Hydrogen-deuterium Kinetic Isotope Effect Research Articles

First-principles based microkinetic modeling of transient kinetics of CO hydrogenation on cobalt catalysts

Computational efforts towards a fundamental understanding of the underlying mechanistic pathways in synthesis gas conversion processes such as Fischer-Tropsch synthesis are exemplary for the developments in heterogeneous catalysis. Advances in transient kinetic analysis methods contribute to unraveling complex reaction pathways over nanoparticle surfaces. Tracing the activity and selectivity of Fischer-Tropsch catalysts to the individual events occurring at the active site remains difficult with experimental techniques. Here we provide simulations of transient kinetics at the scale of the active site by making use of the reaction energetics for CO hydrogenation to methane on stepped and terrace cobalt surfaces that are suitable models for cobalt FT nanoparticle catalysts. We investigate the hydrogen-deuterium kinetic isotope effect and simulate common steady-state and chemical isotopic transients. Comparison to experimental literature leads to important mechanistic insights. Direct CO dissociation is the main pathway for breaking the CO bond and it occurs exclusively on step-edge sites. While the experimentally observed hydrogen-deuterium kinetic isotopic effect is often used as evidence for H-assisted CO dissociation, we show that hydrogenation of C and O as partly rate-controlling steps provides an alternative explanation. The simulations of the chemical transients provide significant insight into the importance of the changing surface coverages that strongly affect the reaction rate. The reversibility of CO dissociation on cobalt step-edges is evident from simulations of 12C16O/13C18O scrambling being in good agreement with experimental data.

Branching Ratios and Vibrational Distributions in Water-Forming Reactions of OH and OD Radicals with Methylamines.

Reactions of OH and OD radicals with (CH3)3N, (CH3)2NH, and CH3NH2 were studied by Fourier transform infrared emission spectroscopy (FTIR) of the water product molecules from a fast-flow reactor at 298 K. The rate constants (4.4 ± 0.5) × 10(-11), (5.2 ± 0.8) × 10(-11), and (2.0 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) were determined for OD + (CH3)3N, (CH3)2NH, and CH3NH2, respectively, by comparing the HOD emission intensities to the HOD intensity from the OD reaction with H2S. Abstraction from the nitrogen site competes with abstraction from the methyl group, as obtained from an analysis of the HOD and D2O emission intensities from the OD reactions with the deuterated reactants, (CD3)2NH and CD3NH2. After adjustment for the hydrogen-deuterium kinetic isotope effect, the product branching fractions of the hydrogen abstraction from the nitrogen for di- and monomethylamine were found to be 0.34 ± 0.04 and 0.26 ± 0.05, respectively. Vibrational distributions of the H2O, HOD, and D2O molecules are typical for direct hydrogen atom abstraction from polar molecules, even though activation energies are negative because of the formation of pre-transition-state complexes. Comparison is made to the reactions of hydroxyl radicals with ammonia and with other compounds with primary C-H bonds to discuss specific features of disposal of energy to water product.

Theoretical study of the thermal unimolecular rearrangement of fluoroethylidenes

AbstractThe thermal unimolecular isomerization of fluoroethylidenes to the corresponding fluoroethylenes has been studied by the MNDO method. It has been shown that fluorine substitution on the carbene carbon increases the activation energy in comparison with the ethylidene rearrangement. To understand the reason for this increase in the activation energy, the charge‐transfer effects have been analyzed. Fluorine substitution at other positions does not significantly affect the activation energies. The thermodynamic parameters for the reaction have been evaluated, using vibrational and rotational spectral data calculated in this work. RRKM calculations have been performed and high‐pressure Arrhenius parameters calculated. Hydrogen–deuterium kinetic isotope effects indicate that the reaction rates are altered considerably on isotopic substitution, and the change in reaction rates depends upon the position of deuterium substitution, as well as on the number of hydrogens replaced by deuterium atoms. © 1992 John Wiley & Sons, Inc.

