Proton-transfer reactions between nitroalkanes and hydroxide ion under non-steady-state conditions. Apparent and real kinetic isotope effects.

The kinetics of the proton-transfer reactions between 1-nitro-1-(4-nitrophenyl)ethane (NNPE(H(D))) and hydroxide ion in water/acetonitrile (50/50 vol %) were studied at temperatures ranging from 289 to 319 K. The equilibrium constants for the reactions are large under these conditions, ensuring that the back reaction is not significant. The extent of reaction/time profiles during the first half-lives are compared with theoretical data for the simple single-step mechanism and a 2-step mechanism involving initial donor/acceptor complex formation followed by unimolecular proton transfer and dissociation of ions. In all cases, the profiles for the reactions of both NNPE(H) and NNPE(D) deviate significantly from those expected for the simple single-step mechanism. Excellent fits of experimental data with theoretical data for the complex mechanism, in the pre-steady-state time period, were observed in all cases. At all base concentrations (0.5 to 5.0 mM) and at all temperatures the apparent kinetic isotope effects (KIE(app)) were observed to increase with increasing extent of reaction. Resolution of the kinetics into microscopic rate constants at 298 K resulted in a real kinetic isotope effect (KIE(real)) for the proton-transfer step equal to 22. Significant proton tunneling was further indicated by the temperature dependence of the rate constants for proton and deuteron transfers: KIE(real) ranging from 17 to 26, E(a)(D) -- E(a)(H) equal 2.8 kcal/mol, and A(D)/A(H) equal to 4.95.     

Pressure effects on enzyme-catalyzed quantum tunneling events arise from protein-specific structural and dynamic changes

The rate and kinetic isotope effect (KIE) on proton transfer during the aromatic amine dehydrogenase-catalyzed reaction with phenylethylamine shows complex pressure and temperature dependences. We are able to rationalize these effects within an environmentally coupled tunneling model based on constant pressure molecular dynamics (MD) simulations. As pressure appears to act anisotropically on the enzyme, perturbation of the reaction coordinate (donor-acceptor compression) is, in this case, marginal. Therefore, while we have previously demonstrated that pressure and temperature dependences can be used to infer H-tunneling and the involvement of promoting vibrations, these effects should not be used in the absence of atomistic insight, as they can vary greatly for different enzymes. We show that a pressure-dependent KIE is not a definitive hallmark of quantum mechanical H-tunneling during an enzyme-catalyzed reaction and that pressure-independent KIEs cannot be used to exclude tunneling contributions or a role for promoting vibrations in the enzyme-catalyzed reaction. We conclude that coupling of MD calculations with experimental rate and KIE studies is required to provide atomistic understanding of pressure effects in enzyme-catalyzed reactions.     

Proton transfer reactions of methylanthracene radical cations with pyridine bases under non-steady-state conditions. Real kinetic isotope effect evidence for extensive tunneling.

The kinetics of the proton transfer reactions between the 9-methyl-10-phenylanthracene radical cation (MPA(+)(.)) with 2,6-lutidine were studied in acetonitrile-Bu(4)NBF(4) (0.1 M) using derivative cyclic voltammetry. Comparisons of extent of reaction-time profiles with theoretical data for both the simple single-step proton transfer and a mechanism involving the formation of a donor-acceptor complex prior to unimolecular proton transfer were made. The experimental extent of reaction-time profiles deviated significantly from those simulated for the single-step mechanism, while excellent fits of experimental to theoretical data, in the pre-steady-state period, for the complex mechanism were observed. In this time period, the apparent deuterium kinetic isotope effects (KIE(app)) were observed to vary significantly with the extent of reaction as predicted by the complex mechanism. Resolution of the apparent rate constants into the microscopic rate constants for the complex mechanism resulted in a real kinetic isotope effect (KIE(real)) equal to 82 at 291 K. Arrhenius activation parameters (252-312 K) for the reactions of MPA(+)(*) with 2,6-lutidine in acetonitrile-Bu(4)NBF(4) (0.1 M) revealed E(a)(D) - E(a)(H) equal to 2.89 kcal/mol and A(D)/A(H) equal to 2.09. In this temperature range, KIE(real) varied from 46 at the highest temperature to 134 at the lowest. The large KIE(real), along with the Arrhenius parameters, are indicative of extensive tunneling for the proton transfer steps.     

