Kinetic Isotope Effect Studies Research Articles

Effects of Appended Poly(ethylene glycol) on Electrochemical CO2 Reduction by an Iron Porphyrin Complex

Electrochemical carbon dioxide (CO2) reduction is a sustainable approach for transforming atmospheric CO2 into chemical feedstocks and fuels. To overcome the kinetic barriers of electrocatalytic CO2 reduction, catalysts with high selectivity, activity, and stability are needed. Here, we report an iron porphyrin complex, FePEGP, with a poly(ethylene glycol) unit in the second coordination sphere, as a highly selective and active electrocatalyst for the electrochemical reduction of CO2 to carbon monoxide (CO). Controlled-potential electrolysis using FePEGP showed a Faradaic efficiency of 98% and a current density of -7.8 mA/cm2 at -2.2 V versus Fc/Fc+ in acetonitrile using water as the proton source. The maximum turnover frequency was calculated to be 1.4 × 105 s-1 using foot-of-the-wave analysis. Distinct from most other catalysts, the kinetic isotope effect (KIE) study revealed that the protonation step of the Fe-CO2 adduct is not involved in the rate-limiting step. This model shows that the PEG unit as the secondary coordination sphere enhances the catalytic kinetics and thus is an effective design for electrocatalytic CO2 reduction.

Brønsted Acid Organocatalyzed Three-Component Hydroamidation Reactions of Vinyl Ethers.

Hydroamidation of carbon-carbon double bonds is an attractive strategy for installing nitrogen functionality into molecular scaffolds and, with it, increasing molecular complexity. To date, metal-based approaches have dominated this area of chemical synthesis, despite the drawbacks of air and moisture sensitivity, limited functional group tolerance, toxicity, and/or high cost often associated with using metals. Here, in offering an alternative solution, we disclose an operationally simple, metal-free, one-pot, regioselective, multicomponent synthetic procedure for the hydroamidation of carbon-carbon double bonds. This method features mild reaction conditions and utilizes isocyanides and vinyl ethers for the rapid and modular synthesis of privileged α-oxygenated amide scaffolds. In unraveling the mechanistic underpinning of this non-metal-based reactivity, we present kinetic solvent isotope effect studies, variable time normalization analysis, and density functional theory computations offering insight into the mechanism of this multistep catalytic hydroamidation process.

Proton Removal Kinetics That Govern the Hydrogen Peroxide Oxidation Activity of Heterogeneous Bioinorganic Platforms.

Precise regulation of proton-coupled electron-transfer (PCET) rates holds the key to simultaneously optimizing the turnover frequency and product selectivity of redox reactions that are central to the realization of renewable energy schemes in a sustainable future. In this work, a self-assembled monolayer (SAM) of a Ru complex electrografted onto a glassy carbon (GC) electrode was prepared as a heterogeneous electrocatalytic interface to facilitate the hydrogen peroxide (H2O2) oxidation half-cell reaction of a direct hydrogen peroxide/hydrogen peroxide fuel cell. A functional lipid membrane embedded with catalytic amounts of proton carriers was appended on top of the Ru SAM to construct a hybrid bilayer membrane (HBM) platform that can modulate the thermodynamics and kinetics of proton- and electron-transfer steps independently. The performances of the as-prepared Ru SAMs and HBMs toward H2O2 oxidation were investigated using electrochemical means, kinetic isotope effect (KIE) studies, and Tafel analyses. Proton carriers featuring borate, phosphate, and nitrile headgroups were found to dictate the transmembrane proton removal rate, thereby controlling the H2O2 oxidation activity. The first significance of this work was the expansion of HBM platforms to GC substrates to overcome the limited redox potential window on gold thiol systems, thereby enabling electrochemical investigations of anodic reactions at the SAM-lipid interface. The second highlight of this work was demonstrating for the first time that deprotonation kinetics can be taken advantage of to enhance the electrocatalytic oxidation performance of a metal complex anchored at the SAM-lipid interface of a HBM platform. When the knowledge gaps regarding how PCET steps govern redox pathways are closed, the advances achieved using our unique bioinorganic platform are envisioned to accelerate the understanding and optimization of electrocatalytic processes involving proton- and electron- transfer steps that are fundamental to the development of high-performance energy devices.

Temperature-Dependent Kinetic Isotope Effects in R67 Dihydrofolate Reductase from Path-Integral Simulations.

