18O Tracer Studies Research Articles

Synthesis and Stereochemistry of Chiral Selenoxides and Telluroxides

Optically pure selenoxides stabilized by bulky substituents and/or intramolecular coordination with amino group were isolated by optical resolution of a diastereomeric mixture of selenoxide and chromatographic separation of a racemic mixture of selenoxide using an optically active column. Optically pure telluroxides stabilized by similar techniques were also isolated by chromatographic resolution. The absolute configurations were determined by X-ray crystallographic analysis and circular dichroism spectra. Kinetic studies on the racemization of these chiral chalcogen oxides were examined, and a mechanism for the racemization via achiral hydrate was proposed based on the H2 18O tracer studies.

Isotope Studies of Photocatalysis: Dual Mechanisms in the Conversion of Anisole to Phenol

The partial TiO2-mediated photocatalytic degradation of anisole has been studied using isotopically labeled substrates and comparisons to Fenton and hν/H2O2 chemistry. An H/D isotope selectivity of 2.7 is observed for hydrogen abstraction from the methyl group in all cases. An 18O tracer study shows that anisole is simultaneously converted to phenol by both ipso attack and degradation of the methyl. The isotope selectivity for hydroxylation of anisole is reported to be about 1.3 under all conditions. This is attributed to an O2-dependent competition along the reaction pathway that can be avoided by use of benzoquinone instead of O2 as an electron acceptor with TiO2. Under those conditions, the H/D isotope selectivities are between 0.9 and 1.0, as would be expected for inverse secondary kinetic isotope effects.

In Situ Real-Time Studies of Oxygen Incorporation in Complex Oxide Thin Films Using Spectroscopic Ellipsometry and Ion Scattering and Recoil Spectrometry

ABSTRACTThe surface termination of c-axis oriented YBa2Cu3O7-δ (YBCO) and the oxygen incorporation mechanism has been investigated using a unique combination of spectroscopic ellipsometry (SE) and time of flight ion scattering and recoil spectrometry (ToF-ISARS). The high surface sensitivity of the ToF-ISARS technique combined with the bulk oxygen sensitivity of SE are shown to yield complimentary information. The SE provided the film orientation and quality, while ToF-ISARS supplied surface compositional and structural information and enabled isotopic 18O tracer studies. It was determined that the 0 content of the film had little effect on the surface termination of the film, indicating a lack of labile Cu(l) sites at the c-axis oriented YBCO surface. Also, strong evidence for a Ba or BaO terminated structure is shown. The data related to the 18O tracer studies indicate that O from the reaction ambient incorporates only into the labile Cu(1) sites during both deposition and annealing, while stable O sites were populated with O from the sputtered target, indicating either the need for sputtered atomic O or sputtered YCuO complexes to occupy the stable Cu(2) sites.

18O-Tracer Studies of Fe(II)-Induced Decomposition of 1,2,4-Trioxolanes (Ozonides) Derived from Cyclopentenes and Indenes. Inner-Sphere Electron Transfer Reduction of the Peroxide Linkage

Fe(II)-induced decomposition of 1,2,4-trioxolanes (ozonides) 1 and 2 is studied in detail. The inner-sphere electron transfer reduction, which is sensitive to steric effects, is proposed to be a reasonable mechanism for the peroxide decomposition, i.e., the selective generation of one of the two possible Fe(III)-complexed oxy radicals. The fate of the oxy radical species is revealed in detail by using 18O-tracer studies in the reduction of 18O-labeled ozonides 1a and 2a, in which the ethereal oxygens are labeled by 18O atom (18O, 77%) and also by the reduction in the presence of H218O (18O, 10%). The results of the 18O-tracer studies are consistent with the regioselective generation of the Fe(III)-complexed oxy radical species.