Large Isotope Effects in Organometallic Chemistry

The kinetic isotope effect (KIE) is key to understanding reaction mechanisms in many areas of chemistry and chemical biology, including organometallic chemistry. This ratio of rate constants, k_H/k_D, typically falls between 1–7. However, KIEs up to 105 have been reported, and can even be so large that reactivity with deuterium is unobserved. We collect here examples of large KIEs across organometallic chemistry, in catalytic and stoichiometric reactions, along with their mechanistic interpretations. Large KIEs occur in proton transfer reactions such as protonation of organometallic complexes and clusters, protonolysis of metal–carbon bonds, and dihydrogen reactivity. C−H activation reactions with large KIEs occur with late and early transition metals, photogenerated intermediates, and abstraction by metal-oxo complexes. We categorize the mechanistic interpretations of large KIEs into the following three types: (a) proton tunneling, (b) compound effects from multiple steps, and (c) semi-classical effects on a single step. This comprehensive collection of large KIEs in organometallics provides context for future mechanistic interpretation.     

Rate-promoting vibrations and coupled hydrogen-electron transfer reactions in the condensed phase: a model for enzymatic catalysis

A model is presented for coupled hydrogen-electron transfer reactions in condensed phase in the presence of a rate promoting vibration. Large kinetic isotope effects (KIEs) are found when the hydrogen is substituted with deuterium. While these KIEs are essentially temperature independent, reaction rates do exhibit temperature dependence. These findings agree with recent experimental data for various enzyme-catalyzed reactions, such as the amine dehydrogenases and soybean lipoxygenase. Consistent with earlier results, turning off the promoting vibration results in an increased KIE. Increasing the barrier height increases the KIE, while increasing the rate of electron transfer decreases it. These results are discussed in light of other views of vibrationally enhanced tunneling in enzymes.     

Proton-coupled electron transfer in soybean lipoxygenase: dynamical behavior and temperature dependence of kinetic isotope effects

The dynamical behavior and the temperature dependence of the kinetic isotope effects (KIEs) are examined for the proton-coupled electron transfer reaction catalyzed by the enzyme soybean lipoxygenase. The calculations are based on a vibronically nonadiabatic formulation that includes the quantum mechanical effects of the active electrons and the transferring proton, as well as the motions of all atoms in the complete solvated enzyme system. The rate constant is represented by the time integral of a probability flux correlation function that depends on the vibronic coupling and on time correlation functions of the energy gap and the proton donor-acceptor mode, which can be calculated from classical molecular dynamics simulations of the entire system. The dynamical behavior of the probability flux correlation function is dominated by the equilibrium protein and solvent motions and is not significantly influenced by the proton donor-acceptor motion. The magnitude of the overall rate is strongly influenced by the proton donor-acceptor frequency, the vibronic coupling, and the protein/solvent reorganization energy. The calculations reproduce the experimentally observed magnitude and temperature dependence of the KIE for the soybean lipoxygenase reaction without fitting any parameters directly to the experimental kinetic data. The temperature dependence of the KIE is determined predominantly by the proton donor-acceptor frequency and the distance dependence of the vibronic couplings for hydrogen and deuterium. The ratio of the overlaps of the hydrogen and deuterium vibrational wavefunctions strongly impacts the magnitude of the KIE but does not significantly influence its temperature dependence. For this enzyme reaction, the large magnitude of the KIE arises mainly from the dominance of tunneling between the ground vibronic states and the relatively large ratio of the overlaps between the corresponding hydrogen and deuterium vibrational wavefunctions. The weak temperature dependence of the KIE is due in part to the dominance of the local component of the proton donor-acceptor motion.     