The effect of the nature of the amine leaving group on the nature of the E2 transition state for the reaction of 1-phenylethylammonium ions with sodium ethoxide in ethanol

To investigate the effect of the leaving group on the elimination reaction of 1-phenylethylammonium ions with sodium ethoxide in ethanol at 60 °C, the reaction of seven different quaternary ammonium salts and their β-deuterated analogues with trimethylamine, N-methylpiperidine, N-methyldiethylamine, triethylamine, N,N-dimethylbenzylamine, tripropylamine, and N,N-diethylbenzylamine as leaving groups has been studied. In all cases the elimination, which was shown to proceed via the concerted E2 process, was accompanied by competing substitution reactions. Although a significant dependence of the rate of the elimination process on the nature of the leaving group was noted, there was not any linear correlation with the basicity of the amine leaving group. The primary hydrogen–deuterium kinetic isotope effect for the elimination process, (kH/kD)E, was found to increase initially with an increase of reaction rate, [Formula: see text] for substrates containing the leaving groups trimethylamine, N-methylpiperidine, N-methyldiethylamine, triethylamine, and N,N-dimethylbenzylamine; i.e., (kH/kD)E = 5.03, 5.26, 5.40, 5.83, and 5.85, respectively. A further increase in rate, using substrates with tripropylamine and N,N-diethylbenzylamine as leaving groups resulted in a decrease of the magnitude of the hydrogen–deuterium isotope effect; i. e., (kH/kD)E = 5.42 and 4.67, respectively. It is concluded that steric effects mainly determine leaving group ability. As well, it is concluded that the leaving group ability of the amine determines the structure of the E2 transition state. For the reaction of the poorer leaving groups, trimethylamine, N-methylpiperidine, and N-methyldiethylamine, the proton is morethan one-half transferred at the transition state while for reaction involving the two best leaving groups, tripropylamine and N,N-diethylbenzylamine, the Cβ—H bond is lessthan one-half broken at the transition state. The conclusions are considered in the light of the More O'Ferrall – Jencks potential energy surface diagram. Keywords: elimination mechanism, transition state, isotope effects, leaving group, quaternary salts.

Secondary α hydrogen–deuterium kinetic isotope effects in the syn-elimination reaction of 2-phenylethyldimethylamine oxide

The secondary α-deuterium kinetic isotope effect has been measured for the thermal reaction of 2-phenylethyldimethylamine oxide in 90 mol% DMSO–H2O at 60.0 °C. A large secondary α-deuterium isotope effect of 1.158 per α-D was found, which indicates significant [Formula: see text] bond rupture and very little double bond formation at the transition state for this concerted syn-elimination process. The observed large normal value for (kH/kD)α is discussed in terms of the use of secondary α-D isotope effects for the determination of stereochemistry for a concerted elimination process. Various isotope effects found for the syn elimination of the above amine oxide and those for the anti elimination of 2-arylethyltrimethylammonium salts with ethoxide are considered with respect to the relative transition state structures for syn- and anti-elimination reactions.

Tunnel effects in hydrogen-atom-transfer reactions as studied by the temperature dependence of the hydrogen deuterium kinetic isotope effects

The gas-phase kinetic effects k/sub H//k/sub D/ for the hydrogen abstraction from some n-alkanes and cycloalkanes by hydrogen atoms, H + RH(R'D) K/sub H//k/sub D/ H/sub 2/(HD) + R(R'), were recently determined in their laboratory over the temperature range 363-463 K. The data are characterized by the small preexponential factor ratios A/sub H/A/sub D/ ranging from 0.43 to 0.54 and the large activation energy differences E/sub D/ - E/sub H/ from 9.0 to 9.7 kJ mol/sup -1/. Theoretical interpretation of the kinetic isotope effects was made for abstraction of a hydrogen atom from cyclohexane based on traditional transition-state theory. Some characteristics of the tunnel effect were discussed in some detail in an elementary way. Good agreement between experiment and theory was obtained when tunnel correction factors were taken into consideration.

