Search

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 120 K, 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 18–24 kJ/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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present a theory of the effect of quantum tunneling on the basic parameter that characterizes the effect of pressure on the rate constant of chemical reactions in a dense phase, the activation volume. This theory results in combining, on the one hand, the extreme pressure polarizable continuum model, a quantum chemical method to describe the effect of pressure on the reaction energy profile in a dense medium, and, on the other hand, the semiclassical version of the transition state theory, which includes the effect of quantum tunneling through a transmission coefficient. The theory has been applied to the study of the activation volume of the model reaction of hydrogen transfer between methyl radical and methane, including the primary isotope substitution of hydrogen with deuterium (H/D). The analysis of the numerical results offers, for the first time, a clear insight into the effect of quantum tunneling on the activation volume for this hydrogen transfer reaction: this effect results from the different influences that pressure has on the competing thermal and tunneling reaction mechanisms. Furthermore, the computed kinetic isotope effect (H/D) on the activation volume for this model hydrogen transfer correlates well with the experimental data for more complex hydrogen transfer reactions. [ABSTRACT FROM AUTHOR]

[Display omitted] 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 Ni 3 S 2 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 Ni 3 S 2 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 Ni 3 S 2 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hollow core–shell structured Co 3 O 4 @CuO-NiO was synthesized through a simple route. • Charge transfer among Co 3 O 4, CuO, and NiO optimizes catalyst's electronic structures. • TOF of the optimal catalyst in NH 3 BH 3 methanolysis is as high as 67.1 min−1. • Possible mechanism for NH 3 BH 3 methanolysis is proposed based on in-situ FTIR results. Catalytic methanolysis of ammonia borane is an integrated hydrogen production/storage technology with bright prospects, while its wide application is impeded by the high-cost of catalysts. In this work, hollow core@shell structured (HCSS) Co 3 O 4 @CuO-NiO with a size of 300–500 nm and a shell thickness of ca. 100 nm has been designed for ammonia borane (NH 3 BH 3) methanolysis for rapid hydrogen release. The possible formation mechanism of HCSS-Co 3 O 4 @CuO-NiO is proposed based on a series of characterization results, which is crucial for the design and preparation of nanocatalysts with similar architectures. Benefiting from the optimized compositions and electronic structures, the best HCSS-Co 3 O 4 @CuO-NiO sample exhibits high catalytic activity in NH 3 BH 3 methanolysis with a turnover frequency of 67.1 min−1, surpassing that of all the noble-metal-free catalysts in previous reports. A possible reaction mechanism of NH 3 BH 3 methanolysis is put forward based on the in-situ Fourier transform infrared spectrometer (FTIR) characterization and kinetic isotope effect (KIE) experimental results. This work supplies a novel avenue for developing cheap and robust catalysts towards NH 3 BH 3 methanolysis by tailoring the catalysts' morphology to hollow core@shell structures. [ABSTRACT FROM AUTHOR]

The one-step oxidation of benzene to phenol represents a significant and promising advancement in modern industries focused on the production of high-value-added chemical products. Nevertheless, challenges persist in achieving sufficient catalytic selectivity and preventing over-oxidation. Inspired by copper enzymes, we present a nonsymmetric dicopper complex ([Cu II2 (TPMAN)(μ-OH)(H2O)]3+, 1) for the selective oxidation of benzene to phenol. Utilizing H2O2 as the oxidant, complex 1 demonstrates remarkable catalytic activity (a TON of 14 000 within 29 hours) and selectivity exceeding 97%, comparable to the finest homogeneous catalyst derived from first-row transition metals. It is noteworthy that the significant substituent effect, alongside a negligible kinetic isotope effect (KIE = 1.05), radical trapping experiments, and an inconsistent standard selectivity test of the ˙OH radicals, all contradict the conventional Fenton mechanism and rebound pathway. Theoretical investigations indicate that the active CuII(μ-O˙)CuII–OH species generated through the cleavage of the O–O bond in the CuII(μ-1,1-OOH)CuI intermediate facilitates the hydroxylation of benzene via an electrophilic attack mechanism. The nonsymmetric coordination geometry is crucial in activating H2O2 and in the process of O–O bond cleavage. [ABSTRACT FROM AUTHOR]

[Display omitted] • Novel yolk-shell Pd 4 Ir 1 /YSMSNs-NH 2 catalyst with highly dispersed PdIr alloy NPs was prepared. • The radially oriented mesoporous channels of Pd 4 Ir 1 /YSMSNs-NH 2 catalyst can promote mass transfer and increase accessibility. • The amine groups on the YSMSNs-NH 2 support can improve the dispersion and reduce the particle size of the PdIr active phases. • The ultrasmall size (1.4 nm) and high dispersion of PdIr alloy NPs could expose more catalytic active sites. • The Pd 4 Ir 1 /YSMSNs-NH 2 catalyst exhibits an initial TOF value of 5818 h−1 at 50 °C, with 100 % FA conversion and H 2 selectivity. Developing heterogeneous catalysts with exceptional catalytic activity over formic acid (HCOOH, FA) dehydrogenation is imperative to employ FA as an effective hydrogen (H 2) carrier. In this work, ultrasmall (1.4 nm) and well-dispersed PdIr nanoparticles (NPs) immobilized on amine-functionalized yolk-shell mesoporous silica nanospheres (YSMSNs) with radially oriented mesoporous channels have been synthesized by a co-reduction strategy. The optimized catalyst Pd 4 Ir 1 /YSMSNs-NH 2 (Pd/Ir molar ratio = 4:1) exhibited a remarkable turnover frequency (TOF) of 5818 h−1 and remarkable stability at 50 °C with the addition of sodium formate (SF), resulting in complete FA conversion and H 2 selectivity, exceeding most of the solid heterogeneous catalysts in previous reports under similar circumstances. Kinetic isotope effect (KIE) exploration indicates the cleavage of the C H bond is regarded as the rate-determining step (RDS) during the FA dehydrogenation process. Such excellent catalytic properties arise from the ultrafine and well-dispersed PdIr NPs supported on the nanosphere support YSMSNs-NH 2 , the electronic synergistic effect of PdIr alloy NPs, and the strong metal-support interaction (MSI) effect between the introduced PdIr NPs and YSMSNs-NH 2 support. This work offers a new paradigm for exploiting the highly effective silica-supported Pd-based heterogeneous catalysts over the dehydrogenation of FA. [ABSTRACT FROM AUTHOR]