Alkaline hydrolysis of [Co(cyclen)(O 2CO)] + and direct observation of deprotonated S N1CB intermediates

A stopped-flow/rapid-scan study of the hydrolysis of the carbonate chelate [Co(cyclen)(O 2CO)] + in alkaline solution ([OH −] = 0.05 to 1.0 M, 25.0 °C, I = 1.0 M (NaClO 4)) has shown that the reaction follows consecutive first order steps, the first ( r 1 2 = 66 ms at [OH −] = 1.0 M ) corresponding to the OH − promoted reversible ring-opening of the chelate and the second ( I 1 2 = 4.1 s at [OH −] = 1.0 M ) to irreversible loss of CO 3 2− from the cis-[Co(cyclen)(OH)(OCO 2)] product. Results from 18O tracer studies show that steps occur via CoO bond cleavage. Deprotonation of amine centres in the cyclen ligand is important in both processes, with significant concentrations of the amido complex [Co(cyclen-H)(O 2CO)] being generated initially, and with cis-[Co(cyclen-H)(OH)(OCO 2)] − also being directly observed subsequently. The experimentally determined rate laws are interpreted in terms of S N1CB mechanisms for the ring-opening and hydrolysis reactions, with the doubly deprotonated intermediate cis-[Co(cyclen-2H)(OH)(OCO 2)] 2− being an important contributor to the latter.

Reactivity of nitroso oxides: effect of polar substituents and reaction mechanism

The structure and reactivity of nitroso oxide intermediates has been studied by trapping and tracer experiments in the photooxidation of substituted phenyl azides. O-Transfers with p-methoxyphenyl nitroso oxide were efficient and showed the reactivity order of PhNO > Ph2S > Ph2SO > PhH. Sulfides and sulfoxides were oxidized electrophilically and the partial rate factor for the hydroxylation of benzenes afforded a negative ρ-value of –1.40 (σ+). In the case of the p-mtrophenyl isomer, the radical reactivity was much more significant as exemplified in the hydrogen abstraction from toluene. Both isomers behave as electrophilic radicals as shown by the relative rates of hydrogen abstraction from aliphatic C–H bonds, i.e., 1°:2°:3°= 1:7:50–60. An 18O-tracer study revealed that the intramolecular cyclization of aryl nitroso oxides is competitive with intermolecular O-transfers.

18O tracer study of the oxidation of zircaloy‐4 in steam

AbstractThe corrosion resistance of Zircaloy‐4, used as fuel cladding in pressurized water reactors, in steam at 400°C is related to the annealing temperature used in fabrication. This change in corrosion behaviour appears to be related to the size and composition of intermetallic compound precipitates, which vary with annealing temperature.A SIMS study has been undertaken to clarify the mechanisms responsible for these variations in the initial stages of corrosion of Zircaloy‐4. A set of samples subjected to different annealing temperatures have been oxidized, initially in normal steam and subsequently in steam enriched in H2 18O. Corrosion mechanisms have been followed using depth profiling in a Cameca IMS 3F microscope together with three‐dimensional depth profiles and imaging of taper sections using a gallium microprobe (VG Scientific).Iron and chromium, present in intermetallics in the alloy, remain localized within the oxide scale. The size and frequency of these local concentrations of iron and chromium reflect the distribution of these elements in the alloy. There is, however, some diffusion of these elements into the surrounding oxide, the iron diffusion being greater than that for chromium. The 18O tracer studies indicate that grain boundary transport of oxygen dominates the initial oxide growth process for Zircaloy‐4.

Kinetic features and lattice-oxygen participation in propene oxidation over Bi–Mo oxide and some Mo oxide catalysts

The rates of propene conversion in the absence of oxygen have been compared with those in the presence of oxygen at the same propene pressures. Over Bi–Mo oxides which are mainly composed of the β- or γ-phase, the rates for propenal formation were 3.5–4.5 times bigger in the presence of oxygen than in the absence of oxygen. In contrast, some Mo-containing oxide catalysts such as MoO3, Co–Mo (1/1), Co–Te–Mo (1/1/1) and other Bi–Mo oxides exhibit little or no difference in the rates of propenal formation in the presence or absence of oxygen. In the case of but-1-ene and (E)-but-2-ene oxidation, both rates were nearly the same even on β- and γ-phase Bi–Mo oxides. Such a sensitive effect of oxygen on propenal formation over β- and γ-phase oxides seems to originate from an appreciable increase of active sites owing to a reoxidation step. Using 18O tracer studies in propene oxidation, the extents of lattice-oxygen participation were determined over various Mo oxide catalysts. The number of active sites for propene oxidation was found to be closely related to the extent of lattice-oxygen participation. A modified redox mechanism was proposed on the basis of a different active site model for the reduction and reoxidation steps. The kinetic features were discussed using this model.

