Oxygen Scrambling Research Articles

Oxidation of a phosphinidene oxide: formation of a dioxaphosphirane oxide with oxygen scrambling.

The oxidation of a prototypical phosphinidene oxide FP[double bond, length as m-dash]O has been studied in O2-doped Ar and N2 matrices at 10 K. Upon 266 nm laser irradiation, FP[double bond, length as m-dash]O combines with O2 and yields the cyclic peroxide, dioxaphosphirane oxide FP([double bond, length as m-dash]O)(O2). Unexpected oxygen scrambling occurs during the oxygenation as evidenced by the observation of a 1 : 2 mixture of FP([double bond, length as m-dash]16O)(18O18O) and FP([double bond, length as m-dash]18O)(16O18O) when 18O2 was used. Quantum chemical calculations suggest that the scrambling happens via the intermediacy of the low-lying triplet FPO3 by passing minimum energy crossing points (MECPs). In addition, inorganic dioxophosphorane FP([double bond, length as m-dash]O)2 has been also identified among the oxidation products of FP[double bond, length as m-dash]O.

Oxygen Activation in Oxidative Coupling of Methane on Calcium Oxide

A pulsed isotope exchange technique was applied to study the oxygen scrambling activity of polycrystalline calcium oxide under temperatures and pressures relevant for the oxidative coupling of methane (OCM). Oxygen exchange was observed above 400 °C. The onset was attributed to the removal of impurities on the catalyst surface. By trapping impurities in the gas feed, the scrambling could already be observed at room temperature. An activation energy of 80 kJ/mol was determined for the oxygen scrambling of O2 on the surface of polycrystalline CaO powder in absence of other gases. Presence of water and carbon dioxide shift the onset of the reaction to higher temperatures and increase the activation energy significantly to 110 and 150 kJ/mol, respectively. The OCM activity could be directly linked to the oxygen scrambling activity of the material in pulsed OCM operation. It is proposed that the same sites are responsible for oxygen scrambling and OCM reaction and that the rate is dictated by desorption of CO2 and H2O. The high reaction temperatures in OCM in case of CaO are only required to regenerate the active sites, which may apply to basic OCM catalysts in general. In situ Raman and thermogravimetric experiments verified the formation of a bulk calcite phase below 750 °C, which is inactive in OCM and oxygen scrambling. Above 750 °C no surface oxygen species or adsorbates were found by Raman spectroscopy suggesting that only surface defects are responsible for catalytic activity of CaO.

Oxygen Scrambling of CO2 Adsorbed on CaO(001)

The adsorption of CO2 on CaO(001) is investigated by density functional theory and infrared reflection absorption spectroscopy (IRAS). The calculations show that isolated CO2 adsorbates on terraces as monodentate carbonates can freely rotate at room temperature, while rotation within carbonate aggregates has some hindrance. Rotation and other motions are important to facilitate oxygen atom exchange between the CO2 adsorbate and CaO lattice. The calculated intrinsic barrier to oxygen scrambling is 114 kJ/mol for an isolated carbonate species and 148 kJ/mol within a long carbonate chain. However, due to the large adsorption energy for CO2 on a defect-free CaO terrace site, the apparent barrier becomes −39 kJ/mol for an isolated carbonate. At lower coordinated sites with higher degrees of freedom, the calculated intrinsic barrier to oxygen atom exchange is 80 kJ/mol at filled monatomic step sites and 26.9 kJ/mol at corner sites. IRAS studies are performed by adsorbing C18O2 on well-ordered Ca16O films grown ...

