- D2O product angular and translational energy distributions from the oxidation of deuterium on Pt(111)
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The angular and translational energy distributions of D2O produced from the oxidation of deuterium on the (111) crystal face of platinum have been measured over the surface temperature range of 440-913 K.Although the angular distributions are described by a cosine function, the translational energy distributions deviate from the corresponding Maxwell-Boltzmann distributions.At the normal angle, the D2O mean translational energy /2k varies from 220 to 400 K over the temperature range investigated.Two mechanisms for the production of translationally cold product molecules are discussed.
- Ceyer, S. T.,Guthrie, W. L.,Lin, T.-H.,Somorjai, G. A.
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- Hot water from cold. the dissociative recombination of water cluster ions
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Dissociative recombination of the Zundel cation D5O2+ almost exclusively produces D + 2 D2O with a maximum kinetic energy release of 5.1 eV. An imaging technique is used to investigate the distribution of the available reaction energy among these products. Analysis shows that as much as 4 eV can be stored internally by the molecular fragments, with a preference for producing highly excited molecular fragments, and that the deuteron shows a nonrandom distribution of kinetic energies. A possible mechanism and the implications for these observations are addressed.
- Thomas,Zhaunerchyk,Hellberg,Ehlerding,Geppert,Bahati,Bannister,Fogle,Vane,Petrignani,Andersson,Oejekull,Pettersson,Van Der Zande,Larsson
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- Time-resolved in situ neutron diffraction studies of gas hydrate: Transformation of structure II (sII) to structure I (sI)
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We report the in situ observation from diffraction data of the conversion of a gas hydrate with the structure II (sII) lattice to one with the structure I (sI) lattice. Initially, the in situ formation, dissociation, and reactivity of argon gas clathrate
- Halpern,Thieu,Henning,Wang,Schultz
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- Oxygen Reduction Mechanism of Monometallic Rhodium Hydride Complexes
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The reduction of O2 to H2O mediated by a series of electronically varied rhodium hydride complexes of the form cis,trans-RhIIICl2H(CNAd)(P(4-X-C6H4)3)2 (2) (CNAd = 1-adamantylisocyanide; X = F (2a), Cl (2b), Me (2c), OMe (2d)) was examined through synthetic and kinetic studies. Rhodium(III) hydride 2 reacts with O2 to afford H2O with concomitant generation of trans-RhIIICl3(CNAd)(P(4-X-C6H4)3)2 (3). Kinetic studies of the reaction of the hydride complex 2 with O2 in the presence of HCl revealed a two-term rate law consistent with an HX reductive elimination (HXRE) mechanism, where O2 binds to a rhodium(I) metal center and generates an η2-peroxo complex intermediate, trans-RhIIICl(CNAd)(η2-O2)(P(4-X-C6H4)3)2 (4), and a hydrogen-atom abstraction (HAA) mechanism, which entails the direct reaction of O2 with the hydride. Experimental data reveal that the rate of reduction of O2 to H2O is enhanced by electron-withdrawing phosphine ligands. Complex 4 was independently prepared by the addition of O2 to trans-RhICl(CNAd)(P(4-X-C6H4)3)2 (1). The reactivity of 4 toward HCl reveals that such peroxo complexes are plausible intermediates in the reduction of O2 to H2O. These results show that the given series of electronically varied rhodium(III) hydride complexes facilitate the reduction of O2 to H2O according to a two-term rate law comprising HXRE and HAA pathways and that the relative rates of these two pathways, which can occur simultaneously and competitively, can be systematically modulated by variation of the electronic properties of the ancillary ligand set.
- Halbach, Robert L.,Teets, Thomas S.,Nocera, Daniel G.
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- HYDROGEN-DEUTERIUM INVERSE ISOTOPE EFFECT MEASURED FOR THE C-O BOND DISSOCIATION PROCESS IN THE METHANATION ON SUPPORTED NOCKEL CATALYSTS
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A hydrogen-deuterium inverse isotope effect was found for the C-O bond dissociation process in the methanation of adsorbed CO on a Ni catalyst using a pulse technique; the average value of kH/kD was 0.75.Such an effect (0.77) was also found for the steady-state reaction of the CO methanation.It was shown that the adsorbed CO molecule is partially hydrogenated before the C-O bond dissociation.
- Mori, Toshiaki,Masuda, Hiroyuki,Imai, Hisao,Miyamoto, Akira,Baba, Shigeo,Murakami, Yuichi
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- Stoichiometry of oxidation reactions involving α-oxygen on FeZSM-5 zeolite
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The stoichiometry of the low-temperature reaction between surface α-oxygen formed by decomposing N2O over Fe-containing ZSM-5 zeolite and methane, hydrogen (deuterium), and CO was studied. Methane and hydrogen reacted with α-oxygen in stoichiometric ratios of 1:1.8 and 1:1.6, respectively. The observed stoichiometry was due to the mechanisms of the corresponding reactions. According to a mechanism proposed for the interaction of α-oxygen with methane and hydrogen, this reaction was accompanied by the dissociation of CH4 and H2 molecules. α-Oxygen reacted with CO in ratio of 1:1 to form CO2 in amounts equal to those of α-oxygen on the surface. In contrast to the oxidation of methane and hydrogen, the product of CO oxidation by α-oxygen was weakly bound to the zeolite surface and easily desorbed in the gas phase upon heating. Thus, the effect of blocking could be entirely limited by the performance of the reaction at a higher temperature and by the additional removal of the product by freezing.
- Dubkov,Paukshtis,Panov
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- Catalytic Oxidation and Isotopic Exchange of Hydrogen over 12-Molybdophosphoric Acid
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The reactions of H2 such as H2-D2 isotopic equilibration and exchange, reduction of catalyst by H2 (noncatalytic oxidation of H2 by a catalyst), and catalytic oxidation of H2, have been studied mostly at 573 K over 12-molybdophosphoric acid (PMo12) and it
- Mizuno, Noritaka,Watanabe, Tetsuji,Misono, Makoto
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- Picosecond Kinetics of the Reaction H3O(1+) + SO4(2-) -> HSO4(1-) + H2O
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The kinetics of the reaction H3O(1+) + SO4(2-) -> HSO4(1-) + H2O in aqueous solution have been studied over the temperature range 10-60 deg C by IR line broadening.Rate constants and activation energy have been determined.A large change of ionic strength, brought about by addition of 4 M NaCl, had no noticeable effect on the rate constants.The significance of this is disscussed.Deuteration was found to lower the rate constants and to somewhat increase the activation energy.