Isotope effects in nucleophilic substitution reactions. V. The mechanism of the decomposition of 1-phenylethyldimethylphenylammonium halides in chloroform

Secondary α and β hydrogen–deuterium kinetic isotope effects have been used together to show that the SN reaction between 1-phenylethyldimethylphenylammonium ion and bromide or iodide ion in chloroform occurs by way of an SN2 mechanism within a triple ion in spite of the fact that it reacts faster than the primary substrate, benzyldimethylphenylammonium bromide. The very loose transition state and steric effects in the ground state appear to be responsible for the unusually fast SN2 reactions between 1-phenylethyldimethylphenylammonium ion and halide ions in chloroform.

Temperature-dependent stereoselectivity and hydrogen deuterium kinetic isotope effect for .gamma.-hydrogen transfer to 2-hexyloxy radical. The transition state for the Barton reaction

The diastereomers of 5-deuterio-2-hexanol, prepared stereospecifically from acetol and lactic acid by using fermentation procedures, have been subjected to the Barton reaction with Ag/sub 2/CO/sub 3//Br/sub 2/ in pentane over the temperature range -8.65 to 30.05/sup 0/C. The derived 2,5-dimethyltetrahydrofuran deuterium incorporation yields both the C-5 diastereotopic hydrogen stereoselectivity and the isotope effect (k/sub H//k/sub D/) for the intramolecular hydrogen-transfer step. The isotope effect is classically temperature dependent, 5.82 (+- 0.07), 30.05; 6.01 (+- 0.22), 20.55; 6.81 (+- 0.17), 10.55; 6.90 (+- 0.08), 1.65; 7.47 (+- 0.49), -8.65/sup 0/C, while the stereoselectivity is small, 1.23 (1.65/sup 0/C), and temperature independent. Molecular mechanics calculations show that conformations with linear arrangements of C/sub 5/, H, and O show small steric energy differences between the diastereotropic hydrogens at C-5. Calculations of isotope effects and their temperature dependencies for model transition states match the experimental data for only C/sub 5/, H, and O angles greater than 150/sup 0/.

Hydrogen-deuterium kinetic isotope effects of 16 and 22 in the oxidation of hydrogen peroxide

Mechanisms are suggested for the oxidation of H/sub 2/O/sub 2/ by both (bpy)/sub 2/(py)Ru/sup IV/O/sup 2 +/ and (bpy)/sub 2/(py)Ru/sup III/OH/sub 2//sup 2 +/ over the pH range 2 to 10. Stopped-flow or simple mixing techniques were used to obtain rate constants for two reaction pathways. It was demonstrated that the Ru(IV) reactions, which could proceed via one- or two-electron steps, involves initially one-electron steps as do the oxidations involving Ru(III). Isotope effect studies using D/sub 2/O as a solvent yielded striking observation that the reactions of acid-independent pathways show the existence of H-D isotope effects of considerable magnitude. Isotope effect data for the inverse acid path support the earlier idea of the importance of proton content in the determination of the redox mechanism. (BLM)

Origin of the hydrogen-deuterium kinetic isotope effect for porphyrin metalation

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTOrigin of the hydrogen-deuterium kinetic isotope effect for porphyrin metalationDavid K. Lavallee and Gary M. OnadyCite this: Inorg. Chem. 1981, 20, 3, 907–909Publication Date (Print):March 1, 1981Publication History Published online1 May 2002Published inissue 1 March 1981https://doi.org/10.1021/ic50217a053RIGHTS & PERMISSIONSArticle Views95Altmetric-Citations13LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (371 KB) Get e-Alerts Get e-Alerts

Hydrogen-deuterium kinetic isotope effects for .gamma.-hydrogen rearrangement in 2-hexanone following photochemical excitation, electron impact ionization, and anodic oxidation