Mechanistic Insights into RNA Transphosphorylation from Kinetic Isotope Effects and Linear Free Energy Relationships of Model Reactions

Phosphoryl transfer reactions are ubiquitous in biology and the understanding of the mechanisms whereby these reactions are catalyzed by protein and RNA enzymes is central to reveal design principles for new therapeutics. Two of the most powerful experimental probes of chemical mechanism involve the analysis of linear free energy relations (LFERs) and the measurement of kinetic isotope effects (KIEs). These experimental data report directly on differences in bonding between the ground state and the rate‐controlling transition state, which is the most critical point along the reaction free energy pathway. However, interpretation of LFER and KIE data in terms of transition‐state structure and bonding optimally requires the use of theoretical models. In this work, we apply density‐functional calculations to determine KIEs for a series of phosphoryl transfer reactions of direct relevance to the 2′‐O‐transphosphorylation that leads to cleavage of the phosphodiester backbone of RNA. We first examine a well‐studied series of phosphate and phosphorothioate mono‐, di‐ and triesters that are useful as mechanistic probes and for which KIEs have been measured. Close agreement is demonstrated between the calculated and measured KIEs, establishing the reliability of our quantum model calculations. Next, we examine a series of RNA transesterification model reactions with a wide range of leaving groups in order to provide a direct connection between observed Brønsted coefficients and KIEs with the structure and bonding in the transition state. These relations can be used for prediction or to aid in the interpretation of experimental data for similar non‐enzymatic and enzymatic reactions. Finally, we apply these relations to RNA phosphoryl transfer catalyzed by ribonuclease A, and demonstrate the reaction coordinate–KIE correlation is reasonably preserved. A prediction of the secondary deuterium KIE in this reaction is also provided. These results demonstrate the utility of building up knowledge of mechanism through the systematic study of model systems to provide insight into more complex biological systems such as phosphoryl transfer enzymes and ribozymes.     

Tunneling and classical paths for proton transfer in an enzyme reaction dominated by tunneling: oxidation of tryptamine by aromatic amine dehydrogenase

Proton tunneling dominates the oxidative deamination of tryptamine catalyzed by the enzyme aromatic amine dehydrogenase. For reaction with the fast substrate tryptamine, a H/D kinetic isotope effect (KIE) of 55 +/- 6 has been reported-one of the largest observed in an enzyme reaction. We present here a computational analysis of this proton-transfer reaction, applying combined quantum mechanics/molecular mechanics (QM/MM) methods (PM3-SRP//PM3/CHARMM22). In particular, we extend our previous computational study (Masgrau et al. Science 2006, 312, 237) by using improved energy corrections, high-level QM/MM methods, and an ensemble of paths to estimate the tunneling contributions. We have carried out QM/MM molecular dynamics simulations and variational transition state theory calculations with small-curvature tunneling corrections. The results provide detailed insight into the processes involved in the reaction. Transfer to the O2 oxygen of the catalytic base, Asp128beta, is found to be the favored reaction both thermodynamically and kinetically, even though O1 is closer in the reactant complex. Comparison of quantum and classical models of proton transfer allows estimation of the contribution of hydrogen tunneling in lowering the barrier to reaction in the enzyme. A reduction of the activation free energy due to tunneling of 3.1 kcal mol-1 is found, which represents a rate enhancement due to tunneling by 2 orders of magnitude. The calculated KIE of 30 is significantly elevated over the semiclassical limit, in agreement with the experimental observations; a semiclassical value of 6 is obtained when tunneling is omitted. A polarization of the C-H bond to be broken is observed due to the close proximity of the catalytic aspartate and the (formally) positively charged imine nitrogen. A comparison is also made with the related quinoprotein methylamine dehydrogenase (MADH)-the much lower KIE of 11 that we obtain for the MADH/methylamine system is found to arise from a more endothermic potential energy surface for the MADH reaction.     