Calculation of temperature-dependent kinetic isotope effects (KIE) in enzymes presents a significant theoretical challenge. Additionally, it is not trivial to identify enzymes with available experimental accurate intrinsic KIEs in a range of temperatures. In the current work, we present a theoretical study of KIEs in the primitive R67 dihydrofolate reductase (DHFR) enzyme and compare with experimental work. The advantage of R67 DHFR is its significantly lower kinetic complexity compared to more evolved DHFR isoforms. We employ mass-perturbation-based path-integral simulations in conjunction with umbrella sampling and a hybrid quantum mechanics–molecular mechanics Hamiltonian. We obtain temperature-dependent KIEs in good agreement with experiments and ascribe the temperature-dependent KIEs primarily to zero-point energy effects. The active site in the primitive enzyme is found to be poorly preorganized, which allows excessive water access to the active site and results in loosely bound reacting ligands.

Ruthenium (II) Catalyzed C(sp2 )-H Bond Alkenylation of 2-Arylbenzo[d]oxazole and 2-Arylbenzo[d]thiazole with Unactivated Olefins.

Functionalization of the bio-relevant heterocycles 2-arylbenzo[d]oxazole and 2-arylbenzo[d]thiazole has been achieved through Ru(II)-catalyzed alkenylation with unactivated olefins leading to selective formation of the mono-alkenylated products. This approach has a broad substrate scope with respect to the coupling partners, affords high yields, and works for gram scale synthesis using a readily available Ru-based catalyst. Mechanistic studies reveal a C-H activation pathway for the dehydrogenative coupling leading to the alkenylation. However, the results of the ESI-MS-guided deuterium kinetic isotope effect studies indicate that the C-H activation stage may not be the rate-determining step of the reaction. The use of a radical scavenging agent such as TEMPO did not show any detrimental effect on the reaction outcome, eliminating the possibility of the involvement of a free-radical pathway.

A Calix[4]arene-Based Cyclic Dinuclear Ruthenium Complex for Light-Driven Catalytic Water Oxidation.

A cyclic dinuclear ruthenium(bda) (bda: 2,2’‐bipyridine‐6,6’‐dicarboxylate) complex equipped with oligo(ethylene glycol)‐functionalized axial calix[4]arene ligands has been synthesized for homogenous catalytic water oxidation. This novel Ru(bda) macrocycle showed significantly increased catalytic activity in chemical and photocatalytic water oxidation compared to the archetype mononuclear reference [Ru(bda)(pic)2]. Kinetic investigations, including kinetic isotope effect studies, disclosed a unimolecular water nucleophilic attack mechanism of this novel dinuclear water oxidation catalyst (WOC) under the involvement of the second coordination sphere. Photocatalytic water oxidation with this cyclic dinuclear Ru complex using [Ru(bpy)3]Cl2 as a standard photosensitizer revealed a turnover frequency of 15.5 s−1 and a turnover number of 460. This so far highest photocatalytic performance reported for a Ru(bda) complex underlines the potential of this water‐soluble WOC for artificial photosynthesis.

Exploring the Shuffling Roles of Interfacial Water for the HER/HOR of Pt-Surfaces in Aqueous Solutions