Hydrogen-deuterium kinetic isotope effects for .gamma.-hydrogen rearrangement in 2-hexanone following photochemical excitation, electron impact ionization, and anodic oxidation

Hydrogen-deuterium isotope effect in the λ-initiated chain reaction between carbon tetrachloride and cyclohexane in the liquid phase

The hydrogen-deuterium kinetic isotope effect for the chain reaction between carbon-tetrachloride and cyclohexane was measured in the liquid phase, in the temperature range from 302 to 413 K. The rate expression for the two following reactions was obtained by competitive experiments in ternary solutions of CCl 4 in c- C 6 H 12/ c- C 6 D 12 mixtures (2) ·CCl 3+c-C 6H 12→CHCl 3+c-C 6H 11 . (2′) ·CCl 3+ c-C 6D 12→CDCl 3+c-C 6D 11 . log 10 ( k 2/ k 2′)=-0.38±0.04-(10.1±0.3) kJ· mol -1/2.303 RT. The high value for the difference in activation energies between (2) and (2′) suggests that tunneling is important in the proton abstraction reaction by ·CCl 3.

A Kinetic Study of the Reaction of Arylthiotrimethylsilane with Carboxylic Acid Giving Acyloxytrimethylsilane and Arenethiol

Abstract The mechanism of the reactions of arylthiotrimethylsilanes with carboxylic acids has been studied kinetically. The reaction was found to obey a second-order kinetic equation. The substituent effects of the aryl moiety (ρ=−2.4) and of the silicon atom, the hydrogen-deuterium kinetic isotope effect of the acid (kH/kD=3) were observed. A mechanism involving the 5-coordination of the silicon atom prior to the rate-determining protona-tion of the sulfur atom has been suggested for the reaction. A remarkable base catalysis was observed. The formation of the coordinated intermediate has been suggested to be the slow step in the presence of a bese.

Kinetic Studies for the Acyloxy Exchange Reactions of Acyloxytrimethylsilanes with Carboxylic Acids

Abstract The acyloxy exchange reactions of acyloxysilane with carboxylic acids have been studied kinetically. The rate of reaction has been found to proceed faster with a stronger attacking acid and more basic leaving acyloxy group. The rate was markedly accelerated by the addition of base. The steric effect due to substituents on the silicon atom and hydrogen-deuterium kinetic isotope effects have been examined. A mechanism involving the 5-coordination of the silicon atom prior to the rate-determining proton transfer has been suggested.

ChemInform Abstract: PRIMARY HYDROGEN‐DEUTERIUM KINETIC ISOTOPE EFFECTS IN THE REDUCTION OF TRIARYLCARBONIUM IONS BY FORMATE ANION

Chemischer InformationsdienstVolume 10, Issue 10 Preparative Organic Chemistry ChemInform Abstract: PRIMARY HYDROGEN-DEUTERIUM KINETIC ISOTOPE EFFECTS IN THE REDUCTION OF TRIARYLCARBONIUM IONS BY FORMATE ANION R. STEWART, R. STEWARTSearch for more papers by this authorT. W. TOONE, T. W. TOONESearch for more papers by this author R. STEWART, R. STEWARTSearch for more papers by this authorT. W. TOONE, T. W. TOONESearch for more papers by this author First published: March 6, 1979 https://doi.org/10.1002/chin.197910132Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume10, Issue10March 6, 1979 RelatedInformation

Stereochemical and kinetic studies on the Pummerer reaction of arylsulphinylcyclopropanes and phenylsulphinylmethylcycloalkanes with acetic anhydride

