Temperature Dependence Of Reaction Rate Research Articles

Do vibrationally excited OH molecules affect middle and upper atmospheric chemistry?

Abstract. Except for a few reactions involving electronically excited molecular or atomic oxygen or nitrogen, atmospheric chemistry modelling usually assumes that the temperature dependence of reaction rates is characterized by Arrhenius' law involving kinetic temperatures. It is known, however, that in the upper atmosphere the vibrational temperatures may exceed the kinetic temperatures by several hundreds of Kelvins. This excess energy has an impact on the reaction rates. We have used upper atmospheric OH populations and reaction rate coefficients for OH(v=0...9)+O3 and OH(v=0...9)+O to estimate the effective (i.e. population weighted) reaction rates for various atmospheric conditions. We have found that the effective rate coefficient for OH(v=0...9)+O3 can be larger by a factor of up to 1470 than that involving OH in its vibrational ground state only. At altitudes where vibrationally excited states of OH are highly populated, the OH reaction is a minor sink of Ox and O3 compared to other reactions involving, e.g., atomic oxygen. Thus the impact of vibrationally excited OH on the ozone or Ox sink remains small. Among quiescent atmospheres under investigation, the largest while still small (less than 0.1%) effect was found for the polar winter upper stratosphere and mesosphere. The contribution of the reaction of vibrationally excited OH with ozone to the OH sink is largest in the upper polar winter stratosphere (up to 4%), while its effect on the HO2 source is larger in the lower thermosphere (up to 1.5% for polar winter and 2.5% for midlatitude night conditions). For OH(v=0...9)+O the effective rate coefficients are lower by up to 11% than those involving OH in its vibrational ground state. The effects on the odd oxygen sink are negative and can reach −3% (midlatitudinal nighttime lowermost thermosphere), i.e. neglecting vibrational excitation overestimates the odd oxygen sink. The OH sink is overestimated by up to 10%. After a solar proton event, when upper atmospheric OH can be enhanced by an order of magnitude, the excess relative odd oxygen sink by consideration of vibrational excitation in the reaction of OH(v=0...9)+O3 is estimated at up to 0.2%, and the OH sink by OH(v=0...9)+O can be reduced by 12% in the thermosphere by vibrational excitation.

Conversion degree in the head of spinning waves propagating in gas-solid SHS systems

The paper is a continuation of earlier studies by the author and coworkers on combustion in heterogeneous gas-solid systems. Since spinning combustion is usually treated as 2D thermal instability of planar combustion front, the previous studies took into account basically temperature dependence of reaction rate. In the present study, the reaction rate was also assumed to depend on the conversion degree of solid reagent (parabolic law of retardation). The domain of existence for spinning combustion mode was plotted on the parametric plane “initial temperature-spin head conversion degree”. Over a wide parametric range, the conversion degree in the spin head was found to vary between 0.08 and 0.54. Suggested is a new interpretation of multi-head spinning combustion in the systems under consideration.

Kinetics of the reaction of Cl atoms with CHCl 3 over the temperature range 253–313 K

The reaction CHCl 3 + Cl → CCl 3 + HCl was studied in the atmospherically relevant temperature range from 253 to 313 K and in 930 mbar of N 2 diluent using the relative rate method. A temperature dependent reaction rate constant, valid in the temperature range 220–330 K, was determined by a fit to the result of the present study and that of Orlando (1999); k = (3.77 ± 0.32) × 10 −12 exp((−1011 ± 24)/T) cm 3 molecule −1 s −1.

Kinetics of ochratoxin A destruction during coffee roasting

In the present study, Coffea arabica was artificially contaminated with spores of toxigenic Aspergillus westerdijkiae. The contaminated coffee was roasted in a vertical spouted bed roaster at four different temperatures (180 °C, 200 °C, 220 °C and 240 °C) and three different time periods (5, 8 and 12 min), in order to obtain more accurate results for the development of the kinetic model for ochratoxin A (OTA). Chlorogenic acids (CGA) content during coffee roasting was also evaluated to investigate the effect of the heat employed to destroy OTA in these health promoting compounds. Coffee treated with spouted bed roasting significantly reduced the OTA level from 8% to 98%. The spouted bed roasting proved to be a very efficient procedure for OTA reduction in coffee, and its reduction depended directly on the degree of roasting. OTA degradation during coffee roasting followed first order reaction kinetics. Using the apparent activation energy of OTA degradation and the temperature-dependent reaction rate, there was a compliance with the Arrhenius equation. This model was capable of predicting the thermal induced degradation of OTA and may become an important predicting tool in the coffee industry. The present study was also able to propose roasting conditions appropriate to destroy OTA and maintain most of the CGA at the same time.

