The magnetic isotope effect and oxygen isotope selection in chain processes of polymer oxidation

Experimental evidence based on the dependence of molecular oxygen isotope enrichment on the oxygen conversion, temperature and kinetic chain length indicate that, in chain processes of polymer oxidation, the elementary, reactions (recombination or disproportionation) of peroxy radicals are responsible for the selection of both ¹⁷O and ¹⁸O isotopes. The ¹⁷O selection is induced by a magnetic isotope effect and is sensitive to the molecular dynamics, while ¹⁸O selection is due to a classical mass-dependent isotope effect and is much less effective.     

The kinetic isotope effect for carbon and oxygen in the reaction CO + OH

The kinetic isotope effect (KIE) for carbon and oxygen in the reaction CO + OH has been measured over a range of pressures of air and at 0.2 and 1.0 atm of oxygen, argon, and helium. The reaction was carried out with 21–86% conversion under static conditions, utilizing the photolysis of H₂O₂ as a source of OH radicals. The value of the KIE for carbon varies with pressure and the kind of ambient gas; for air the ratio of the reaction rates ¹²k/¹³k has the value 1.007 at 1.00 atm and decreases to 0.997 at 0.2 atm; for oxygen and argon over the same pressure range the values are 1.002–0.994 and 1.000–0.991, respectively. The value of the KIE for the CO oxygen atom is ¹⁶k/¹⁸k = 0.990 over the pressure range 0.2–1.0 atm and is independent of the kind of ambient gas. No exchange of the oxygen atoms in the activated complex, followed by decomposition to the starting molecules, was observed. From the mechanistic standpoint the normal KIE observed for carbon at the high pressure is attributed to the initial formation of the activated HOCO radical, whereas the inverse KIE observed at low pressures is a result of the KIE for the reverse reaction HOCO? → CO + OH being greater than that for the forward reaction HOCO^? → CO₂ + H. The derived isotopic equilibrium constant for HOCO ?CO favors the enrichment of ¹³C in the more strongly bound HOCO.     

Intramolecular chain reaction of artemisinin oxidation

The intramolecular chain oxidation of artemisinin was analyzed using the parabolic model. The competition of the mono- and bimolecular peroxy radicals formed from artemisinin was considered. Artemisinin is predominantly oxidized via the intramolecular chain mechanism to form polyatomic hydroperoxides. This results in the situation when, under aerobic conditions, artemisinin is transformed from the monofunctional into polyfunctional initiator with several hydroperoxide groups. The enthalpy was calculated, and the activation energies and rate constants of the intramolecular reactions of the artemisinin peroxy radicals, as well as those of their bimolecular reactions with C-H, S-H, and O-H bonds of biological substrates and their analogs, were calculated in the framework of the parabolic model. A new kinetic scheme for artemisinin oxidation was proposed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 267–275, February, 2008.     

Deuterium isotope effect and oxidation of labelled alcohols

Oxidation of alcoholsR-CHD-OH by CrO₃/pyridine gave the aldehydesR-CDO with a relatively small loss of D thus indicating a pronounced isotope effect.     

Origin of the oxygen in the oxidation of triphenylphosphine by potassium persulfate—application of the 18O isotope effect in 31P NMR

Potassium persulfate oxidizes triphenylphosphine to triphenylphosphine oxide in 60% aqueous acetonitrile. It has been suggested that the oxygen of the product, triphenylphosphine oxide, might originate from solvent water, following nucleophilic attack on an intermediate phosphonium ion. We have investigated the origin of the oxygen in the oxidation of triphenylphosphine by potassium persulfate in 60% aqueous acetonitrile containing 20% [¹⁸O]water. The product was analyzed by using the ¹⁸O isotope effect in ³¹P NMR spectroscopy. The magnitude of the ¹⁸O isotope-induced shift was determined by synthesizing triphenylphosphine [¹⁸O]oxide and was found to be 0.038 ppm upfield. The product of the oxidation reaction in 20% [¹⁸O]water displayed no ¹⁸O isotope effect. The origin of the oxygen in the oxidation reaction is the persulfate ion, consistent with an alternative mechanism involving nucleophilic attack by water at the sulfur atom of a phosphonium peroxysulfate intermediate.     

