Kinetic Isotope Effect Research Articles

Glycosylium Ions in Superacid Mimic the Transition State of Enzyme Reactions.

The hydrolysis of glycosides is a biochemical transformation that occurs in all living organisms, catalyzed by a broad group of enzymes, including glycoside hydrolases. These enzymes cleave the glycosidic bond via a transition state with substantial oxocarbenium ion character, resulting in short-lived oxocarbenium ion-like species. While such transient species have been inferred through theoretical studies and kinetic isotope effect measurements, their direct spectroscopic characterization remains challenging. In this study, we exploit a superacid environment to generate, accumulate, and fully characterize nonprotected 2-deoxy glycosyl cations in the d-glucopyranose, d-galactopyranose, and l-arabinofuranose series using low-temperature NMR spectroscopy, supported by DFT calculations. Additionally, QM/MM MD simulations reveal that the properties of these glycosyl cations in superacid closely resemble those within the active sites of glycosidase enzymes, particularly in terms of conformation and anomeric charge distribution. These findings highlight a parallel between the stabilizing effect of counterions in superacid media and the network of multidentate noncovalent interactions within glycosidase active sites, which stabilize transition states with pronounced oxocarbenium ion character.

Mechanistic Investigations of Chiral Lithium Binaphtholate Catalysis for Asymmetric Aldol-Tishchenko Reaction of α-Fluoroarylketones.

In this study, we analyzed the asymmetric aldol-Tishchenko reaction of α-fluoroarylketones with aldehydes in the presence of chiral lithium binaphtholate, which was readily prepared from a chiral BINOL derivative and lithium tert-butoxide. This tandem reaction afforded enantiomerically enriched 2-fluoro-1,3-diols with three contiguous stereogenic centers in high yield and with high diastereo- and enantioselectivities. Moreover, mechanistic investigations of the lithium binaphtholate-catalyzed enantioselective aldol-Tishchenko reaction were performed based on the kinetic isotope effect and computational analyses.

Dehydrogenative Photocyclization of 3-Styryl Indoles to Fused Indole Systems.

The synthesis of 5-aryl- or 5,6-diaryl-11H-benzo[a]carbazoles has been achieved from 3-styryl indoles through catalyst-free photoinitiated dehydrogenative cyclization transformation, providing a range of structurally diverse products in excellent yields under mild conditions. The protocol is also applicable to furan and thiophene samples. This Mallory-type reaction takes place in an argon atmosphere without external oxidants. Finally, detailed mechanistic studies were performed, and kinetic isotope effect experiments indicate that dehydrogenative annulation reaction involves photoinduced 6π-electrocyclic ring-closing and hydrogen evolution cascade processes.

Insights into C-H Activation Reactivity of Fe (IV)O Porphyrinoid Complexes: A Computational Investigation.

This work presents a detailed comparative analysis of C-H activations catalyzed by three different Fe(IV)O porphyrinoid complexes. The study considers the usual heme porphyrin (complex I) as the base compound, porphyrazine (complex II), which is obtained by replacing carbon with nitrogen at the meso position, and phthalocyanine (complex III), which is obtained through the peripheral benzoannulation of porphyrazine. The focus here is to explore the impact of bridging groups and peripheral functionalization in heme systems on reactivity. Factors such as distortion energy and electron acceptor orbitals significantly affect the overall reactivity. The effect of substitution on quantum mechanical tunneling, using H/D kinetic isotope effect studies, is also included. The results reveal a fascinating reactivity order: meso nitrogen substitution enhances reactivity, while additional benzo-annulation hinders reactivity, leading to the order complex II > complex I > complex III. In comparison to the usual Cpd I, which is Fe(IV)O-porphyrin π cation radical with an -SH axial ligand, complex II was found to be more reactive. The electron affinity of the Fe(IV)O complexes and the dissociation energy of the forming Fe(IV)O-H bond aligns with observed reactivity trend. These findings support the use of accessible iron frameworks derived from porphyrin in C-H activation processes.