- Cohen, Benyamin,Weiss, Shmuel
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- Pure ice IV from high-density amorphous ice
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High-density amorphous ice (HDA), made by compression of hexagonal ice at 77 K, was heated at a constant pressure of 0.81 GPa up to 195 K and its phase transition followed by displacement-temperature curves. The crystalline phases recovered at 77 K and 1
- Salzmann, Christoph G.,Loerting, Thomas,Kohl, Ingrid,Mayer, Erwin,Hallbrucker, Andreas
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- Vibrational dynamics of amorphous ice
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Using inelastic neutron scattering, we have measured a range of amorphous forms of ice, including high-density and low-density amorphous ices, in the energy transfer region from 2 to 500 meV (16-4033 cm-1). The measured spectra show that the pressure-produced high-density amorphous ice is significantly different from the two low-density amorphous forms of ice (obtained by vapor deposition and by annealing at 120 K of the high-density amorphous ice), as we expected. However, considerable differences were also observed between the vapor deposits and the low-density amorphous ice obtained from annealing the high-density amorphous ice (the latter has a spectrum very similar to ice-Ih and ice-Ic), indicating that the hydrogen bonding in the two systems are markedly different.
- Kolesnikov,Li, Jichen,Parker,Eccleston,Loong
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- Insights into photoexcited electron scavenging processes on TiO2 obtained from studies of the reaction of O2 with OH groups adsorbed at electronic defects on TiO2(110)
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In this study we show that molecular oxygen reacts with bridging OH (OHbr) groups formed as a result of water dissociation at oxygen vacancy defects on the surface of rutile TiO2(110). The electronic structure of an oxygen vacancy defect on TiO2(110) is essentially the same as that of electron trap states detected on photoexcited or sensitized TiO2 photocatalysts, being Ti3+ in nature. Electron energy loss spectroscopy (EELS) measurements, in agreement with valence band photoemission results in the literature, indicate that water dissociation at oxygen vacancy sites has little or no impact on the electronic structure of these sites. Temperature programmed desorption (TPD) measurements show that O2 adsorbed at 120 K reacts with near unity reaction probability with OHbr groups on TiO2(110) to form an unidentified intermediate that decomposes to generate terminal OH groups at nondefect sites. Commensurate with this process, the electronic defect associated with the original oxygen vacancy defect (Ti3+) is oxidized. Vibrational EELS results indicate that the reaction between O2 and OHbr occurs at about 230 K, whereas electronics EELS results suggest that charge is transferred away from the vacancies at 90 K. Detailed TPD experiments in which the precoverage of water was varied indicate that chemisorption of O2 at cation sites on the TiO2(110) surface is not required in order for the reaction between O2 and OHbr to occur, which implies a direct interaction between weakly bound (physisorbed) O2 and the OHbr groups. In agreement with this conclusion, we find that second-layer water, which selectively hydrogen-bonds to bridging O2- sites and bridging OH groups, blocks the reaction of O2 with OHbr groups and prevents oxidation of the vacancy-related Ti3+ electronic state. These results suggest that the electron scavenging role of O2 in photocatalysis may involve a direct reaction between O2 and trapped electrons located at bridging OH groups. Our studies suggest that the negative influence of high water concentrations in gas-phase heterogeneous photocatalysis studies results from hydrogen-bonded water blocking access of O2 to trapped electrons located at surface OH groups.
- Henderson, Michael A.,Epling, William S.,Peden, Charles H.F.,Perkins, Craig L.
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- Model reaction studies on vanadium oxide nanostructures on Pd(111)
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Deuterium desorption and reaction between deuterium and oxygen to water has been studied on ultrathin vanadium oxide structures prepared on Pd(111). The palladium sample was part of a permeation source, thus enabling the supply of atomic deuterium to the sample surface via the bulk. Different vanadium oxide films have been prepared by e-beam evaporation in UHV under oxygen atmosphere. The structure of these films was determined using low energy electron diffraction and scanning tunneling microscopy. The mean translational energy of the desorption and reaction products has been measured with a time-of-flight spectrometer. The most stable phases for monolayer and submonolayer VO x are particular surface-V2O3 and VO phases at 523 and 700 K, respectively. Thicker films grow in the form of bulk V 2O3. The mean translational energy of the desorbing deuterium species corresponds in all cases to the thermalized value. Apparent deviations from this energy distribution could be attributed to different adsorption/desorption and/or accommodation behaviors of molecular deuterium from the gas phase on the individual vanadium oxide films. The water reaction product shows a slightly hyperthermal mean translational energy, suggesting that higher energetic permeating deuterium contributes with higher probability to the water formation.