Roaming reactions involving a neutral fragment of a molecule that transiently wanders around another fragment before forming a new bond are intriguing and peculiar pathways for molecular rearrangement. Such reactions can occur for example upon double ionization of small organic molecules, and have recently sparked much scientific interest. We have studied the dynamics of the -roaming reaction leading to the formation of after two-photon double ionization of ethanol and 2-aminoethanol, using an XUV-UV pump-probe scheme. For ethanol, we find dynamics similar to previous studies employing different pump-probe schemes, indicating the independence of the observed dynamics from the method of ionization and the photon energy of the disruptive probe pulse. Surprisingly, we do not observe a kinetic isotope effect in ethanol- , in contrast to previous experiments on methanol where such an effect was observed. This distinction indicates fundamental differences in the energetics of the reaction pathways as compared to the methanol molecule. The larger number of possible roaming pathways compared to methanol complicates the analysis considerably. In contrast to previous studies, we additionally analyze a broad range of dissociative ionization products, which feature distinct dynamics from that of and allow initial insight into the action of the disruptive UV-probe pulse. [ABSTRACT FROM AUTHOR]

Cyclic dipeptides are produced by organisms across all domains of life, with many exhibiting anticancer and antimicrobial properties. Oxidations are often key to their biological activities, particularly C-C bond oxidation catalysed by tailoring enzymes including cyclodipeptide oxidases. These flavin-dependent enzymes are underexplored due to their intricate three-dimensional arrangement involving multiple copies of two distinct small subunits, and mechanistic details underlying substrate selection and catalysis are lacking. Here, we determined the structure and mechanism of the cyclodipeptide oxidase from the halophile Nocardiopsis dassonvillei (NdasCDO), a component of the biosynthetic pathway for nocazine natural products. We demonstrated that NdasCDO forms filaments in solution, with a covalently bound flavin mononucleotide (FMN) cofactor at the interface between three distinct subunits. The enzyme exhibits promiscuity, processing various cyclic dipeptides as substrates in a distributive manner. The reaction is optimal at high pH and involves the formation of a radical intermediate. Pre-steady-state kinetics, a significant solvent kinetic isotope effect, and the absence of viscosity effects suggested that a step linked to FMN regeneration controlled the reaction rate. Our work elucidates the complex mechanistic and structural characteristics of this dehydrogenation reaction, positioning NdasCDO as a promising biocatalyst and expanding the FMN-dependent oxidase family to include enzyme filaments. Cyclodipeptide oxidases are filamentous enzymes. Here, the authors dissect the mechanism of a promiscuous flavoenzyme from the biosynthesis of cyclodipeptide natural products, unveiling fast catalysis for peptide oxidation in a distinct active site. [ABSTRACT FROM AUTHOR]

The New Delhi metallo‐β‐lactamase 1 (NDM‐1) can hydrolyze nearly all clinically important β‐lactam antibiotics, narrowing the options for effective treatment of bacterial infections. QM/MM MP2 calculations are performed to reveal the mechanism of ampicillin hydrolysis catalyzed by NDM‐1. It is found that the rate‐determining step is the dissociation of hydrolyzed ampicillin from the NDM‐1 active site, which requires a proton transfer from the bridging neutral water molecule to the newly formed carboxylate group. The precedent reaction steps, including the hydroxide nucleophilic addition, CN bond cleavage, and the protonation of the negative lactam N atom by a solvent water molecule, all require insignificant activation free energies. The calculated activation free energy for this rate‐determining proton transfer step is 16.0 kcal/mol, in good agreement with experimental values of 13.7 ~ 14.7 kcal/mol. This proton transfer step exhibits a solvent hydrogen‐deuterium kinetic isotope effect of 3.4, consistent with several experimental kinetic results. [ABSTRACT FROM AUTHOR]

1,4-Azaborine-based arenes are promising electroluminescent emitters with thermally activated delayed fluorescence (TADF), offering narrow emission spectra and high quantum yields due to a multi-resonance (MR) effect. However, their practical application is constrained by their limited operational stability. This study investigates the degradation mechanism of MR-TADF molecules. Electroluminescent devices incorporating these compounds display varied operational lifetimes, uncorrelated with excitonic stability or external quantum efficiency roll-off. Bulk electrolysis reveals significant instability in the radical cationic forms of MR-TADF compounds, with device lifetime linked to the Faradaic yield of oxidation. Comprehensive chemical analyses corroborate that the degradation byproducts originated from intramolecular cyclization of radical cation, followed by hydrogen atom transfer. The mechanism is further supported by enhanced stability observed in a deuterated MR-TADF emitter, attributed to a secondary kinetic isotope effect. These findings provide insights into the stabilizing effects of deuteration and mechanism-driven strategies for designing MR-TADF compounds with improved operational longevity. The limited operational stability of multi-resonance thermally activated delayed fluorescence emitters often constrains their practical application. Here, the authors report a direct correlation between device lifetime and Faradaic yield for oxidative degradation of emitter molecules. [ABSTRACT FROM AUTHOR]