Tantalum-induced reconstruction of nickel sulfide for enhanced bifunctional water splitting: Separate activation of the lattice oxygen oxidation and hydrogen spillover

Designing highly active and stable bifunctional catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under alkaline conditions is crucial for sustainable overall water splitting. Herein, we present a targeted reconstruction of Ni3S2 by introducing tantalum, achieving remarkable overall water splitting performance through the separate activation of the lattice oxygen mechanism and hydrogen spillover. Electrochemical Mass Spectrometry and in-situ Raman spectroscopy reveal that tantalum induces Ni3S2 to reconstruct into nickel hydroxide during OER, thereby enhancing catalytic activity via the activation of the lattice oxygen mechanism. In the corresponding HER, tantalum promotes the reconstruction of Ni3S2 into oxysulfide, facilitates hydrogen spillover, and acts as an anchor to shorten the spillover distance, improving the HER catalytic performance, as verified by the kinetic isotope effect and theoretical calculations. Therefore, the catalyst-based anion exchange membrane water electrolyzer system achieves a current density of 1 A cm−2 at just 1.97 V, maintaining continuous operation for 500 h. This study offers new insights into the design of bifunctional catalysts, advancing the development of efficient and robust overall water splitting catalysts.

Degradation study of δ-hexachlorocyclohexane by iron sulfide nanoparticles: elucidation of reaction pathway using compound specific isotope analysis and pH variation

pH influences the reactivity of iron (II) minerals towards halogenated pollutants like hexachlorocyclohexanes (HCHs). To explore these incompletely understood interactions, we investigated the carbon isotope fractionation of the δ-HCH isomer during dehalogenation by iron sulfide at pHs spanning a pH range across slightly acidic to alkaline domains (5.8 to 9.6). The δ-1,3,4,5,6-pentachlorocyclohex-1-ene (δ-PCCH) was the intermediate degradation product, while benzene, monochlorobenzene (MCB), but especially 1,2-dichlorobenzene (1,2-DCB), and 1,2,4-trichlorobenzene (1,2,4-TCB), were the main degradation products of δ-HCH. These degradation products suggested dehydrochlorination as the main degradation pathway of δ-HCH by iron sulfide. Different kinetic experiments indicate that the rate constants (ka) during dechlorination of δ-HCH by iron sulfide rose with pH: 0.003 d-1 (pH 5.8) < 0.034 d-1 (pH 8) < 0.085 (pH 9.3) < 0.286 d-1 (pH 9.6). Upon Rayleigh model calculations, an enrichment factor (εC) of -7.8 ± 1.0 ‰ was calculated for δ-HCH dehalogenation by FeS at pH 8.0. This suggests an apparent kinetic isotope effect (AKIEC) value of 1.049 ± 0.006 for dehydrohalogenation. The magnitude of the isotope effect from this paper furthermore supports dehydrohalogenation and opens the possibility to study the degradation of HCHs by iron (II) minerals containing FeS as mackinawite in oxygen-deprived environments.

C-H Activation of Pyridines by Boryl Pincer Complexes: Elucidation of Boryl-Directed C-H Oxidative Addition to Ir and Discovery of Transition Metal-Assisted Reductive Elimination from Boron at Rh.

Experimental and theoretical techniques were used to investigate the mechanism of pyridine C-H activation by diarylboryl/bis(phosphine) PBP pincer complexes of Ir. The critical intermediate (PBP)IrCO (4) contains a three-coordinate, Ir-bound boron that retains Lewis acidity in the perpendicular direction. Coordination of pyridine to this boron center in 4 leads to fast insertion of Ir into the 2-CH bond of pyridine, providing a different topology of direction than the conventional directed C-H activation where both the directing group coordination and C-H activation happen at the same metal center. Beyond this critical sequence, the system possesses significant complexity in terms of possible isomers and pathways, which have been thoroughly explored. Kinetic and thermodynamic preferences for the activation of differently substituted pyridines were also investigated. In experimental work, the key intermediate 4 is accessed via elimination of benzene from a phenyl/hydride containing precursor (PBPhP)IrHCO (3). Density functional theory (DFT) investigations of the mechanism of benzene loss from 3 revealed the possibility of a genuinely new type of mechanism, whereby the Ph-H bond is made in a concerted process that is best described as C-H reductive elimination from boron, assisted by the transition metal (TMARE). For Ir, this pathway was predicted to be competitive with the more conventional pathways involving C-H reductive elimination from Ir, but still higher in energy barrier. However, for the Rh analog 3-Rh, TMARE was calculated to be the preferred pathway for benzene loss and this prediction was experimentally corroborated through the study of reaction rates and the kinetic isotope effect.