Previous studies focus mainly on the inherent properties of the catalysts such as the metal-hydrogen binding energy (EM-H) to rationalize their catalytic activities toward the hydrogen evolution and oxidation reactions (HER/HOR) in aqueous solutions, as well as the HER/HOR kinetics. While this strategy proves effective in rationalizing and predicting the HER/HOR activity trend across a wide range of elements in dilute acidic solutions as per the Sabatier Principle, it is inapplicable to account for the sluggish HER/HOR kinetics of many transition metals in alkaline solutions. Recent works suggest that interfacial water plays critical roles in affecting the HER/HOR kinetics of Pt-based surfaces in alkaline solutions.1-4 However, the specific roles of interfacial water for the HER/HOR kinetics remain elusive, partly because of the lack of methods to explore the surface-electrolyte interface.Herein, we probe the Pt electrode/electrolyte interface by depositing a variety of transition metals (TMs) onto Pt surfaces and monitoring their redox-controlled interactions with the HOR/HER intermediates and interfacial water via in situ x-ray absorption spectroscopy. Carbon monoxide stripping experiments with varied alkaline metal cations (AM+) concentrations are conducted to facilitate correlating the redox-controlled interactions to the TM-induced changes of the HOR/HER kinetics of Pt. We found that the TM-induced changes of the HOR/HER kinetics of Pt in alkaline are governed by the TM0/TM(OH)x redox potential or the metal-oxygen binding energy (ETM-O). The TMs with too strong ETM-O or too low redox potentials such as Mn or Co are dominated by TM(OH)x within the HOR potential region and can only improve the HER kinetics of Pt but not the HOR. The TMs with moderate ETM-O or redox potentials located within the HOR kinetic potential region of Pt (0-0.25 V) such as Ni and Ru can simultaneously improve the HOR/HER of Pt in alkaline, but the HOR improvement vanishes as the TM0 transforms to TM(OH)x with increasing potentials. The TMs with too high redox potentials or weak ETM-O(s) such as Cu improve neither. According to these results, we propose that the TM in either the form of TM0 or TM(OH)x can improve the HER of Pt in alkaline by hosting the OHad generated from water dissociation that is subsequently removed by the interfacial water coordinated to AM+ via the HSAB interaction: TM0/TM(OH)x-OHad-[AM(H2O)x]+ + e- ↔ TM0/TM(OH)x + OH--[AM(H2O)x]+;1 whereas only the form of TM0 can improve the HOR by hosting the water molecule with the oxygen facing-down orientation (H2O↓ad) that removes Had from the Pt atoms nearby via the L-H mechanism: Pt-Had + TM0-H2O↓ad ↔ Pt + TM0 + H3O+ + e-. By this new notion, the surface TM promotes the HOR/HER kinetics of Pt by facilitating interfacial water shuffling reaction intermediates against the electric field, rather than by weakening the electric field by negatively shifting the potential of zero free charge (pzfc)2, or by weakening the (apparent) EPt-H,5 or by facilitating the dissociation and formation of water molecules.6,7 The participation of interfacial water in the HOR/HER of Pt is further verified by kinetic isotope effect studies in both acidic and alkaline solutions. Acknowledgements.This work was supported by the Office of Naval Research (ONR) under award number N00014-18-1-2155. The authors declare no competing financial interests. This research used beamline 6-BM, 7-BM, and 8-ID (ISS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. References (1) Liu, E.; Li, J.; Jiao, L.; Doan, H. T. T.; Liu, Z.; Zhao, Z.; Huang, Y.; Abraham, K. M.; Mukerjee, S.; Jia, Q. J. Am. Chem. Soc. 2019, 141, 3232-3239.(2) Ledezma-Yanez, I.; Wallace, W. D. Z.; Sebastián-Pascual, P.; Climent, V.; Feliu, J. M.; Koper, M. T. M. Nat. Energy 2017, 2, 17031.(3) Herranz, J.; Durst, J.; Fabbri, E.; Patru, A.; Cheng, X.; Permyakova, A. A.; Schmidt, T. J.Nano Energy 2016, 29, 4-28.(4) Rebollar, L.; Intikhab, S.; Snyder, J. D.; Tang, M. H. J. Phys. Chem. Lett. 2020, 11, 2308-2313.(5) Schwämmlein, J. N.; Stühmeier, B. M.; Wagenbauer, K.; Dietz, H.; Tileli, V.; Gasteiger, H. A.; El-Sayed, H. A. J. Electrochem. Soc. 2018, 165, H229-H239.(6) Li, J.; Ghoshal, S.; Bates, M. K.; Miller, T. E.; Davies, V.; Stavitski, E.; Attenkofer, K.; Mukerjee, S.; Ma, Z.-F.; Jia, Q. Angew. Chem. Int. Ed. 2017, 56, 15594-15598.(7) Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; Van Der Vliet, D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Nat. Chem. 2013, 5, 300-306.

Mechanistic Insights into Fe Catalyzed α‐C−H Oxidations of Tertiary Amines

AbstractWe report detailed mechanistic investigations of an iron‐based catalyst system, which allows the α‐C−H oxidation of a wide variety of amines. In contrast to other catalysts that effect α‐C−H oxidations of tertiary amines, the system under investigation exclusively employs peroxy esters as oxidants. More common oxidants (e. g. tBuOOH) previously reported to affect amine oxidations via free radical pathways do not provide amine α‐C−H oxidation products in combination with the described catalyst system. The investigations described herein employ initial rate kinetics, kinetic profiling, DFT calculations as well as Eyring, kinetic isotope effect, Hammett, ligand coordination, and EPR studies to shed light on the Fe catalyst system. The obtained data suggest that the catalytic mechanism proceeds through C−H abstraction at a coordinated substrate molecule. This rate‐determining step occurs either through an Fe(IV) oxo pathway or a 2‐electron pathway at an Fe(II) intermediate with bound oxidant. DFT calculations indicate that the Fe(IV) oxo mechanism will be the preferred route of these two possibilities. We further show via kinetic profiling and EPR studies that catalyst activation follows a radical pathway, which is initiated by hydrolysis of PhCO3tBu to tBuOOH. Overall, the obtained mechanistic data support a non‐classical, Fe catalyzed pathway that requires substrate binding, inducing selectivity for α‐C−H functionalization.

