Kinetic Isotope Effect Research Articles

Inverse Isotope Kinetic Effect of the Charge Transfer Reactions of Ar+ with H2O and D2O.

Hydrogen isotopic effect, as the key to revealing the origin of Earth's water, arises from the H/D mass difference and quantum dynamics at the transition state of reaction. The ion-molecule charge-exchange reaction between water (H2O/D2O) and argon ion (Ar+) proceeds spontaneously and promptly, where there is no transition-state or intermediate complex. In this energetically resonant process, we find an inverse kinetic isotope effect (KIE) leading to the higher charge transfer rate for D2O, by the velocity map imaging measurements of H2O+/D2O+ products. Using the average dipole orientation capture model, we estimate the orientation angles of C2v axis of H2O/D2O relative to the Ar+ approaching direction and attribute to the difference of stereodynamics. According to the long-distance Landau-Zener charge transfer model, this inverse KIE could be also attributed to the density-of-state difference of molecular bending motion between H2O+ and D2O+ around the resonant charge transfer.

Dual clumped isotopes (Δ47 and Δ48) reveal non-equilibrium formation of freshwater cements

Low-Mg calcite, precipitating from meteoric fluids, is a common mineral that forms in a variety of near-surface diagenetic environments. However, recent studies, based on a combination of analyses of δ18O and Δ47 values, have suggested that this mineral might form in disequilibrium and consequently yield kinetic bias in Δ47-derived temperatures and fluid δ18O values. Here, we use dual clumped isotope proxies (Δ47 and Δ48) to investigate the influence of kinetic isotope effects within different meteoric diagenetic zones of Holocene and Pleistocene carbonates from the southern Florida and the Dominican Republic. In the Miami Oolite, the primary aragonite ooids and secondary low-Mg calcite cements in the bulk sample were separated from each other and their isotopic compositions (δ13C, δ18O, Δ47 and Δ48 values) were measured. The Δ47 and Δ48 values of the separated aragonite are consistent with the modern ooid sediments and in approximate equilibrium with the surface seawater. In contrast, the low-Mg calcite cement shows the higher Δ48 and lower Δ47 values, than expected, with the disequilibrium arising as a result of CO2 degassing in the vadose zone. Such deviations of Δ47 and Δ48 values are also observed in low-Mg calcite vadose cements in the Dominican Republic. While low-Mg calcites formed in the lower freshwater phreatic zone in the Dominican Republic have the Δ47- and Δ48-derived temperatures close to expected, the same mineral forming near the water-table and upper phreatic zone shows much higher Δ48-derived temperatures (up to ∼ 90 °C). The possible origin of such elevated temperatures can be attributed to non-equilibrium processes caused by changes in pH and pCO2, mediated by microbial sulfate reduction. Such differential kinetic behavior of Δ48 values between vadose and phreatic zones could be used as a proxy marker for the presence and the location of a water-table. This study demonstrates the great potential of dual clumped isotopes in the investigation of meteoric diagenesis and will help understand the alteration of ancient sequences and the interpretation of stable C isotope trends that they contain.

Kinetic isotope effects on hydrogen/deuterium disordering and ordering in ice crystals: A Raman and dielectric study of ice VI, XV, and XIX.

Ice XIX and ice XV are both partly hydrogen-ordered counterparts to disordered ice VI. The ice XIX → XV transition represents the only order-to-order transition in ice physics. Using Raman and dielectric spectroscopies, we investigate the ambient-pressure kinetics of the two individual steps in this transition in real time (of hours), that is, ice XIX → transient ice VI (the latter called VI‡) and ice VI‡ → ice XV. Hydrogen-disordered ice VI‡ appears intermittent between 101 and 120K, as inferred from the appearance and subsequent disappearance of the ice VI Raman marker bands. A comparison of the rate constants for the H2O ices reported here with those from D2O samples [Thoeny et al., J. Chem. Phys. 156, 154507 (2022)] reveals a large kinetic isotope effect for the ice XIX decay, but a much smaller one for the ice XV buildup. An enhancement of the classical overbarrier rate through quantum tunneling for the former can provide a possible explanation for this finding. The activation barriers for both transitions are in the 18-24kJ/mol range, which corresponds to the energy required to break a single hydrogen bond. These barriers do not show an H/D isotope effect and are the same, no matter whether they are derived from Raman scattering or from dielectric spectroscopy. These findings favor the notion that a dipolar reorientation, involving the breakage of a hydrogen bond, is the rate determining step at the order-to-order transition.