Deep tunneling dominates the biologically important hydride transfer reaction from NADH to FMN in morphinone reductase

The temperature dependence of the primary kinetic isotope effect (KIE), combined temperature-pressure studies of the primary KIE, and studies of the alpha-secondary KIE previously led us to infer that hydride transfer from nicotinamide adenine dinucleotide to flavin mononucleotide in morphinone reductase proceeds via environmentally coupled hydride tunneling. We present here a computational analysis of this hydride transfer reaction using QM/MM molecular dynamics simulations and variational transition-state theory calculations. Our calculated primary and secondary KIEs are in good agreement with the corresponding experimental values. Although the experimentally observed KIE lies below the semiclassical limit, our calculations suggest that approximately 99% of the reaction proceeds via tunneling: this is the first "deep tunneling" reaction observed for hydride transfer. We also show that the dominant tunneling mechanism is controlled by the isotope at the primary rather than the secondary position: with protium in the primary position, large-curvature tunneling dominates, whereas with deuterium in this position, small-curvature tunneling dominates. Also, our study is consistent with tunneling being preceded by reorganization: in the reactant, the rings of the nicotinamide and isoalloxazine moieties are stacked roughly parallel to each other, and as the system moves toward a "tunneling-ready" configuration, the nicotinamide ring rotates to become almost perpendicular to the isoalloxazine ring.     

Conversion and origin of normal and abnormal temperature dependences of kinetic isotope effect in hydride transfer reactions

The effects of substituents on the temperature dependences of kinetic isotope effect (KIE) for the reactions of the hydride transfer from the substituted 5-methyl-6-phenyl-5,6-dihydrophenanthridine (G-PDH) to thioxanthylium (TX(+)) in acetonitrile were examined, and the results show that the temperature dependences of KIE for the hydride transfer reactions can be converted by adjusting the nature of the substituents in the molecule of the hydride donor. In general, electron-withdrawing groups can make the KIE to have normal temperature dependence, but electron-donating groups can make the KIE to have abnormal temperature dependence. Thermodynamic analysis on the possible pathways of the hydride transfer from G-PDH to TX(+) in acetonitrile suggests that the transfers of the hydride anion in the reactions are all carried out by the concerted one-step mechanism whether the substituent is an electron-withdrawing group or an electron-donating group. But the examination of Hammett-type free energy analysis on the hydride transfer reactions supports that the concerted one-step hydride transfer is not due to an elementary chemical reaction. The experimental values of KIE at different temperatures for the hydride transfer reactions were modeled by using a kinetic equation formed according to a multistage mechanism of the hydride transfer including a returnable charge-transfer complex as the reaction intermediate; the real mechanism of the hydride transfer and the root that why the temperature dependences of KIE can be converted as the nature of the substituents are changed were discovered.     

Quantum-classical Liouville dynamics of proton and deuteron transfer rates in a solvated hydrogen-bonded complex

Proton and deuteron transfer reactions in a hydrogen-bonded complex dissolved in a polar solution are studied using quantum-classical Liouville dynamics. Reactive-flux correlation functions that involve quantum-classical Liouville dynamics for species operators and quantum equilibrium sampling are used to calculate the rate constants. Adiabatic and nonadiabatic reaction rates are computed, compared, and analyzed. Large variations of the kinetic isotope effect (KIE) for this reaction have been observed in the literature, which depend on the nature of the approximate calculation used to estimate the proton and deuteron transfer rates. Our estimate of the KIE lies at the low end of the range of previously observed values, suggesting a rather small KIE for this reaction.     