- Kratzer,Surnev,Netzer,Winkler
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- Hydrogen transfer reaction on the surface of an oxide catalyst
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RuO2(110) exposes two kinds of active surface species (acidic and basic centers) that govern the interaction of the gas phase in contact with the catalyst's surface. Here we will elucidate the cooperative interplay of these two active surface sites for a simple model reaction, namely the water formation over RuO2 catalysts when supplying hydrogen and oxygen from the gas phase. The bridging O atoms harvest the hydrogen from the gas phase, while the on-top O atoms pick up those adsorbed hydrogen atoms from the bridging O atoms to form water. This mechanism of hydrogen transfer is mediated by a strong hydrogen bond. Hydrogen transfer is expected to play a vital role for the whole class of catalyzed hydrogenation and dehydrogenation reactions of hydrocarbons over RuO2. Copyright
- Knapp, Marcus,Crihan, Daniela,Seitsonen, Ari P.,Over, Herbert
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- Synthesis of OH from reaction of O and H on the Rh(100) surface
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We report the synthesis of the OH intermediate from O and H coadsorbed on the Rh( 100) surface at 90 K and heated in ultrahigh vacuum ( UHV ) to between 140 and 240 K; the species is stable when cooled again to 90 K.When heated to higher than 240 K H2O is formed and evolved from the surface, demonstrating that OH formation is an important step in H20 synthesis.Temperature programmed electron energy loss spectroscopy (TP-EELS), temperature programmed reaction spectroscopy (TPRS), and low energy electron diffraction ( LEED ) were employed in the study of this intermediate.The EEL spectra of the OH species is characterized by a stretch mode at 394 meV, bending mode at 114 meV, frustrated lateral translation at 82 meV, and frustrated vertical translation at 54 meV.Off-specular measurements show that the OH bending mode is entirely dipole active at an impact energy of 6 eV.By monitoring the OH bend intensity normalized to the elastic intensity as the crystal temperature is linearly ramped, the kinetics of both OH synthesis and OH combination to form Hz0 was probed.Kinetics was obtained using the heating rate variation method (from the shift in the temperature of fastest reaction Tp with heating rate) and the coverage variation method ( from the shift in Tp resulting from different reactant concentrations ) .An activation energy of Ef = 4+/-1 kcal mol-1 for OH formation was obtained.We find a half order coverage dependence indicating that OH formation occurs at the perimeters of O islands.An activation energy of Ec = 24+/-1 kcal/mol-1 was obtained for the combination reaction.Isotopic substitution of deuterium ( D ) for hydrogen yielded no OD or D2O under UHV conditions, but an O covered surface heated in a D2 pressure 10-8 Torr formed both surface OD and evolved D2O.This difference in H and D reactivity can be explained by the combination of the observed inverse kinetic isotope effect (KIE) in D2 recombinative desorption (in which D2 desorbs faster than H2) and the observed normal KIE in OD formation ( in which OD forms at a slower rate than OH ) .
- Gurney, Bruce A.,Ho, W.
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- Kinetics of hydrogen oxidation to water on the Rh(111) surface using multiple source modulated molecular beam techniques
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We have examined the kinetics of the oxidation of hydrogen to water on the Rh(111) surface using modulated molecular beam reactive scattering. For reactant pressures below 10-4 Torr and temperatures from 450-1250 K we observe serial steps, with apparent activation energies of 2.5 ± 1 and 10 ± 1 kcal/mol. Pseudo-first-order preexponential factors are 105 and 107 s-1, respectively, varying slightly with oxygen coverage. Reaction is inhibited by excess oxygen. Maximum water production occurs around 650 K. At lower temperatures the reaction becomes nonlinear. We use a new three-molecular-beam arrangement. Two continuous, independently adjustable beams establish steady-state surface concentrations, while a weaker modulated third beam induces small concentration perturbations around the selected steady state. With this technique we varied surface oxygen coverages, used isotopic substitution in the three beams to produce H2O, D2O and HDO, and linearized the HDO reaction.
- Padowitz,Sibener
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- Concerted Multiproton-Multielectron Transfer for the Reduction of O2to H2O with a Polyoxovanadate Cluster
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The concerted transfer of protons and electrons enables the activation of small-molecule substrates by bypassing energetically costly intermediates. Here, we present the synthesis and characterization of several hydrogenated forms of an organofunctionalized vanadium oxide assembly, [V6O13(TRIOLNO2)2]2-, and their ability to facilitate the concerted transfer of protons and electrons to O2. Electrochemical analysis reveals that the fully reduced cluster is capable of mediating 2e-/2H+ transfer reactions from surface hydroxide ligands, with an average bond dissociation free energy (BDFE) of 61.6 kcal/mol. Complementary stoichiometric experiments with hydrogen-atom-accepting reagents of established bond strengths confirm that the electrochemically established BDFE predicts the 2H+/2e- transfer reactivity of the assembly. Finally, the reactivity of the reduced polyoxovanadate toward O2 reduction is summarized; our results indicate a stepwise reduction of the substrate, proceeding through H2O2 en route to the formation of H2O. Kinetic isotope effect experiments confirm the participation of hydrogen transfer in the rate-determining step of both the reduction of O2 and H2O2. This work constitutes the first example of hydrogen atom transfer for small-molecule activation with reduced polyoxometalates, where both electron and proton originate from the cluster.
- Brennessel, William W.,Fertig, Alex A.,Matson, Ellen M.,McKone, James R.
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p. 15756 - 15768
(2021/10/02)
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- Phosphonium Phenolate Zwitterion vs Phosphonium Ylide: Synthesis, Characterization and Reactivity Study of a Trimethylphosphonium Phenolate Zwitterion
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4-Methoxy-3-(trimethylphosphonio)phenolate was obtained from a regioselective addition of PMe3 to p-quinone monoacetal. This compound undergoes hydrogen isotope exchange with D2O or CD3CN, and is capable of catalyzing H/D exchange of CD3CN with substrates bearing weakly acidic hydrogens. It exhibits similar reactivity to phosphorus ylides for olefinations of aldehydes. A possible tautomerization between the phosphonium phenolate zwitterion and phosphonium ylide is proposed for the first time to rationalize the unique reactivity.
- Xiao, Jing,Li, Qiang,Shen, Ruwei,Shimada, Shigeru,Han, Li-Biao
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supporting information
p. 5715 - 5720
(2019/11/22)
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- Temperature-induced polymorphism in methyl stearate
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The crystallisation of methyl stearate under a range of crystallisation conditions has been studied and three new polymorphs have been identified and structurally characterised. Form III (monoclinic, space group Cc, Z = 8) was obtained at room temperature by slow evaporation of a saturated solution in CS2. Form IV (monoclinic, space group C2/c, Z = 4) was obtained by slow cooling of the melt. Both structures were characterised by single crystal X-ray diffraction. Form V (monoclinic, space group Cc, Z = 4) was obtained from the melt by rapid cooling. X-ray and neutron powder diffraction methods were employed in the determination of this structure. Form V shows highly anisotropic thermal expansion, with expansion along the crystallographic b-axis being substantially greater than along the other two axes.