Hydrogenation and dehydrogenation reactions are fundamental in chemistry and essential for all living organisms. We employ density functional theory (DFT) to understand the reaction mechanism of the oxidative dehydrogenation (ODH) of the pyridyl-amine complex [FeIIIL3]3+ (L3, 1,9-bis(2′-pyridyl)-5-[(ethoxy-2″-pyridyl)methyl]-2,5,8-triazanonane) to the mono-imine complex [FeIIL4]2+ (L4, 1,9-bis(2′-pyridyl)-5-[(ethoxy-2″-pyridyl)methyl]-2,5,8-triazanon-1-ene) in the presence of dioxygen. The nitrogen radical [FeIIL3N8•]2+, formed by deprotonation of [FeIIIL3]3+, plays a crucial role in the reaction mechanism derived from kinetic studies. O2 acts as an oxidant and is converted to H2O. Experiments with the deuterated ligand L3 reveal a primary C-H kinetic isotope effect, kCH/kCD = 2.30, suggesting C-H bond cleavage as the rate-determining step. The DFT calculations show that (i) 3O2 abstracts a hydrogen atom from the α-pyridine aliphatic C-H moiety, introducing a double bond regio-selectively at the C7N8 position, via the hydrogen atom transfer (HAT) mechanism, (ii) O2 does not coordinate to the iron center to generate a high-valent Fe oxo species observed in enzymes and biomimetic complexes, and (iii) the experimental activation parameters (ΔH≠ = 20.38 kcal mol−1, ΔS≠ = −0.018 kcal mol−1 K−1) fall within in the range of values reported for HAT reactions and align well with the computational results for the activated complex [FeIIL3N8•]2+···3O2. [ABSTRACT FROM AUTHOR]

Carbon-14 (C-14) has been a major contributor to the human radioactive exposure dose, as it is released into the environment from the nuclear industry in larger quantities compared to other radionuclides. This most abundant nuclide enters the biosphere as organically bound C-14 (OBC-14), posing a potential threat to public health. Yet, it remains unknown how this relatively low radiotoxic nuclide induces health risks via chemical effects, such as isotope effect. By establishing a trophic transfer model involving algae (Scenedesmus obliquus), daphnia (Daphnia magna) and zebrafish (Danio rerio), we demonstrate that rapid incorporation and transformation of inorganic C-14 by algae into OBC-14 facilitates the blending of C-14 into the biomolecules of zebrafish. We find that internalized C-14 is persistently retained in the brain of zebrafish, affecting DNA methylation and causing alterations in neuropathology. Global isotope tracing metabolomics with C-14 exposure further reveals the involvement of C-14 in various critical metabolic pathways, including one-carbon metabolism and nucleotide metabolism. We thus characterize the kinetic isotope effects for 12C/14C in the key reactions of these metabolic pathways through kinetic experiments and density functional theory computations, showing that the isotopic substitution of carbon in biochemicals regulates metabolism by disrupting reaction ratios via isotope effects. Our results suggest that inorganic C-14 discharged by the nuclear industry can be biotransformed into OBC-14 to impact metabolism via isotope effects, providing new insights into understanding the health risk of C-14, which is traditionally considered as a low radiotoxic nuclide. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: Salimabromide, 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 (> 99% ee). [ABSTRACT FROM AUTHOR]

In the present work, we performed calculations of the kinetic isotope effect (KIE) on H/D, 14N/15N, 16O/18O, and 12C/13C isotopic substitution in the dissociation of beta-sheet polyglycine dimers of different lengths into two monomer chains. This dissociation reaction, proceeding via breaking of the interchain hydrogen bonds (H-bonds), is considered to be a model of unfolding of the secondary structure of proteins. The calculated strengthening of the interchain hydrogen bonds N − H ⋯ O = C due to heavy isotope substitution decreases in the row H/D >> 14N/15N > 16O/18O > 12C/13C. The KIE for H/D substitution, defined as the ratio of the rate constants k (H) k (D) , was calculated with the use of a "completely loose" transition state model. The results of the calculations show that a very high H/D isotope effect can be achieved for proteins even with moderately long chains connected by dozens of interchain H-bonds. The results obtained also indicate that the heavy isotope substitution in the internal (interchain) and external H-bonds, located on the periphery of a dimer, can provide comparable effects on secondary structure stabilization. [ABSTRACT FROM AUTHOR]

N-6 methyladenosine is the most abundant nucleic acid modification in eukaryotes and plays a crucial role in gene regulation. The AlkB family of alpha-ketoglutarate-dependent dioxygenases is responsible for nucleic acid demethylation. Recent studies have discovered that a chemical demethylation system using hydrogen peroxide and ammonium bicarbonate can effectively demethylate nucleic acids. The addition of ferrous ammonium sulfate boosts the oxidation rate by forming a Fenton reagent with hydrogen peroxide. However, the specific mechanism and key steps of this process remain unclear. In this study, we investigate the influence of ferrous ammonium sulfate concentration on the kinetic isotope effect (KIE) of the chemical demethylation system using LC-MS. As the concentration of ferrous ions increases, the observed KIE decreases from 1.377 ± 0.020 to 1.120 ± 0.016, indicating a combination of the primary isotope effect and inverse α-secondary isotope effect with the ion pairing effect. We propose that the initial hydrogen extraction is the rate-limiting step and observe a tight transition state structure in the formation of the hm6A process through the analysis of KIE trends. The concentration-dependent KIE provides a novel perspective on the mechanism of chemical demethylation and offers a chemical model for enzyme-catalyzed demethylation. [ABSTRACT FROM AUTHOR]

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 (CO32−) 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 CO32− activation, in which with thermodynamic and kinetic investigations, a compact Langmuir‐Hinshelwood reaction model was established which suggests that the overall rate of CO32− hydrogenation was controlled by a specific C−O bond rupture elementary step within HCOO− and the Ru surface was mainly covered by CO32− or HCOO− at independent conditions. This assumption was further supported by negligible kinetic isotope effects (kH/kD≈1), similarity on reaction barriers of CO32− 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 CO32− hydrogenation. [ABSTRACT FROM AUTHOR]

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 complexes 1–4 have 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 in 1–4 are comparable, electrochemical studies on complexes 1–4 reveal that redox properties of these complexes differ due to the changes in the σ‐donor abilities of the phenolate/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/CuI redox potential. Finally, this study allows mechanism driven development of catalytic nitrite reduction by ArOH in the presence of 10 mol % copper complex (1). [ABSTRACT FROM AUTHOR]