Na-Promoted Bimetallic Hydroxide Nanoparticles for Aerobic C-H Activation: Catalyst Design Principles and Insights into Reaction Mechanism.

A precious metal-free bimetallic FexMn1-x(OH)y hydroxide catalyst was developed that is capable of catalyzing aerobic C-H oxidation reactions at low temperatures, without the need for an initiator, relying sustainably on molecular oxygen. Through a systematic synthetic effort, we scanned a wide nanoparticle synthesis parameter space to lay out a detailed set of catalyst design principles unraveling how the Fe/Mn cation ratio, NaOH(aq) concentration used in the synthesis, catalyst washing procedures, extent of residual Na+ promoters on the catalyst surface, reaction temperature, and catalyst loading influence catalytic C-H activation performance as a function of the electronic, surface chemical, and crystal structure of FexMn1-x(OH)y bimetallic hydroxide nanostructures. Our comprehensive XRD, XPS, BET, ICP-MS, 1H NMR, and XANES structural/product characterization results as well as mechanistic kinetic isotope effect (KIE) studies provided the following valuable insights into the molecular level origins of the catalytic performance of the bimetallic FexMn1-x(OH)y hydroxide nanostructures: (i) catalytic reactivity is due to the coexistence and synergistic operation of Fe3+ and Mn3+ cationic sites (with minor contributions from Fe2+ and Mn2+ sites) on the catalyst surface, where in the absence of one of these synergistic sites (i.e., in the presence of monometallic hydroxides), catalytic activity almost entirely vanishes, (ii) residual Na+ species on the catalyst surface act as efficient electronic promoters by increasing the electron density on the Fe3+ and Mn3+ cationic sites, which in turn, presumably enhance the electrophilic adsorption of organic reactants and strengthen the interaction between molecular oxygen and the catalyst surface, (iii) in the fluorene oxidation reaction the step dictating the reaction rate likely involved the breaking of a C-H bond (kH/kD = 2.4), (iv) reactivity patterns of a variety of alkylarene substrates indicate that the C-H bond cleavage follows a stepwise PT-ET (proton transfer-electron transfer) pathway.

Position-specific kinetic isotope effects for nitrous oxide: a new expansion of the Rayleigh model

Abstract. Nitrous oxide (N2O) is a potent greenhouse gas and the most significant anthropogenic ozone-depleting substance currently being emitted. A major source of anthropogenic N2O emissions is the microbial conversion of fixed nitrogen species from fertilizers in agricultural soils. Thus, understanding the enzymatic mechanisms by which microbes produce N2O has environmental significance. Measurement of the 15N / 14N isotope ratios of N2O produced by purified enzymes or axenic microbial cultures is a promising technique for studying N2O biosynthesis. Typically, N2O-producing enzymes combine nitrogen atoms from two identical substrate molecules (NO or NH2OH). Position-specific isotope analysis of the central (Nα) and outer (Nβ) nitrogen atoms in N2O enables the determination of the individual kinetic isotope effects (KIEs) for Nα and Nβ, providing mechanistic insight into the incorporation of each nitrogen atom. Previously, position-specific KIEs (and fractionation factors) were quantified using the Rayleigh distillation equation, i.e., via linear regression of δ15Nα or δ15Nβ against [-fln⁡f/(1-f)], where f is the fraction of substrate remaining in a closed system. This approach, however, is inaccurate for Nα and Nβ because it does not account for fractionation at Nα affecting the isotopic composition of substrate available for incorporation into the β position (and vice versa). Therefore, we developed a new expansion of the Rayleigh model that includes specific terms for fractionation at the individual N2O nitrogen atoms. By applying this Expanded Rayleigh model to a variety of simulated N2O synthesis reactions with different combinations of normal, inverse, and/or no KIEs at Nα and Nβ, we demonstrate that our new model is both accurate and robust. We also applied this new model to two previously published datasets describing N2O production from NH2OH oxidation in a methanotroph culture (Methylosinus trichosporium) and N2O production from NO by a purified Histoplasma capsulatum (fungal) P450 NOR, demonstrating that the Expanded Rayleigh model is a useful tool in calculating position-specific fractionation for N2O synthesis.