Hydride transfer catalyzed by xylose isomerase: mechanism and quantum effects

We have applied molecular dynamics umbrella-sampling simulation and ensemble-averaged variational transition state theory with multidimensional tunneling (EA-VTST/MT) to calculate the reaction rate of xylose-to- xylulose isomerization catalyzed by xylose isomerase in the presence of two Mg2+ ions. The calculations include determination of the free energy of activation profile and ensemble averaging in the transmission coefficient. The potential energy function is approximated by a combined QM/MM/SVB method involving PM3 for the quantum mechanical (QM) subsystem, CHARMM22 and TIP3P for the molecular mechanical (MM) environment, and a simple valence bond (SVB) local function of two bond distances for the hydride transfer reaction. The simulation confirms the essential features of a mechanism postulated on the basis of kinetics and X-ray data by Whitlow et al. (Whitlow, M.; Howard, A. J.; Finzel, B. C.; Poulos, T. L.; Winborne, E.; Gilliland, G. L. Proteins 1991, 9, 153) and Ringe, Petsko, and coworkers (Labie, A.; Allen, K.-N.; Petsko, G. A.; Ringe, D. Biochemistry 1994, 33, 5469). This mechanism involves a rate-determining 1,2-hydride shift with prior and post proton transfers. Inclusion of quantum mechanical vibrational energy is important for computing the free energy of activation, and quantum mechanical tunneling effects are essential for computing kinetic isotope effects (KIEs). It is found that 85% of the reaction proceeds by tunneling and 15% by overbarrier events. The computed KIE for the ratio of hydride to deuteride transfer is in good agreement with the experimental results. The molecular dynamics simulations reveal that proton and hydride transfer reactions are assisted by breathing motions of the mobile Mg2+ ion in the active site, providing evidence for concerted motion of Mg2+ during the hydride transfer step.     

Proton-coupled hole transfer in X-irradiated doped crystalline cytosine.H2O

Following exposure to X-irradiation at low temperatures, the main reactions taking place in single crystals of cytosine monohydrate doped with minute amounts of 2-thiocytosine are hole transfer (HT) from the electron-loss centers to the dopant and recombination of oxidation and reduction products, assumedly by electron transfer. A huge deuterium kinetic isotope effect (KIE; >102-103) at 100 K, together with the kinetic curves obtained and density functional theory (DFT) calculations of equilibrium energy changes, indicates that these reactions proceed through a concerted proton-coupled electron/hole transfer where the proton transfer occurs between hydrogen-bonded cytosine molecules. The temperature dependence of these reaction rates between 10 and 150 K in normal and partially deuterated samples was investigated by monitoring the growth and decay of the various radical species over time using electron paramagnetic resonance (EPR) spectroscopy. By assuming a random distribution of the hole donors and acceptors in the crystals, the data are consistent with an exponential distance-dependent rate, giving a distance decay constant (beta) around 1 A-1 for the HT, which indicates that a long-range single-step superexchange mechanism mediates the charge transfer. The reactions undergo a transition from a slow, weakly temperature-dependent rate to an Arrhenius-type rate at 40-50 K, presumably being activated by excitation of low-frequency intermolecular vibrations that couple to the process. Below this transition temperature, the transfer probability might be dominated by temperature-independent nuclear tunneling. A similar beta value in both temperature regions suggests that hopping is not activated.     

Kinetic isotope effects of L-Dopa decarboxylase

A mixed centroid path integral and free energy perturbation method (PI-FEP/UM) has been used to investigate the primary carbon and secondary hydrogen kinetic isotope effects (KIEs) in the amino acid decarboxylation of L-Dopa catalyzed by the enzyme L-Dopa decarboxylase (DDC) along with the corresponding uncatalyzed reaction in water. DDC is a pyridoxal 5'-phosphate (PLP) dependent enzyme. The cofactor undergoes an internal proton transfer between the zwitterionic protonated Schiff base configuration and the neutral hydroxyimine tautomer. It was found that the cofactor PLP makes significant contributions to lowering the decarboxylation barrier, while the enzyme active site provides further stabilization of the transition state. Interestingly, the O-protonated configuration is preferred both in the Michaelis complex and at the decarboxylation transition state. The computed kinetic isotope effects (KIE) on the carboxylate C-13 are consistent with that observed on decarboxylation reactions of other PLP-dependent enzymes, whereas the KIEs on the α carbon and secondary proton, which can easily be validated experimentally, may be used as a possible identification for the active form of the PLP tautomer in the active site of DDC.     