Anomalous oxygen isotope enrichment in CO2 produced from O+CO: estimates based on experimental results and model predictions

The oxygen isotope fractionation associated with O+CO-->CO(2) reaction was investigated experimentally where the oxygen atom was derived from ozone or oxygen photolysis. The isotopic composition of the product CO(2) was analyzed by mass spectrometry. A kinetic model was used to calculate the expected CO(2) composition based on available reaction rates and their modifications for isotopic variants of the participating molecules. A comparison of the two (experimental data and model predictions) shows that the product CO(2) is endowed with an anomalous enrichment of heavy oxygen isotopes. The enrichment is similar to that observed earlier in case of O(3) produced by O+O(2) reaction and varies from 70 0/00 to 136 0/00 for (18)O and 41 0/00 to 83 0/00 for (17)O. Cross plot of delta (17)O and delta (18)O of CO(2) shows a linear relation with slope of approximately 0.90 for different experimental configurations. The enrichment observed in CO(2) does not depend on the isotopic composition of the O atom or the sources from which it is produced. A plot of Delta(delta (17)O) versus Delta(delta (18)O) (two enrichments) shows linear correlation with the best fit line having a slope of approximately 0.8. As in case of ozone, this anomalous enrichment can be explained by invoking the concept of differential randomization/stabilization time scale for two types of intermediate transition complex which forms symmetric ((16)O(12)C(16)O) molecule in one case and asymmetric ((16)O(12)C(18)O and (16)O(12)C(17)O) molecules in the other. The delta (13)C value of CO(2) is also found to be different from that of the initial CO due to the mass dependent fractionation processes that occur in the O+CO-->CO(2) reaction. Negative values of Delta(delta (13)C) ( approximately 12.1 0/00) occur due to the preference of (12)C in CO(2)* formation and stabilization. By contrast, at lower pressures (approximately 100 torr) surface induced deactivation makes Delta(delta (13)C) zero or slightly positive.     

Magnetic field as a means to identify initiating reaction in oxidation of organic substances with molecular oxygen

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The NO+O3 reaction: a triple oxygen isotope perspective on the reaction dynamics and atmospheric implications for the transfer of the ozone isotope anomaly

Atmospheric nitrate shows a large oxygen isotope anomaly (Delta 17 O), characterized by an excess enrichment of 17 O over 18 O, similar to the ozone molecule. Modeling and observations assign this specific isotopic composition mainly to the photochemical steady state that exists in the atmosphere between ozone and nitrate precursors, namely, the nitrogen oxides (NOx=NO+NO2). However, this transfer is poorly quantified and is built on unverified assumptions about which oxygen atoms of ozone are transferred to NO(x), greatly weakening any interpretation of the nitrate oxygen isotopic composition in terms of chemical reaction pathways and the oxidation state of the atmosphere. With the aim to improve our understanding and quantify how nitrate inherits this unusual isotopic composition, we have carried out a triple isotope study of the reaction NO+O3. Using ozone intramolecular isotope distributions available in the literature, we have found that the central atom of the ozone is abstracted by NO with a probability of (8+/-5)%(+/-2 sigma) at room temperature. This result is at least qualitatively supported by dynamical reaction experiments, the non-Arrhenius behavior of the kinetic rate of this reaction, and the kinetic isotope fractionation factor. Finally, we have established the transfer function of the isotope anomaly of O3 to NO2, which is described by the linear relationship Delta 17 O(NO2)=A x Delta 17 O(O3)+B, with A=1.18+/-0.07(+/-1 sigma) and B=(6.6+/-1.5)[per thousand](+/-1 sigma). Such a relationship can be easily incorporated into models dealing with the propagation of the ozone isotope anomaly among oxygen-bearing species in the atmosphere and should help to better interpret the oxygen isotope anomaly of atmospheric nitrate in terms of its formation reaction pathways.     