Activating dihydrogen, H2, is a challenging endeavor typically achieved using transition metal centers. Pure main‐group compounds capable of this are rare and have emerged in recent decades. These systems rely on synergistic donor‐acceptor interactions with H2's antibonding σ* and bonding σ orbital. An alternative (hydrocarbon) radical‐mediated activation is problematic because the H−H bond is stronger (104.2 kcal mol−1) than most C−H bonds. Here, we explore using the phenyl radical to tackle this, forming benzene with a C−H bond energy (112.9 kcal mol−1) that provides a thermodynamic driving force. We mainly observe a benzene‐HI complex upon photolysis of iodobenzene in an H2‐doped neon matrix at 4.4 K despite a barrier of 7.6 kcal mol−1, while phenyl radical forms in case of the heavier D2 isotopologue. When D2 molecules are allowed to diffuse, mono‐deuterated benzene accumulates within hours. Computations using path integral‐based instanton theory highlight that primarily the transferred hydrogen atom is moving during the reaction which greatly increases the tunneling probability. In excellent agreement with the experimental results, we predict significant tunneling rate constants for both isotopologues, H2 and D2, featuring a strong kinetic isotope effect of up to four orders of magnitude at the lowest temperatures. [ABSTRACT FROM AUTHOR]

The statistical quantum model (SQM), which assumes that the reactivity is controlled by entrance/exit channel quantum capture probabilities, is well suited for chemical reactions with a long‐lived intermediate complex. In this work, a time‐independent coupled‐channel implementation of the SQM approach is developed for atom‐triatom systems in full dimensionality. As SQM treats the capture dynamics quantum mechanically, it is capable of handling quantum effects such as tunneling. A detailed study of the H/D + O3 capture dynamics was performed by applying the newly developed SQM method on an accurate global potential energy surface. Agreement with previous ring polymer molecular dynamics (RPMD) results on the same potential energy surface is excellent except for very low temperatures. The SQM results are also in reasonably good agreement with available experimental rate coefficients. The strong H/D kinetic isotope effect underscores the dominant role of quantum tunneling under an entrance channel barrier at low temperatures. [ABSTRACT FROM AUTHOR]

A systematic mechanistic study employing rate constant (kobs) kinetics measurements, 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 HCl. 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. [ABSTRACT FROM AUTHOR]

Understanding the dynamics of δ13C and δ18O in modern resin is crucial for interpreting (sub)fossilized resin records and resin production dynamics. We measured the δ13C and δ18O offsets between resin acids and their precursor molecules in the top‐canopy twigs and breast‐height stems of mature Pinus sylvestris trees. We also investigated the physiological and environmental signals imprinted in resin δ13C and δ18O at an intra‐seasonal scale. Resin δ13C was c. 2‰ lower than sucrose δ13C, in both twigs and stems, likely due to the loss of 13C‐enriched C‐1 atoms of pyruvate during isoprene formation and kinetic isotope effects during diterpene synthesis. Resin δ18O was c. 20‰ higher than xylem water δ18O and c. 20‰ lower than δ18O of water‐soluble carbohydrates, possibly caused by discrimination against 18O during O2‐based diterpene oxidation and 35%–50% oxygen atom exchange with water. Resin δ13C and δ18O recorded a strong signal of soil water potential; however, their overall capacity to infer intraseasonal environmental changes was limited by their temporal, within‐tree and among‐tree variations. Future studies should validate the potential isotope fractionation mechanisms associated with resin synthesis and explore the use of resin δ13C and δ18O as a long‐term proxy for physiological and environmental changes. Summary Statement: We studied the poorly understood intra‐seasonal dynamics of resin δ13C and δ18O. We found that resin δ13C was c. 2‰ lower than sucrose δ13C, while resin δ18O was c. 20‰ higher than xylem water δ18O in mature Pinus sylvestris. Both resin δ13C and δ18O recorded a signal of soil water potential. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A lanthanum silicate La9.33Si6O26 (LSO) crystallizes in an apatite-type structure and has been known as a promising oxide-ion conductor. Here, we report the activity of LSO for catalytic partial oxidation of methane (CPOX) to synthesis gas. The LSO catalyst demonstrated relatively high catalytic activity from 500 to 700 °C, with CH4 conversion reaching 22.1% at 700 °C while retaining moderate CO and H2 selectivities of 20–60%. Notably, LSO exhibited higher CPOX activity than non-apatite-type La2SiO5 despite their similar specific surface areas. The higher CPOX activity of LSO is likely attributed to its structural superiority involving mobile oxide ions in the crystal structure. The reaction kinetic study showed that the reaction orders for methane and oxygen in the CPOX reaction over the LSO catalyst were 0.69–0.73 and 0.08–0.21, respectively. Furthermore, the small contribution of adsorbed O species generated from gas-phase O2 molecules indicated that the lattice oxygen may be involved in the reaction mechanism. The kinetic isotope effect (KIE) study using a CD4 suggested that C–H bond breaking is the rate-determining step of CPOX over LSO. [ABSTRACT FROM AUTHOR]

Developing 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. [ABSTRACT FROM AUTHOR]

The reactive singlet state of oxygen (O2) can decay to the triplet ground state nonradiatively in the presence of a solvent. There is a controversy about whether tunnelling is involved in this nonadiabatic spin-crossover process. Semiclassical instanton theory provides a reliable and practical computational method for elucidating the reaction mechanism and can account for nuclear quantum effects such as zero-point energy and multidimensional tunnelling. However, the previously developed instanton theory is not directly applicable to this system because of a branch-point singularity which appears in the flux correlation function. Here we derive a new instanton theory for cases dominated by the singularity, leading to a new picture of tunnelling in nonadiabatic processes. Together with multireference electronic-structure theory, this provides a rigorous framework based on first principles that we apply to calculate the decay rate of singlet oxygen in water. The results indicate a new reaction mechanism that is 27 orders of magnitude faster at room temperature than the classical process through the minimum-energy crossing point. We find significant heavy-atom tunnelling contributions as well as a large temperature-dependent H2O/D2O kinetic isotope effect of approximately 20, in excellent agreement with experiment. Singlet oxygen relaxes in water to its triplet ground state. Here, the authors show that heavy-atom tunnelling speeds up this reaction from time scales longer than the age of the universe to microseconds. [ABSTRACT FROM AUTHOR]