- Liu,Gibbs,Nichol,Tang,Knight,Dowding,More,Pulham
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p. 6885 - 6893
(2018/11/21)
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- Hydrogenation processes from hydrogen peroxide: An investigation in Ne matrix for astrochemical purposes
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Hydrogenation processes are of paramount importance in the interstellar medium. Many laboratory experiments were carried out from unsaturated species. Herein, the hydrogenation of hydrogen peroxide was experimentally investigated step by step by means of the matrix isolation technique. This reaction leads to the formation of water. Moreover, the formation of H3O2 and OH radicals as intermediates was characterized. Such a hydrogenation process should take place on the surface of dust grains in the interstellar medium. This reaction is consistent with the very small amount of interstellar hydrogen peroxide. This hydrogenation process also takes place in solid phase. This journal is the Partner Organisations 2014.
- Zins, Emilie-Laure,Krim, Lahouari
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p. 22172 - 22180
(2014/06/23)
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- Molecular hydrogen formation from proximal glycol pairs on TiO 2(110)
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Understanding hydrogen formation on TiO2 surfaces is of great importance, as it could provide fundamental insight into water splitting for hydrogen production using solar energy. In this work, hydrogen formation from glycols having different numbers of methyl end-groups has been studied using temperature-programmed desorption on reduced, hydroxylated, and oxidized rutile TiO2(110) surfaces. The results from OD-labeled glycols demonstrate that gas-phase molecular hydrogen originates exclusively from glycol hydroxyl groups. The yield is controlled by a combination of glycol coverage, steric hindrance, TiO2(110) order, and the amount of subsurface charge. Combined, these results show that proximal pairs of hydroxyl-aligned glycol molecules and subsurface charge are required to maximize the yield of this redox reaction. These findings highlight the importance of geometric and electronic effects in hydrogen formation from adsorbates on TiO2(110).
- Chen, Long,Li, Zhenjun,Smith, R. Scott,Kay, Bruce D.,Dohnálek, Zdenek
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supporting information
p. 5559 - 5562
(2014/05/06)
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- Reduction of oxygen catalyzed by nickel diphosphine complexes with positioned pendant amines
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Nickel(ii) bis(diphosphine) complexes that contain positioned bases in the second coordination sphere have been found to catalyze the reduction of O 2 with H2 to selectively form water. The complexes also serve as electrocatalysts for the reduction of O2 with the addition of a weak acid. In contrast, a closely related nickel diphosphine complex without the positioned bases is catalytically inactive for O2 reduction. These results indicate that pendant bases in synthetic catalysts for O2 reduction can play a similar role to proton relays in enzymes, and that such relays should be considered in the design of catalysts for multi-electron and multi-proton reactions.
- Yang, Jenny Y.,Bullock, R. Morris,Dougherty, William G.,Kassel, W. Scott,Twamley, Brendan,Dubois, Daniel L.,Rakowski Dubois
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p. 3001 - 3010
(2010/06/14)
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- Formation of hydrogen peroxide and water from the reaction of cold hydrogen atoms with solid oxygen at 10 K
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The reactions of cold H atoms with solid O2 molecules were investigated at 10 K. The formation of H2O2 and H2O has been confirmed by in situ infrared spectroscopy. We found that the reaction proceeds very effici
- Miyauchi,Hidaka,Chigai,Nagaoka,Watanabe,Kouchi
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- Mechanism of the reaction NO + H2 on the Pt(100)-hex surface under conditions of the spatially nonuniform distribution of reacting species
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The interaction of hydrogen with NOads/1 × 1 islands produced by NO adsorption on the reconstructed surface Pt(100)-hex was studied by high-resolution electron energy loss spectroscopy (HREELS) and the temperature-programmed reaction (TPR) method. The islands are areas of the unreconstructed surface Pt(100)-1 × 1 saturated with NOads molecules. The hexagonal phase around these islands adsorbs much more hydrogen near room temperature than does the clean Pt(100)-hex surface. It is assumed that hydrogen is adsorbed on the hexagonal surface areas that are adjacent to, and are modified by, the NOads/1 × 1 islands. The reaction of adsorbed hydrogen atoms with NOads takes place upon heating and has the character of so-called surface explosion. The TPR peaks of the products of this reaction-nitrogen and water-occur at T des ～ 365-370 K, their full width at half-maximum being ～5-10 K. In the case of the NO ads/1 × 1 islands preactivated by heating in vacuo above the NO desorption onset temperature (375-425 K), after the admission of hydrogen at 300 K, the reaction proceeds in an autocatalytic regime and the product formation rate increases monotonically at its initial stage. In the case of activation at 375 K, during the initial, slow stage of the reaction (induction period), hydrogen reacts with nitric oxide molecules bound to structure defects (NOdef). After activation at 425 K, the induction period is characterized by the formation and consumption of imido species (NH ads). It is assumed that NHads formation involves N ads atoms that have resulted from NOads dissociation on defects upon thermal activation. The induction period is followed by a rapid stage of the reaction, during which hydrogen reacts with NO 1 × 1 molecules adsorbed on 1 × 1 areas, irrespective of the activation temperature. After the completion of the reaction, the areas of the unreconstructed phase 1 × 1 are saturated with adsorbed hydrogen. The formation of Hads is accompanied by the formation of a small amount of amino species (NH2ads).