Oxygen-exchange reactions during CO oxidation over titania- and alumina-supported Au nanoparticles

Abstract Gold nanoparticles supported on titania and alumina were used as catalysts for CO oxidation. The oxidation of C 16 O (2 mol%) with 18 O 2 (2 mol%) in He at ambient temperature and 1.2 atm total pressure was performed in a single-pass, fixed-bed reactor. A mixture of CO 2 isotopomers was observed during oxidation over Au/TiO 2 and Au/Al 2 O 3 . However, no oxygen scrambling was observed in and between O 2 and CO. Moreover, no evidence for direct exchange between O 2 and CO 2 was found. Isotopic transient analysis of a switch between C 16 O 2 and C 18 O 2 showed that oxygen exchange occurred between the metal-oxide supports and CO 2 . Therefore, the CO 2 isotopomer distribution observed during CO oxidation could not be attributed solely to the oxidation reaction on Au. Comparison of the isotopic transients from 13 C and 18 O studies suggests the primary product of C 16 O oxidation with 18 O 2 is C 16 O 18 O. The presence of 0.16 mol% H 2 O co-fed with the reactants significantly promoted the rate of CO oxidation by factors of 2.5 for Au/TiO 2 and 10 for Au/Al 2 O 3 . Despite the promotional effect of H 2 O on the rate of oxidation, no significant differences in the level of oxygen exchange in the CO 2 were observed. In addition, the presence of H 2 O did not result in scrambling of oxygen in O 2 . Labeled oxygen found in the H 2 O exiting the reactor appeared to originate from 18 O associated with the CO 2 .

Dynamic Behavior of Carbonate Species on Metal Oxide Surface: Oxygen Scrambling between Adsorbed Carbon Dioxide and Oxide Surface

The oxygen exchange between surface carbonate species and oxide surfaces was investigated by the temperature-programmed desorption (TPD) of C18O2 adsorbed on metal oxides, MgO, CaO, and ZrO2 at room temperature. Although desorption plots were quite different for these metal oxides, extensive oxygen exchange was commonly observed. The fraction of C16O2 in the total desorbed CO2 increased monotonically with desorption temperature, and each desorption peak in the plots for the total CO2 could not be characterized by the isotopic distribution. The difference in proportion of incorporated lattice oxygen in the total CO2 at each desorption temperature was small between the low and high concentrations of C18O2. Among these metal oxides, ZrO2 showed the highest fraction of C16O2 in the total CO2 desorbed at 400 °C when C18O2 was adsorbed on these metal oxide with same surface concentration. The value of [C16O18O]2/{[C16O2][C18O2]} was close to 4 over almost all the temperature region of desorption for all these metal oxides, indicating that the three oxygen atoms composing carbonate were entirely scrambled before desorption as CO2. The amount of fixed monodentate carbonate with no exchange of oxygen was quite small. The manner of surface lattice oxygen incorporation into carbon dioxide is discussed on the basis of the mechanism proposed in the previous study. Successive adsorption experiments using 13CO2 with MgO demonstrated that the carbonate species switches from the weak basic sites to the strong basic sites during the heating process. It was also suggested that the dynamic behavior of carbonate species varies the coordinative environment and the adsorption strength of basic sites, which results in their further migration and desorption as CO2.

FTIR Study of Adsorption and Photoassisted Oxygen Isotopic Exchange of Carbon Monoxide, Carbon Dioxide, Carbonate, and Formate on TiO2

In-situ FTIR has been employed to investigate adsorption and photoassisted oxygen scrambling of CO, CO2, CO3, and HCOO on powdered TiO2 with 18O2. As clean TiO2 is exposed to CO at 35 °C, the IR frequencies of adsorbed CO shows that CO is bonded to Ti ions with an oxidation state +4 or less. The CO molecules that are bonded to two metal centers are more stable than those to only one center. In the case of CO2 adsorption, it produces adsorbed CO3 and CO2 with Ti−O−C−O bonding. Adsorbed CO does not exchange oxygen with 18O2, while adsorbed CO2, CO3, and HCOO do under UV irradiation. In the case of CO3 with bidentate configuration, only the oxygen bonded to the carbon with carbonyl character is involved in the exchange. Possible mechanisms for the oxygen scrambling of CO2, CO3, and HCOO are discussed.