Kinetic isotopic effect for methanol oxidation on Pt(111), Pt(100), Pt(775) and Pt(755) electrodes

Methanol oxidation is studied on well-defined Pt single-crystal surfaces to obtain information about the rate-determining step of the mechanism. For that, a voltammetric analysis of the kinetic isotopic effect using CH3OH and CD3OH at two different concentrations (10–4 and 10–2 M) has been carried out. For both concentrations, a significant diminution of the currents is observed when CD3OH is used, implying that the break of the C—H bond is involved in the rate-determining step of the reaction. The quantification of the Kinetic Isotope Effect (KIE) using the voltammetric currents shows values between 2.5 and 3.5, depending on the potential and surface orientation. This complex behavior is a consequence of the existence of several paths in the oxidation mechanism, whose rates strongly depend on the interfacial conditions. The different values are discussed considering the mechanism. On the other hand, when signals leading to the formation of CO on the Pt(100) electrode are analyzed, a KIE factor of 4.7 is obtained. All these numbers imply that in the rate-determining step of the mechanism, a C—H bond is broken.

Toward mending the marine mass balance model for nickel: Experimentally determined isotope fractionation during Ni sorption to birnessite

In fewer than fifteen years, the study of Ni stable isotopes has advanced from early method development to application of a powerful tool for resolving a long-standing question: why does it appear that output fluxes of Ni from the global oceans far exceed input fluxes? The seawater concentration of Ni, a bioessential trace metal, is almost certainly at steady state on timescales comparable to its residence time of ∼20 kyr, so some of the current flux estimates must be inaccurate. Just as the input and output fluxes should balance, so should the flux-weighted isotopic compositions of the inputs and outputs. Thus, isotopic characterization of inputs and outputs provide an additional constraint on a balanced model of the marine Ni budget. Here, we report on experiments designed to elucidate fractionation mechanisms and magnitudes for sorption of Ni to Mn oxyhydroxide (birnessite), because Mn-rich sediments accumulating on abyssal plains represent the largest sink flux of Ni from seawater to marine sediments. Our results show remarkably large fractionations at low ionic strength (average Δ60/58Nidissolved-sorbed = +1.38 ‰). Neither closed-system equilibrium trends nor Rayleigh curves fit the data well. Fractionations are even larger at high ionic strength (Δ60/58Nidissolved-sorbed ranging from +2.0 to +4.0 ‰), and they decrease with experimental duration from 2 d (49 h) to 27 d. The high ionic strength data fit Rayleigh trends well. Here, we use X-ray absorption fine-structure spectroscopy (EXAFS) and results from previous studies to support interpretation of our data as combinations of kinetic and equilibrium isotope effects that vary in their proportional contributions to the total fractionation with time and with surface loading. One important consequence of this study is that none of the experimental results reported thus far, including ours, are directly applicable to building steady-state models of the Ni cycle. Even our longest duration experiments did not achieve equilibrium, which is likely to be manifest in the very slowly accumulating sediments on abyssal plains. Our work constrains further the mechanisms of Ni sorption to birnessite and clearly indicates that determination of equilibrium fractionation in this system, although challenging, will be a crucial step toward resolving the apparent marine Ni imbalance.