Nonlinear phenomena in heterogeneous catalytic reactions with a branched-chain mechanism of formation of active centers

The branched-chain mechanism of multiplication of active centers on the surface of a catalyst results in a hysteresis temperature dependence of reaction rate. Domain structures appear because of local disturbances in the case of diffusion instability of uniform stationary states under certain conditions for transfer coefficients. Autowave transition processes are characterized by a “plateau“ of zero front velocities with the formation of standing waves.

Modelling and experimental results of heat transfer in a metal hydride store during hydrogen charge and discharge

A numerical model for the transient hydrogen charge/discharge rates and thermal behaviour of metal hydride stores was developed and verified against experiments using a cylindrical reactor filled with AB5-type metal hydride. The model assumes local thermal equilibrium between the gas and solid phases, and incorporates the pressure and temperature-dependent hydrogen reaction rates, as well as heat transfer in the porous metal hydride bed. The model was verified through experimental data. The experiments were performed using a unit with hydrogen storage capacity of 130 Nl H2; the store was submerged in an isothermal water bath. Experiments at different water bath temperatures and charge/discharge hydrogen pressures indicated a relation between charge/discharge time and these parameters. The reactor’s ability to deliver a constant hydrogen flow at different water bath temperatures was experimentally investigated. During simulations it was found that the model applied is sensitive to perturbations of some of its parameters; activation energy of absorption, effective conductivity and heat of reaction were found to be the most important ones. The charge and discharge performances of the store are controlled by the reaction rate in the first half-part of the H absorption/desorption experiments and by a heat transfer in the second half-part of charge/discharge. a 2009 Published by Elsevier Ltd on behalf of International Association for Hydrogen Energy.

Structural, Spectroscopic, and Kinetic Investigation of the Molybdenum Dialkylhydrazido Complexes [MoBr(NNC5H10)(dppe)2]Br and [Mo(NNC5H10)(dppe)2]: Activation Parameters and Revised Mechanism for N−N Cleavage†

The reaction of [Mo(NNC(5)H(10))(dppe)(2)] (B(Mo)) with an excess of acid, HNEt(3)BPh(4), is investigated applying temperature-dependent stopped-flow measurements. The kinetic data indicate a biphasic process with rate constants k(obs(1)) and k(obs(2)) which are both slower than the single rate constant reported by Henderson et al. (Henderson, R. A.; Leigh, G. J.; Pickett, C. J., J. Chem. Soc., Dalton Trans. 1989, 425-430). Moreover, both rate constants exhibit a linear dependence on the acid concentration with a large intercept which is attributed to acid-dependent and acid-independent components of each reaction phase, respectively. All four reaction channels exhibit temperature-dependent reaction rates. Furthermore, B(Mo) and its Mo(IV) analogue [MoBr(NNC(5)H(10))(dppe)(2)]Br (A(Mo)) are characterized structurally and spectroscopically. Density-functional theory calculations are performed to locate possible barriers in the overall reaction scheme and determine their energies, providing additional information for the formulation of a mechanism. The temperature-dependent rate of N-N cleavage is explained by a revised mechanism which involves an alpha-protonated intermediate that is inert with respect to N-N cleavage and is generated from its beta-protonated counterpart by a rapid 1,2-proton shift. The implications of these results with respect to N(2) reduction in the Chatt cycle and the enzyme nitrogenase are discussed.

Determination of Arrhenius and Thermodynamic Parameters for the Aqueous Reaction of the Hydroxyl Radical with Lactic Acid