As nitrogen analogues of iron‐oxo species, high‐valent iron‐imido species have attracted great interest in the past decades. FeV‐alkylimido species are generally considered to be key reaction intermediates in Fe(III)‐catalyzed C(sp3)─H bond aminations of alkyl azides but remain underexplored. Here, it is reported that iron‐corrole (Cor) complexes can catalyze a wide range of intramolecular C─H amination reactions of alkyl azides to afford a variety of 5‐, 6‐ and 7‐membered N‐heterocycles, including alkaloids and natural product derivatives, with up to 3880 turnover numbers (TONs) and excellent diastereoselectivity (>99:1 d.r.). Mechanistic studies including density functional theory (DFT) calculations and intermolecular hydrogen atom abstraction (HAA) reactions reveal key reactive FeV‐alkylimido intermediates. The [FeV(Cor)(NAd)] (Ad = adamantyl) complex is independently prepared and characterized through electron paramagnetic resonance (EPR), resonance Raman (rR) measurement, and X‐ray photoelectron spectroscopy (XPS). This complex is reactive toward HAA reactions with kinetic isotope effects (KIEs) similar to [Fe(Cor)]‐catalyzed intramolecular C─H amination of alkyl azides. [ABSTRACT FROM AUTHOR]

Understanding enzyme catalysis as connected to protein motions is a major challenge. Here, based on temperature kinetic studies combined with isotope effect measurements, we obtain energetic description of C-H activation in NAD-dependent UDP-glucuronic acid C4 epimerase. Approach from the ensemble-averaged ground state (GS) to the transition state-like reactive conformation (TSRC) involves, alongside uptake of heat ( Δ H ‡ = 54 kJ mol−1), significant loss in entropy (− T Δ S ‡ = 20 kJ mol−1; 298 K) and negative activation heat capacity ( Δ C p ‡ = −0.64 kJ mol−1 K−1). Thermodynamic changes suggest the requirement for restricting configurational freedom at the GS to populate the TSRC. Enzyme variants affecting the electrostatic GS preorganization reveal active-site interactions important for precise TSRC sampling and H-transfer. Collectively, our study captures thermodynamic effects associated with TSRC sampling and establishes rigid positioning for C-H activation in an enzyme active site that requires conformational flexibility in fulfillment of its natural epimerase function. Enzymes involve structural flexibility in their function, but understanding enzyme catalysis as connected to protein motions is a major challenge. Here, the authors obtain energetic description of C-H activation in nicotinamide coenzyme-dependent UDP-glucuronic acid C4 epimerase based on temperature kinetic studies and isotope effect measurements. [ABSTRACT FROM AUTHOR]

The role of quantum tunnelling (QT) in the proton transfer kinetics of the uranyl tetrahydroxide (UTH, [UO2(OH)4]2−) cis to trans isomerization was computationally studied under three possible reaction pathways. The first pathway involved a direct proton transfer from the hydroxide ligand to the oxo atom. In the other two pathways, one or two water molecules were added to the second sphere. The first H2O, bound by hydrogen bonds to the ligands, acts as a bridge enabling a proton shuttling, a concerted hopping of a proton from the hydroxide to the oxo atom similar to the Grotthuss mechanism. In the third pathway, the second water molecule does not participate in the H-transfer chain, but works as an anchor for the first water molecule, limiting its movement and therefore enhancing the QT. Since experimentally the reaction occurs in water, the first two pathways (no water or one H2O) serve only as models of the gas phase behaviour, while the third pathway will always be thermodynamically and kinetically preferred. The effects were investigated in the gas phase as well as in a continuum aqueous model, including the H/D Kinetic Isotope Effect (KIE). The results indicate that at very low temperatures, QT is the only mechanism that permits the reaction kinetics, consistent with the large computed KIE. At higher temperatures, thermally activated tunnelling competes with the classical crossing over the potential barrier. [ABSTRACT FROM AUTHOR]

Development of heterogeneous catalyst is significant key for achieving high performance of NaBH 4 hydrolysis to produce hydrogen. The stability of sodium borohydride in the presence of suitable catalyst is the efficient method to generate hydrogen. In this work, styrene-imid PIILP, polymer 1 decorated styrene, imidazolium ionic liquid, and cross-coupling is successfully synthesised by AIBN initiated radical polymerisation, then precipitation with 3,3′,3″phosphinetriyltribenzene sulfonate with ratio (1:0.3) to isolate the first styrene-imid- sulphonated PIILP, polymer 2. RuNP@styrene-imid-sulphonated PIILP, RuNPs catalyst 3 was synthesised by impregnation with RuCl 3 , then reduction by NaBH 4. Moreover, several analytical techniques such as, 31P, 1H, 13C solid-state NMR, CHN, IR, XPS, TEM, TGA-DTA, and SEM were examined to explore the obtained catalyst 3. The hydrogen generation from NaBH 4 utilizing catalyst 3 was exhibited the catalytic behaviour studying several parameters such as, temperature, catalytic loading, and NaBH 4 concentrations this work was deducted the activation energy (30.32 kJmol-1), and the turnover frequency (TOF) reached up 134.9, 112.77, and 98.38 moleH 2.mol cat −1.min−1 at 40, 35 and 30 °C. The effectiveness of Ruthenium nanoparticle catalyst 3 for the hydrolysis reaction of NaBH 4 was examined in H 2 O and D 2 O resulting kinetic isotope effect (KIE) k H /k D = 3. In addition, RuNPs demonstrated a promising efficient for reuse catalyst 3 after five cycles for the catalytic hydrolysis. [Display omitted] • RuNPs was synthesised and characterised by 31P and 13C Solid-State NMR, CHN, XPS, TEM, SEM, TGA and FT-IR. • A TOF 134.9 mole H2.mol cat −1.min−1 for RuNPs@styrene-imid-sulphonated PIILP was achieved. • E a of 30.32 for RuNPs@styrene-imid-sulphonated PIILP were achieved. • RuNPs demonstrated a promising efficiency for reuse RuNPs after five cycles for the catalytic hydrolysis. [ABSTRACT FROM AUTHOR]