Additive-Free Radical Cascade Reaction of Oxime Esters: Synthesis of Pyrroline-Functionalized Phenanthridines.

A variety of dihydropyrrole-functionalized phenanthridines were efficiently synthesized by the metal-free, radical cascade cyclization reaction of 2-isocyanobiphenyls with γ,δ-unsaturated oxime esters. The C-N/C-C/C-C bonds were formed via the oil bath method in a one-pot procedure with broad substrate applicability. The radical process was supported by kinetic isotope effect studies and radical inhibition studies.

Enantioselective Aryl-Iodide-Catalyzed Wagner-Meerwein Rearrangements.

We report a strategy for effecting catalytic, enantioselective carbocationic rearrangements through the intermediacy of alkyl iodanes as stereodefined carbocation equivalents. Asymmetric Wagner-Meerwein rearrangements of β-substituted styrenes are catalyzed by the C2-symmetric aryl iodide 1 to provide access to enantioenriched 1,3-difluorinated molecules possessing interesting and well-defined conformational properties. Hammett and kinetic isotope effect studies, in combination with computational investigations, reveal that two different mechanisms are operative in these rearrangement reactions, with the pathway depending on the identity of the migrating group. In reactions involving alkyl-group migration, intermolecular fluoride attack is product- and enantio-determining. In contrast, reactions in which aryl rearrangement occurs proceed through an enantiodetermining intramolecular 1,2-migration prior to fluorination. The fact that both pathways are promoted by the same chiral aryl iodide catalyst with high enantioselectivity provides a compelling illustration of generality across reaction mechanisms in asymmetric catalysis.

Dioxygen Activation by Internally Aromatic Metallacycle: Crystallographic Structure and Mechanistic Investigations.

SummaryMononuclear metal-peroxo species are invoked as the key intermediates in metalloenzymatic or synthetic catalysis. However, either transience or sluggishness reactivity of synthetic analogs of metal-peroxo species impedes our understanding of oxygen activation mechanism. Herein, we designed and characterized a dioxygen-derived mononuclear osmium-peroxo complex, in which the peroxo ligand is stabilized by internally aromatic metallacycle. We demonstrate that the osmium-peroxo species shows catalytic activity toward promoterless alcohol dehydrogenations. Furthermore, computational studies provide a new mechanism for the osmium-peroxo-mediated alcohol oxidation, starting with the concerted double-hydrogen transfer and followed by the generation of osmium-oxo species. Interestingly, the internally aromatic metallacycle also plays a vital role in catalysis, which mediates the hydrogen transfer from osmium center to the distal oxygen atom of Os–OOH moiety, thus facilitating the Os–OOH→Os=O conversion. We expect that these insights will advance the development of aromatic metallacycle toward aerobic oxidation catalysis.

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

Recent experimental studies of kinetic isotope effects (KIE-s) and hydrogen tunnelling comprising three proton-coupled electron transfer (PCET) oxidations of ascorbate monoanion, (a) in aqueous reaction solutions, (b) in the mixed water-organic cosolvent systems, (c) in aqueous solutions of various salts and (d) in fairly diluted aqueous solutions of the various partial hydrophobes are reviewed. A number of new insights into the wealth of the kinetic isotope phenomena in the PCET reactions have been obtained. The modulation of KIE-s and hydrogen tunnelling observed when partially hydrophobic solutes are added into water reaction solution, in the case of fairly diluted solutions is revealed as the strong linear correlation of the isotopic ratios of the Arrhenius prefactors Ah/Ad and the isotopic differences in activation energies ΔEa (D,H). The observation has been proposed to be a signature of the involvement of the collective intermolecular excitonic vibrational dynamics of water in activation processes and aqueous chemistry.

A Selenourea-Thiourea Brønsted Acid Catalyst Facilitates Asymmetric Conjugate Additions of Amines to α,β-Unsaturated Esters.

β-Amino esters are obtained with high levels of enantioselectivity via the conjugate addition of cyclic amines to unactivated α,β-unsaturated esters. A related strategy enables the kinetic resolution of racemic cyclic 2-arylamines, using benzyl acrylate as the resolving agent. Reactions are facilitated by an unprecedented selenourea-thiourea organocatalyst. As elucidated by DFT calculations and 13C kinetic isotope effect studies, the rate-limiting and enantiodetermining step of the reaction is the protonation of a zwitterionic intermediate by the catalyst. This represents a rare case in which a thiourea compound functions as an asymmetric Brønsted acid catalyst.

