Experimental and theoretical multiple kinetic isotope effects for an SN2 reaction. An attempt to determine transition-state structure and the ability of theoretical methods to predict experimental kinetic isotope effects

Theoretical Kinetic Isotope Effects in Establishing the Precise Biodegradation Mechanisms of Organic Pollutants

Compound-specific isotope analysis (CSIA) for natural isotope ratios has been recognized as a promising tool to elucidate biodegradation pathways of organic pollutants by microbial enzymes by relating reported kinetic isotope effects (KIEs) to apparent KIEs (AKIEs) derived from bulk isotope fractionations (ε_bulk). However, for many environmental reactions, neither are the reference KIE ranges sufficiently narrow nor are the mechanisms elucidated to the point that rate-determining steps have been identified unequivocally. In this work, besides providing reference KIEs and rationalizing AKIEs, good relationships have been explained by DFT computations for diverse biodegradation pathways with known enzymatic models between the theoretical isotope fractionations (ε_bulk^') from intrinsic KIEs on the rate-determining steps and the observed ε_bulk. (1) To confirm the mechanistic details of previously reported pathway-dependent CSIA, it includes isotope changes in MTBE biodegradation between hydroxylation by CYP450 and S_N2 reaction by cobalamin-dependent methyltransferase, the regioselectivity of toluene biodegradation by CYP450, and the rate-determining step in toluene biodegradation by benzylsuccinate synthase. (2) To yield new fundamental insights into some unclear biodegradation pathways, it consists of the oxidative function of toluene dioxygenase in biodegradation of TCE, the epoxidation mode in biodegradation of TCE by toluene 4-monooxygenase, and the weighted average mechanism in biodegradation of cDCE by CYP450.

The development of 11C-carbonylation chemistry: A systematic view

The prospects for using carbon-11 labelled compounds in molecular imaging has improved with the development of diverse synthesis methods, including ¹¹C-carbonylations and refined techniques to handle [¹¹C]carbon monoxide at a nanomole scale. Facilitating biological research and molecular imaging was the driving force when [¹¹C]carbon monoxide was used in the first in vivo application with carbon-11 in human (1945) and when [¹¹C]carbon monoxide was used for the first time as a chemical reagent in the synthesis of [¹¹C]phosgene (1978). This review examines a rich plethora of labelled compounds synthesized from [¹¹C]carbon monoxide, their chemistry and use in molecular imaging. While the strong development of the ¹¹C-carbonylation chemistry has expanded the carbon-11 domain considerably, it could be argued that the number of ¹¹C-carbonyl compounds entering biological investigations should be higher. The reason for this may partly be the lack of commercially available synthesis instruments designed for ¹¹C-carbonylations. But as this review shows, novel and greatly simplified methods to handle [¹¹C]carbon monoxide have been developed. The next important challenge is to make full use of these technologies and synthesis methods in PET research. When there is a PET-tracer that meets a more general need, the incentive to implement ¹¹C-carbonylation protocols will increase.

Role of Microsolvation and Quantum Effects in the Accurate Prediction of Kinetic Isotope Effects: The Case of Hydrogen Atom Abstraction in Ethanol by Atomic Hydrogen in Aqueous Solution

Hydrogen abstraction from ethanol by atomic hydrogen in aqueous solution is studied using two theoretical approaches: the multipath variational transition state theory (MP-VTST) and a path-integral formalism in combination with free-energy perturbation and umbrella sampling (PI-FEP/UM). The performance of the models is compared to experimental values of H kinetic isotope effects (KIE). Solvation models used in this study ranged from purely implicit, via mixed–microsolvation treated quantum mechanically via the density functional theory (DFT) to fully explicit representation of the solvent, which was incorporated using a combined quantum mechanical-molecular mechanical (QM/MM) potential. The effects of the transition state conformation and the position of microsolvating water molecules interacting with the solute on the KIE are discussed. The KIEs are in good agreement with experiment when MP-VTST is used together with a model that includes microsolvation of the polar part of ethanol by five or six water molecules, emphasizing the importance of explicit solvation in KIE calculations. Both, MP-VTST and PI-FEP/UM enable detailed characterization of nuclear quantum effects accompanying the hydrogen atom transfer reaction in aqueous solution.

