic states believed to be involved in the hydrogen redox chemistry catalyzed by [NiFe] hydrogenase. However, the characterization and isolation of intermediates involved in mononuclear Ni electrocatalysts which are reported to operate through a NiI/III cycle have largely remained elusive. Herein, we report a NiII complex (NCHS2)Ni(OTf)2, where NCHS2 is 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane, that is an efficient electrocatalyst for the hydrogen evolution reaction (HER) with turnover frequencies of ~3,000 s−1 and a overpotential of 670 mV in the presence of trifluoroacetic acid. This electrocatalyst follows a hitherto unobserved HER mechanism involving C-H activation, which manifests as an inverse kinetic isotope effect for the overall hydrogen evolution reaction, and NiI/NiIII intermediates, which have been characterized by EPR spectroscopy. We further validate the possibility of the involvement of NiIII intermediates by the independent synthesis and characterization of organometallic NiIII complexes.

Contrasting Mechanisms of Aromatic and Aryl-Methyl Substituent Hydroxylation by the Rieske Monooxygenase Salicylate 5-Hydroxylase.

The hydroxylase component (S5HH) of salicylate-5-hydroxylase catalyzes C5 ring hydroxylation of salicylate but switches to methyl hydroxylation when a C5 methyl substituent is present. The use of 18O2 reveals that both aromatic and aryl-methyl hydroxylations result from monooxygenase chemistry. The functional unit of S5HH comprises a nonheme Fe(II) site located 12 Å across a subunit boundary from a one-electron reduced Rieske-type iron-sulfur cluster. Past studies determined that substrates bind near the Fe(II), followed by O2 binding to the iron to initiate catalysis. Stopped-flow-single-turnover reactions (STOs) demonstrated that the Rieske cluster transfers an electron to the iron site during catalysis. It is shown here that fluorine ring substituents decrease the rate constant for Rieske electron transfer, implying a prior reaction of an Fe(III)-superoxo intermediate with a substrate. We propose that the iron becomes fully oxidized in the resulting Fe(III)-peroxo-substrate-radical intermediate, allowing Rieske electron transfer to occur. STO using 5-CD3-salicylate-d8 occurs with an inverse kinetic isotope effect (KIE). In contrast, STO of a 1:1 mixture of unlabeled and 5-CD3-salicylate-d8 yields a normal product isotope effect. It is proposed that aromatic and aryl-methyl hydroxylation reactions both begin with the Fe(III)-superoxo reaction with a ring carbon, yielding the inverse KIE due to sp2 → sp3 carbon hybridization. After Rieske electron transfer, the resulting Fe(III)-peroxo-salicylate intermediate can continue to aromatic hydroxylation, whereas the equivalent aryl-methyl intermediate formation must be reversible to allow the substrate exchange necessary to yield a normal product isotope effect. The resulting Fe(III)-(hydro)peroxo intermediate may be reactive or evolve through a high-valent iron intermediate to complete the aryl-methyl hydroxylation.

Inverse kinetic isotope effects in the oxygen reduction reaction at platinum single crystals.

Although the oxygen reduction reaction (ORR) involves multiple proton-coupled electron transfer processes, early studies reported the absence of kinetic isotope effects (KIEs) on polycrystalline platinum, probably due to the use of unpurified D2O. Here we developed a methodology to prepare ultra-pure D2O, which is indispensable for reliably investigating extremely surface-sensitive platinum single crystals. We find that Pt(111) exhibits much higher ORR activity in D2O than in H2O, with potential-dependent inverse KIEs of ~0.5, whereas Pt(100) and Pt(110) exhibit potential-independent inverse KIEs of ~0.8. Such inverse KIEs are closely correlated to the lower *OD coverage and weakened *OD binding strength relative to *OH, which, based on theoretical calculations, are attributed to the differences in their zero-point energies. This study suggests that the competing adsorption between *OH/*OD and *O2 probably plays an important role in the ORR rate-determining steps that involve a chemical step preceding an electrochemical step (CE mechanism).

What It Takes for Imidazolium Cations to Promote Electrochemical Reduction of CO2.

Imidazolium cations enhance the performance of several electrodes in converting CO2 to CO in non-aqueous media. In this publication, we elucidate the origin of the function of imidazolium cations when exposed to Au electrodes in anhydrous acetonitrile in CO2 atmosphere. We demonstrate that imidazolium cations lead to unprecedentedly low overpotentials for CO2 reduction to CO on Au, with ∼100% Faradaic efficiency. By modification of the N1 and N3 functionality of the imidazolium cation, we show a direct correlation between the performance in CO2 reduction and the C2–H acidity of the cation. Based on NMR analyses, DFT calculations, and isotopic labeling, showing an inverse kinetic isotope effect, we demonstrate that the mechanism involves a concerted proton–electron transfer to the electrode-adsorbed CO2 intermediate. The demonstrated mechanism provides guidelines for improvement in the energy efficiency of non-aqueous electrochemical CO2 reduction, by a tailored design of electrolyte cations.