Acyloxyl radical pair intermediate for the initial stage of the thermal decomposition of diacyl peroxide: a density functional study

To examine the reaction mechanism for the thermal reorganization, or more specifically the oxygen scrambling, in diacyl peroxide, we have carried out a hybrid density functional study using formyl peroxide as a model compound. The B3LYP calculations suggest that the oxygen scrambling in diacyl peroxide is most likely to occur via a σ-acyloxyl radical pair species: the competitive pathways of the [3,3]- and [1,3]-sigmatropic shifts are highly improbable. Thus, the mechanism for the thermal oxygen scrambling in diacyl peroxide should be completely different from those for the carbon counterparts of diacyl peroxide (the Cope and Claisen rearrangement).

Preparation and Reactions of Unsymmetrically Labeled Hydroperoxides: Solvent Viscosity Dependent Oxygen Scrambling of Cumylperoxyl Free Radicals

Preparation and Reactions of Unsymmetrically Labeled Hydroperoxides: Solvent Viscosity Dependent Oxygen Scrambling of Cumylperoxyl Free Radicals

Solvolysis of 1-(3-noradamantyl)ethyl sulfonates

The title esters solvolyze in aqueous ethanol (a) almost entirely to the rearranged substitution products 2- methyl-1-adamantyl alcohol and ether, (b) about 1000 times faster than unstrained analogues, (c) by a non-first-order rate law, (d) without oxygen scrambling, and (e) with production of large proportions of the rearranged tertiary 2-methyl-1-adamantyl sulfonate esters as reactive intermediates. The tertiary esters solvolyze (a) to arranged substitution products, (b) 2-7 times faster than the noradamantyl isomers, (c) in clean first-order reactions, and (d) with oxygen scrambling. The formation of rearranged tertiary esters as reactive intermediates in the solvolyses of the secondary esters and the oxygen scrambling during solvolysis of the tertiary esters both show that the solvolyses of the tertiary esters involve large proportions of internal return and are therefore not 'k, processes. In addition, solvent effects on the partitioning of the tertiary ester's tight ion pair between covalent substrate and products are significant and lead to a Grunwald-Winstein m value for internal return that is about 0.5 less than that for solvent separation; this result provides an explanation for the larger-than-average m values observed for 1-adamantyl systems. The &d3 rate effects for solvolysis of the 1-(3-noradamantyl)ethyl esters are in the narrow range of 1.14-1.15, smaller than the value of 01.20 shown by 3,3-dimethyl-2-butyl sulfonates and indicative of carbon a-participation and strong carbon hyperconjugation. Thus, the accelerated solvolysis rates, the absence of internal return, and the lowered isotope effects clearly establish this solvolysis as a kA process. Plots of log k values vs YoT, for the title esters give slopes (m values) around 0.7, while the m values for the adamantyl isomers are around 1. The differences between these plots for structurally very similar reactants solvolyzing with clearly different rate-determining steps are not large and indicate the hazards in using rate correlations to establish solvolytic reaction mechanisms, especially when comparing reactants with greater structural differences. The higher homologues, 1-(3-noradamantyl)propyl sulfonate esters, were found to behave similarly.

ChemInform Abstract: Oxygen Scrambling and Stereochemistry During the Trifluoroethanolysis of Optically Active 2‐Butyl 4‐Bromobenzenesulfonate (I).

AbstractDuring the trifluoroethanolysis of (I), containing 18O in the nonbridging oxygens, scrambling of the oxygen label occurs.

ChemInform Abstract: Facile Conversion of a Coordinated Nitro Group into an Aqua Group: Acid‐Induced Nitro‐to‐Nitrito Rearrangement.

Abstract(NH3)5CoNO22+ reacts rapidly with pure anhydrous CF3SO3H yielding the aqua species (NH3)5CoOH23+.