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 δ15 Nα or δ15 Nβ 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. [ABSTRACT FROM AUTHOR]

Synthesis of novel benzothiazoles via intramolecular CS bond formation reactions is increasingly being explored since they have been found in a wide range of natural products and pharmaceutical agents. Sharma et al. reported the ruthenium‐catalyzed preparation of novel benzothiazole derivatives from N‐arylthiourea precursors, with a range of reaction yields and selectivity being observed. We have employed a density functional theory‐based computational model to investigate the reaction mechanism leading to the benzothiazole product and help uncover the origin of the differing experimental yields and substrate specificities. We proposed a modified mechanistic scheme where the rate‐determining step to be the synchronized breaking of the peroxide bond of the oxidizing agent with the concomitant proton‐coupled electron transfer from the haloarene urea and a Ru‐bound water molecule, not electrophilic RuC bond activation. Evidence for this being the rate‐determining step is (a) the barrier is consistent with a lack of kinetic isotope effects associated with the ortho‐H atom and (b) the computed rate‐determining barriers for 10 N‐arylthiourea substrates show good correlation with the observed yield. [ABSTRACT FROM AUTHOR]

Heavy water, containing the heavy hydrogen isotope, is toxic to cells, although the underlying mechanism remains incompletely understood. In addition, certain enzymatic proton transfer reactions exhibit kinetic isotope effects attributed to hydrogen isotopes and their temperature dependencies, indicative of quantum tunneling phenomena. However, the correlation between the biological effects of heavy water and the kinetic isotope effects mediated by hydrogen isotopes remains elusive. In this study, we elucidated the kinetic isotope effects arising from hydrogen isotopes of water and their temperature dependencies in vitro, focusing on deacetylation, DNA cleavage, and protein cleavage, which are crucial enzymatic reactions mediated by hydrolysis. Intriguingly, the intracellular isotope effects of heavy water, related to the in vitro kinetic isotope effects, significantly impeded multiple DNA double-strand break repair mechanisms crucial for cell survival. Additionally, heavy water exposure enhanced histone acetylation and associated transcriptional activation in cells, consistent with the in vitro kinetic isotope effects observed in histone deacetylation reactions. Moreover, as observed for the in vitro kinetic isotope effects, the cytotoxic effect on cell proliferation induced by heavy water exhibited temperature-dependency. These findings reveal the substantial impact of heavy water-induced isotope effects on cellular functions governed by hydrolytic enzymatic reactions, potentially mediated by quantum-level mechanisms underlying kinetic isotope effects. [ABSTRACT FROM AUTHOR]

Theoretical studies of the diffusional isotope effect in solids are still stuck in the 1960s and 1970s. With the development of high spatial resolution mass spectrometers, isotopic data of mineral grains are rapidly accumulated. To dig up information from these data, molecular-level theoretical models are urgently needed. Based on the microscopic definition of the diffusion coefficient (D), a new theoretical framework for calculating the diffusional isotope effect (DIE(v)) (in terms of D*/D) for vacancy-mediated impurity diffusion in solids is provided based on statistical mechanics formalism. The newly derived equation shows that the DIE(v) can be easily calculated as long as the vibration frequencies of isotope-substituted solids are obtained. The calculated DIE(v) values of 199Au/195Au and 60Co/57Co during diffusion in Cu and Au metals are all within 1% of errors compared to the experimental data, which shows that this theoretical model is reasonable and precise. [ABSTRACT FROM AUTHOR]

Deuteration has emerged as a significant method for investigating kinetic isotope effects in chemical reactions. In recent years, research on deuteration has revealed that this reaction often alters the pharmacokinetic or toxicological properties of drug molecules, thereby enhancing their therapeutic effect. Therefore, deuteration is considered as a potential avenue for drug research direction. Since the approval of the first deuterated drug, "deutetrabenazine," by the American Food and Drug Administration (FDA) in 2017, deuteration has been applied in the development of a variety of novel drugs. Research on deuteration technology has matured with the publication of numerous deuteration strategies. This article reviews recently reported strategies for late‐stage selective deuteration, discusses the challenges associated with current deuteration strategies, and presents future prospects for the synthesis of deuterated drugs. [ABSTRACT FROM AUTHOR]

A new approach to attenuating pathological inflammatory reactions by buffering the eicosanoid pathways with oxidation-resistant hexadeuterated arachidonic acid (D-ARA) is discussed. Enzymatic processing of ARA, released by phospholipase A2, by lipoxygenases, cyclooxygenases, and cytochromes yields a wide range of bioactive eicosanoids, including pro-inflammation, pro-angiogenesis and pro-thrombosis species that, when produced in excess, are an underlying cause of pathology. Conversely, some products of ARA oxidation possess pro-resolving properties. Non-enzymatic free radical oxidation of ARA generates another large group of products such as isoprostanes and their metabolites, associated with inflammation, ischemia-reperfusion stress, and atherosclerosis. A separate group comprises reactive carbonyl derivatives that irreversibly damage diverse biomolecules. Being resistant to both enzymatic and non-enzymatic oxidation pathways due to large kinetic isotope effects, D-ARA may play a role in mitigating inflammation-related disorders and conditions, including inflammaging. [ABSTRACT FROM AUTHOR]