- Smirnov,Zemlyanov,Vovk
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p. 853 - 863
(2008/10/09)
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- Homogeneous catalytic reduction of dioxygen using transfer hydrogenation catalysts
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Solutions of Cp*IrH(rac-TsDPEN) (TsDPEN = H 2NCHPhCHPhN(SO2C6H4CH 3)-) (1H(H)) with O2 generate Cp*Ir(TsDPEN-H) (1) and 1 equiv of H2O. Kinetic analysis indicates a third-order rate law (second order in [1H(H)] and first order in [O2]), resulting in an overall rate constant of 0.024 ± 0.013 M-2 s-1. Isotopic labeling revealed that the rate of the reaction of 1H(H) + O2 was strongly affected by deuteration at the hydride position (kHH2/kDH2 = 6.0 ± 1.3) but insensitive to deuteration of the amine (kHH2/kHD2 = 1.2 ± 0.2); these values are more disparate than for conventional transfer hydrogenation (Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998-2001). The temperature dependence of the reaction rate indicated Δ ? = 82.2 kJ/mol, ΔS? = 13.2 J/mol·K, and a reaction barrier of 85.0 kJ/mol. A CH2Cl 2 solution under 0.30 atm of H2 and 0.13 atm of O 2 converted to H2O in the presence of 1 and 10 mol % of H(OEt2)2BArF4 (BArF 4- = B(C6H3-3,5-(CF 3)2)4-). The formation of water from H2 was verified by 2H NMR for the reaction of D 2 + O2. Solutions of 1 slowly catalyze the oxidation of amyl alcohol to pentanal; using 1,4-benzoquinone as a cocatalyst, the conversion was faster. Complex 1 also catalyzes the reaction of O2 with RNH2BH3 (R = H, t-Bu), resulting in the formation of water and H2. The deactivation of the catalyst 1 in its reactions with O2 was traced to degradation of the Cp* ligand to a fulvene derivative. This pathway is not observed in the presence of amine-boranes, which were shown to reduce fulvenes back to Cp*. This work suggests the potential of transfer hydrogenation catalysts in reactions involving O 2.
- Heiden, Zachariah M.,Rauchfuss, Thomas B.
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p. 14303 - 14310
(2008/04/06)
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- Dissociative recombination of H+ (H2 O)3 and D+ (D2 O)3 water cluster ions with electrons: Cross sections and branching ratios
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Dissociative recombination (DR) of the water cluster ions H+ (H2 O)3 andD+ (D2 O)3 with electrons has been studied at the heavy-ion storage rin g CRYRING (Manne Siegbahn Laboratory, Stockholm University). For the first time, absolute DR cross sections have been measured for H+ (H2 O)3 inthe energy range of 0.001-0.8 eV, and relative cross sections have been measured for D+ (D2 O)3 in the energy range of 0.001-1.0 eV. The DR cro ss sections for H+ (H2 O)3 are larger than previously observed for H+ (H2 O)n (n=1,2), which is in agreement with the previously observed trend indicating that the DR rate coefficient increases with size of the watercluster ion. Branching ratios have been determined for the dominating p roduct channels. Dissociative recombination of H+ (H2 O)3 mainly resultsin the formation of 3 H2 O+H (probability of 0.95±0.05) and with a possible minor channel resulting in 2 H2 O+OH+ H2 (0.05±0.05). The dominating channels for DR of D+ (D2 O)3 are 3 D2 O+D (0.88± 0.03) and 2 D2 O+OD+ D2 (0.09±0.02). The branching ratios are comparable to earlier DR results for H+ (H2 O)2 and D+ (D2 O)2, which gave 2 X2 O+X (X=H,D) with a probability of over 0.9.
- Oejekull, J.,Andersson, P. U.,Naagaard, M. B.,Pettersson, J. B. C.,Markovic, N.,et al.
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- Vibrational overtone spectrum of matrix isolated cis, cis-HOONO
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Cis, cis-peroxynitrous acid is known to be an intermediate in atmospheric reactions between OH and N O2 as well as HOO and NO. The infrared absorption spectra of matrix-isolated cc-HOONO and cc-DOONO in argon have been observed in the range of 500-8000 cm-1. Besides the seven fundamentalvibrational modes that have been assigned earlier for this molecule [Zh ang, J. Chem. Phys. 124, 084305 (2006)], more than 50 of the overtone and combination bands have been observed for cc-HOONO and cc-DOONO. Ab initio CCSD(T)/atomic natural orbital anharmonic force field calculations were used to help guide the assignments. Based on this study of the vibrational overtone transitions of cis, cis-HOONO that go as high as 8000 cm-1 and the earlier paper on the vibrational fundamentals, we conclude that the CCSD(T)/ANO anharmonic frequencies seem to correct to ±35 cm-1. The success of the theoretically predicted anharmonic frequencies {} in assigning overtone spectra of HOONO up to 8000 cm-1 suggests that the CCSD(T)/ANO method is producing a reliable potential energy surface for this reactive molecule.
- Zhang, Xu,Nimlos, Mark R.,Ellison, G. Barney,Varner, Mychel E.,Stanton, John F.
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- Surface-nitrogen removal in a steady-state NO + H2 reaction on Pd(110)
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Surface-nitrogen removal steps were analyzed in the course of a catalyzed NO + H2 reaction on Pd(110) by angle-resolved mass spectroscopy combined with cross-correlation time-of-flight techniques. Four removal steps, i.e., (i) the associative process of nitrogen atoms, 2N(a) → N 2(g), (ii) the decomposition of the intermediate, NO(a) + N(a) → N2O(a) → N2(g) + O(a), (iii) its desorption, N 2O(a) → N2O(g), and (iv) the desorption as ammonia, N(a) + 3H(a) → NH3(g), are operative in a comparable order. Above 600 K, process (i) is predominant, whereas the others largely contribute below 600 K. Process (iv) becomes significant at H2 pressures above a critical value, about half the NO pressure. Hydrogen was a stronger reagent than CO toward NO reduction and relatively enhanced the N(a) associative process.
