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Nitrogen-15 Kinetic Isotope Effects in the Decomposition of Nitric Oxide on Copper

Polič S., Senegačnik M., Kobal I., Zieliński M.

Polish Journal of Chemistry

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2002

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Vol. 76 / nr 7

1009-1018

EN

Isotopic rate constant ratios, k14/k15, have been determined for the decomposition of nitric oxide over copper wire at initial NO pressures 40-60 kPa over temperature range 725-825 K. They were interpreted using the Bigeleisen formalism. Under these experimental conditions the reaction was found to be second order in NO. On the basis of the temperature independence, k14/k15 = 1.010 š 0.001 at all temperatures studied, a cis ONNO transition state was shown to account satisfactorily for the experimental k14/k15 values.

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Carbon and Chlorine Kinetic Isotope Effects and Solvent Effects on the Hydrolysis of Chloroformates

Swanson S., Rudziński J., Paneth P., O'Leary M. H.

Polish Journal of Chemistry

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2002

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Vol. 76 / nr 12

1721-1731

EN

Carbon and chlorine kinetic isotope effects and pseudo-first order rate constants were measured for the hydrolysis of ethyl, benzyl, and isopropyl chloroformate in various mixtures of the binary solvent water-1,4-dioxane. Rate constants decreased as the percentage of 1,4-dioxane increased for all three chloroformates. Carbon-13 kinetic isotope effects (k12/k13) for hydrolysis of these compounds were 1.039-1.042 for ethyl chloroformate (0-75% 1,4-dioxane), 1.034-1.039 for benzyl chloroformate (15-75% 1,4-dioxane), and 1.025-1.037 for isopropyl chloroformate (25-75% 1,4-dioxane). Chlorine-37 kinetic isotope effects (k35/k37) were measured for benzyl and isopropyl chloroformates in 25% 1,4-dioxane (1.0088, benzyl; 1.0080 isopropyl) and 75% 1,4-dioxane (1.0090, benzyl; 1.0102 isopropyl). These data are consistent with an associative mechanism for benzyl and ethyl chloroformates, but hydrolysis of isopropyl chloroformate appears to be dissociative in polar solvents.

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Elucidation of reaction mechanisms using kinetic isotope effects of short-lived radionuclides

Matsson O.

Nukleonika

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2002

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Vol. 47,suppl.1

43-45

EN

The use of the short-lived radionuclides 11C and 18F for the elucidation of organic reaction mechanisms is described. Examples of the different mechanistic problems that are discussed include concerted vs. stepwise base catalysed elimination (E2 or E1cB) and finding the rate limiting step for nucleophilic aromatic substitution (SNAr). The use of radionuclides to learn details about transition state structure for concerted nucleophilic aliphatic substitution (SN2) is also described.

4

DFT/PM3 study of the enoyl-CoA hydratase catalyzed reaction

Pawlak J., Bahnson B., Anderson V.

Nukleonika

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2002

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Vol. 47,suppl.1

33-36

EN

The enoyl-CoA hydratase catalyzed hydration of alfa,beta-unsaturated thiolesters has been modeled by using the crystal structure of 4-(N,N-dimethylamino)cinnamoyl-CoA bound at the active site. The quantum chemical calculation used the ONIOM mixed level procedure to permit the substrate thiolester and water molecule to be modeled using B3LYP/6-31G(d) level of theory and the active site residues modeled at a semiempirical level using the PM3 Hamiltonian. The results permitted the identification of a stable thiolester enolate intermediate, supporting a stepwise reaction mechanism. The calculation also suggests that the same proton removed from the nucleophilic water molecule is transferred to C alfa in the subsequent protonation of the enolate intermediate. This observation reconciles the stepwise mechanism with the previously reported double isotope effect study [3].

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Carbon-14 kinetic isotope effects in the debromination of p-nitrodibromocinnamic acid

Bukowski J., Kanski R., Kanska M.

Nukleonika

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2002

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Vol. 47, nr 4

133-136

EN

The kinetic isotope effect (KIE) method was applied to study the mechanism of elimination of bromine from p-nitro-erythro-alfa,beta-dibromocinnamic acid labeled at the alfa-carbon. This compound was obtained starting from [2-14C]malonic acid via [2-14C]cinnamic acid and subsequent addition of bromide. The value of 14C KIE determined for alfa-position of side chain of p-nitro-erythro-alfa,beta-dibromocinamic acid proves that elimination of bromine leading to formation of (E)-p-nitrocinnamic acid proceeds via E2 mechanism.