Kinetic nitrogen isotope effects of 18 amino acids degradation during burning processes

Understanding the nitrogen isotopic variations of individual amino acids (AAs) is essential for utilizing the nitrogen isotope values of individual amino acids (δ15N-AA) as source indicators to identify proteinaceous matter originating from biomass combustion processes. However, the nitrogen isotope effects (ε) associated with the degradation of individual amino acids during combustion processes have not been previously explored. In this study, we measured the nitrogen isotope values of residual free amino acids -following a series of controlled combustion experiments at temperatures of 160–240 °C and durations of 2 min to 8 h, as described in Part 1. δ15N values of proline, aspartate, alanine, valine, glycine, leucine, and isoleucine are more positive than their initial δ15N values after prolonged combustion. Variations in δ15N values of the most AAs conform to the Rayleigh fractionation during combustion and their nitrogen isotope effects (ε) are greatly impacted by their respective combustion degradation pathways. This is the first time the ε values associated with the degradation pathways of AAs during combustion have been characterized. Only the ε values associated with Pathway 1 (dehydration to form dipeptide) and 2 (simultaneous deamination and decarboxylation) are found to be significant and temperature-dependent, ranging from + 2.9 to 6.4‰ and + 0.9‰ to + 3.8‰, respectively. Conversely, ε values associated with other pathways are minor. This improves the current understanding on the degradation mechanisms of protein nitrogen during biomass burning.

Hydrogen Tunneling Exhibiting Unexpectedly Small Primary Kinetic Isotope Effects

Hydrogen Tunneling Exhibiting Unexpectedly Small Primary Kinetic Isotope Effects

Hexa-Fe(III) Carboxylate Complexes Facilitate Aerobic Hydrocarbon Oxidative Functionalization: Rh Catalyzed Oxidative Coupling of Benzene and Ethylene to Form Styrene.

Fe(II) carboxylates react with dioxygen and carboxylic acid to form Fe6(μ-OH)2(μ3-O)2(μ-X)12(HX)2 (X = acetate or pivalate), which is an active oxidant for Rh-catalyzed arene alkenylation. Heating (150-200 °C) the catalyst precursor [(η2-C2H4)2Rh(μ-OAc)]2 with ethylene, benzene, Fe(II) carboxylate, and dioxygen yields styrene >30-fold faster than the reaction with dioxygen in the absence of the Fe(II) carboxylate additive. It is also demonstrated that Fe6(μ-OH)2(μ3-O)2(μ-X)12(HX)2 is an active oxidant under anaerobic conditions, and the reduced material can be reoxidized to Fe6(μ-OH)2(μ3-O)2(μ-X)12(HX)2 by dioxygen. At optimized conditions, a turnover frequency of ∼0.2 s-1 is achieved. Unlike analogous reactions with Cu(II) carboxylate oxidants, which undergo stoichiometric Cu(II)-mediated production of phenyl esters (e.g., phenyl acetate) as side products at temperatures ≥150 °C, no phenyl ester side product is observed when Fe carboxylate additives are used. Kinetic isotope effect experiments using C6H6 and C6D6 give k H/k D = 3.5(3), while the use of protio or monodeutero pivalic acid reveals a small KIE with k H/k D = 1.19(2). First-order dependencies on Fe(II) carboxylate and dioxygen concentration are observed in addition to complicated kinetic dependencies on the concentration of carboxylic acid and ethylene, both of which inhibit the reaction rate at a high concentration. Mechanistic studies are consistent with irreversible benzene C-H activation, ethylene insertion into the formed Rh-Ph bond, β-hydride elimination, and reaction of Rh-H with Fe6(μ-OH)2(μ3-O)2(μ-X)12(HX)2 to regenerate a Rh-carboxylate complex.

Converting CO2 Into Natural Gas Within the Autoclave: A Kinetic Study on Hydrogenation of Carbonates in Aqueous Solution.