Magnetic isotope effect of tin and isotope exchange in the reactions of organotin compounds

Conclusions Magnetic isotope effects from tin are not observed during the isotope analysis of the products from reactions involving trimethylstannyl radicals. One of the possible reasons for this is the demonstrated presence of degenerate exchange processes between the initial compounds and the products during the reaction.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2207–2211, October, 1985.     

A new method to determine the 17O isotopic abundance in CO2 using oxygen isotope exchange with a solid oxide.

This paper discusses a simple method to determine 17O isotope excess or deficiency ('mass-independent isotopic composition') in CO2 gas. When applying conventional mass spectrometry of CO2 (m/z 44, 45 and 46) to determine the 17O/16O ratio, the 13C/12C ratio has to be established separately. This can be achieved by analysing an aliquot of sample CO2 before and after subjecting it to oxygen isotope exchange with a pool of oxygen with 'normal' 17O/16O ratio, i.e. with Delta17O approximately equal to delta17O-0.516 x delta18O = 0. Cerium oxide has been shown to be practically well suited for the exchange of CO2 oxygen; the reagent is safe and does not produce any contamination. The CO2-CeO2 exchange reaction has 99.8 +/- 0.7% recovery yield. At 650 degrees C this reaction reaches equilibrium in 30 min and, as tested, results in complete oxygen replacement. Delta17O determinations depend on accuracy of CO2 delta measurements: the repeatability of +/-0.015 per thousand (1sigma) in delta(45)R and delta(46)R determination relative to the working reference results in an error of Delta17O as small as +/-0.33 per thousand. Such a precision is sufficient for Delta17O determination in stratospheric CO2. The calculated Delta17O value systematically depends on absolute 17R and 13R ratios in isotopic reference materials, which are presently not yet known with certainty (the 17R value is most important), and may be inadequate for 17O-correction with a = 0.516. Within the present uncertainty, Delta17O determined in 17O-enriched CO2 agrees with the value directly measured in the enriched O2 from which this CO2 was produced. Besides Delta17O determination, investigated CO2-CeO2 equilibration may have several other implications. Fast, complete isotopic exchange of CO2 by reaction with CeO2 may also be employed to get reproducible 17O-correction and, hence, to better monitor small delta13C shifts and to isotopically equilibrate mixtures of CO2 gases.     

Magnetic isotope effect in the photolysis of organotin compounds

Photolysis of organotin molecules RSnMe3 is shown to be a spin selective radical reaction accompanied by fractionation of magnetic, (117,119)Sn, and nonmagnetic, (118,120)Sn, isotopes between starting reagents and products. A primary photolysis process is a homolytic cleavage of the C-Sn bond and generation of a triplet radical pair as a spin-selective nanoreactor. Nuclear spin dependent triplet-singlet conversion of the pair results in the tin isotope fractionation. Experimentally detected isotope distribution unambiguously demonstrates that the classical, mass-dependent isotope effect is negligible in comparison with magnetic, spin-dependent isotope effect.     

Magnetic field effects on the initiation of chain oxidation

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Magnetic isotope effects in biradicals: Substantial 13C enrichment and a novel mechanistic probe

Photolysis of 2,n-diphenylcycloalkanones (n-membered ring, n = 10, 11, 12, 15) produces products that are significantly enriched in ¹³C. The enrichments are different for each product, and this allows assignment of the dynamic pathways through which each product is formed.     

Membrane oxygen enrichment : I. Demonstration of membrane oxygen enrichment for natural gas combustion

Combustion with oxygen-enriched air can substantially reduce fuel consumption in certain industrial applications. The use of membranes is a potentially attractive approach for producing oxygen-enriched air. A variety of membrane materials and operating conditions can be used to produce oxygen concentrations in the range of interest, which is generally 25 to 35% O₂. To determine the optimum system, a trade-off must be made between capital cost (membrane area) and operating cost (pumping power). A silicone-based membrane oxygen enricher was fabricated and factory tested. As expected, a substantial reduction in natural gas consumption was achieved. During the test period some membrane degradation was experienced due to pinhole formation. It was determined that pinholes were not a result of the factory environment, and that use of a membrane oxygen enricher to achieve fuel savings is technically feasible.