Facile conversion of a coordinated nitro group into an aqua group: acid-induced nitro-to-nitrito rearrangement

The nitro complex (NH/sub 3/)/sub 5/CoNO/sub 2//sup 2 +/ reacts rapidly and completely with neat anhydrous trifluoromethanesulfonic acid to generate the aqua species (NH/sub 3/)/sub 5/CoOH/sub 2//sup 3 +/. Oxygen-17 NMR results show that the oxygen in the bound water is derived from the original nitro group. A mechanism involving acid-catalyzed nitrogen-to-oxygen nitrite rearrangement is considered. The relationship between the mechanisms for oxygen scrambling and acid-catalyzed loss of NO/sup +/ from the nitrito linkage isomer is discussed, together with the mechanism for the present reaction. 20 references, 1 figure.

Origin of oxygen atoms in cantharidin biosynthesized by beetles

Biosynthesis by blister beetles (Co1eoptera:Meloidae) of the defensive substance cantharidin (2), an apparent monoterpene, proceeds by unprecedented degradation of farnesol (l), a sesquiterpenoid precursor. To obtain chemical insight into this transformation, we examined the origin of the four oxygen atoms in 2. Labeling studies used adult male Epicauta pestifera that were exposed to either I80-enriched O2 or H20. Analysis of the mass spectrometric data of the resulting 2 indicated that the tetrahydrofuranyl oxygen atom and two, but not three, of the anhydride oxygen atoms are derived from 02, whereas the third anhydride oxygen atom comes from H20. Examination of maximally labeled 2 using mass spectrometry-mass spectrometry, which obviated complications owing to isotope dilution, revealed that the H20-derived oxygen atom is located in the anhydride ring in some molecules and in a carbonyl group in others, implicating intramolecular oxygen scrambling. Results indicate that 0,-derived incorporated oxygen atoms undergo no appreciable exchange with the medium. The possibility that 2 is a juvenile hormone metabolite is suggested. Insects not only are the largest and most diverse group of organisms on earth but also are evolutionarily distant from vertebrates, plants, and bacteria. For these reasons, the study of insect metabolism promises to reveal novel biological chemistry. For example, use by some insects of homomevalonate to synthesize their juvenile hormone,I a homosesquiterpenoid, is unique among all types of organisms studied. An understanding of the unique metabolic features of insects may reveal insights regarding biological chemistry and could provide the foundation for new approaches for the control of pest insects. Cantharidin (2), an apparent monoterpene that serves as a defensive substance in blister beetles,2 is formed by an unprecedented degradation of the CIS farnesyl skeleton. Following early experiment^,^ an extensive series of I4C radiolabeling studies4 by Schmid and his collaborators demonstrated that the carbon atoms in 2 are derived from farnesol (l), which itself is derived from (1) (a) Schooley, D. A,; Judy, K. J.; Bergot, B. J.; Hall, M. S.; Siddall, J. B. Proc. Natl. Acad. Sci. U.S.A. 1973, 70, 2921-2925. (b) Bergot, B. J.; Jamieson, G. C.; Ratcliff, M. A.; Schooley, D. A. Science (Washington, D. C. ) 1980, 210, 336-338. (2) Carrel, J. E.; Eisner, T . Science (Wushington, D . C.) 1974, 183, 155-751. (3) (a) Schlatter, C.; Waldner, E. E.; Schmid, H. Experientia 1968, 24, 994-995. (b) Guenther, H.; Ramstad, E.; Floss, H. G. J . Pharm. Sci. 1969, 58, 1274. (4) (a) Peter, M. G.; Waespe, H.-R.; Woggon, W.-D.; Schmid, H. Helu. Chim. Acto 1977,60, 1262-1272. (b) Woggon, W.-D.; Peter, M. G.; Schmid, H. Helu. Chim. Acta 1977.60, 2288-2294. (c) Peter, M. G.; Woggon, W.-D.; Schmid, H. Helv. Chim. Acta 1977, 60, 2756-2762. (d) Woggon, W.-D.; Hauffe, S. A,; Schmid, H. J. Chem. Soc., Chem. Commun. 1983, 212-274. mevalonate in the normal manner. Intriguingly, the two terminal methyl groups of 1 are almost completely randomized during its conversion into Z3** Tritium labeling experiments demonstrated that all of the hydrogen atoms in 2, with the exception of one attached to C-6, are derived from m e ~ a l o n a t e . ~ ~ . ~ Unfortunately, knowledge of the prigins of the carbon and hydrogen atoms in 2 fails to unveil chemical details regarding its biosynthesis beyond