Measurement and Prediction of Chlorine Kinetic Isotope Effects in Enzymatic Systems

Approaches to determine chlorine kinetic isotope effects (Cl-KIEs) on enzymatic dehalogenations are discussed and illustrated by representative examples. Three aspects are considered. First methodology for experimental measurement of Cl-KIEs, with stress being on FAB-IRMS technique developed in our laboratory, is described. Subsequently, we concentrate our discussion on the consequences of reaction complexity in the interpretation of experimental values, a problem especially important in cases of polychlorinated reactants. The most fruitful studies of enzymatic dehalogenations by Cl-KIEs require their theoretical evaluation, hence the computational focus of the second part of this chapter.

A Benchmark Study of Kinetic Isotope Effects and Barrier Heights for the Finkelstein Reaction

Herein, we present a combined (experimental and computational) study of the Finkelstein reaction in condensed phase, where bromine is substituted by iodine in 2-bromoethylbenzene, in the presence of either acetone or acetonitrile as a solvent. Performance of various density functional theory and ab initio methods were tested for reaction barrier heights as well as for bromine and carbon kinetic isotope effects (KIEs). Two different implicit solvation models were examined (PCM and SMD). Theoretically predicted KIEs were compared with experimental values, while reaction barrier heights were assessed using the CCSD(T)-level and experimental energies as reference. In general, although the tested parameters (energies and KIEs) do not exhibit any substantial difference upon a change of the solvent, the different behavior of the theoretical methods was observed depending on the solvent. With respect to isotope effects, both PCM and SMD seem to perform very similarly, though results obtained with PCM are slightly closer to the experimental values. For predicting reaction barriers, utilization of either PCM or SMD solvation models yielded different results. Functionals from the ωB97 family: ωB97, ωB97X, and ωB97X-D provide the most accurate results for the studied system.

Cytochrome P450-catalyzed dealkylation of atrazine by Rhodococcus sp. strain NI86/21 involves hydrogen atom transfer rather than single electron transfer

Cytochrome P450 enzymes are responsible for a multitude of natural transformation reactions. For oxidative N-dealkylation, single electron (SET) and hydrogen atom abstraction (HAT) have been debated as underlying mechanisms. Combined evidence from (i) product distribution and (ii) isotope effects indicate that HAT, rather than SET, initiates N-dealkylation of atrazine to desethyl- and desisopropylatrazine by the microorganism Rhodococcus sp. strain NI86/21. (i) Product analysis revealed a non-selective oxidation at both the αC and βC-atom of the alkyl chain, which is expected for a radical reaction, but not SET. (ii) Normal (13)C and (15)N as well as pronounced (2)H isotope effects (εcarbon: -4.0‰ ± 0.2‰; εnitrogen: -1.4‰ ± 0.3‰, KIEH: 3.6 ± 0.8) agree qualitatively with calculated values for HAT, whereas inverse (13)C and (15)N isotope effects are predicted for SET. Analogous results are observed with the Fe(iv)[double bond, length as m-dash]O model system [5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-iron(iii)-chloride + NaIO4], but not with permanganate. These results emphasize the relevance of the HAT mechanism for N-dealkylation by P450.

Solvent effects on isotope effects: methyl cation as a model system

The isotopic sensitivity (CH3(+) vs CD3(+)) of the equilibrium between the methyl cation in vacuum and in solution has been investigated. Two alternative options for describing the shape of the solute cavity within the widely used polarized continuum model for implicit solvation were compared; the UFF and UA0 methods give equilibrium isotope effects (EIEs) that vary as a function of the dielectric constant in opposite directions. The same isotope effect was also obtained as the average over 40 structures from a hybrid quantum mechanical/molecular mechanical molecular dynamics simulation for the methyl cation explicitly solvated by many water molecules; the inverse value of the EIE agrees with UFF but not UA0. The opposing trends may be satisfactorily explained in terms of the different degrees of exposure of the atomic charges to the dielectric continuum in cavities of different shapes.

δ-Deuterium isotope effects as probes for transition-state structures of isoprenoid substrates

The biosynthetic pathways to isoprenoid compounds involve transfer of the prenyl moiety in allylic diphosphates to electron-rich (nucleophilic) acceptors. The acceptors can be many types of nucleophiles, while the allylic diphosphates only differ in the number of isoprene units and stereochemistry of the double bonds in the hydrocarbon moieties. Because of the wide range of nucleophilicities of naturally occurring acceptors, the mechanism for prenyltransfer reactions may be dissociative or associative with early to late transition states. We have measured δ-secondary kinetic isotope effects operating through four bonds for substitution reactions with dimethylallyl derivatives bearing deuterated methyl groups at the distal (C3) carbon atom in the double bond under dissociative and associative conditions. Computational studies with density functional theory indicate that the magnitudes of the isotope effects correlate with the extent of bond formation between the allylic moiety and the electron-rich acceptor in the transition state for alkylation and provide insights into the structures of the transition states for associative and dissociative alkylation reactions.