The Pummerer reactions of arylsulphinylcyclopropanes and phenylsulphinylmethylcycloalkanes with acetic anhydride containing anhydrous sodium acetate afforded 1-phenylthio-1-acetoxycyclopropanes and [acetoxy-(phenylthio)methyl]cycloalkanes, respectively, in high yields but no ring opening products. The Pummerer reactions of trans- and cis-1-phenyl-2-phenylsulphinylcyclopropanes and cis,anti- and cis,syn-1,2-dimethyl-3-(phenylsulphinyl)cyclopropanes gave the corresponding stereoselective products. The pseudo-first-order rate constants for the Pummerer reaction were determined using a large excess of acetic anhydride. A Hammett correlation of the rates with σ values gives a ∪ shape curve, whereas the enthalpy and the entropy of activation for the reaction of phenylsulphinylcyclopropane are ΔH‡ 41.3 kcal mol–1 and ΔS‡+ 10.4 cal mol–1 K–1, respectively. Hydrogen–deuterium kinetic isotope effects were found to be rather small (kH/kD 1.13, 1.24, and 1.49 for p-MeOH-, and m-CF3-phenylsulphinyl[1-2H]cyclopropanes, respectively). An 18O tracer study indicated that there was marked oxygen exchange during the reaction but the products were found to retain a substantial proportion of the 18O of the original sulphoxides. These observations suggest that the reaction proceeds via an ylide–ion pair intermediate, after initial formation of acetoxysulphonium salts.

Primary hydrogen–deuterium kinetic isotope effects in the reduction of triarylcarbonium ions by formate anion

The rates of reduction of 16 triarylcarbonium ions to the corresponding triarylmethanes by HCO2Na and DCO2Na have been measured in 71.0% aqueous trifluoroacetic acid containing 2.50M-formate. Bronsted plots of log kH and log kD against pKR+ of the carbonium ion are slightly curved with the curvature of the former being the more pronounced. The kinetic isotope effect kH/kD varies between 1.8 and 3.2 and shows a maximum near pKR+ of –4, the first such maximum observed for a hydride transfer process.

Kinetic and Equilibrium Studies of the Reactions of 2,4,6-Trinitrotoluene and 2,4,6-Trinitrotoluene-d3 with Sodium and Potassium t-Butoxide in t-Butanol

The reactions of 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrotoluene fully deuterated at the methyl position (TNT-d3) with sodium and potassium t-butoxide in t-butanol have been studied. With TNT as the substrate, proton abstraction by ion-paired sodium or potassium t-butoxide appears to be the predominant reaction in solution. With sodium t-butoxide as base, the forward rate constant for proton abstraction at 30.0 °C (Kf,ip) is 6000 ± 400 M−1 s−1 while ΔH≠ and ΔS≠ for the reaction are 4.2 ± 0.3 kcal mol−1 and −27 ± 2 cal deg−1 mol−1, respectively. With TNT-d3 as the substrate, formation of a TNT-d3-t-butoxide ion σ-complex occurs simultaneously with deuteron abstraction. Specific rate constants for the two processes have been determined at 30.0 °C. Initial rate studies establish a hydrogen-deuterium kinetic isotope effect of 8 ± 1 for the formation of the anion in t-butanol.

Implication of small hydrogen-deuterium kinetic isotope effect in the reaction of quinaldine and 1-methylisoquinoline N-oxides with acetic anhydride

Implication of small hydrogen-deuterium kinetic isotope effect in the reaction of quinaldine and 1-methylisoquinoline N-oxides with acetic anhydride

A MODEL FOR THE HYDROGEN BOND AND PROTON TRANSFER REACTIONS

A simple extension of Platt's model for determining the force constants in diatomic hydrides is used to investigate the properties of transition states for proton transfer reactions. The model is first tested by comparing the predicted and observed changes in bond lengths and stretching and bending frequencies for [Formula: see text] systems. The quantities predicted for the transition state are the geometry and the perpendicular bending frequencies. The implications of the results with regard to the interpretation of hydrogen–deuterium kinetic isotope effects are discussed. In particular, the variations in both the isotope effect and the geometry of the transition state with changing acid or base strength are considered in detail.