Catalytic conversion of carbon dioxide (CO2) into value-added chemicals is of pivotal importance, well the cost of capturing CO2 from dilute atmosphere is super challenge. One promising strategy is combining the adsorption and transformation at one step, such as applying alkali solution that could selectively reduce carbonate (CO3 2-) as consequences of CO2 adsorption. Due to complexity of this system, the mechanistic details on controlling the hydrogenation have not been investigated in depth. Herein, Ru/TiO2 catalyst was applied as a probe to elucidate the mechanism of CO3 2- activation, in which with thermodynamic and kinetic investigations, a compact Langmuir-Hinshelwood reaction model was established which suggests that the overall rate of CO3 2- hydrogenation was controlled by a specific C-O bond rupture elementary step within HCOO- and the Ru surface was mainly covered by CO3 2- or HCOO- at independent conditions. This assumption was further supported by negligible kinetic isotope effects (kH/kD≈1), similarity on reaction barriers of CO3 2- and HCOO- hydrogenation (ΔH≠ hydr,Na2CO3 and ΔH≠ hydr,HCOONa) and a non-variation of entropy (ΔS≠ hydr≈0). More interestingly, the alkalinity of the solution is certainly like a two sides in a sword and could facilitate the adsorption of CO2 while hold back catalysis during CO3 2- hydrogenation.

Reactions dynamics for X + H2 insertion reactions (X = C(1D), N(2D), O(1D), S(1D)) with Cayley propagator ring-polymer molecular dynamics.

In this work, rate coefficients of four prototypical insertion reactions, X + H2 → H + XH (X = C(1D), N(2D), O(1D), S(1D)), and associated isotope reactions are calculated based on ring polymer molecular dynamics (RPMD) with Cayley propagator (Cayley-RPMD). The associated kinetic isotope effects are systematically studied too. The Cayley propagator used in this work increases the stability of numerical integration in RPMD calculations and also supports a larger evolution time interval, allowing us to reach both high accuracy and efficiency. So, our results do not only provide chemical kinetic data for the title reactions in an extended temperature range but also consist of experimental results, standard RPMD, and other theoretical methods. The results in this work also reflect that Cayley-RPMD has strong consistency and high reliability in its investigations of chemical dynamics for insertion reactions.

Modeling the Initiation Phase of the Catalytic Cycle in the Glycyl-Radical Enzyme Benzylsuccinate Synthase.

The reaction of benzylsuccinate synthase, the radical-based addition of toluene to a fumarate cosubstrate, is initiated by hydrogen transfer from a conserved cysteine to the nearby glycyl radical in the active center of the enzyme. In this study, we analyze this step by comprehensive computer modeling, predicting (i) the influence of bound substrates or products, (ii) the energy profiles of forward- and backward hydrogen-transfer reactions, (iii) their kinetic constants and potential mechanisms, (iv) enantiospecificity differences, and (v) kinetic isotope effects. Moreover, we support several of the computational predictions experimentally, providing evidence for the predicted H/D-exchange reactions into the product and at the glycyl radical site. Our data indicate that the hydrogen transfer reactions between the active site glycyl and cysteine are principally reversible, but their rates differ strongly depending on their stereochemical orientation, transfer of protium or deuterium, and the presence or absence of substrates or products in the active site. This is particularly evident for the isotope exchange of the remaining protium atom of the glycyl radical to deuterium, which appears dependent on substrate or product binding, explaining why the exchange is observed in some, but not all, glycyl-radical enzymes.

Visible Light-promoted C(sp3)-H α-Carbamoylation of Cyclic Ethers with Isocyanides.

A protocol exploiting isocyanides as carbamoylating agents for the α-C(sp3)-H functionalization of cyclic ethers has been optimized via a combined visible light-driven hydrogen atom transfer/Lewis acid-catalyzed approach. The isocyanide substrate scope revealed an exquisite functional group compatibility (18 examples, with yields up to 99 %). Both radical and polar trapping, kinetic isotopic effect and real-time NMR studies support the mechanistic hypothesis and provide insightful details for the design of new chemical processes involving the generation of oxocarbenium ions.

Theoretical study of kinetic isotope effects for vacancy diffusion of impurity in solids

Theoretical study of kinetic isotope effects for vacancy diffusion of impurity in solids

Kinetic isotope effect reveals rate-limiting step in green-to-red photoconvertible fluorescent proteins.