Benzothiazoles with tunable electron-withdrawing strength and reverse polarity: a route to triphenylamine-based chromophores with enhanced two-photon absorption

A series of dipolar and octupolar triphenylamine-derived dyes containing a benzothiazole positioned in the matched or mismatched fashion have been designed and synthesized via palladium-catalyzed Sonogashira cross-coupling reactions. Linear and nonlinear optical properties of the designed molecules were tuned by an additional electron-withdrawing group (EWG) and by changing the relative positions of the donor and acceptor substituents on the heterocyclic ring. This allowed us to examine the effect of positional isomerism and extend the structure-property relationships useful in the engineering of novel heteroaromatic-based systems with enhanced two-photon absorption (TPA). The TPA cross-sections (δ(TPA)) in the target compounds dramatically increased with the branching of the triphenylamine core and with the strength of the auxiliary acceptor. In addition, a change from the commonly used polarity in push-pull benzothiazoles to a reverse one has been revealed as a particularly useful strategy (regioisomeric control) for enhancing TPA cross-sections and shifting the absorption and emission maxima to longer wavelengths. The maximum TPA cross-sections of the star-shaped three-branched triphenylamines are ∼500-2300 GM in the near-infrared region (740-810 nm); thereby the molecular weight normalized δ(TPA)/MW values of the best performing dyes within the series (2.0-2.4 GM·g(-1)·mol) are comparable to those of the most efficient TPA chromophores reported to date. The large TPA cross-sections combined with high emission quantum yields and large Stokes shifts make these compounds excellent candidates for various TPA applications, including two-photon fluorescence microscopy.

Kinetic isotope effects on dehalogenations at an aromatic carbon

In order to interpret the observed isotopic fractionation it is necessaryto understand its relationship with the isotope effect(s) on steps that occur during the conversion of the initial reactant to the final product. We examine this relationship from the biochemical point of view and elaborate on the consequences of the assumptions that it is based on. We illustrate the discrepancies between theoretical and experimental interpretation of kinetic isotope effects on examples of dehalogenation reactions that occur at an aromatic carbon atom. The examples include 4-chlorobenzoyl-CoA dehalogenase-catalyzed conversion of 4-chlorobenzoyl-CoA to 4-hydroxybenzoyl-CoA, dehaloperoxidase-catalyzed conversion of 2,4,6-trichlorophenol to 2,6-dichloroquinone, and spontaneous hydrolysis of atrazine at pH 12. For this latter reaction we have measured the chlorine kinetic isotope effect and estimated its value theoretically at the DFT level of theory. Results of chlorine kinetic isotope effects suggest that the studied dechlorination reactions proceed in a single step with significant weakening of the carbon-chlorine bond in the transition state.

Determining the transition-state structure for different SN2 reactions using experimental nucleophile carbon and secondary alpha-deuterium kinetic isotope effects and theory

Nucleophile (11)C/ (14)C [ k (11)/ k (14)] and secondary alpha-deuterium [( k H/ k D) alpha] kinetic isotope effects (KIEs) were measured for the S N2 reactions between tetrabutylammonium cyanide and ethyl iodide, bromide, chloride, and tosylate in anhydrous DMSO at 20 degrees C to determine whether these isotope effects can be used to determine the structure of S N2 transition states. Interpreting the experimental KIEs in the usual fashion (i.e., that a smaller nucleophile KIE indicates the Nu-C alpha transition state bond is shorter and a smaller ( k H/ k D) alpha is found when the Nu-LG distance in the transition state is shorter) suggests that the transition state is tighter with a slightly shorter NC-C alpha bond and a much shorter C alpha-LG bond when the substrate has a poorer halogen leaving group. Theoretical calculations at the B3LYP/aug-cc-pVDZ level of theory support this conclusion. The results show that the experimental nucleophile (11)C/ (14)C KIEs can be used to determine transition-state structure in different reactions and that the usual method of interpreting these KIEs is correct. The magnitude of the experimental secondary alpha-deuterium KIE is related to the nucleophile-leaving group distance in the S N2 transition state ( R TS) for reactions with a halogen leaving group. Unfortunately, the calculated and experimental ( k H/ k D) alpha's change oppositely with leaving group ability. However, the calculated ( k H/ k D) alpha's duplicate both the trend in the KIE with leaving group ability and the magnitude of the ( k H/ k D) alpha's for the ethyl halide reactions when different scale factors are used for the high and the low energy vibrations. This suggests it is critical that different scaling factors for the low and high energy vibrations be used if one wishes to duplicate experimental ( k H/ k D) alpha's. Finally, neither the experimental nor the theoretical secondary alpha-deuterium KIEs for the ethyl tosylate reaction fit the trend found for the reactions with a halogen leaving group. This presumably is found because of the bulky (sterically hindered) leaving group in the tosylate reaction. From every prospective, the tosylate reaction is too different from the halogen reactions to be compared.

