Carbon-13 Kinetic Isotope Effects Research Articles

Deuterium and carbon-13 kinetic isotope effects for the reaction of OH with CH4

Interpolated variational transition state theory calculations with centrifugal-dominant, small-curvature tunneling coefficients have been carried out for the case of the deuterium kinetic isotope effect (KIE) in the reaction OH+12CD4→HDO+12CD3 and for the 13C KIE for the reaction OH+13CH4→H2O+13CH3. The interpolated variationally optimized generalized transition states predict notably different nontunneling KIEs than the conventional ones, and factorization analyses of the KIEs are presented to illustrate the origin of the differences. The zero-point energies at the variational transition states differ from those at the saddle point by up to 0.19 kcal/mol for the OH+12CD4 reaction and by up to 0.34 kcal/mol for the OH+13CH4 reaction. The incorporation of multidimensional tunneling effects partly cancels the effect of variational optimization of the transition state.

Carbon-13 kinetic isotope effects in the reactions of Grignard reagents with benzophenone

The rate-determining step for the reaction of benzophenone with Grignard reagents is an inner sphere electron transfer (ET) with a simultaneous transfer of magnesium to produce the magnesium benzophenone ketyl and the alkyl radical. For tertiary and secondary reagents the alkyl radical is free to escape from the cage, while for primary reagents and methyl partial bond formation between the radicals stabilizes the transition state and prevents escape. Substitution in the carbonyl group with isotopic carbon gives to a kinetic isotope effect (KIE), which for 14 C has been reported to be near unity for the reaction of tert-butylmagnesium chloride

Carbon-14 kinetic isotope effect in the decarbonylation of lactic acid[1-14C

The carbon-14 kinetic isotope effect for the decarbonylation of lactic acid[1-14C] in sulfuric acid has been measured in the temperature interval of 20–90°C. The experimental values of (k12C/k14C) are compared with the theoretical14C kinetic isotope effect calculated assuming that one carbon-oxygen stretching vibration is lost in the rate-determining step. The discrepancy between experimentally observed temperature dependence of the14C kinetic isotope effect and the theoretical one is explained by the possible side reactions which change the apparent degrees of decarbonylation and isotopic composition of CH3CHOHCOOH[1-14C] used in experiments aiming at the determination of carbon-14 kinetic isotope effect in the decarbonylation process itself.

Carbon-13 kinetic isotope effects in the decarbonylations of lactic acid containing13C at the natural abundance level

The13C kinetic isotope fractionation in the decarbonylation of lactic acid of natural isotopic composition by sulfuric acid has been studied in the temperature range of 20–80°C. The13C(1) isotope separation in the decarbonylation of lactic acid by concentrated sulfuric acid depends strongly on the temperature above 40°C. Below this temperature the13C isotope effect in the decarbonylation of lactic acid by concentrated sulfuric acid is normal similarly as has been found inthe decarbonylation of lactic [1-14C] acid. The experimental values of k(12C)/k(13C) ratios of isotopic rate constants for12C and13C are close to, but slightly higher than theoretical13C-kinetic isotope effects calculated (neglecting tunneling) under the asumption that the C(1)-OH bond is broken in the rate-controlling step of the dehydration reaction. Dilution of concentrated sulfuric acid with water up to 1.4 molar (H2O)/(H2SO4) ratio caused the increase of the13C isotope fractionation from 1.0273 found in concentrated sulfuric acid at 80.5°C to 1.0536±0.0008 (at 80.6°C). A discussion of the abnormally high temperature dependence of14C and13C isotope fractionation in this reaction and the discussion of the problem of relative14C/13C kinetic isotope effects is given.

Relative reactivity and stereoselectivity in the Wittig reactions of substituted benzaldehydes with benzylidenetriphenylphosphorane

Carbonyl carbon-14 kinetic isotope effects and substituent effects on the relative reactivity and on the cis-trans product ratio were determined in the Wittig reaction of XC 6 H 4 CHO with Ph 3 P=CHC 6 H 4 Y in THF at 0 o C. The isotope effect and the Hammett ρ value were positive under both Li salt-free ( 12 k/ 14 k=1.060±0.003 and ρ x =2.77±0.15) and Li salt-present ( 12 k/ 14 k=1.015±0.004 and ρ x =1.38±0,12) conditions, although they were much larger in the former case

Carbon-11/carbon-14 kinetic isotope effects in enzyme mechanism studies. 11C/14C Kinetic isotope effect of the tyrosine phenol-lyase catalyzed .alpha.,.beta.-elimination of L-tyrosine