Nonperfect synchronization of reaction center rehybridization in the transition state of the hydride transfer catalyzed by dihydrofolate reductase

It has been suggested that the magnitudes of secondary kinetic isotope effects (2 degrees KIEs) of enzyme-catalyzed reactions are an indicator of the extent of reaction-center rehybridization at the transition state. A 2 degrees KIE value close to the corresponding secondary equilibrium isotope effects (2 degrees EIE) is conventionally interpreted as indicating a late transition state that resembles the final product. The reliability of using this criterion to infer the structure of the transition state is examined by carrying out a theoretical investigation of the hybridization states of the hydride donor and acceptor in the Escherichia coli dihydrofolate reductase (ecDHFR)-catalyzed reaction for which a 2 degrees KIE close to the 2 degrees EIE was reported. Our results show that the donor carbon at the hydride transfer transition state resembles the reactant state more than the product state, whereas the acceptor carbon is more productlike, which is a symptom of transition state imbalance. The conclusion that the isotopically substituted carbon is reactant-like disagrees with the conclusion that would have been derived from the criterion of 2 degrees KIEs and 2 degrees EIEs, but the breakdown of the correlation with the equilibrium isotope effect can be explained by considering the effect of tunneling.     

Temperature dependence of carbon-13 kinetic isotope effects of importance to global climate change

We report here a theoretical study of the 13C kinetic isotope effect (KIE) and its temperature dependence for the reaction OH + CH4 --> H2O + CH3, the major sink of atmospheric methane in the troposphere. The KIE values at various atmospherically significant temperatures were determined by direct dynamics using variational transition state theory with multidimensional tunneling contributions (VTST/MT). The potential energy surfaces (PESs) were generated by hybrid density functional theory as well as by recently developed doubly hybrid density functional theory methods. Comparisons of our calculated KIEs with experimental data and theoretical values in the literature reveal the critical contributions due to multidimensional tunneling and torsion anharmonicity as well as the critical issue of the choice of internal rotational axis.     

Primary semiclassical kinetic hydrogen isotope effects in identity carbon-to-carbon proton- and hydride-transfer reactions, an ab initio and DFT computational study

Enthalpies of activation, transition state (ts) geometries, and primary semiclassical (without tunneling) kinetic isotope effects (KIEs) have been calculated for eleven bimolecular identity proton-transfer reactions, four intramolecular proton transfers, four nonidentity proton-transfer reactions, eleven identity hydride transfers, and two 1,2-intramolecular hydride shifts at the HF/6-311+G, MP2/6-311+G, and B3LYP/6-311++G levels. We find the KIEs to be systematically smaller for hydride transfers than for proton transfers. This outcome is not the result of "bent" transition states, although extreme bending can lower the KIE. Rather, it is a consequence of generally greater total bonding in a hydride-transfer ts than in a proton-transfer ts, most prominently manifested as a reduced contribution from the zero-point vibrational energy difference between reactant and transition states (the DeltaZPVE factor) for hydride transfers relative to proton transfers. This and other differences between proton and hydride transfers are rationalized by modeling the central .C...H...C unit of a proton-transfer ts as a 4-electron, 3-center (4-e 3-c) system and the same unit of a hydride-transfer ts as a 2-e 3-c system. Inclusion of tunneling is most likely to magnify the observed differences between proton-transfer and hydride-transfer KIEs, leaving our qualitative conclusions unchanged.     