- Ma, Yunsheng,Matsushima, Tatsuo
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p. 1256 - 1261
(2007/10/03)
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- Temperature dependence and kinetic isotope effects for the OH + HBr reaction and H/D isotopic variants at low temperatures (53-135 K) measured using a pulsed supersonic laval nozzle flow reactor
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The reactions of OH + HBr and all isotopic variants have been measured in a pulsed supersonic Laval nozzle flow reactor between 53 and 135 K, using a pulsed DC discharge to create the radical species and laser induced fluorescence on the A 2∑ ←X 2π(v′ = 1 ←v″ = 0) transition. All reactions are found to possess an inverse temperature dependence, in accord with previous work, and are fit to the form k = A(T/298)-n, with k1 (OH + HBr) = (10.84 ±0.31) × 10-12(T/298)(-0.67±0-02) cm3/s, k2 (OD + HBr) = (6.43 ±2.60) × 10-12(T/298) (-1.19±026) cm3/s, k3 (OH + DBr) = (5.89 ±1.93) × 10-12(T/298)(-0.76±22) cm3/s, and k4 (OD + DBr) = (4.71 ±1.56) × 10-12(T/298)(-1.09±0.21) cm3/s. A global fit of k vs T over the temperature range 23-360 K, including the new OH + HBr data, yields k(T) = (1.06 ±0.02) × 10-11(T/298) (-0.90±0.11) cm3/s, and (0.96 ±0.02) × 10-11(T/298)(-0.90±0.03)exp(((-2.88±1.82K)/T) cm 3/s, in accord with previous fits. In addition, the primary and secondary kinetic isotope effects are found to be independent of temperature within experimental error over the range investigated and take on the value of (kH/kD)AVG =1.64 for the primary effect and (kH/kD)AVG =0.87 for the secondary effect. These results are discussed within the context of current experimental and theoretical work.
- Mullen, Christopher,Smith, Mark A.
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p. 3893 - 3902
(2008/10/09)
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- Infrared spectrum of Hg(OH)2 in solid neon and argon
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Mercury(II) hydroxide molecules have been prepared upon mercury arc lamp irradiation of Hg, H2, and O2 mixtures in solid neon and argon. The strongest three infrared absorptions are identified through isotopic substitution (D2, HD, 18O2,16O 18O) and comparison to frequencies from DFT calculations. The isolated Hg(OH)2 molecule is stable and has a linear O-Hg-O linkage in a C2 structure with an 86° dihedral angle. However, in aqueous solution Hg2+ and 2OH- may form an Hg(OH)2 intermediate, which eliminates water and precipitates solid HgO: The solid Hg(OH)2 compound is not known.
- Wang, Xuefeng,Andrews, Lester
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p. 108 - 113
(2008/10/09)
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- Mechanism of the low-temperature interaction of hydrogen with α-oxygen on FeZSM-5 zeolite
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The mechanism of a low-temperature reaction of hydrogen with the radical anion surface oxygen species (α-oxygen, Oα) formed by decomposing N2O over FeZSM-5 zeolite was studied using kinetic and isotope techniques. It was found that the reaction is of first order with respect to hydrogen and the rate of the reaction is proportional to the concentration of Oα. The activation energy of the reaction, which was measured for H2 or D2 over a temperature range from +20 to -100°C, is equal to 3.2 or 5.3 kcal/mol, respectively. The reaction occurs with a considerable kinetic isotope effect (kH/k D), which varies over the range of 3.4-41 depending on the temperature. This fact indicates that the rate-limiting step of the reaction includes the dissociation of the hydrogen molecule. The temperature dependence of the isotope effect gave a value of 2.1 kcal/mol, which is close to the difference between the zero bond energies in the molecules of H2 and D2; this fact suggests that a tunnel effect does not significantly contribute to the reaction. The dissociative mechanism is consistent with data obtained by in situ IR spectroscopy. The interaction of hydrogen with α-oxygen is accompanied by the formation of new hydroxyl groups O αH (absorption bands at 3635 and 3674 cm-1) at the surface of the zeolite. The identification of these groups was supported by an isotope shift either on the replacement of H2 by D2 or on the replacement of 16Oα by Oα,. The stoichiometric ratio H2:Oα, is consistent with the previously drawn conclusion on the paired arrangement of α-sites.
- Dubkov,Starokon',Paukshtis,Volodin,Panov
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p. 202 - 208
(2008/10/09)
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- Isotopic and kinetic assessment of the mechanism of reactions of CH 4 with CO2 or H2O to form synthesis gas and carbon on nickel catalysts
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Kinetic and isotopic measurements for catalysts and conditions that rigorously excluded transport and thermodynamic artifacts led to a common sequence of elementary steps for reactions of CH4 with CO2 or H2O and for its stoichiometric decomposition on Ni/MgO catalysts. Turnover rates for forward reactions of CH4/CO2 and CH4/H2O mixtures were proportional to CH4 pressure (5-450 kPa) and independent of the partial pressure of the CO 2 or H2O coreactants (5-450 kPa). These turnover rates and their first-order rate constants and activation energies are also similar to those measured for CH4 decomposition, indicating that these reactions are mechanistically equivalent and that C-H bond activation is the sole kinetically relevant step in all three reactions. These conclusions were confirmed by identical CH4/CD4 kinetic isotope effects (kH/kD=1.62-1.71) for reforming and decomposition reactions and by undetectable H2O/D2O isotopic effects. The kinetic relevance of C-H bond activation is consistent with the relative rates of chemical conversion and isotopic mixing in a CH4/CD 4/CO2 mixture and with the isotopic evidence for the quasi-equilibrated nature of coreactant activation and H2 and H 2O desorption obtained from reactions of CH4/CO 2/D2 and 12CH4/12CO 2/13CO mixtures. These quasi-equilibrated steps lead to equilibrated water-gas-shift reactions during CH4 reforming, a finding confirmed by measurements of the effluent composition. These elementary steps provide also a predictive model for carbon filament growth and identify a rigorous dependence of the carbon thermodynamic activity on various kinetic and thermodynamic properties of elementary steps and on the prevalent concentrations of reactants and products, specifically given by PCH4P CO/PCO2 (or PCH4PH2/PH2O) ratios. These mechanistic features on Ni surfaces resemble those previously established for supported noble metal catalysts (Rh, Pt, Ir, Ru). These direct measurements of C-H bond activation turnover rates allowed the first direct and rigorous comparison of the reactivity of Ni and noble metal catalysts for CH4-reforming reactions, under conditions of strict kinetic control and relevant commercial practice and over a wide range of compositions and metal dispersions.