Carbon-11/carbon-14 kinetic isotope effects in enzyme mechanism studies. 11C/14C Kinetic isotope effect of the tyrosine phenol-lyase catalyzed .alpha.,.beta.-elimination of L-tyrosine

Investigation of the enzymic mechanism of yeast orotidine-5'-monophosphate decarboxylase using carbon-13 kinetic isotope effects

Orotidine-5'-monophosphate decarboxylase (ODCase) from Saccharomyces cerevisiae displays an observed 13C kinetic isotope effect of 1.0247 +/- 0.0008 at 25 degrees C, pH 6.8. The observed isotope effect is sensitive to changes in the reaction medium, such as pH, temperature, or glycerol content. The value of 1.0494 +/- 0.0006 measured at pH 4.0, 25 degrees C, is not altered significantly by temperature or glycerol, and thus the intrinsic isotope effect for the reaction is apparently being observed under these conditions and decarboxylation is almost entirely rate-determining. These data require a catalytic mechanism with freely reversible binding and one in which a very limited contribution to the overall rate is made by chemical steps preceding decarboxylation; the zwitterion mechanism of Beak and Siegel [Beak, P. & Siegel, B. (1976) J. Am. Chem. Soc. 98, 3601-3606], which involves only protonation of the pyrimidine ring, is such a mechanism. With use of an intrinsic isotope effect of 1.05, a partitioning factor of less than unity is calculated for ODCase at pH 6.0, 25 degrees C. A quantitative kinetic analysis using this result excludes the possibility of an enzymatic mechanism involving covalent attachment of an enzyme nucleophile to C-5 of the pyrimidine ring. The observed isotope effect does not rise to the intrinsic value above pH 8.5; instead, the observed isotope effects at 25 degrees C plotted against pH yield an asymmetric curve that at high pH plateaus at about 1.035. These data, in conjunction with the pH profile of Vmax/km, fit a kinetic model in which an enzyme proton necessary for catalysis is titrated at high pH, thus providing evidence for the catalytic mechanism of Beak and Siegel (1976).

A secondary carbon-11/carbon-14 kinetic isotope effect in the base-catalyzed prototropic rearrangement of 1- to 3-methylindene

The secondary 11 C/ 14 C kinetic isotope effect (KIE) has been determined to 1.010±0.005 for the base-catalyzed rearrangement of 1-methylindene to 3-methylindene isotopically substituted in the methyl group in the solvent benzene at 20°C by using the tertiary amine DABCO as catalyst. The result is discussed in connection with previously reported values of the corresponding primary 2 H and secondary β- 2 H 3 KIEs (5.03 and 1.103, respectively). The isotope effects are consistent with a late TS, structurally close to the ion-pair intermediate postulated in the rate-determining proton abstraction step of the reaction. The influence of hyperconjugation and rehybridization on the magnitude of the secondary carbon KIE is discussed

Deuterium and carbon-13 kinetic isotope effects for the isomerization of 5,5-dimethylbicyclo[2.1.0]pent-2-ene to 5,5-dimethylcyclopentadiene

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDeuterium and carbon-13 kinetic isotope effects for the isomerization of 5,5-dimethylbicyclo[2.1.0]pent-2-ene to 5,5-dimethylcyclopentadieneJohn E. Baldwin and Naresh D. GhatliaCite this: J. Am. Chem. Soc. 1989, 111, 9, 3319–3325Publication Date (Print):April 1, 1989Publication History Published online1 May 2002Published inissue 1 April 1989https://doi.org/10.1021/ja00191a032RIGHTS & PERMISSIONSArticle Views386Altmetric-Citations12LEARN 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 (929 KB) Get e-Alerts Get e-Alerts

Electron transfer in reactions of ketones with organolithium reagents. A carbon-14 kinetic isotope effect probe

Kinetic isotope effects have been determined for reactions of ketones labeled with carbon-14 at the carbonyl carbon with MeLi and Me/sub 2/CuLi in diethyl ether at 0/sup 0/C. Observed isotope effects were as follows: (C/sub 6/H/sub 5/)/sub 2/C double bonds O + MeLi, /sup 12/k//sup 14/k = 1.000 +/- 0.002; (C/sub 6/H/sub 5/)/sub 2/C double bonds O + Me/sub 2/CuLi, 1.029 +/- 0.005; 2,4,6-Me/sub 3/C/sub 6/H/sub 2/COC/sub 6/H/sub 5/ + MeLi, 1.023 +/- 0.004. The relative reactivities of ortho-, meta-, and para-substituted benzophenones with these reagents were also determined by the competition experiments. These results are consistent with an electron-transfer step which is followed by a carbon-carbon bond-forming step that is or is not rate determining depending on the structure of ketones and reagents. The reaction of benzophenone with MeLi proceeds via rate-determining electron transfer; the change in nucleophile from MeLi to Me/sub 2/CuLi shifts the rate-determining step from electron transfer to recombination; the change in ketone from benzophenone to 2,4,6-trimethylbenzophenone also shifts the rate-determining step from electron transfer to recombination because the latter step becomes slower for the more hindered ketone. The extent of the geometrical change of the substrate at the electron-transfer transition state of the reaction ofmore » benzophenone with MeLi was estimated to be small on the basis of the magnitude of the KIE and the rho value of the Hammett correlation.« less