Ab initio path‐integral calculations of kinetic and equilibrium isotope effects on base‐catalyzed RNA transphosphorylation models

Detailed understandings of the reaction mechanisms of RNA catalysis in various environments can have profound importance for many applications, ranging from the design of new biotechnologies to the unraveling of the evolutionary origin of life. An integral step in the nucleolytic RNA catalysis is self‐cleavage of RNA strands by 2′‐O‐transphosphorylation. Key to elucidating a reaction mechanism is determining the molecular structure and bonding characteristics of transition state. A direct and powerful probe of transition state is measuring isotope effects on biochemical reactions, particularly if we can reproduce isotope effect values from quantum calculations. This article significantly extends the scope of our previous joint experimental and theoretical work in examining isotope effects on enzymatic and nonenzymatic 2′‐O‐transphosphorylation reaction models that mimic reactions catalyzed by RNA enzymes (ribozymes), and protein enzymes such as ribonuclease A (RNase A). Native reactions are studied, as well as reactions with thio substitutions representing chemical modifications often used in experiments to probe mechanism. Here, we report and compare results from eight levels of electronic‐structure calculations for constructing the potential energy surfaces in kinetic and equilibrium isotope effects (KIE and EIE) computations, including a “gold‐standard” coupled‐cluster level of theory [CCSD(T)]. In addition to the widely used Bigeleisen equation for estimating KIE and EIE values, internuclear anharmonicity and quantum tunneling effects were also computed using our recently developed ab initio path‐integral method, that is, automated integration‐free path‐integral method. The results of this work establish an important set of benchmarks that serve to guide calculations of KIE and EIE for RNA catalysis. © 2014 Wiley Periodicals, Inc.     

Tunneling Assists the 1,2‐Hydrogen Shift in N‐Heterocyclic Carbenes

At room temperature, 1,2‐hydrogen‐transfer reactions of N‐heterocyclic carbenes, like the imidazol‐2‐ylidene to give imidazole is shown to occurr almost entirely (>90 %) by quantum mechanical tunneling (QMT). At 60 K in an Ar matrix, for the 2, 3‐dihydrothiazol‐2‐ylidene→thiazole transformation, QMT is shown to increase the rate about 10⁵ times. Calculations including small‐curvature tunneling show that the barrier for intermolecular 1,2‐hydrogen‐transfer reaction is small, and QMT leads to a reduced rate of the forward reaction because of nonclassical reflections even at room temperature. A small barrier also leads to smaller kinetic isotope effects because of efficient QMT by both H and D. QMT does not always lead to faster reactions or larger KIE values, particularly when the barrier is small.     

Insights Gained by Modeling the Kinetics of Archetypal Hydrogen Atom Transfer Reactions: NH3 + ·H ⇆ H2N· + H2 and CH4 + ·H ⇆ H3C· + H2

Experimental measurements of the kinetics of the title reactions extend to temperature ranges of 1360 K for the ammonia‐hydrogen reaction and of 1602 K for the methane‐hydrogen reaction. Curved plots of ln(k) versus 1/T are obtained. Many theoretical calculations modeling these reactions routinely use tunneling corrections to match experiment. The steepness and curvatures of the plots are modeled successfully in this work and are shown to be caused solely by changes in the bond dissociation energies of the bonds involved in the reactions without invoking tunneling or any other adjustable parameters. The conclusion that tunneling does not contribute significantly to the rates in the temperature range of the measurements is in stark contrast with those theoretical calculations invoking large tunneling factors in the experimental temperature range. Support for the conclusion is provided by theoretical calculations of harmonic quantum transition state theory implementing instanton theory. There is direct experimental evidence that significant tunneling occurs in some H atom transfers, as with isotopomers of H₂ + ·H and other H transfers at very low temperatures. However, there is no direct experimental evidence of significant tunneling contributions to the rates of the title reactions in the temperature range of the measurements. Insights are gained into what specific forces must be overcome by the enthalpy of activation for reaction to occur.