Ab Initio, Density Functional Theory, and Continuum Solvation Model Prediction of the Product Ratio in the SN2 Reaction of NO2- with CH3CH2Cl and CH3CH2Br in DMSO Solution

The reaction pathways for the interaction of the nitrite ion with ethyl chloride and ethyl bromide in DMSO solution were investigated at the ab initio level of theory, and the solvent effect was included through the polarizable continuum model. The performance of BLYP, GLYP, XLYP, OLYP, PBE0, B3PW91, B3LYP, and X3LYP density functionals has been tested. For the ethyl bromide case, our best ab initio calculations at the CCSD(T)/aug-cc-pVTZ level predicts product ratio of 73% and 27% for nitroethane and ethyl nitrite, respectively, which can be compared with the experimental values of 67% and 33%. This translates to an error in the relative DeltaG* of only 0.17 kcal mol(-1). No functional is accurate (deviation <0.5 kcal mol(-1)) for predicting relative DeltaG*. The hybrid X3LYP functional presents the best performance with deviation 0.82 kcal mol(-1). The present problem should be included in the test set used for the evaluation of new functionals.

Dynamic effects on the periselectivity, rate, isotope effects, and mechanism of cycloadditions of ketenes with cyclopentadiene

The cycloadditions of cyclopentadiene with diphenylketene and dichloroketene are studied by a combination of kinetic and product studies, kinetic isotope effects, standard theoretical calculations, and trajectory calculations. In contrast to recent reports, the reaction of cyclopentadiene with diphenylketene affords both [4 + 2] and [2 + 2] cycloadducts directly. This is surprising, since there is only one low-energy transition structure for adduct formation in mPW1K calculations, but quasiclassical trajectories started from this single transition structure afford both [4 + 2] and [2 + 2] products. The dichloroketene reaction is finely balanced between [4 + 2] and [2 + 2] cycloaddition modes in mPW1K calculations, as the minimum-energy path (MEP) leads to different products depending on the basis set. The MEP is misleading in predicting a single product, as trajectory studies for the dichloroketene reaction predict that both [4 + 2] and [2 + 2] products should be formed. The periselectivity does not reflect transition state orbital interactions. The (13)C isotope effects for the dichloroketene reaction are well-predicted from the mPW1K/6-31+G** transition structure. However, the isotope effects for the diphenylketene reaction are not predictable from the cycloaddition transition structure and transition state theory. The isotope effects also appear inconsistent with kinetic observations, but the trajectory studies evince that nonstatistical recrossing can reconcile the apparently contradictory observations. B3LYP calculations predict a shallow intermediate on the energy surface, but trajectory studies suggest that the differing B3LYP and mPW1K surfaces do not result in qualitatively differing mechanisms. Overall, an understanding of the products, rates, selectivities, isotope effects, and mechanism in these reactions requires the explicit consideration of dynamic trajectories.

A New Insight into Using Chlorine Leaving Group and Nucleophile Carbon Kinetic Isotope Effects To Determine Substituent Effects on the Structure of SN2 Transition States