Foam Ti Supported Pd Catalysts for the Selective Hydrogenation of Nitroaromatics.

The selective hydrogenation of nitroaromatics plays an essential role in the chemical industry for the synthesis of anilines and their derivatives, which are known as crucial fine chemicals and pharmaceuticals. In this study, we demonstrate the preparation of Pd/Ti monolith catalyst containing well-isolated metallic Pd sites on Ti substrate through a simple impregnation method, showing remarkable catalytic properties in the selective hydrogenation of nitroaromatics containing various functional groups. Kinetic analyses reveal an apparent activation energy of 61 kJ/mol and the kinetic isotope effect (KH2/KD2) of ~1.7 in the hydrogenation of 3-chloronitrobenzene over Pd/Ti-200 ppm catalyst, indicating the facile dissociation of dihydrogen and the subsequent efficient hydrogenation. The Pd/Ti-200 ppm catalyst also demonstrates good stability and recyclability, maintaining its performance over multiple cycles. This simple but innovative approach not only enhances the efficiency of Pd catalysts in the selective hydrogenation of nitroaromatics but also offers significant potential for industrial applications in aniline production.

A Mechanistic Study of the Oxidation of Substituted Benzyl Alcohols with Trichloroisocyanuric Acid

A systematic mechanistic study employing kobs kineticsmeasurements, linear free‐energy relationship, hydrogen kinetic isotope effect and rate law, as well as the Density Functional Theory (DFT) approach (M06‐2x/6‐311G(d,p)) for the oxidation of diverse substituted benzyl alcohols with trichloroisocyanuric acid (TCCA) / H2O in 50 % MeCN / CH2Cl2 at 25.0 °C is present. The kinetic results showed an autocatalytic behavior and a primary kinetic isotope effect (4.22). The linear free‐energy relationship (σ, ρ = ‐1.22) indicated a mechanism change for 4‐CF3, 3‐NO2 and 4‐NO2 substituted benzyl alcohols and a fractional rate law (3.25) for benzyl alcohol. The DFT results indicated Cl2, formed in situ from TCCA and the benzyl alcohol, as the actual oxidizing species. Furthermore, calculations support the kinetic results with high agreement through a transition state that performs a hydride abstraction by autocatalytic Cl2 in the induction zone. In the case of substrates bearing strong electron withdrawing groups, the mechanism changes to hydrogen abstraction from the corresponding benzyl hypochlorite as the main pathway.

Revealing OH species in situ generated on low-valence Cu sites for selective carbonyl oxidation

Catalytic oxidation through the transfer of lattice oxygen from metal oxides to reactants, namely the Mars-van Krevelen mechanism, has been widely reported. In this study, we evidence the overlooked oxidation route that features the in situ formation of surface OH species on Cu catalysts and its selective addition to the reactant carbonyl group. We observed that glucose oxidation to gluconic acid in air (21% O2) was favored on low-valence Cu sites according to X-ray spectroscopic analyses. Molecular O2 was activated in situ on Cu0/Cu+ forming localized, adsorbed hydroxyl radicals (*OH) which played the primary reactive oxygen species as confirmed by the kinetic isotope effect (KIE) study in D2O and in situ Raman experiments. Combined with DFT calculations, we proposed a mechanism of O2-to-*OH activation through the *OOH intermediate. The localized *OH exhibited higher selectivity toward glucose oxidation at C1HO to form gluconic acid (up to 91% selectivity), in comparison with free radicals in bulk environment that emerged from thermal, noncatalytic hydrogen peroxide decomposition (40% selectivity). The KIE measurements revealed a lower glucose oxidation rate in D2O than in H2O, highlighting the role of water (H2O/D2O) or its derivatives (e.g., *OH/*OD) in the rate-determining step. After proving the C1-H activation step kinetically irrelevant, we proposed the oxidation mechanism that was characterized by the rate-limiting addition of *OH to C1=O in glucose. Our findings advocate that by maneuvering the coverage and activity of surface *OH, high-performance oxidation of carbonyl compounds beyond biomass molecules can be achieved in water and air using nonprecious metal catalysts.