Photoconvertible fluorescent proteins (pcFPs) undergo a slow photochemical transformation when irradiated with blue light. Since their emission is shifted from green to red, pcFPs serve as convenient fusion tags in several cutting-edge biological imaging technologies. Here, a pcFP termed the Least Evolved Ancestor (LEA) was used as a model system to determine the rate-limiting step of photoconversion. Perdeuterated histidine residues were introduced by isotopic enrichment and chromophore content was monitored by absorbance. pH-dependent photoconversion experiments were carried out by exposure to 405-nm light followed by dark equilibration. The loss of green chromophore correlated well with the rise of red, and maximum photoconversion rates were observed at pH 6.5 (0.059 ± 0.001 min-1 for red color acquisition). The loss of green and the rise of red provided deuterium kinetic isotope effects (DKIEs) that were identical within error, 2.9 ± 0.9 and 3.8 ± 0.6, respectively. These data indicate that there is one rate-determining step in the light reactions of photoconversion, and that CH bond cleavage occurs in the transition state of this step. We propose that these reactions are rate-limited on the min time scale by the abstraction of a proton at the His62 beta-carbon. A conformational intermediate such as a twisted or isomerized chromophore is proposed to slowly equilibrate in the dark to generate the red form. Additionally, His62 may shuttle protons to activate Glu211 to serve as a general base, while also facilitating beta-elimination. This idea is supported by a recent X-ray structure of methylated His62.

Built‐in Electric Field in Yolk Shell CuO‐Co3O4@Co3O4 with Modulated Interfacial Charge to Facilitate Hydrogen Production from Ammonia Borane Methanolysis Under Visible Light

AbstractDeveloping highly efficient and low‐cost catalysts is an endless challenge in the field of producing H2 from ammonia borane (AB). Herein, the manufacture of yolk‐shell CuO‐Co3O4@Co3O4 nanocomposites are reported by using Cu2O@CuO as a template, in which CuO‐Co3O4 is encapsulated into the Co3O4 hollow nanocubes. Due to the unique morphology and built‐in electric field (BIEF) induced by the CuO‐Co3O4 interface, the CuO‐Co3O4@Co3O4 nanocomposites display remarkable catalytic activity in AB methanolysis. The turnover frequency (TOF) is 24.8 min‐1 in the absence of light and significantly increases to 33.9 min‐1 when exposed to visible light. The experimental and theoretical calculations demonstrate that charge migration from CuO to Co3O4 results in the formation of dual active sites (Cu and Co sites) in charge of adsorption and activation of CH3OH and AB, respectively. Visible light‐induced acceleration is likely caused by type‐II heterojunction, which allows a large number of photogenerated electrons to accumulate in the CuO conduction band. This effectively activates the adsorbed CH3OH on the Cu site, rendering it easier to break the O−H bond. A plausible reaction mechanism involved the activation of the O−H bond of CH3OH, as the RDS is proposed according to the FT‐IR and kinetic isotope effect (KIE) experiments. This work offers an avenue to rationally design high‐performance yolk‐shell catalyst for rapid hydrogen production from AB methanolysis reaction under visible light.

Br, O‐Modified Cu(111) Interface Promotes CO2 Reduction to Multicarbon Products

AbstractElectrochemical reduction of CO2 to multicarbon (C2+) products with added value represents a promising strategy for achieving a carbon‐neutral economy. Precise manipulation of the catalytic interface is imperative to control the catalytic selectivity, particularly toward C2+ products. In this study, a unique Cu/UIO‐Br interface is designed, wherein the Cu(111) plane is co‐modified simultaneously by Br and O from UIO‐66‐Br support. Such Cu/UIO‐Br catalytic interface demonstrates a superior Faradaic efficiency of ≈53% for C2+ products (ethanol/ethylene) and the C2+ partial current density reached 24.3 mA cm−2 in an H‐cell electrolyzer. The kinetic isotopic effect test, in situ attenuated total reflection Fourier transform infrared spectroscopy and density functional theory calculations have been conducted to elucidate the catalytic mechanism. The Br, O co‐modification on the Cu(111) interface enhanced the adsorption of CO2 species. The hydrogen‐bond effect from the doped Br atom regulated the kinetic processes of *H species in CO2RR and promoted the formation of *COH intermediate. The formed *COH facilitates the *CO–*COH coupling and promotes the C2+ selectivity finally. This comprehensive investigation not only provides an in‐depth study and understanding of the catalytic process but also offers a promising strategy for designing efficient Cu‐based catalysts with exceptional C2+ products.