Sulfate ionic liquids promoted FeCl3-catalyzed dehydration of propargyl alcohols

Conjugated enynes are an important class of compounds that could be widely used in medicinal chemistry and organic synthesis. Herein, various sulfate ionic liquids with FeCl3 were used to catalyze the dehydration of propargyl alcohols to prepare conjugated enynes. Sulfate ionic liquids with different substituent chain lengths or cations exhibited high catalytic activity, affording the highest conjugated enyne yield of 96 % with selectivity of 97 %. At the same time, the by-product naphthofuran derivative was isolated and verified by the single crystal X-ray diffraction for the first time. Kinetic studies found that the addition of sulfate ionic liquids increased the rate of formation of desired product conjugated enyne and inhibited the production of by-product naphthofuran derivative. Deuterated kinetic isotope studies were carried out by using the deuterated substrate PhCC(CD3)2OH and PhCC(CH3)2OD. A large value of H/D kinetic isotope effects (KIE1 = 5.5) suggested that the βC—H bond cleavage was involved in the rate-determining step and the formation of desired conjugated enyne followed the E2 elimination mechanism. An inverse kinetic isotope effects (KIE3 = 0.66) suggested that the formation of by-product naphthofuran derivative followed the SN2 mechanism. Density functional theory (DFT) calculations were conducted to reveal the activation of sulfate anion on the methyl group of propargyl alcohol. Based on the crystal structure of by-product, kinetic studies, deuterated kinetic isotope studies and DFT calculations, a cooperative mechanism of dehydration of propargyl alcohol over sulfate ionic liquids and FeCl3 was proposed.

Heteroatom modified polymer immobilized ionic liquid stabilized ruthenium nanoparticles: Efficient catalysts for the hydrolytic evolution of hydrogen from sodium borohydride

Ruthenium nanoparticles stabilised by polymer immobilized ionic liquids catalyse the hydrolytic release of hydrogen from sodium borohydride. The composition of the polymer influences performance and ruthenium nanoparticles stabilised by an amine-decorated imidazolium-based polymer immobilised ionic liquid (RuNP@NH2-PIILS) was the most efficient with a maximum initial turnover frequency (TOF) of 177 moleH2.molRu−1.min−1, obtained at 30°C with a catalyst loading of 0.08 mol%; markedly higher than that of 69 molH2.molRu−1.min−1 obtained with 5 wt% Ru/C and one of the highest to be reported for a RuNP catalyst. The apparent activation energy (Ea) of 38.9 kJ mol−1 for the hydrolysis of NaBH4 catalysed by RuNP@NH2-PIILS is lower than that for the other polymer immobilized ionic liquid stabilised RuNPs, which is consistent with its efficacy. Comparison of the initial rates of hydrolysis in H2O and D2O catalysed by RuNP@NH2-PIILS gave a primary kinetic isotope effect (kH/kD) of 2.3 which supports a mechanism involving rate limiting oxidative addition of one of the O-H bonds in a strongly hydrogen-bonded surface-coordinated [BH3H−]—-H2O ensemble. The involvement of a surface-coordinated borohydride is further supported by an inverse kinetic isotope effect of 0.65 obtained from a comparison of the initial rates for the hydrolysis of NaBH4 and NaBD4 under the conditions of catalysis i.e., at a high hydride/catalyst mole ratio. Interestingly though, when the comparison of the initial rates of hydrolysis of NaBH4 and NaBD4 was conducted in dilute solution with a hydride/catalyst mole ratio of 1 a kinetic isotope effect (kH/kD) of 2.72 was obtained; this would be more consistent with concerted activation of both an O-H and B-H bond in the rate limiting step, possibly via a concerted oxidative addition-hydride transfer in the surface-coordinated hydrogen-bonded ensemble. Catalyst stability and reuse studies showed that RuNP@NH2-PIILS retained 71% of its activity over five runs; the gradual drop in the initial TOF with run number appears to be due to passivation of the catalyst by the sodium borate by-product as well as an increase in viscosity of the reaction mixture rather than leaching of the catalyst.

Oxidation of Hypophosphorous Acid by a Ruthenium(VI) Nitrido Complex in Aqueous Acidic Solution. Evidence for a Proton-Coupled N-Atom Transfer Mechanism.