Hierarchical Equations of Motion for Quantum Chemical Dynamics: Recent Methodology Developments and Applications.

ConspectusQuantum effects are critical to understanding many chemical dynamical processes in condensed phases, where interactions between molecules and their environment are usually strong and non-Markovian. In this Account, we review recent progress from our group in development and application of the hierarchical equations of motion (HEOM) method, highlighting its ability to address some challenging problems in quantum chemical dynamics.In the HEOM method, the bath degrees of freedom are represented using effective modes from exponential decomposition of the bath correlation function. Complex spectral densities and low temperature simulations often require a larger number of modes, making the simulations very expensive. Recent advances, such as the barycentric spectral decomposition (BSD) technique, can significantly reduce the number of effective modes, allowing to handle complex spectral densities and enabling simulations at very low temperatures, including near-zero temperature dynamics.Another key improvement in the computational efficiency is the use of tensor network methods like matrix product states and hierarchical tensor networks. These techniques allow for efficient HEOM propagation with thousands of effective modes, crucial for simulating large molecular systems interacting with multiple baths. This combination enables simulations of excitation energy transfer (EET) in systems like the Fenna-Matthews-Olson (FMO) complex and even larger systems with experimentally determined spectral densities.The versatility of the HEOM method is demonstrated through applications to a wide range of chemical dynamics problems. Simulations of EET and related ultrafast spectroscopy are first briefly covered. Applications of the HEOM to quantum tunneling effects in proton transfer reactions are then presented. Early works have studied the non-Kramers dependence of the rate constant as a function of bath friction due to deep tunneling and revealed vibrationally nonadiabatic dynamics within the so-called nontraditional view of proton transfer reactions. A recent work on the large kinetic isotope effects in soybean lipoxygenase also indicated that many quantum correction approximations to classical transition-state theory may fall short in describing deep tunneling effects.Charge transport and separation dynamics in organic semiconductors are another area where the HEOM method has been instrumental. We first demonstrate that the HEOM provides a unified description of both band-like and thermally assisted charge carrier transport in organic materials. The effect of non-nearest neighbor transitions is then investigated by combining generalized master equations with exact memory kernels. The HEOM method also enables simulation of charge separation in organic photovoltaics (OPVs) and reveals how factors such as external electric fields, entropy, and charge delocalization influence the charge separation barrier and dynamics.Moreover, HEOM has been applied to investigate hydrogen atom scattering on the Au(111) surface and vibrational energy relaxation at molecule-metal interfaces. These studies provide deeper insights into how electron-hole pair excitations and temporary charge transfer states influence the nuclear motion, offering a new framework for simulating nonadiabatic dynamics on metal surfaces.In summary, the HEOM method has developed into a robust tool for simulating quantum effects in condensed phases. Future developments in algorithm efficiency and computational power will likely expand its applicability to even more complex systems.

Highly enriched carbon and oxygen isotopes in carbonate-derived CO2 at Gale crater, Mars

Carbonate minerals are of particular interest in paleoenvironmental research as they are an integral part of the carbon and water cycles, both of which are relevant to habitability. Given that these cycles are less constrained on Mars than they are on Earth, the identification of carbonates has been a point of emphasis for rover missions. Here, we present carbon (δ13C) and oxygen (δ18O) isotope data from four carbonates encountered by the Curiosity rover within the Gale crater. The carbon isotope values range from 72 ± 2‰ to 110 ± 3‰ Vienna Pee Dee Belemnite while the oxygen isotope values span from 59 ± 4‰ to 91 ± 4‰ Vienna Standard Mean Ocean Water (1 SE uncertainties). Notably, these values are isotopically heavy (13C- and 18O-enriched) relative to nearly every other Martian material. The extreme isotopic difference between the carbonates and other carbon- and oxygen-rich reservoirs on Mars cannot be reconciled by standard equilibrium carbonate-CO2 fractionation, thus requiring an alternative process during or prior to carbonate formation. This paper explores two processes capable of contributing to the isotopic enrichments: 1) evaporative-driven Rayleigh distillation and 2) kinetic isotope effects related to cryogenic precipitation. In isolation, each process cannot reproduce the observed carbonate isotope values; however, a combination of these processes represents the most likely source for the extreme isotopic enrichments.