Lactic acid is a major component of the TALSPEAK process planned for use in the separation of trivalent lanthanide and actinide elements. This acid acts both as a buffer and to protect the actinide complexant from radiolytic damage. However, there is little kinetic information on the reaction of water radiolysis species with lactic acid, particularly under the anticipated process conditions of aerated aqueous solution at pH approximately 3, where oxidizing reactions are expected to dominate. Here we have determined temperature-dependent reaction rate constants for the reactions of the hydroxyl radical with lactic acid and the lactate ion. For lactic acid this rate constant is given by the following equation: ln k(1) = (23.85 +/- 0.19) - (1120 +/- 54)/T, corresponding to an activation energy of 9.31 +/- 0.45 kJ mol(-1) and a room temperature reaction rate constant of (5.24 +/- 0.35) x 10(8) M(-1) s(-1) (24.0 degrees C). For the lactate ion, the temperature-dependent rate constant is given by ln k(2) = (24.83 +/- 0.14) - (1295 +/- 42)/T, for an activation energy of 10.76 +/- 0.35 kJ mol(-1) and a room temperature value of (7.77 +/- 0.50) x 10(8) M(-1) s(-1) (22.2 degrees C). These kinetic data have been combined with autotitration measurements to determine the temperature-dependent behavior of the lactic acid pK(a) value, allowing thermodynamic parameters for the acid dissociation to be calculated as DeltaH(o) = -10.75 +/- 1.77 kJ mol(-1), DeltaS(o) = -103.9 +/- 6.0 J K(-1) mol(-1) and DeltaG(o) = 20.24 +/- 2.52 kJ mol(-1) at low ionic strength.

High‐Throughput Kinetic Study of Hydrogenation over Palladium Nanoparticles: Combination of Reaction and Analysis

The hydrogenation of 1-acetylcyclohexene, cyclohex-2-enone, nitrobenzene, and trans-methylpent-3-enoate catalyzed by highly active palladium nanoparticles was studied by high-throughput on-column reaction gas chromatography. In these experiments, catalysis and separation of educts and products is integrated by the use of a catalytically active gas chromatographic stationary phase, which allows reaction rate measurements to be efficiently performed by employing reactant libraries. Palladium nanoparticles embedded in a stabilizing polysiloxane matrix serve as catalyst and selective chromatographic stationary phase for these multiphase reactions (gas-liquid-solid) and are coated in fused-silica capillaries (inner diameter 250 microm) as a thin film of thickness 250 nm. The palladium nanoparticles were prepared by reduction of palladium acetate with hydridomethylsiloxane-dimethylsiloxane copolymer and self-catalyzed hydrosilylation with methylvinylsiloxane-dimethylsiloxane copolymer to obtain a stabilizing matrix. Diphenylsiloxane-dimethylsiloxane copolymer (GE SE 52) was added to improve film stability over a wide range of compositions. Herein, we show by systematic TEM investigations that the size and morphology (crystalline or amorphous) of the nanoparticles strongly depends on the ratio of the stabilizing polysiloxanes, the conditions to immobilize the stationary phase on the surface of the fused-silica capillary, and the loading of the palladium precursor. Furthermore, hydrogenations were performed with these catalytically active stationary phases between 60 and 100 degrees C at various contact times to determine the temperature-dependent reaction rate constants and to obtain activation parameters and diffusion coefficients.

Further studies of flow and reactive pollutant dispersion in a street canyon with bottom heating

This study numerically investigates how flow and reactive pollutant dispersion in a street canyon with a canyon aspect ratio of one vary with the street-bottom heating intensity. For this, numerical simulations are performed over a wide range of street-bottom heating intensities (Δ T=0–15 °C in 1 °C intervals) using a Reynolds-averaged Navier–Stokes equations (RANS) model with NO–NO 2–O 3 photochemistry. The pollutants NO and NO 2 are emitted from near the street bottom in the presence of background O 3. A primary vortex is formed in the street canyon in all the simulated cases, but the location of the vortex center at the cross-canyon plane becomes quasi-stationary or meanders in the street canyon, depending on the street-bottom heating intensity. The time series of the street canyon-averaged NO concentration at the cross-canyon plane exhibits a quasi-steady, oscillatory, or fluctuating pattern, depending on the street-bottom heating intensity. As the street-bottom heating intensity increases, the averaged NO and NO 2 concentrations tend to decrease and the magnitude of the roof-level area-integrated and time-averaged vertical mean (turbulent) flux between Δ T=2 and 13 °C tends to increase (decrease). Some peculiar features are found in the cases of Δ T=11, 12, and 13 °C in which there are strong downward motion near the downwind building wall, downward motion below the roof level near the upwind building wall, and strong reverse flow in the lower region of the street canyon. Moreover, the vortex center is shifted toward the upwind building wall and does not meander in the street canyon after a certain period of time. Following the downward motions below the roof level, the O 3-containing ambient air is considerably entrained into the street canyon, resulting in a large increase in the O 3 concentration in the street canyon. In the cases of Δ T=11, 12, and 13 °C, the magnitude of the vertical mean flux is larger than that of the vertical turbulent flux. The direct effect of the inhomogeneous temperature distribution on the O 3 concentration via the temperature-dependent photolysis rate and reaction rate constant increases with the street-bottom heating intensity, although the averaged fractional difference in O 3 concentrations is small.