Proton-Coupled Electron Transfer and Hydrogen Tunneling in Olive Oil Phenol Reactions.

Olive oil phenols are recognized as molecules with numerous positive health effects, many of which rely on their antioxidative activity, i.e., the ability to transfer hydrogen to radicals. Proton-coupled electron transfer reactions and hydrogen tunneling are ubiquitous in biological systems. Reactions of olive oil phenols, hydroxytyrosol, tyrosol, oleuropein, oleacein, oleocanthal, homovanillyl alcohol, vanillin, and a few phenolic acids with a DPPH• (2,2-diphenyl-1-picrylhydrazyl) radical in a 1,4-dioxane:water = 95:5 or 99:1 v/v solvent mixture were studied through an experimental kinetic analysis and computational chemistry calculations. The highest rate constants corresponding to the highest antioxidative activity are obtained for the ortho-diphenols hydroxytyrosol, oleuropein, and oleacein. The experimentally determined kinetic isotope effects (KIEs) for hydroxytyrosol, homovanillyl alcohol, and caffeic acid reactions are 16.0, 15.4, and 16.7, respectively. Based on these KIEs, thermodynamic activation parameters, and an intrinsic bond orbital (IBO) analysis along the IRC path calculations, we propose a proton-coupled electron transfer mechanism. The average local ionization energy and electron donor Fukui function obtained for the phenolic compounds show that the most reactive electron-donating sites are associated with π electrons above and below the aromatic ring, in support of the IBO analysis and proposed PCET reaction mechanism. Large KIEs and isotopic values of Arrhenius pre-exponential factor AH/AD determined for the hydroxytyrosol, homovanillyl alcohol, and caffeic acid reactions of 0.6, 1.3, and 0.3, respectively, reveal the involvement of hydrogen tunneling in the process.

Geochemistry of oils and condensates from the lower Eagle Ford formation, south Texas. Part 6: Carbon isotopes

Compound specific carbon isotope analyses were performed on a well-characterized suite of unconventional oils and condensates produced from the lower Eagle Ford Formation. Thermal maturity is the major influence on the δ13C values of C3–C19 hydrocarbons. From ∼0.8 to ∼1.1 %Ro, the main stage of oil generation, the δ13C values of most measured hydrocarbons remain relatively constant: n-alkanes are ∼ −30 ‰: light aromatic hydrocarbons, cycloalkanes and isoprenoids are ∼ −29 to −28 ‰. Only the C4 to C10 methylalkanes exhibit high variance with carbon number with δ13C values ranging from ∼ −32 to −28 ‰, respectively. Hydrocarbons from samples ∼1.1 to 1.3 %Ro, the late oil generation stage, are 13C-enriched by ∼1–∼2 ‰. This is the range typically assigned to the influence of thermal maturity on oil from many conventional petroleum systems. The onset of significant oil cracking occurs at ∼1.3 %Ro and all measured hydrocarbons systematically become progressively 13C-enriched although the degree varies by compound class and carbon number. The δ13C values of individual hydrocarbons shift by as much as ∼ +9 ‰ for propane and butane to only ∼ +3 ‰ for methylcyclopentane from <1.1 to 1.7 %Ro. The 13C-enrichments observed in the Eagle Ford oils/condensates with increasing maturity are attributed to the kinetic isotope effect (KIE), whereby 12C–12C bonds cleave faster than 12C–13C bonds. No evidence for isotopic equilibrium was found. The consistency of the sums of the weighted δ13C values for C4–C7n-alkane/methylalkane isomers provides support for Mango's (2000a) theory that these hydrocarbons are generated from precursors with a common chemical structure that pass through a similar transition state.Source facies exert only a minor influence on the carbon isotopic composition of Eagle Ford oils/condensates. Small variances in the 13C values of C6+n-alkanes, cyclohexane and methylcyclohexane are consistent with oils produced east of the San Marco Arch being generated from shales with a higher influx of terrestrial high plant matter compared to the shales west of the Arch. Phase fractionation was shown to have a measurable impact on the δ13C values of the most volatile hydrocarbons in evaporated samples and on the least volatile hydrocarbon that underwent phase separation in the subsurface during production.