The Cedars is an area in Northern California with a chain of highly alkaline springs resulting from CO2-charged meteorological water interacting with a peridotite body. Serpentinization resulting from this interaction at depth leads to the sequestration of various carbonate minerals into veins accompanied by a release of Ca2+ and OH– enriched water to the surface, creating an environment which promotes rapid precipitation of CaCO3 at surface springs. This environment enables us to apply the recently developed Δ47-Δ48 dual clumped isotope analysis to probe kinetic isotope effects (KIEs) and timescales of CO2 transformation in a region with the potential for geological CO2 sequestration. We analyzed CaCO3 recovered from various localities and identified significant kinetic fractionations associated with CO2 absorption in a majority of samples, characterized by enrichment in Δ47 values and depletion in Δ48 values relative to equilibrium. Surface floes exhibited the largest KIEs (ΔΔ47: 0.163‰, ΔΔ48: −0.761‰). Surface floe samples begin to precipitate out of solution within the first hour of CO2 absorption, and the dissolved inorganic carbon (DIC) pool requires a residence time of >100 h to achieve isotopic equilibria. The Δ48/Δ47 slope of samples from the Cedars (−3.223 ± 0.519) is within the range of published theoretical values designed to constrain CO2 hydrolysis-related kinetic fractionation (−1.724 to −8.330). The Δ47/δ18O slope (−0.009 ± 0.001) and Δ47/δ13C slope (−0.009 ± 0.001) are roughly consistent with literature values reported from a peridotite in Oman of −0.006 ± 0.002 and −0.005 ± 0.002, respectively. The consistency of slopes in the multi-isotope space suggests the Δ47-Δ48 dual carbonate clumped isotope framework can be applied to study CO2-absorption processes in applied systems, including sites of interest for geological sequestration.

Abstract We present a new clumped isotope calibration relating temperature to the Δ47 composition of the bioapatite scales of gar fish (Lepisosteidae family). Modern gars live at

In this work, the dynamics of a prototypical heavy–light–heavy abstract reaction, Cl(2P) + HCl → HCl + Cl(2P), is investigated both by constructing a new potential energy surface (PES) and by rate coefficient calculations. Both the permutation invariant polynomial neural network method and the embedded atom neural network (EANN) method, based on ab initio MRCI-F12+Q/AVTZ level points, are used for obtaining globally accurate full-dimensional ground state PES, with the corresponding total root mean square error being only 0.043 and 0.056 kcal/mol, respectively. In addition, this is also the first application of the EANN in a gas-phase bimolecular reaction. The saddle point of this reaction system is confirmed to be nonlinear. In comparison with both the energetics and rate coefficients obtained on both PESs, we find that the EANN is reliable in dynamic calculations. A full-dimensional approximate quantum mechanical method, ring-polymer molecular dynamics with a Cayley propagator, is employed to obtain the thermal rate coefficients and kinetic isotopic effects of the title reaction Cl(2P) + XCl→ XCl + Cl(2P) (H, D, Mu) on both new PESs, and the kinetic isotope effect (KIE) is also obtained. The rate coefficients reproduce the experimental results at high temperatures perfectly but with moderate accuracy at lower temperatures, but the KIE is with high accuracy. The similar kinetic behavior is supported by quantum dynamics using wave packet calculations as well. [ABSTRACT FROM AUTHOR]

We use the modified Bigeleisen–Mayer equation to compute kinetic isotope effect values for non-enzymatic phosphoryl transfer reactions from classical and path integral molecular dynamics umbrella sampling. The modified form of the Bigeleisen–Mayer equation consists of a ratio of imaginary mode vibrational frequencies and a contribution arising from the isotopic substitution's effect on the activation free energy, which can be computed from path integral simulation. In the present study, we describe a practical method for estimating the frequency ratio correction directly from umbrella sampling in a manner that does not require normal mode analysis of many geometry optimized structures. Instead, the method relates the frequency ratio to the change in the mass weighted coordinate representation of the minimum free energy path at the transition state induced by isotopic substitution. The method is applied to the calculation of 16/18O and 32/34S primary kinetic isotope effect values for six non-enzymatic phosphoryl transfer reactions. We demonstrate that the results are consistent with the analysis of geometry optimized transition state ensembles using the traditional Bigeleisen–Mayer equation. The method thus presents a new practical tool to enable facile calculation of kinetic isotope effect values for complex chemical reactions in the condensed phase. [ABSTRACT FROM AUTHOR]

2,4,6-triamino-1,3,5-trinitrobenzene (TATB) is an insensitive High Explosive (HE) that is widely studied to better understand the physical properties of safety and sensitivity of HE. A dominant initial decomposition mechanism of TATB is believed to be a dehydration reaction that forms mono- and di-furazans, although other mechanisms have been reported. In this work, seven initial decomposition mechanisms were modeled with ab initio simulations to calculate its free energy barriers, decomposition rates, and kinetic isotope effects. The energy barrier for mono-benzofurazan mechanisms was found to be high, > --> 61 kcal/mol in the gas phase; however, the reaction energy can decrease significantly in a disordered condensed state. The predicted kinetic isotope effect ratio of the furazan mechanism was found to be k h /k d ≈ 1.41 at 600 K, in agreement with the experiment. The NO 2 scission mechanism was found to be an entropy-driven mechanism because the free energy barrier decreased significantly with temperature, making it the most energetically favorable mechanism at high temperatures in the gas phase. The results provide a better understanding of the atomistic decomposition mechanisms of TATB and may be useful for improving models of safety and sensitivity. [ABSTRACT FROM AUTHOR]

The debate surrounding the roles of thermal and non-thermal pathways in plasmonic catalysis has captured the attention of researchers and sparked vibrant discussions within the scientific community. In this review, we embark on a thorough exploration of this intriguing discourse, starting from fundamental principles and culminating in a detailed understanding of the divergent viewpoints. We probe into the core of the debate by elucidating the behavior of excited charge carriers in illuminated plasmonic nanostructures, which serves as the foundation for the two opposing schools of thought. We present the key arguments and evidence put forth by proponents of both the non-thermal and thermal pathways, providing a perspective on their respective positions. Beyond the theoretical divide, we discussed the evolving methodologies used to unravel these mechanisms. We discuss the use of Arrhenius equations and their variations, shedding light on the ensuing debates about their applicability. Our review emphasizes the significance of localized surface plasmon resonance (LSPR), investigating its role in collective charge oscillations and the decay dynamics that influence catalytic processes. We also talked about the nuances of activation energy, exploring its relationship with the nonlinearity of temperature and light intensity dependence on reaction rates. Additionally, we address the intricacies of catalyst surface temperature measurements and their implications in understanding light-triggered reaction dynamics. The review further discusses wavelength-dependent reaction rates, kinetic isotope effects, and competitive electron transfer reactions, offering an all-inclusive view of the field. This review not only maps the current landscape of plasmonic photocatalysis but also facilitates future explorations and innovations to unlock the full potential of plasmon-mediated catalysis, where synergistic approaches could lead to different vistas in chemical transformations. The authors present a comprehensive review exploring the mechanistic understanding of plasmonic photocatalysis through examining the evidence for and interplay of thermal and non-thermal effects in this broad class of reactions. [ABSTRACT FROM AUTHOR]