Comparative clumped isotope temperature relationships in freshwater carbonates

AbstractLacustrine, riverine and spring carbonates represent archives of terrestrial climates and their geochemistry has been used to study palaeoenvironments. Clumped isotope thermometry is an emerging tool that has been applied to freshwater carbonates. Limited work has been done to evaluate comparative relationships between clumped isotopes and temperature in different types of modern freshwater carbonates. This study assembles an extensive calibration data set with 135 samples of modern freshwater carbonates from 96 sites and constrains the relationship between independent observations of water temperature and the clumped isotopic composition of carbonates (denoted by Δ47), including new measurements, and recalculates published data in accordance with current community‐defined standard values. For temperature reconstruction, the study reports a composite freshwater calibration and material‐specific calibrations for biogenic carbonates (freshwater gastropods and bivalves), fine‐grained carbonate (e.g. micrites), biologically mediated carbonates (microbialites and tufas) and travertines. Material‐specific calibration trends show a convergence of slopes that are in agreement with recently published syntheses, but statistically significant differences in intercepts occur between some materials (e.g. some biogenics, fine‐grained carbonates). These differences may arise due to unresolved seasonal biases, kinetic isotope effects and/or varying degrees of biological influence. The impact of different calibrations is shown through application to new data for glacial and deglacial age travertines from Austria and published data sets. While material‐specific calibrations may yield more accurate results for biogenic and fine‐grained carbonate samples, the use of material‐specific and the composite freshwater calibrations generally produces values within 1.0–1.5°C of each other, and typically fall within calibration uncertainty given limitations of precision.

Highly Acidic Electron-Rich Brønsted Acids Accelerate Asymmetric Pictet-Spengler Reactions by Virtue of Stabilizing Cation-π Interactions.

Electron-rich heteroaromatic imidodiphosphorimidates (IDPis) catalyze the asymmetric Pictet-Spengler reaction of N-carbamoyl-β-arylethylamines with high stereochemical precision. This particular class of catalysts furthermore provides a vital rate enhancement compared to related Brønsted acids. Here we present experimental studies on the underlying reaction kinetics that shed light on the specific origins of rate acceleration. Analysis of Hammett plots, kinetic isotope effects, reaction orders, Eyring plots, and isotopic scrambling experiments, allowed us to gather insights into the molecular interactions between the chiral Brønsted acid and catalytically formed intermediates. Based on rigorously determined pKa values as well as the experimental evidence, we propose that attractive intermolecular forces offered by electron-rich π-surfaces of the chiral counteranion enthalpically stabilize cationic intermediates and transition states by way of cation-π interactions. This view is furthermore supported by in-depth density functional theory calculations. Our deepened understanding of the reaction mechanism allowed us to develop a method for accessing 1-aryltetrahydroisoquinolines from aromatic dimethyl acetals, a substrate class that was thus far inaccessible via catalytic asymmetric Pictet-Spengler reactions.