Carbon-14 Kinetic Isotope Effect in Reduction of Benzophenone with Lithium Isopropoxide

Abstract The carbon-14 kinetic isotope effect (KIE) was determined for the reduction of benzophenone-carbonyl-14C with lithium isopropoxide in THF at 25 °C. The observed KIE (12k⁄14k=1.045) suggested that there is considerable bonding change at the carbonyl carbon in the transition state. This is the first example of carbon KIE in a reaction which proceeds via initial electron transfer followed by a slow radical process.

Kinetic Isotope Effect and Substituent Effect Study on the Grignard Reaction of MeMgI with Benzophenone. A Rate-Determining C–C Bond Formation

Abstract The carbonyl carbon-14 kinetic isotope effect and substituent effects were determined for the reaction of MeMgI with benzophenone. The observed large carbon isotope effect (14k⁄12k=1.056) together with the large steric effect on reactivity introduced by o-substituents on benzophenone indicated that the C–C bond formation is the rate-determining step of the Grignard reaction.

Kinetic isotope effect study of reductions of benzophenone with complex metal hydrides

Carbon-14 and deuterium kinetic isotope effects were determined for reductions of benzophenone with NaBH/sub 4/, LiBH/sub 4/, and LiAlH/sub 4/ at 25.0/sup 0/C. The observed /sup 12/k//sup 14/k at the carbonyl carbon and k/sub H4//k/sub D4/ of the reducing agents were 1.066 and 0.75, 1.043 and 1.089 and 1.024 and 1.10, respectively, for NaBH/sub 4/, LiBH/sub 4/ and LiAlH/sub 4/ reductions. Kinetic isotope effect calculations with various transition-state models suggested that the extent of hydride transfer from Al or B to the carbonyl carbon is increasing in the order LiAlH/sub 4/ (0.35) < LiBH/sub 4/ (0.55) < NaBH/sub 4/ (0.75). Nucleophilic solvent participation to the BH/sub 4/ moiety is weak if present for the SBH reduction.

Carbon-13 and oxygen-18 kinetic isotope effects on methanolysis of p-nitrostyrene oxide

Carbon-13 and oxygen-18 kinetic isotope effects on methanolysis of p-nitrostyrene oxide

Carbon-14 kinetic isotope effects and mechanisms of addition of 2,4-dinitrobenzenesulfenyl chloride to substituted styrenes in the presence of lithium perchlorate

The k/ αk isotope effect for addition of ArSCI to p -methylstyrene is small and unchanged by addition of LiClO ; k/ βk is high and unchanged. For styrene and p -chlorostyrene k/ βk values are high and unchanged, while k/ αk increases from ∼ 1.025 to ∼ 1.04. p -Methylstyrene reacts through an open carbenium ion and the other two have cyclic intermediates.

.alpha.-Deuterium and carbon-13 kinetic isotope effects associated with the SN2 displacement of iodide and tosylate by lithium organocuprates

Etude des effets isotopiques secondaires du deuterium et du carbone 13 associes a une methylation concurrente de deux cuprates, (n-C 8 H 17 ) 2 CuLi(PBu 3 ) et (n-C 8 H 17 ) 4 CuLi 3 (PBu 3 ), par CH 3 X-CD 3 X et 12,13 CH 3 X (X=I ou OTs)

Carbon isotope effect on dehydration of bicarbonate ion catalyzed by carbonic anhydrase.

The carbon-13 kinetic isotope effect on the dehydration of HCO3- by bovine carbonic anhydrase has been measured. To accomplish this, bicarbonate was added to a buffer solution at pH 8 containing carbonic anhydrase under conditions where purging of the product CO2 from the solution is rapid. Measurement of the isotopic composition of the purged CO2 as a function of the concentration of carbonic anhydrase permits calculation of the isotope effect on the enzymic reaction. The isotope effect on the dehydration is k12/k13 = 1.0101 +/- 0.0004. This effect is most consistent with a ping-pong mechanism for carbonic anhydrase action, in which proton transfer to or from the enzyme occurs in a step separate from the dehydration step. Substrate and product dissociation steps are at least 2-3-fold faster than the hydration/dehydration step.