- Wei, Junmei,Iglesia, Enrique
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p. 370 - 383
(2008/10/09)
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- Reaction pathways and site requirements for the activation and chemical conversion of methane on Ru-based catalysts
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Kinetic and isotopic tracer and exchange measurements were used to determine the identity and reversibility of elementary steps required for CH4 reforming reactions on Ru-based catalyst. CH4 reactions were limited by C-H bond activat
- Wei, Junmei,Iglesia, Enrique
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p. 7253 - 7262
(2007/10/03)
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- Mechanism and Site Requirements for Activation and Chemical Conversion of Methane on Supported Pt Clusters and Turnover Rate Comparisons among Noble Metals
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The mechanism and site requirements for activation and chemical conversion of methane on supported Pt clusters and turnover rate comparisons among noble metals were presented. Isotopic trace and kinetic measurements led to a simple mechanistic picture and a unifying kinetic treatment of CH4-CO2, CH4-H2O, and CH4 decomposition reactions, as well as water-gas shift, on Pt-based catalysts. Reforming and decomposition rates were first-order on CH4 concentration and independent of the concentration or identity of the co-reactants. The normal CH4/CD4 kinetic isotope effects measured were similar for all three CH4 reactions and thus also independent of co-reactant identity. Forward CH4 turnover rates increased monotonically with increasing Pt dispersion for CO2 reforming, H2O reforming, and CH4 decomposition reactions. The rates of structure-insensitive CO oxidation reactions were similar before and after CH4 reforming.
- Wei, Junmei,Iglesia, Enrique
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p. 4094 - 4103
(2007/10/03)
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- Reactivity of stoichiometric and defective TiO2 (110) surfaces toward DCOOD decomposition
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The effect of defects on TiO2 (110) on the decomposition of DCOOD was investigated by temperature-programmed reaction spectroscopy (TPRS). Different concentrations of anion vacancy defects were created on the TiO2 (110) surface by el
- Wang, Qinggen,Biener, Jurgen,Guo, Xing-Cai,Farfan-Arribas, Enrique,Madix, Robert J.
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p. 11709 - 11720
(2007/10/03)
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- Mechanism of formation of hydrogen trioxide (HOOOH) in the ozonation of 1,2-diphenylhydrazine and 1,2-dimethylhydrazine: An experimental and theoretical investigation
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Low-temperature (-78 °C) ozonation of 1,2- diphenylhydrazine in various oxygen bases as solvents (acetone-d6, methyl acetate, tert-butyl methyl ether) produced hydrogen trioxide (HOOOH), 1,2-diphenyldiazene, 1,2-diphenyldiazene-N-oxide, and hydrogen peroxide. Ozonation of 1,2-dimethylhydrazine produced besides HOOOH, 1,2-dimethyldiazene, 1,2-dimethyldiazene-N-oxide and hydrogen peroxide, also formic acid and nitromethane. Kinetic and activation parameters for the decomposition of the HOOOH produced in this way, and identified by 1H, 2H, and 17O NMR spectroscopy, are in agreement with our previous proposal that water participates in this reaction as a bifunctional catalyst in a polar decomposition process to produce water and singlet oxygen (O2, 1Δg). The possibility that hydrogen peroxide is, besides water, also involved in the decomposition of hydrogen trioxide is also considered. The half-life of HOOOH at room temperature (20 °C) is 16 ± 1 min in all solvents investigated. Using a variety of DFT methods (restricted, broken-symmetry unrestricted, self-interaction corrected) in connection with the B3LYP functional, a stepwise mechanism involving the hydrotrioxyl (HOOO.) radical is proposed for the ozonation of hydrazines (RNHNHR, R = H, Ph, Me) that involves the abstraction of the N-hydrogen atom by ozone to form a radical pair, RNNHR..OOOH. The hydrotrioxyl radical can then either abstract the remaining N(H) hydrogen atom from the RNNHR. radical to form the corresponding diazene (RN=NR), or recombines with RNNHR. in a solvent cage to form the hydrotrioxide, RN(OOOH)NHR. The decomposition of these very labile hydrotrioxides involves the homolytic scission of the RO-OOH bond with subsequent in cage formation of the diazene-N-oxide and hydrogen peroxide. Although 1,2-diphenyldiazene is unreactive toward ozone under conditions investigated, 1,2-dimethyldiazene reacts with relative ease to yield 1,2-dimethyldiazene-N-oxide and singlet oxygen (O2, 1Δg). The subsequent reaction sequence between these two components to yield nitromethane as the final product is discussed. The formation of formic acid and nitromethane in the ozonolysis of 1,2-dimethylhydrazine is explained as being due to the abstraction of a methyl H atom of the CH3NNHCH3. radical by HOOO . in the solvent cage. The possible mechanism of the reaction of the initially formed formaldehyde methylhydrazone (and HOOOH) with ozone/oxygen mixtures to produce formic acid and nitromethane is also discussed.
- Plesnicar, Bozo,Tuttle, Tell,Cerkovnik, Janez,Koller, Joze,Cremer, Dieter
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p. 11553 - 11564
(2007/10/03)
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- Investigation of NO reduction by H2 on Pd monolith with transient and isotopic exchange techniques: II. H2/D2 exchange in the reduction of NO
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The kinetics and mechanism of catalytic reduction of nitrogen oxide (NO) by hydrogen on an alumina-based palladium monolith were studied under atmospheric pressure at 155°C. Transient kinetic experiments, as well as isotopic exchange techniques, were appl
- Rahkamaa-Tolonen,Salmi,Murzin,Barreto Dillon,Lassi,Keiski
-
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- Reaction of hydroxyl radical with nitric acid: Insights into its mechanism
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The rate constant for the reaction of hydroxyl radicals with nitric acid has an unusual pressure and temperature dependence. To explore the mechanism for this reaction, we have measured rate constants for reactions of isotopically substituted species OD+DNO3, OH+DNO3, OD+HNO3, and 18OH+HNO3 and the yield of NO3 product. Deuterium substitution on nitric acid results in more than a 10-fold reduction in the rate constant, removes the pressure dependence (over the observed range of 20-200 Torr in He and SF6), and leads to a strongly curved Arrhenius temperature dependence. Deuterium substitution on hydroxyl increases the rate constant slightly but does not change the pressure dependence. There is no evidence for exchange reactions in the isotopically mixed reactions. Absorption measurements of the NO3 product yield show that the title reaction produces nitrate radical with unit efficiency over all temperatures and pressures studied. We discuss the implications of the measured rate constants, product yields, and lack of isotopic exchange in terms of a mechanism that involves formation of a hydroxyl radical-nitric acid complex and its subsequent reaction to give NO3 and H2O.