The direct auration of arenes is a key step in numerous gold‐catalyzed reactions. Although reported more than 100 years ago, understanding of its underlying mechanism has been hampered by the difficulties in the isolation of relevant intermediates given the propensity of gold(III) species to undergo reductive elimination. Here, we report the synthesis and isolation of a new family of intriguing zwitterionic [C(sp3)^C(sp2)]‐auracyclopentanes, as well as of their alkyl‐gold(III) precursors and demonstrate their value as mechanistic probes to study the C(sp2)‐Au bond‐forming event. Experimental investigations employing Kinetic Isotope Effects (KIE), Hammett plot, and Eyring analysis provided important insights into the formation of the auracycle. The data suggest a SEAr mechanism wherein the slowest step might be the π‐coordination between the arene and the gold(III) center, en route to the Wheland intermediate. We also show that these auracyclopentanes can work as catalysts in several gold‐promoted transformations. [ABSTRACT FROM AUTHOR]

Whole microorganisms are rarely preserved in the fossil record but actively silicifying environments like hot springs provide an opportunity for microbial preservation, making silicifying environments critical for the study of microbial life through time on Earth and possibly other planetary bodies. Yet, the changes that biosignatures may undergo through lithification and burial remain unconstrained. At Steep Cone Geyser in Yellowstone National Park, we collected microbial material from (1) the living system across the active outflows, (2) the silicified areas adjacent to flows, and (3) lithified and buried material to assess the preservation of biosignatures and their changes across the lithification transect. Five biofabrics, built predominantly by CyanobacteriaGeitlerinema, Pseudanabaenaceae, and Leptolyngbya with some filamentous anoxygenic phototrophs contributions, were identified and tracked from the living system through the process of silicification/lithification. In the living systems, δ30Si values decrease from +0.13‰ in surficial waters to −2‰ in biomat samples, indicating a kinetic isotope effect potentially induced by increased association with actively growing biofabrics. The fatty acids C16:1 and iso‐C14:0 and the hydrocarbon C17:0 were disentangled from confounding signals and determined to be reliable lipid biosignatures for living biofabric builders and tenant microorganisms. Builder and tenant microbial biosignatures were linked to specific Cyanobacteria, anoxygenic phototrophs, and heterotrophs, which are prominent members of the living communities. Upon lithification and burial, silicon isotopes of silicified biomass began to re‐equilibrate, increasing from δ30Si −2‰ in living biomats to −0.55‰ in lithified samples. Active endolithic microbial communities were identified in lithified samples and were dominated by Cyanobacteria, heterotrophic bacteria, and fungi. Results indicate that distinct microbial communities build and inhabit silicified biofabrics through time and that microbial biosignatures shift over the course of lithification. These findings improve our understanding of how microbial communities silicify, the biomarkers they retain, and transitionary impacts that may occur through lithification and burial. [ABSTRACT FROM AUTHOR]

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 Δ 47 values, have suggested that this mineral might form in disequilibrium and consequently yield kinetic bias in Δ 47 -derived temperatures and fluid δ18O values. Here, we use dual clumped isotope proxies (Δ 47 and Δ 48) 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, Δ 47 and Δ 48 values) were measured. The Δ 47 and Δ 48 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 Δ 48 and lower Δ 47 values, than expected, with the disequilibrium arising as a result of CO 2 degassing in the vadose zone. Such deviations of Δ 47 and Δ 48 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 Δ 47 - and Δ 48 -derived temperatures close to expected, the same mineral forming near the water-table and upper phreatic zone shows much higher Δ 48 -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 pCO 2 , mediated by microbial sulfate reduction. Such differential kinetic behavior of Δ 48 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. [ABSTRACT FROM AUTHOR]

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 Δ60/58Ni dissolved-sorbed = +1.38 ‰). Neither closed-system equilibrium trends nor Rayleigh curves fit the data well. Fractionations are even larger at high ionic strength (Δ60/58Ni dissolved-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. [ABSTRACT FROM AUTHOR]

Herein, a series of pincer–Mn complexes based on bis(imino)pyridine ligands of the type R2NNN (R = tBu, iPr, Cy and Ph) were synthesized and characterized using various spectroscopic techniques. SCXRD studies revealed a trigonal bipyramidal geometry around the metal center in all the complexes. EPR spectroscopy confirmed the presence of high-spin Mn(II) centers with the consistent observation of sextets in EPR spectra. Additionally, solution magnetic moment measurement exhibited values ranging from 5.8 to 6.2 BM for all the complexes, which are in accordance with the theoretical value of 5.92 BM. HRMS analysis complemented structural characterization, showing fragments corresponding to various solvent adducts and derivatives of the complexes. Subsequently, the synthesized complexes were investigated for their catalytic activity in the transfer dehydrogenation of glycerol to lactic acid in the presence of acetone. Among the considered complexes, the catalyst Ph2NNNMnCl2 was found to be highly efficient. Remarkably, a yield of 92% LA was observed with >99% selectivity at 0.5 mol% loading of Ph2NNNMnCl2 in the presence of 1 equivalent of NaOH at 140 °C in 24 h, surpassing the yield obtained from its precursor MnCl2·4H2O, where a yield of 72% LA was observed with 96% selectivity under similar reaction conditions. This catalytic system was further investigated with a range of acceptors, and good to moderate yields were observed in most cases. Moreover, several control experiments, including reaction with PPh3, CS2 and Hg, highlighted the major involvement of molecular species in the reaction medium. Deuterium labelling studies indicated the involvement of C–H bond activation in the catalytic cycle but not in the rate-determining step (RDS), with a secondary kinetic isotope effect (KIE) of 1.25. [ABSTRACT FROM AUTHOR]