Kinetics of Water Gas Shift Reaction on Au/CeZrO4: A Comparison Between Conventional Heating and Dielectric Barrier Discharge (DBD) Plasma Activation

The kinetics of the low-temperature forward water gas shift (LT-WGS) reaction have been studied over a 2 wt% Au/CeZrO4 (Au/CZO) catalyst using both thermal and dielectric barrier discharge plasma heterogeneous catalyst systems. Using the energy density (e), the apparent activation energy has been calculated under plasma and thermally activated conditions. A substantially lower apparent activation energy is observed in the plasma activated system (9.5 kJ/mol) compared with the thermal-catalysed reaction (132.9 kJ/mol). Different kinetic isotope effect (KIE) values for water were found in thermal (1.43) and plasma (1.89) activated catalytic systems which infer different mechanisms between the two activation processes and also shows the importance of water activation. Furthermore, negative and positive reaction orders with respect to CO and H2O are found for both conditions which are − 1.30, 0.28 under thermal and − 1.53, 0.35 under plasma processes, respectively. The reaction order with respect to H2O and KIE studies demonstrate that the bond cleavage in H2O molecule is a rate determining step in the plasma-assisted LT-WGS, similar to that in the thermal-assisted reaction.

Mechanistic and Kinetic Studies of the Ring Opening Metathesis Polymerization of Norbornenyl Monomers by a Grubbs Third Generation Catalyst.

The mechanism of ring-opening metathesis polymerization (ROMP) for a set of functionalized norbornenyl monomers initiated by a Grubbs third generation precatalyst [(H2IMes)(pyr)2(Cl)2Ru═CHPh] was investigated. Through a series of 12C/13C and 1H/2H kinetic isotope effect studies, the rate-determining step for the polymerization was determined to be the formation of the metallacyclobutane ring. This experimental result was further validated through DFT calculations showing that the highest energy transition state is metallacyclobutane formation. The effect of monomer stereochemistry (exo vs endo) of two types of ester substituted monomers was also investigated. Kinetic and spectroscopic evidence supporting the formation of a six-membered chelate through coordination of the proximal polymer ester to the Ru center is presented. This chelation and its impact on the rate of polymerization are shown to vary based on the monomer employed and its stereochemistry. The combination of this knowledge led to the derivation of a generic rate law describing the rate of polymerization of norbornene monomers initiated by a Grubbs third generation catalyst.

Kinetic Isotope Effect of the Oxygen Reduction Reaction on Carbon-Supported Platinum Electrocatalysts

Proton exchange membrane fuel cells (PEMFC) can partially satisfy the energy need with the least anthropogenic impact on nature. However, they suffer from great costs, partly induced by their expensive catalytic material, platinum, necessary to catalyze the oxygen reduction reaction (ORR) occurring at the cathode. Hence, it becomes essential to either synthesize electrocatalysts without platinum1, or to maximize the performance of the fuel cell per unit mass of platinum2 by increasing the exposure of its active sites for the ORR.3 We studied the oxygen reduction reaction (ORR) mechanism on Pt nanoparticles (NPs) supported on several Vulcan-type carbon blacks with various physicochemical characteristics. Those materials were thoroughly investigated by various microscopic, spectroscopic and electrochemical methods, such as kinetic isotope effect (KIE) studies. KIE studies were used to assess an eventual change of the rate determining step (RDS). The rate constants of the RDS were calculated in 0.5M H2SO4 and 0.5M D2SO4 at the onset potential. The KIE values are reported in Figure 1, as a function of Scarbon/SPt,H . Scarbon/SPt,H corresponds to the ratio between the carbon specific surface and the platinum electrochemically active surface and acts as an indicator of the Pt NPs density onto the carbon surface. A low Scarbon/SPt,H shows KIE = 1, which corresponds to what is commonly observed on Pt electrocatalysts for the ORR and implies that the rate determining step is proton independent.4 An identical value was observed on the commercial Pt/C (40 wt. % on Vulcan XC72R carbon, i.e. with the high density of Pt NPs), i.e. KIE = 1.11 ± 0.08. At high Scarbon/SPt,H a KIE = ca. 1.5 is observed, hence, providing evidence for a shift from a proton independent to a proton dependent mechanism with the increase of the Scarbon/SPt,H : the involvement of the proton in the ORR RDS is dependent on the density of Pt NPs on the electrocatalyst surface. (1)Chen, Y.; Asset, T.; Lee, R.; Artyushkova, K.; Atanassov, P. J. Phys. Chem. C 2019, acs.jpcc.9b01480. (2)Robson, M. H.; Serov, A.; Artyushkova, K.; Atanassov, P. Electrochim. Acta 2013, 90, 656–665. (3)Matanović, I.; Garzon, F. H.; Henson, N. J. J. Phys. Chem. C 2011, 115 (21), 10640–10650. (4)Tse, E. C. M.; Varnell, J. A.; Hoang, T. T. H.; Gewirth, A. A. J. Phys. Chem. Lett. 2016, 7 (18), 3542–3547. Figure 1