ChemInform Abstract: ACYL‐ENZYME EXCHANGE DETECTION BY INTERMOLECULAR OXYGEN SCRAMBLING: AN APPLICATION OF THE OXYGEN‐18 ISOTOPE EFFECT IN CARBON‐13 NMR

Chemischer InformationsdienstVolume 15, Issue 28 Physical Organic Chemistry ChemInform Abstract: ACYL-ENZYME EXCHANGE DETECTION BY INTERMOLECULAR OXYGEN SCRAMBLING: AN APPLICATION OF THE OXYGEN-18 ISOTOPE EFFECT IN CARBON-13 NMR T. G. WOOD, T. G. WOODSearch for more papers by this authorO. A. WEISZ, O. A. WEISZSearch for more papers by this authorJ. W. KOZARICH, J. W. KOZARICHSearch for more papers by this author T. G. WOOD, T. G. WOODSearch for more papers by this authorO. A. WEISZ, O. A. WEISZSearch for more papers by this authorJ. W. KOZARICH, J. W. KOZARICHSearch for more papers by this author First published: July 10, 1984 https://doi.org/10.1002/chin.198428043AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume15, Issue28July 10, 1984 RelatedInformation

Decomposition of diacyl peroxide—VII : Oxygen scrambling in 1-apocamphoryl peroxide and related diacyl peroxides-general remarks on the relationship between the stability of acyloxy radical and amounts of cage recombinations and ester formation in the decomposition of diacyl peroxide

Thermal decomposition of 1-apocamphoryl peroxide has been investigated in CCl 4 using the 18O-labelled peroxide. 1-Apocamphoryl peroxide is the first example which undergoes radical decomposition, carboxy-inversion and oxygen scrambling reaction between carbonyl and peroxidic O atoms in the peroxide in comparable rates. The major product of the decomposition was the inversion product, 1-apocamphoryl 1-apocamphyl carbonate (52·5%), and only a minute amount of 1-apocamphyl 1-apocamphorate (2·2%) was formed. The rates of oxygen scrambling were found to be 2·70±0·21 × 10 −6 (55°), 1·85±0·12 × 10 −1 sec −1 (70°) and 9·33±0·18 × 10 −5 sec −1 (84·3°) (E a, 27·5 Kcal/mol, ΔS‡, −2·3 e.u.). The cage recombination mechanism was suggested for the oxygen scrambling and the amounts of cage recombination of 1-apocamphoryloxy radical pair were calculated as 65% (55°), 60% (70°) and 52% (84·3°). The yield of the ester and the amount of cage recombination of geminate acyloxy radical pair were rationalized in terms of the stability of acyloxy radicals formed in the cage.

Photolysis of NO2in the presence of SO2at 3660 Å

Nitrogen dioxide was irradiated at 3660 Å in the presence of SO2. Quantum yields were measured as a function of SO2 pressure, and the specific rate constant for O+SO2→SO*3 was determined to be (1.1±0.1)× 109 l. mole–1 sec–1. The ratio of rate constants for the proposed reactions: SO*3→O+SO2 and SO*3+M→SO3+M, was estimated to be 0.077 mole l.–1 by the combination of the results of quantum yields with those of isotopic oxygen scrambling. A dark reaction was also observed which, if attributed to a simple bimolecular process, yields kD= 3.7 × 10–3 l. mole–1 sec–1.