Modelling of 18O tracer studies of the oxygen redistribution during formation of SiO 2 layers by high dose implantation

The modelling of buried SiO 2 layer formation by high dose implantation is extended to simulate new experimental data. The experiments involved the implantation of two different oxygen isotopes ( 16O and 18O) followed by subsequent analysis by secondary ion mass spectroscopy (SIMS) to determine the isotope distributions. The model contains all the essential features of the original formulation such as the inclusion of volume swelling due to silicon oxidation, sputtering and the diffusion of excess oxygen within the SiO 2, but has now been enhanced to include (a) the two oxygen isotopes and (b) the observed phenomenon of perfect isotopic exchange between migrating oxygen and matrix oxygen within the layer. Using the model, isotope depth distributions have been generated for comparison with the experimental data. For doses lower than the critical dose needed for layer formation, profiles have been calculated showing the distribution of the two isotopes. For the case where the combined dose is greater than this critical dose good agreement is found between the experimental data and the distributions calculated. Some discrepancies are apparent especially at the edges of the buried layer which originate in the non-linearity of the SIMS signals.

ChemInform Abstract: The Role of Oxygen in the Hydroxylation Reaction of Benzene with Fenton′s Reagent. 18O Tracer Study.

AbstractIn order to study the role of O2 and H2O2 in the oxidation of benzene, labeled O2 and nonlabeled H2O2 are applied.

Molybdate Catalysts

AbstractIt is possible to classify the solid state catalyst systems into two categories; structure static catalyst and structure dynamic catalyst. The former is the catalyst in which the original structural circumstances of surface active center, such as coordinatively unsaturated state of active element, length and order of metal-ligand bond or of a bond between structural component, and their bond angle, are maintained unchanged during catalysis. The example is molybdenum sulfid which is a good catalyst for the isomerization and hydrogenation of olefins. On the contrary, the catalysts, in which the above circumstances of surface active center are changing dynamically during catalysis, are classified into the latter category. Molybdate catalysts for olefin oxidation by molecular oxygen at high temperature belong to this category. Main difference between two categories is whether the originally coordinated ligand on metal cation, such as sulfur anion of molybdenum sulfid and oxygen anion of metal molybdate, can participate in the catalysis directly or not. When the coordinated ligand is involved in the reaction, resulting abnormal coordinatively unsaturated state of surface active center must be sustained by the dynamic structure changes of both surface and bulk for the steady state catalysis. The bulk structure, therefore, affects strongly the properties of structure dynamic catalysts, whereas the properties of structure static catalysts depend on the surface structure like coner and edge mainly. The relationship between catalytic properties and structural stability of molybdate catalysts in the selective oxidation of propylene was discussed on the basis of18O tracer study.

AN 18O-TRACER STUDY ON THE TiO2-SENSITIZED PHOTOOXIDATION OF AROMATIC COMPOUNDS

Abstract TiO2-sensitized photooxidation of benzenes has been studied to disclose the origin of the ring hydroxylating species. An 18O-tracer study using 18O2 gas revealed that the hydroxyl group of phenols comes essentially from molecular oxygen at higher pH region. At lower pH, however, the oxygen atom in phenols originates mostly from solvent water. These findings are compatible with two different mechanisms proposed By Osa et al. and Tokumaru et al.; that is, the major pathway changes with pH.

SIMS and 18O tracer studies of the redistribution of oxygen in buried SiO 2 layers formed by high dose implantation

The formation of buried SiO 2 layers by high dose oxygen implantation has been monitored by the implantation of 18O tracer atoms and subsequent analysis by negative ion SIMS. Exchange of 18O with matrix oxygen in the buried layer occurred for all experiments. Rapid redistribution of the implanted tracer trace to the edges of the SiO 2 layer was observed. The lower SiSiO 2 interface appears to be a barrier to further redistribution of the tracer oxygen.