Diels‐Alder Cycloaddition of C60 with Photochemically Generated Hydroxy to o‐quinodimethanes Governed by Steric Factors: A Mechanistic Study

Photoexcited o‐alkyl‐substituted benzaldehydes add to C60through their photoenol reactive intermediates producing stable [4 + 2] fullerene adducts. A mechanistic approach for this reactivity of C60 is provided, based mainly on intra‐ and intermolecular kinetic isotope effects (KIEs) studies. It is demonstrated that the rotation of Caromatic–Ccarbonyl single bond in the excited triplet state of o‐alkyl carbonyl compounds, is the rate‐determining step (rds), whereas C‐H(D) bond scission during the photoenolization step, occurs relatively fast.

Cavity Catalysis by Cooperative Vibrational Strong Coupling of Reactant and Solvent Molecules.

Here, we report the catalytic effect of vibrational strong coupling (VSC) on the solvolysis of para‐nitrophenyl acetate (PNPA), which increases the reaction rate by an order of magnitude. This is observed when the microfluidic Fabry–Perot cavity in which the VSC is generated is tuned to the C=O vibrational stretching mode of both the reactant and solvent molecules. Thermodynamic experiments confirm the catalytic nature of VSC in the system. The change in the reaction rate follows an exponential relation with respect to the coupling strength of the solvent, indicating a cooperative effect between the solvent molecules and the reactant. Furthermore, the study of the solvent kinetic isotope effect clearly shows that the vibrational overlap of the C=O vibrational bands of the reactant and the strongly coupled solvent molecules is critical for the catalysis in this reaction. The combination of cooperative effects and cavity catalysis confirms the potential of VSC as a new frontier in chemistry.

Cavity Catalysis by Cooperative Vibrational Strong Coupling of Reactant and Solvent Molecules

AbstractHere, we report the catalytic effect of vibrational strong coupling (VSC) on the solvolysis of para‐nitrophenyl acetate (PNPA), which increases the reaction rate by an order of magnitude. This is observed when the microfluidic Fabry–Perot cavity in which the VSC is generated is tuned to the C=O vibrational stretching mode of both the reactant and solvent molecules. Thermodynamic experiments confirm the catalytic nature of VSC in the system. The change in the reaction rate follows an exponential relation with respect to the coupling strength of the solvent, indicating a cooperative effect between the solvent molecules and the reactant. Furthermore, the study of the solvent kinetic isotope effect clearly shows that the vibrational overlap of the C=O vibrational bands of the reactant and the strongly coupled solvent molecules is critical for the catalysis in this reaction. The combination of cooperative effects and cavity catalysis confirms the potential of VSC as a new frontier in chemistry.

Hydrogen by Deuterium Substitution in an Aldehyde Tunes the Regioselectivity by a Nonheme Manganese(III)-Peroxo Complex.

Mononuclear nonheme MnIII -peroxo complexes are important intermediates in biology, and take part in oxygen activation by photosystem II. Herein, we present work on two isomeric biomimetic side-on MnIII -peroxo intermediates with bispidine ligand system and reactivity patterns with aldehydes. The complexes are characterized with UV/Vis and mass spectrometric techniques and reaction rates with cyclohexane carboxaldehyde (CCA) are measured. The reaction gives an unusual regioselectivity switch from aliphatic to aldehyde hydrogen atom abstraction upon deuteration of the substrate, leading to the corresponding carboxylic acid product for the latter, while the former gives a deformylation reaction. Mechanistic details are established from kinetic isotope effect studies and density functional theory calculations. Thus, replacement of C-H by C-D raises the hydrogen atom abstraction barriers and enables a regioselectivity switch to a competitive pathway that is slightly higher in energy.