Transient yielding of CO 2 over oxidized ruthenium surfaces: Monte Carlo simulations

We develop Monte Carlo simulations to study the catalytic oxidation of CO over a surface of ruthenium. The catalyst is exposed to a continuous flux of CO molecules and its surface is pre-covered with an amount of oxygen atoms. Recent experiments performed on this system [R. Blume, W. Christen, H. Niehus, J. Phys. Chem. B 110 (2006) 13912] have shown that three different reaction mechanisms can account for the experimental results. Two of them are based on the Langmuir–Hinshelwood mechanism, where CO molecules are adsorbed at oxygen-free defect sites before reactions take place. The third one proceeds via the Eley–Rideal mechanism, which is almost time independent, and reactions occur at non-defect sites. In our model, we consider a semi-infinite cubic lattice to mimic the surface of the catalyst and oxygen atoms are incorporated into the layers below the surface. A fraction of defects is created at the topmost layer and at the first subsurface layer. Oxygen atoms can diffuse over the surface as well as between adjacent layers of the system. We also assumed a temperature dependent reaction rate that is related to the residence time of CO at the surface. Comparisons are made between the CO 2 yielding at defect-rich and smooth surfaces as a function of temperature.

Nonequilibrium Zeldovich-von Neumann-Doring theory and reactive flow modeling of detonation

This paper discusses the Nonequilibrium Zeldovich-von Neumann-Doring (NEZND) theory of self-sustaining detonation waves and the Ignition and Growth reactive flow model of shock initiation and detonation wave propagation in solid explosives. The NEZND theory identified the nonequilibrium excitation processes that precede and follow the exothermic decomposition of a large high explosive molecule into several small reaction product molecules. The thermal energy deposited by the leading shock wave must be distributed to the vibrational modes of the explosive molecule before chemical reactions can occur. The induction time for the onset of the initial endothermic reactions can be calculated using high pressure-high temperature transition state theory. Since the chemical energy is released well behind the leading shock front of a detonation wave, a physical mechanism is required for this chemical energy to reinforce the leading shock front and maintain its overall constant velocity. This mechanism is the amplification of pressure wavelets in the reaction zone by the process of de-excitation of the initially highly vibrationally excited reaction product molecules. This process leads to the development of the three-dimensional structure of detonation waves observed for all explosives. For practical predictions of shock initiation and detonation in hydrodynamic codes, phenomenological reactive flow models have been developed. The Ignition and Growth reactive flow model of shock initiation and detonation in solid explosives has been very successful in describing the overall flow measured by embedded gauges and laser interferometry. This reactive flow model uses pressure and compression dependent reaction rates, because time-resolved experimental temperature data is not yet available. Since all chemical reaction rates are ultimately controlled by temperature, the next generation of reactive flow models will use temperature dependent reaction rates. Progress on a statistical hot spot ignition and growth reactive flow model with multistep Arrhenius chemical reaction pathways is discussed.

Modeling reactive pollutant dispersion in an urban street canyon

Reactive pollutant dispersion in an urban street canyon with a street aspect ratio of one is numerically investigated using a computational fluid dynamics (CFD) model. The CFD model developed is a Reynolds-averaged Navier–Stokes equations (RANS) model with the renormalization group (RNG) k–ε turbulence model and includes transport equations for NO, NO 2, and O 3 with simple photochemistry. An area emission source of NO and NO 2 is considered in the presence of background O 3 and street bottom heating (Δ T=5 °C) with an ambient wind perpendicular to the along-canyon direction. A primary vortex is formed in the street canyon and the line connecting the centers of cross-sectional vortices meanders over time and in the canyon space. The cross-canyon-averaged temperature and reactive pollutant concentrations oscillate with a period of about 15 min. The averaged temperature is found to be in phase with NO and NO 2 concentrations but out of phase with O 3 concentration. The photostationary state defect is small in the street canyon except for near the roof level and the upper downwind region of the canyon and its local minimum is observed near the center of the primary vortex. The budget analysis of NO (NO 2) concentration shows that the magnitude of the advection or turbulent diffusion term is much larger (larger) than that of the chemical reaction term and that the advection term is largely balanced by the turbulent diffusion term. On the other hand, the budget analysis of O 3 concentration shows that the magnitude of the chemical reaction term is comparable to that of the advection or turbulent diffusion term. The inhomogeneous temperature distribution itself affects O 3 concentration to some extent due to the temperature-dependent photolysis rate and reaction rate constant.