The oxidation of hypophosphorous acid (H3PO2) by a ruthenium(VI) nitrido complex, [(L)RuVI(N)(OH2)]+ (RuVIN; L = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion), has been studied in aqueous acidic solutions at pH 0-2.50. The reaction has the following stoichiometry: 2[(L)RuVI(N)(OH2)]+ + 3H3PO2 + H2O → 2[(L)RuIII(NH2P(OH)2)(OH2)]+ + H3PO3. The pseudo-first-order rate constant, kobs, depends linearly on [H3PO2], and the second-order rate constant k2 depends on [H+] according to the relationship k2 = k[H+]/([H+] + Ka), where k is the rate constant for the oxidation of H3PO2 molecule and Ka is the dissociation constant of H3PO2. At 298.0 K and I = 1.0 M, k = (2.04 ± 0.19) × 10-2 M-1 s-1 and Ka = (6.38 ± 0.63) × 10-2 M. A kinetic isotope effect (KIE) of 2.9 ± 0.1 was obtained when kinetic studies were carried out with D3PO2 at pH 1.16, suggesting P-H bond cleavage in the rate-determining step. On the other hand, when the kinetics were determined in D2O, an inverse KIE of 0.21 ± 0.03 (H3PO2 in H2O vs H3PO2 in D2O) was found. On the basis of experimental results and DFT calculations, the proposed mechanism involves an acid-catalyzed tautomerization of H2P(O)(OH) to HP(OH)2; the latter molecule is the reacting species which reacts with RuVIN via a proton-coupled N-atom transfer pathway.

Enhancement in the metal efficiency of Ru/TiO2 catalyst for guaiacol hydrogenation via hydrogen spillover in the liquid phase

Decrease in the metal dispersion of noble-metal-loaded catalysts with high metal loading is a major factor reducing noble-metal efficiency. Herein, we investigated the enhancement in metal efficiency of Ru/TiO2 catalyst by utilizing hydrogen spillover in the liquid-phase. Ru was highly dispersed at low loadings (0.1 and 0.5 wt%), while larger nanoparticles were formed at higher loadings (1–5 wt%). The hydrogen spillover in liquid phase was activated at reaction temperature (100 °C) as the Ru dispersion decreased, which was confirmed through physical dilution experiments, hydrogen temperature-programmed-reduction, and kinetic analysis. Isotope experiment was conducted using D2O, observing inverse kinetic isotope effect (IKIE) for the high-Ru-loading catalysts. Based on the understanding of the hydrogen spillover in the liquid phase, the low metal efficiency of high-Ru-loading catalysts resulting from low dispersion could be compensated simply by physically mixing pristine TiO2, which played a role as new active sites when liquid-phase hydrogen spillover was activated.

Density functional theory calculations of nitrogen and oxygen equilibrium isotope fractionations in NO3−–NO2−–H2O aqueous system reveal inverse kinetic isotope effects during nitrite oxidation

Nitrate and nitrite play key roles in the nitrogen cycle on Earth's surface. The isotope fractionation during nitrate reduction on enzymatic level involves multiple steps, including transfer of free NO3− to the activate site on nitrate reductase and NO2− equilibration with ambient water. However, the isotope fractionation factors of 15N and 18O among NO3−, NO2−, and H2O molecules in aqueous phases are poorly constrained. It strongly impedes the understanding of the involved processes and using stable isotopes to quantitatively examine the biogeochemical nitrogen cycle. In this contribution, we employ the density functional theory method with the Urey-Bigeleisen-Goeppert-Mayer model to predict the nitrogen and oxygen equilibrium isotope fractionation factors of NO3−, NO2−, and H2O molecules in gaseous and aqueous phases. Our calculation results show that the solvent effect has a large influence on equilibrium isotope fractionation for oxygen in water (+10.1‰ between liquid and vapor water at 25 °C), which is different to the little solvent effects on both oxygen and nitrogen in nitrate and nitrite. The calculated temperature-dependent equilibrium isotope fractionations between nitrogen of NO3− and NO2−, and oxygen of H2O and NO2− are consistent with previous laboratory experiments. Our results confirm that the oxygen equilibrium isotope fractionations between NO3− and NO2− should be +9.4‰ at 25 °C. Integrating the new results and previously reported kinetic isotope effects of nitrate reduction, we demonstrate inverse kinetic isotope effects for both nitrogen (+15.4‰) and oxygen (+5.2‰) during nitrite oxidation, which falls in the range of previous experiments. The new results enable us to use both nitrogen and oxygen isotopes as a bonded isotope tool to quantitatively assess the nitrogen cycle in low-temperature environments.