Chlorine leaving group k(35)/k(37), nucleophile carbon k(11)/k(14), and secondary alpha-deuterium [(kH/kD)alpha] kinetic isotope effects (KIEs) have been measured for the SN2 reactions between para-substituted benzyl chlorides and tetrabutylammonium cyanide in tetrahydrofuran at 20 degrees C to determine whether these isotope effects can be used to determine the substituent effect on the structure of the transition state. The secondary alpha-deuterium KIEs indicate that the transition states for these reactions are unsymmetric. The theoretical calculations at the B3LYP/aug-cc-pVDZ level of theory support this conclusion; i.e., they suggest that the transition states for these reactions are unsymmetric with a long NC-C(alpha) and reasonably short C(alpha)-Cl bonds. The chlorine isotope effects suggest that these KIEs can be used to determine the substituent effects on transition state structure with the KIE decreasing when a more electron-withdrawing para-substituent is present. This conclusion is supported by theoretical calculations. The nucleophile carbon k(11)/k(14) KIEs for these reactions, however, do not change significantly with substituent and, therefore, do not appear to be useful for determining how the NC-C(alpha) transition-state bond changes with substituent. The theoretical calculations indicate that the NC-C(alpha) bond also shortens as a more electron-withdrawing substituent is placed on the benzene ring of the substrate but that the changes in the NC-C(alpha) transition-state bond with substituent are very small and may not be measurable. The results also show that using leaving group and nucleophile carbon KIEs to determine the substituent effect on transition-state structure is more complicated than previously thought. The implication of using both chlorine leaving group and nucleophile carbon KIEs to determine the substituent effect on transition-state structure is discussed.

The Effect of Solvent on the Structure of the Transition State for the SN2 Reaction between Cyanide Ion and Ethyl Chloride in DMSO and THF Probed with Six Different Kinetic Isotope Effects

The secondary alpha- and beta-deuterium, the alpha-carbon, the nucleophile carbon, the nucleophile nitrogen, and the chlorine leaving group kinetic isotope effects for the S(N)2 reaction between cyanide ion and ethyl chloride were determined in the very slightly polar solvent THF at 30 degrees C. A comparison of these KIEs with those reported earlier for the same reaction in the polar solvent DMSO shows that the transition state in THF is only slightly tighter with very slightly shorter NC-C(alpha) and C(alpha)-Cl bonds. This minor change in transition state structure does not account for the different transition structures that were earlier suggested by interpreting the experimental KIEs and the gas-phase calculations, respectively. It therefore seems unlikely that the different transition states suggested by the two methods are due to the lack of appropriate solvent modeling in the theoretical calculations. Previously it was predicted that the transition state of S(N)2 reactions where the nucleophile and the leaving group have the same charge would be unaffected by a change in solvent. The experimental KIEs support this view.

Activation barriers for DNA alkylation by carcinogenic methane diazonium ions

Methylation reactions of the DNA bases with the methane diazonium ion, which is the reactive intermediate formed from several carcinogenic methylating agents, were examined. The SN2 transition states of the methylation reactions at N7, N3, and O6 of guanine; N7, N3, and N1 of adenine; N3 and O2 of cytosine; and O2 and O4 of thymine were calculated using the B3LYP density functional method. Solvation effects were examined using the conductor-like polarizable continuum method and the combined discrete/SCRF method. The transition states for reactions at guanine N3, adenine N7, and adenine N1 are influenced by steric interactions between the methane diazonium ion and exocyclic amino groups. Both in the gas phase and in aqueous solution, the methylation reactions at N atoms have transition states that are looser, and generally occur earlier along the reaction pathways than reactions at O atoms. The forming bonds in the transition states in water are 0.03 to 0.13 A shorter than those observed in the gas phase, and the activation energies are 13 to 35 kcal/mol higher. The combined discrete/SCRF solvation energy calculations using base-water complexes with three water molecules yield base solvation energies that are larger than those obtained from the CPCM continuum method, especially for cytosine. Reactivities calculated using barriers obtained with the discrete/SCRF method are consistent with the experimentally observed high reactivity at N7 of guanine.

Evidence for Different Types of Water Participation in the Solvolysis of 1-Adamantyl, tert-Butyl, and Methyl Chlorides from Density Functional Theory Computations

[structure: see text] The activation energy in the gas phase (deltaE(double dagger)) and the free energy of activation (deltaG(double dagger)) in water solution for the hydrolysis of the monohydrates of methyl chloride (MeCl), tert-butyl chloride (t-BuCl), and 1-adamantyl chloride (AdCl) have been computed with the B3LYP/631-G(d) method and the polarizable continuum (PCM) solvation model. There is a fair agreement between the deltaG(double dagger) values computed by us and the experimental data. The mechanistic implications of our computations are in severe contradiction with conventional representations. Thus, the computed nucleophilic solvent assistance (NSA) for the backside attack of a water molecule in the hydrolysis of MeCl is slightly lower than the corresponding NSA for t-BuCl. Hence, the hydrolysis of both MeCl and t-BuCl takes place mainly according to the classical S(N)2 mechanism. The most relevant difference is that deltaG(double dagger) for the frontside attack of water to t-BuCl is disfavored only by ca. 2 kcal/mol with regard to the backside attack but by ca. 23 kcal/mol in the case of MeCl. The higher solvolysis rate in water of t-BuCl in relation to AdCl is not due to steric factors affecting the specific solvation of the corresponding transition states, but to differential bulk solvent effects, which are accounted for by the PCM model.