Evolution of the Short Enantioselective Total Synthesis of the Unique Marine Myxobacteria Polyketide Salimabromide

Comprehensive SummarySalimabromide, a unique and scarce marine tetracyclic polyketide, was synthesized in both racemic and optically active forms. A novel carboxylic acid‐directed method for tandem oxidative difunctionalization of olefins was developed, whereby the formation of bridged butyrolactone and enone moieties occurs concurrently. Density functional theory (DFT) calculations indicate that this reaction follows a [3+2] process rather than the [2+2] process. In the meantime, the distinctive benzo‐fused [4.3.1] carbon skeleton and highly hindered vicinal quaternary stereocenters were simultaneously constructed through a challenging intramolecular Giese‐Baran radical cyclization. Furthermore, deuterium kinetic isotopic effects were utilized to enhance the efficacy of this pivotal step. This represents a new illustration of the application of kinetic isotope effects in natural product synthesis. Then, the short asymmetric synthesis of (+)‐salimabromide (13 or 15 steps) was accomplished by combing this method with rhodium‐catalyzed enantioselective hydrogenation of a cycloheptenone derivative (97% ee) or conjugate addition of an aryl boronic acid with 2‐cyclohepten‐1‐one (&gt; 99% ee).

Impact of Nuclear Motion on Light-Induced Bimolecular Interaction Dynamics

In chemical reactions, the nuclear motion of the molecules plays a crucial role in determining the reaction rates and outcomes. Employing the cold target recoil ion momentum spectroscopy and femtosecond pump-probe techniques, we perform a molecular-level study into the influence of nuclear vibrations on light-induced bimolecular reactions within H2−D2 dimers. The study focuses on the formation dynamics of D2H+ and H2D+ cations, shedding light on the interplay between translational and vibrational motions of the nuclei steering the bimolecular reactions. Our observations reveal a notable yield ratio of 1:1.6 between H2D+ and D2H+ channels, accompanied with a faster formation of D2H+ compared to H2D+. Molecular dynamics simulations unveil that the faster vibrational motion of H2+ than that of D2+ upon single ionization within the dimer accounts for these differences. Our findings provide new insight into the time-resolved kinetic isotope effect on the bimolecular reactions, highlighting the critical relationship between nuclear vibrational motions and reaction dynamics. Published by the American Physical Society 2024

Tuning the Reactivity of Copper(II)-Nitrite Core Towards Nitric Oxide Generation.

Insights into the molecular mechanism and factors affecting nitrite-to-NO transformation at transition metal sites are essential for developing sustainable technologies relevant to NO-based therapeutics, waste water treatment, and agriculture. A set of copper(II)-nitrite complexes1-4have been isolated employing tridentate pincer-type ligands (quL,pyL,ClArOL-,PhOL-) featuring systematically varied donors. Although the X-ray crystal structures of the copper(II)-nitrite cores in1-4are comparable, electrochemical studies on complexes1-4reveal that redox properties of these complexes differ due to the changes in thes-donor abilities of thephenolate / N-heterocycle based donor sites. Reactivity of these nitrite complexes with oxygen-atom-transfer (OAT) reagent (e.g. triphenyl phosphine Ph3P) and H+/e-donor reagent (e.g. substituted phenols ArOH) show the reduction of nitrite to NO gas. Detailed kinetic investigations including kinetic isotope effect (KIE), Eyring analyses for determining the activation parameters unfold that reduction of nitrite at copper(II) by Ph3P or ArOH are influenced by the CuII/CuIredox potential. Finally, this study allows mechanism driven development of catalytic nitrite reduction by ArOH in the presence of 10 mol% copper complex (1).

Hydrogen activation and surface exchange over La0.8Sr0.2Ga0.8Mg0.2O3-z

Hydrogen mass-transfer between molecular hydrogen and La0.8Sr0.2Ga0.8Mg0.2O3-z (LSGM) has been studied by means of hydrogen isotope exchange with gas phase equilibration. The measurements were carried out within a temperature range of 300–700 °C, with molecular hydrogen pressures of 100, 150 and 200 Pa. It was found that LSGM is capable of consuming hydrogen from the gas phase. The relative number of hydrogen atoms in the LSGM structure varied between 1 and 4 mol.%. The kinetics of hydrogen isotope redistribution revealed that the mass-transfer mechanism between molecular hydrogen in the gas phase and LSGM includes at least three steps. The first two relate to dissociative and associative/molecular adsorption of molecular hydrogen, while the third, identified as the rate-determining step in the exchange process, relates to hydrogen incorporation into LSGM structure. The kinetic and thermodynamic isotope effects of the exchange were estimated.