18O-tracer study on the rearrangement of carbonyl oxide intermediates to esters

A tracer study for the rearrangement of carbonyl oxides to esters using doubly-labeled oxygen lead to a conclusion that carbonyl oxides do not isomerize directly to dioxyranes and the rearrangement to esters proceeds via a cyclic tetroxide.

Source of the oxygen atoms of nitrate in the oxidation of nitrite by Nitrobacter agilis and evidence against a PON anhydride mechanism in oxidative phosphorylation

15N, 18O Tracer studies were applied to the aerobic oxidation of nitrite to nitrate by the chemolithotrophic bacterium, Nitrobacter agilis. It was established that, in conversion of nitrite to nitrate, one oxygen atom of nitrate arose from water and none from O 2 or inorganic phosphate. This result confirms that of Kumar et al. [(1983) FEBS Lett. 152, 71–74]. Oxygen exchange between water and inorganic phosphate was small and that between water and nitrite or nitrate or any reaction intermediates between these two was not detected. Oxidation of nitrite was, therefore, effectively irreversible under the conditions employed. The uptake of extracellular phosphate was sufficient to allow significant transfer of 18O from phosphate to nitrate if oxidative phosphorylation had occurred by way of a PON anhydride between phosphate (or ADP) and nitrate. The results are, therefore, inconsistent with the occurrence of a reaction of this type during nitrite oxidation.

Catalytic Activity of MoO3 and V2O5 Highly Dispersed on TiO2 for Oxidation Reactions

Abstract The structure of Mo–Ti oxide catalysts has been investigated by using IR and XRD techniques. At a low Mo content, MoO3 is highly dispersed on TiO2, an amorphous phase being formed. Simultaneously, a shift of the Mo=O band from 995 cm−1 to 950 and 905 cm−1 takes place. The maximum rate of the oxidation of C2H5OH and C3H6 over Mo–Ti-6(atom% of Mo) is attributable to the weakening of the Mo=O bond. The 18O tracer and kinetic studies of the oxidation of C3H6 over Mo–Ti and V–Ti oxides suggest that a redox mechanism is applicable, the rate constants of reduction and reoxidation step being determined. The promoter action of TiO2 support is attributed mainly to the increase in the rate of reduction step.

18O Tracer studies of Al2O3 scale formation on NiCrAl alloys

Diffusion processes in Al2O3scales formed on NiCrAl + Zr alloys were studied by the proton activation technique employing the18O isotope as a tracer. The18O profiles identified a zone of oxide penetration beneath the external scale. Both this subscale formation and the outer Al2O3scale thickness were shown by this technique to increase with Zr content in the alloy. Estimated kp's from scale thicknesses were in agreement with gravimetric measurements for various Zr levels. Alternate exposures in O and18O revealed that oxygen inward transport was the primary growth mechanism. A qualitative analysis of these18O profiles indicated that the oxygen transport was primarily via short-circuit paths, such as grain boundaries.

18O tracer study of the active species of oxygen on Bi2MoO6 catalyst

The species of oxygen on Bi2MoO6 catalyst which are active in the oxidation of propene and but-1-ene have been investigated using an 18O tracer. Both on the catalyst surface and in the bulk oxide the active species of oxygen are classified into two independent types. One species abstracts hydrogen from the olefin and the other adds exclusively to the intermediate to form the oxygenated product.

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.

PARTIAL ASYMMETRIC PUMMERER REACTION OF ARYL PROP-2-YNYL SULFOXIDE WITH ACETIC ANHYDRIDE

Abstract The Pummerer reaction of aryl prop-2-ynyl sulfoxides with acetic anhydride was found to be stereospecific. From the 18O-tracer studies, the asymmetric induction at α-carbon is considered to be caused by an intramolecular and somewhat synchronized 1,2-acetoxy migration within the ylide intermediate.