Kinetics of Base‐Catalyzed Dehydrochlorination of Hexachlorocyclohexanes: I. Homogeneous Systems

Base-catalyzed dehydrochlorination of alpha-, beta-, and gamma-hexachlorocyclohexane (HCH) was examined at different pH and temperature conditions. No reaction was observed for beta-HCH under all tested conditions likely due to the fact that all chlorines are at the equatorial positions. Highly pH- and temperature-dependent reaction rates were observed for alpha- and gamma-HCH, and pentachlorocyclohexenes (PCCHs) and 1,2,4- and 1,2,3-trichlorobenzene (TCB) were detected as the intermediates and final products of the transformation of both alpha- and gamma-HCH. The pseudo-first-order rate constants of each step of the reactions were calculated at different T and pH conditions. For the first step, the rate constants were at approximately 0.0005 d-1 in solutions below pH 7.0, and increased by about an order of magnitude per pH unit from pH 7.01 to pH 12.02. The second-order reaction rate constants (kb) of this step were 3.57+/-0.03 and 3.19+/-0.19 M-1 d-1, respectively, and the associated activation energies (Ea) at pH 9.26 were 60.4+/-7.8 and 67.7+/-8.7 kJ mol-1, respectively, for alpha- and gamma-HCH. The rate constants for the formation of 1,2,3-TCB (k21) and 1,2,4-TCB (k22) were 0.0032+/-0.0001 d-1 and 0.051+/-0.003 d-1 from alpha-HCH, and 0.0017+/-0.0001 d-1 and 0.0041+/-0.0002 d-1 from gamma-HCH at pH 8.28 and 25 degrees C. Both k21 and k22 also showed an increase by about one order of magnitude per pH unit from pH 8.28 to pH 12.02. The Ea values were 64.4+/-6.8 and 88.8+/-5.2 kJ mol-1, respectively, for the formation of 1,2,4-TCB and 1,2,3-TCB from alpha-HCH, and 70.6+/-8.7 and 92.0+/-4.9 kJ mol-1, respectively, for the formation of 1,2,4- and 1,2,3-TCB from gamma-HCH at pH 9.26. Data provided by this study may be used for calculation of the percentages of the TCBs formed at different environmental conditions.

PREPARATION AND CHARACTERIZATION OF HYDROXYAPATITE BY A REACTION OF CALCIUM TUNGSTATE WITH PHOSPHATE

Hydroxyapatite was prepared by a reaction of CaWO4 with phosphate at temperatures from 90 °C to 210 °C. High concentration of NaOH was appropriate for the formation processes of hydroxyapatite. The temperature dependence of reaction rate was related to the mass transport through a reaction layer. Very small activation energy of reaction, 25.5 kJ/mol, was attributed to the diffusion of Ca2+ ions in liquid among reaction products.

Crystallization Kinetics of Cubic Boron Nitride in the B-N-Mg-H-F System

The kinetics of the h-BN → c-BN transformation in a mixture of composition 78% h-BN + 5% NH4F + 2% B + 15% Mg are studied at 5 GPa and temperatures from 1670 to 1910 K. The activation energy of c-BN formation, temperature-dependent reaction rate constant, strength of the forming c-BN particles, and density of paramagnetic defects in c-BN are evaluated.

Hydrogen tunneling in quinoproteins

It is now widely accepted that substrate C–H bond breakage by quinoprotein enzymes occurs by quantum mechanical tunneling. This paradigm shift in the conceptual framework for these reactions away from semi-classical transition state theory (i.e., including zero-point energy but with no tunneling correction) has been driven over recent years by experimental studies of the temperature dependence of kinetic isotope effects for these reactions in the TTQ-dependent enzymes methylamine dehydrogenase and aromatic amine dehydrogenase, which produced observations also inconsistent with the simple Bell correction model of tunneling. However, these data—specifically, the strong temperature dependence of reaction rates and the variable temperature dependence of kinetic isotope effects—are consistent with other tunneling models (denoted full tunneling models) in which protein and/or substrate fluctuations generate a configuration compatible with tunneling. These models accommodate substrate/protein (environment) fluctuations required to attain a configuration with degenerate quantum states and, when necessary, motion required to increase the probability of tunneling in these states. Furthermore, tunneling mechanisms in quinoproteins are supported by computational studies employing variational transition state theory with multidimensional tunneling corrections; these studies are also discussed in this review. Potential pitfalls in analyzing the temperature dependence of kinetic isotope effects as probes of tunneling are also discussed with reference to PQQ-dependent methanol dehydrogenase.

