Kinetic isotope effects oxygen

The kinetic parameters are E = 6.3 kcal.mole" and AS = —38.4 eu, and at 25 °C the reaction exhibits a primary kinetic isotope effect of 6.6. When 0-labelled MnO was employed, no labelled oxygen appeared in the benzophenone. The mechanism involves abstraction of hydrogen, either as a hydride ion or a hydrogen atom, from the anion of the alcohol... 

The oxidations of formic acid by Co(III) and V(V) are straightforward, being first-order with respect to both oxidant and substrate and acid-inverse and slightly acid-catalysed respectively. The primary kinetic isotope effects are l.Sj (25°C)forCo(IU)and4.1 (61.5 C°)for V(V). The low value for Co(lII) is analogous to those for Co(IIl) oxidations of secondary alcohols, formaldehyde and m-nitrobenzaldehyde vide supra). A djo/ h20 for the Co(III) oxidation is about 1.0, which is curiously high for an acid-inverse reaction . The mechanisms clearly parallel those for oxidation of alcohols (p. 376) where Rj and R2 become doubly bonded oxygen. 

One-step hydroxylation of aromatic nucleus with nitrous oxide (N2O) is among recently discovered organic reactions. A high eflSciency of FeZSM-5 zeolites in this reaction relates to a pronounced biomimetic-type activity of iron complexes stabilized in ZSM-5 matrix. N2O decomposition on these complexes produces particular atomic oj gen form (a-oxygen), whose chemistry is similar to that performed by the active oxygen of enzyme monooxygenases. Room temperature oxidation reactions of a-oxygen as well as the data on the kinetic isotope effect and Moessbauer spectroscopy show FeZSM-5 zeolite to be a successfiil biomimetic model. 

Relatively detailed study has been done for the reaction pathways over Au/Ti02 catalysts mainly because of simplicity in catalytic material components. The rate of PO formation at temperatures around 323 K does not depend on the partial pressure of C3H6 up to 20vol% and then decreases with an increase, while it increases monotonously with the partial pressure of O2 and H2 [57]. A kinetic isotope effect of H2 and D2 was also observed [63]. These rate dependencies indicate that active oxygen species are formed by the reaction of O2 and H2 and that this reaction is rate-determining [57,63,64]. 

A kinetic isotope effect 160/180 of 2% in the spontaneous hydrolysis of the 2,4-dinitrophenyl phosphate dianion, whose ester oxygen is labeled, suggests a P/O bond cleavage in the transition state of the reaction, and thus also constitutes compelling evidence for formation of the metaphosphate 66,67). The hydrolysis behavior of some phosphoro-thioates (110) is entirely analogous 68). 

The reactions depicted in Fig. 32 are most often carried out at low temperatures. The incursion of a thermal process at elevated temperatures has occasionally been observed. In some cases the thermal oxygenation products are identical to the photochemical products and in other cases are different. For example, when 2,3-dimethyl-2-butene/02 NaY is warmed above — 20 °C a reaction was observed which led to pinacolone (3,3-dimethyl-2-butanone) as the major product.98,110 Pin-acolone is not formed in the photochemical reaction at the same temperature. On the other hand, identical products were observed in the thermal and photochemical intrazeolite oxygenations of cyclohexane.114,133 135 These intrazeolite thermal processes occur at temperatures well below that necessary to induce a classical autooxidation process in solution. Consequently, the strong electrostatic stabilization of oxygen CT complexes may also play a role in the thermal oxygenations. Indeed, the increase in reactivity of the thermal oxygenation of cyclohexane with increasing intrazeolite electrostatic field led to the conclusion that initiation of both the thermal and photochemically activated processes occur by the same CT mechanism.134 Identical kinetic isotope effects (kH/kD — 5.5+0.2) for the thermal and photochemical processes appears to support this conclusion.133... 

Methanol still proceeds through an initial C H bond scission, but reacts with water before the OH bond breaks. Alternatively, formaldehyde formation likely occurs along the same pathway as CO formation. This is true if HCO is an intermediate in the decomposition pathway. Furthermore, the lack of a kinetic isotope effect for CH3OD indicates that formaldehyde is not the product of an initial O-H scission.94 Because formaldehyde and formic acid are not the thermodynamically favored products of methanol oxidation, they must be the result of kinetic limitations preventing the full oxidation to C02, analogous to the production of H202 for the reduction of oxygen (see next section). 

Abstract The theoretical framework needed for interpretation of kinetic isotope effects on unimolecular reactions is reviewed. Application to the satisfactory rationalization of the theoretically puzzling mass independent isotope effect observed for oxygen isotope fractionation in extraterrestrial samples is described. 

Tian, G., Berry, J.A. and Khnman, J.P. (1994). Oxygen-18 kinetic isotope effects in the dopamine heta-monooxygenase reaction evidence for a new chemical mechanism in non-heme metaUomonooxygenases. Biochemistry 33, 226-234... 

The oxygen rebound mechanism was supported by experimental evidence including (1) high kinetic isotope effects, (2) partial positional or stereochemical scrambling, and (3) allylic rearrangements. For instance, in the presence of [Fe(TPP)Cl] and PhIO, dx-stilbene was stereospecihcally epoxidized. In addition, it was found that cis-stilbene was 15 times more reactive than trans-stilbene in competitive epoxidations. (see Figure 7.20). " ... 

As assumed, the small and positive valne of H/D kinetic isotope effect may be used as a criterion for an electron-transfer pathway. For example, anion-radicals of a-benzoyl-co-haloalkanes can react in two routes (Kimura and Takamnkn 1994). The first ronte is the common one—an electron is transferred from the oxygen anion of the carbonyl gronp to a terminal halogen. The transfer provokes fission of the carbon-halogen bond. The second ronte is the S 2 reaction, leading to a cyclic product as shown in Scheme 2.37. 

McConnaughey T (1989b) C and 0 disequilibrium in biological carbonates. II. In vitro simulation of kinetic isotope effects. Geochim Cosmochim Acta 53 163-171 McCorkle DC, Emerson SR (1988) The relationship between pore water isotopic composition and bottom water oxygen concentration. Geochim Cosmochim Acta 52 1169-1178 McCorkle DC, Emerson SR, Quay P (1985) Carbon isotopes in marine porewaters. Earth Planet Sci Lett 74 13-26... 

These results, as well as rate studies " and kinetic isotope effects ", support a concerted, 5ptra-structured oxenoid-type transition state for the CH oxidations". The original oxygen-rebound mechanism has been discounted (see the computational work in Section I.D). Recently, however, the stepwise radical mechanism was revived in terms of the so-called molecule-induced homolysis , but such radical-type reactivity was severely criticized on the basis of experimental" and theoretical grounds. 

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