Carbon-14 kinetic isotope effects and mechanisms of addition of 2,4-dinitrobenzenesulfenyl chloride to substituted styrenes-1-14C and styrenes-2-14C

As the first reported examples of carbon isotope effects in simple electrophilic addition reactions we have measured the carbon-14 kinetic isotope effects in the addition of 2,4-dinitrobenzenesulfenyl chloride to a series of para-substituted ..cap alpha.. and ..beta..-labeled styrenes in acetic acid at 30.1 /sup 0/C: for para substituents Cl, H, and CH/sub 3/ the k//sup 14/K values for ..cap alpha.. labeling are 1.027, 1.022, and 1.004, and the k//sup 14/k values for ..beta.. labeling are 1.035, 1.032, and 1.037, all +/-approx.0.004. The kinetics of the reaction were measured for the p-CH/sub 3/O,p-CH/sub 3/, unsubstituted, p-Cl, and m-NO/sub 2/ styrenes; electron-donating groups strongly accelerate the reaction, and electron-withdrawing groups retard it. The Hammett plot is curved with p/sup +/ values ranging from about -4.6 at the electron-donating group (EDG) end to about -1.8 at the electron-withdrawing group (EWG) end. Both the isotope effect and kinetic data, and related data from the literature, are interpreted in terms of a changing mechanism, with the activated complexes of the rate-determining steps having much open carbenium ion (ion pair) character for EDG-substituted styrenes and much cyclic thiiranium io (ion par) character for EWG-substituted styrenes. 1 figure, 2 tables.

Heavy atom isotope effect studies of elimination reaction mechanisms. 1. A kinetic and carbon-14 kinetic isotope effect study of the base-promoted dehydrochlorination of substituted 1-phenylethyl-2-14C chlorides

Upon treatment with the solvent conjugate base, the primary reaction of substituted 1-phenylethyl chlorides is elimination to substituted styrenes in tert-butanol (t-BuOH)-10% v/v dimethyl sulfoxide (Me/sub 2/SO) at 60/sup 0/C and in bis(2-hydroxyethyl) ether-10% v/v Me/sub 2/SO at 45/sup 0/C. Kinetic studies using eight substituted chlorides show that these reactions are strongly accelerated by both electron-donating and electron-withdrawing substituents, probably indicating a fairly reactant-like E2 transition state with the mechanism changing from El-like E2 to ElcB-like E2 as the substituents change from electron donating group (EDG) to electron withdrawing (EWG). In this first carbon isotope effect study of an elimination reaction of an alkyl chloride, carbon-14 kinetic isotope effects have been measured in the alkoxide/bis(2-hydroxyethyl) ether-Me/sub 2/SO system for 1-(4-methylphenyl)ethyl-2-/sup 14/C chloride, k/sup ..beta../k = 1.038, 1-phenylethyl-2-/sup 14/C chloride, k/sup ..beta../k = 1.058, and 1-(4-chlorophenyl)ethyl-2-/sup 14/C chloride, k/sup ..beta../k = 1.068. Clearly none of these compounds reacts by an irreversible El mechanism, for which no ..beta..-carbon isotope effect should be observed. The trend in the results is what would be expected from increased C/sub ..beta../-H bond rupture as the E2 transition state shifts from El-like to ElcB-like. Theoretical calculations involving El-like E2 transition-state models show that in this region onlymore » for a reaction coordinate motion tht strongly couples C/sub ..beta../-H bond rupture, C/sub ..cap alpha../-C/sub ..beta../ double bond formation, and C/sub ..cap alpha../-Cl bond rupture does the calculated value for k/sup ..beta../k come into the experimentally observed range, and then only for reactant-like models. 2 figures, 5 tables.« less

Carbon-14 kinetic isotope effects and kinetic studies in the syn-elimination reactions of (2-phenylethyl)dimethylamine oxides

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTCarbon-14 kinetic isotope effects and kinetic studies in the syn-elimination reactions of (2-phenylethyl)dimethylamine oxidesDaniel R. Wright, Leslie B. Sims, and Arthur FryCite this: J. Am. Chem. Soc. 1983, 105, 11, 3714–3716Publication Date (Print):June 1, 1983Publication History Published online1 May 2002Published inissue 1 June 1983https://doi.org/10.1021/ja00349a063RIGHTS & PERMISSIONSArticle Views116Altmetric-Citations8LEARN 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 (437 KB) Get e-Alerts Get e-Alerts