RhRu Bimetallic Oxide Cluster Catalysts for Cross-Dehydrogenative Coupling of Arenes and Carboxylic Acids.

Noble-metal-based bimetallic oxide clusters are promising novel catalysts. In this study, we developed carbon-supported RhRu bimetallic oxide clusters (RhRuOx/C) with a mean diameter of 1.2 nm, which showed remarkable catalytic activity for the cross-dehydrogenative coupling (CDC) of arenes and carboxylic acids with O2 as the sole oxidant. RhRu bimetallic oxide cluster formation was confirmed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and synchrotron X-ray absorption spectroscopy. Kinetic isotope and substituent effects indicated that arene C-H bond cleavage was the rate-determining step and proceeded via electrophilic concerted metalation-deprotonation mechanism, with a carboxylate as an internal base. Density functional theory calculations supported the proposed mechanism and indicated that the active center for C-H bond activation was Rh(V) rather than Rh(III), while Ru enhanced the electrophilicity of the Rh(V) site by decreasing the negative charge of the surrounding oxygen atoms. Electron-rich arenes showed relatively high reactivity for the RhRuOx/C-catalyzed CDC reaction, and both aliphatic and aromatic carboxylic acids were applicable to the reaction. The RhRuOx/C catalyst is promising for the CDC reaction of arenes and carboxylic acids to produce aryl esters. This work promotes the development of noble-metal-based bimetallic oxide clusters for C-H bond activation reactions.

Trajectory-Based Time-Resolved Mechanism for Benzene Reductive Elimination from Cyclopentadienyl Mo/W Phenyl Hydride Complexes.

Calculated potential energy structures and landscapes are very often used to define the sequence of reaction steps in an organometallic reaction mechanism and interpret kinetic isotope effect (KIE) measurements. Underlying most of this structure-to-mechanism translation is the use of statistical rate theories without consideration of atomic/molecular motion. Here we report direct dynamics simulations for an organometallic benzene reductive elimination reaction, where nonstatistical intermediates and dynamic-controlled pathways were identified. Specifically, we report single spin state as well as mixed spin state quasiclassical direct dynamics trajectories in the gas phase and explicit solvent for benzene reductive elimination from Mo and W bridged cyclopentadienyl phenyl hydride complexes ([Me2Si(C5Me4)2]M(H)(Ph), M = Mo and W). Different from the energy landscape mechanistic sequence, the dynamics trajectories revealed that after the benzene C-H bond forming transition state (often called reductive coupling), σ-coordination and π-coordination intermediates are either skipped or circumvented and that there is a direct pathway to forming a spin flipped solvent caged intermediate, which occurs in just a few hundred femtoseconds. Classical molecular dynamics simulations were then used to estimate the lifetime of the caged intermediate, which is between 200 and 400 picoseconds. This indicates that when the η2-π-coordination intermediate is formed, it occurs only after the first formation of the solvent-caged intermediate. This dynamic mechanism intriguingly suggests the possibility that the solvent-caged intermediate rather than a coordination intermediate is responsible (or partially responsible) for the inverse KIE value experimentally measured for W. Additionally, this dynamic mechanism prompted us to calculate the kH/kD KIE value for the C-H bonding forming transition states of Mo and W. Surprisingly, Mo gave a normal value, while W gave an inverse value, albeit small, due to a much later transition state position.