Energy Transfer from NH(aΔ,v) to O2(X3Σg-): Kinetics of Single Vibrational States

This experimental investigation reports the temperature-dependent reaction rate constants for electronically excited, metastable NH(a) and ND(a) radicals in the v = 0−3 levels that are transferring electronic energy to O2 to yield O2(b). The covered temperature range is 209−421 K. All measured Arrhenius activation energies (Ea) are essentially independent of v and fall into the range of 6.7−9.0 kJ/mol, with an average value of 7.6 kJ/mol, except for NH(a,v = 3), which has an Ea value of 3.5 ±1.6 kJ/mol. Obviously, for the lowest levels, vibrational excitation does not markedly accelerate the energy transfer and does not seem to be favorable to overcome the small activation barrier. From this and previous work, we propose that a vibrationally adiabatic process generates NH(X,v). In the case of NH(a,v = 3), we suggest that an additional channel opens.

Laboratory and Homework Exercises in the Geochemical Kinetics of Mineral-Water Reaction: Rate Law, Arrhenius Activation Energy, and the Rate-Determining Step in the Dissolution of Halite

This exercise for mineralogy/geochemistry courses uses optical microscopy to measure the grain sizes of equant halite crystals, and the time for complete dissolution of each grain. The required equipment is readily available in most science departments: microscopes, thermometers, stopwatches, glassware, and table salt. Using hot plates, the procedure can be repeated at a second, elevated temperature, allowing determination of the temperature dependence (Arrhenius activation energy) of the reaction. The data generated are used in homework exercises, to calculate (1) a simple rate law for the dissolution of halite in pure water; (2) the rate coefficient in that rate law, at two different temperatures; and (3) the activation energy of the halite dissolution reaction. The lab and homework illustrate several basic principles of chemical kinetics directly relevant to geology, including diffusion, the temperature dependence of reaction rates, and the relationship between rate-determining mechanisms and crystal-surface morphology.

H-tunneling in the Multiple H-transfers of the Catalytic Cycle of Morphinone Reductase and in the Reductive Half-reaction of the Homologous Pentaerythritol Tetranitrate Reductase

The mechanism of flavin reduction in morphinone reductase (MR) and pentaerythritol tetranitrate (PETN) reductase, and flavin oxidation in MR, has been studied by stopped-flow and steady-state kinetic methods. The temperature dependence of the primary kinetic isotope effect for flavin reduction in MR and PETN reductase by nicotinamide coenzyme indicates that quantum mechanical tunneling plays a major role in hydride transfer. In PETN reductase, the kinetic isotope effect (KIE) is essentially independent of temperature in the experimentally accessible range, contrasting with strongly temperature-dependent reaction rates, consistent with a tunneling mechanism from the vibrational ground state of the reactive C-H/D bond. In MR, both the reaction rates and the KIE are dependent on temperature, and analysis using the Eyring equation suggests that hydride transfer has a major tunneling component, which, unlike PETN reductase, is gated by thermally induced vibrations in the protein. The oxidative half-reaction of MR is fully rate-limiting in steady-state turnover with the substrate 2-cyclohexenone and NADH at saturating concentrations. The KIE for hydride transfer from reduced flavin to the alpha/beta unsaturated bond of 2-cyclohexenone is independent of temperature, contrasting with strongly temperature-dependent reaction rates, again consistent with ground-state tunneling. A large solvent isotope effect (SIE) accompanies the oxidative half-reaction, which is also independent of temperature in the experimentally accessible range. Double isotope effects indicate that hydride transfer from the flavin N5 atom to 2-cyclohexenone, and the protonation of 2-cyclohexenone, are concerted and both the temperature-independent KIE and SIE suggest that this reaction also proceeds by ground-state quantum tunneling. Our results demonstrate the importance of quantum tunneling in the reduction of flavins by nicotinamide coenzymes. This is the first observation of (i) three H-nuclei in an enzymic reaction being transferred by tunneling and (ii) the utilization of both passive and active dynamics within the same native enzyme.