A theoretical investigation of alpha-carbon kinetic isotope effects and their relationship to the transition-state structure of S(N)2 reactions

[reaction: see text] The transition structures and alpha-carbon 12C/13C kinetic isotope effects for 22 S(N)2 reactions between methyl chloride and a wide variety of nucleophiles have been calculated using the B1LYP/aug-cc-pVDZ level of theory. Anionic, neutral, and radical anion nucleophiles were used to give a wide range of S(N)2 transition states so the relationship between the magnitude of the alpha-carbon kinetic isotope effect and transition-state structure could be determined. The results suggest that the alpha-carbon 12C/13C kinetic isotope effects for S(N)2 reactions will be large (near the experimental maximum) and that the curve relating the magnitude of the KIE to the percent transfer of the alpha-carbon from the nucleophile to the leaving group in the transition state has a broad maximum. This means very similar KIEs will be found for early, symmetric, and late transition states and that one cannot use the magnitude of these KIEs to estimate transition-state structure.

A computational study of the effect of bending on secondary kinetic isotope effects in SN2 transition states

Using conventional transition state theory, the secondary deuterium kinetic isotope effect (KIE) in the inversion SN2 reaction of CH3F and F- is calculated to be small, 0.98 (T = 298 K). This is shown to be the result of a balance among opposing entropy and enthalpy terms. By contrast, KIE in the retention SN2 mechanism is calculated to be large (1.5). Accordingly, KIE is a potential observable for discriminating between the two mechanisms. Large KIE's are also found for the inversion and retention mechanisms of the ion pair reactions between CH3F and LiF. All of the transition structures leading to large KIE's have a bent FCF angle and an imaginary frequency that is sensitive to deuterium labeling.

Ab Initio Study of the SN2 and E2 Mechanisms in the Reaction between the Cyanide Ion and Ethyl Chloride in Dimethyl Sulfoxide Solution

[reaction: see text] Reliable theoretical calculations predict a free energy barrier for nitrile formation from the reaction between the cyanide ion and ethyl chloride in DMSO solvent of 24.1 kcal/mol, close to the experimental value of 22.6 kcal/mol. We have also predicted that the isonitrile formation is less favorable by 4.7 kcal/mol, while the elimination mechanism is less favorable by more than 10 kcal/mol. These results indicate that isonitrile formation and bimolecular elimination are not significant side reactions for primary alkyl chloride reactions.

A new interpretation of chlorine leaving group kinetic isotope effects; a theoretical approach

The chlorine leaving group kinetic isotope effects (KIEs) for the S(N)2 reactions between methyl chloride and a wide range of anionic, neutral, and radical anion nucleophiles were calculated in the gas phase and, in several cases, using a continuum solvent model. In contrast to the expected linear dependence of the chlorine KIEs on the C(alpha)-Cl bond order in the transition state, the KIEs fell in a very small range (1.0056-1.0091), even though the C(alpha)-Cl transition state bond orders varied widely from approximately 0.32 to 0.78, a range from reactant-like to very product-like. This renders chlorine KIEs, and possibly other leaving-group KIEs, less useful for studies of reaction mechanisms than commonly assumed. A partial explanation for this unexpected relationship between the C(alpha)-Cl transition state bond order and the magnitude of the chlorine KIE is presented.

Calculations of Substituent and Solvent Effects on the Kinetic Isotope Effects of Menshutkin Reactions

Nitrogen, deuterium, halogen, and carbon kinetic isotope effects have been modeled for the Menshutkin reaction between methyl halides and substituted N,N-dimethylaniline at the HF/6-31G(d) level of theory augmented by the C-PCM continuum solvent model for several solvents. Systematic changes in geometries of the transition states and Gibbs free energies of activation have been found with phenyl ring substituents, solvent, and the leaving group. Kinetic isotope effects also change systematically; however, these changes are predicted to be small, inside the usual precision of the experimental measurements. On the contrary, no correlation has been found between the kinetic isotope effects and the Hammett constants for para substituents. Thus opposite to previous assumptions, our results indicate that kinetic isotope effects on the Menshutkin reaction cannot be used to predict the position of the transition state on the reaction coordinate.