- Brown, Steven S.,Burkholder, James B.,Talukdar, Ranajit K.,Ravishankara
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p. 1605 - 1614
(2007/10/03)
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- Mechanism of dihydrogen cleavage by high-valent metal oxo compounds: Experimental and computational studies
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The oxidation of dihydrogen by metal tetraoxo compounds was investigated. Kinetic measurements of the oxidations of H2 by MnO4- and RuO4, performed by UV-vis spectroscopy, showed these reactions to be quite rapid at 25 °C (k1 ≈ (3-6) x 10-2 M-1 s-1). Rates measured for H2 oxidation by MnO4- in aqueous solution (using KMnO4) and in chlorobenzene (using nBu4NMnO4) revealed only a minor solvent effect on the reaction rate. Substantial kinetic isotope effects [(kH2/kD2 = 3.8(2) (MnO4- aq), 4.5(5) (MnO4-, C6H5C1 soln) and 1.8(6) (RuO4, CCl4 soln)] indicated that H-H bond cleavage is rate determining and that the mechanism of dihydrogen cleavage is likely similar in aqueous and organic solutions. Third-row transition-metal oxo compounds, such as OsO4, ReO4-, and MeReO3, were found to be completely unreactive toward H2. Experiments were performed to probe for a catalytic hydrogen/deuterium exchange between D2 and H20 as possible evidence of dihydrogen σ-complex intermediates, but no H/D exchange was observed in the presence of various metal oxo compounds at various pH values. In addition, no inhibition of RuO4-catalyzed hydrocarbon oxidation by H2 was observed. On the basis of the available evidence, a concerted mechanism for the cleavage of H2 by metal tetraoxo compounds is proposed. Theoretical models were developed for pertinent MnO4- + H2 transition states using density functional theory in order to differentiate between concerted [2 + 2] and [3 + 2] scissions of H2. The density functional theory calculations strongly favor the [3 + 2] mechanism and show that the H2 cleavage shares some mechanistic features with related hydrocarbon oxidation reactions. The calculated activation energy for the [3 + 2] pathway (ΔH? = 15.4 kcal mol-1) is within 2 kcal mol-1 of the experimental value.
- Collman,Slaughter,Eberspacher,Strassner,Brauman
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p. 6272 - 6280
(2008/10/08)
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- Kinetics and mechanism of the OH and OD reactions with BrO
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The kinetics and mechanism of the reactions OH + BrO → products (1) and OD + BrO → products (2) have been studied in the temperature ranges of 230-355 K and 230-320 K, respectively, and at total pressure of 1 Torr of helium using the discharge-flow mass s
- Bedjanian, Yuri,Riffault, Veronique,Le Bras, Georges,Poulet, Gilles
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p. 6154 - 6166
(2007/10/03)
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- Temperature-dependent kinetic isotope effects in the gas-phase reaction: OH + HBr
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The temperature dependence of the hydrogen transfer rate coefficients for the reactions: OH + HBr (Reaction 1), OD + HBr (Reaction 2), OH + DBr (Reaction 3), and OD + DBr (Reaction 4) have been investigated at temperatures between 120 and 224K using a pulsed uniform supersonic flow monitoring hydroxyl reactive loss. The lack of observed isotopic scrambling indicates the reaction occurs by H/D atom transfer from HBr/ DBr at all temperatures. The rate coefficients demonstrate little temperature dependence above 200 K, but strong inverse temperature behavior below 200 K. The current work provides unequivocal experimental evidence of temperature dependent and inverse primary and secondary kinetic isotope effects (kH/kD H/kD, at 120 K are for primary substitution on HBr; k1/k3 = 1.00 (±0.17) and k2/k4 = 0.46 (±0.08), while for secondary substitution on OH; k1/k2 = 0.94 (±0.20) and k3/k4 = 0.43 (±0.05). At the lowest temperature employed (120 K), deuterated reactants react as fast or faster than their natural hydrogen isotopomer and there is no significant difference between the primary and secondary kinetic isotope effect. The results are discussed within the framework of recent theoretical models.
- Jaramillo, Veronica I.,Smith, Mark A.
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p. 5854 - 5859
(2007/10/03)
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- Temperature programmed desorption studies of OD coadsorbed with H2 on Pt(111)
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A molecular beam source of pure hydroxyl radicals has been developed and used to explore the water reaction catalyzed over Pt(111). An electrostatic hexapole selectively focused OD radicals from a supersonic corona discharge source onto a Pt target at a surface temperature of TS=143 K. Subsequent D2O temperature programmed desorption (TPD) spectra revealed two major features, one near TS ca. 170 K from desorption of molecular water overlayer and a second near TS ca. 210 K from the decomposition of an adsorbed OD intermediate. The latter feature was isolated and analysis of TPD spectra revealed that the D2O production reaction was approximately half-order in total oxygen coverage with a pre-exponential factor ranging from υd=4+/-1*1016 to 5+/-2*1018 molecules1/2 cm-1 s-1 and activation energy Ea=9.7+/-0.1 to 11.5+/-0.1 kcal mol-1 for initial coverage ranging from θ0=0.04 to 0.25 ML. Coadsorption studies of OD and H2 revealed that H atoms drive reactions with adsorbed OD at TS ca. 180 K to form all three water isotopes: D2O, HDO, and H2O. Oxygen (O2) TPD spectra contained three desorption features (TS=700 K, 735 K, and 790 K). The relative abundance of O2 from these three features was virtually the same in all low temperature (TS=143 K) TPD experiments. At elevated dosing temperatures (TS=223 K) the two features at TS=700 K and 790 K could be selectively titrated from the surface by hydrogen. The presence of hydrogen prior to OD exposure at this elevated temperature prevented the accumulation of oxygen on the surface. The implications of these observations on our mechanistic understanding of the low temperature (TS210K) water reaction are discussed.
- Backstrand, Kyle M.,Weibel, Michael A.,Moision, Robert M.,Curtiss, Thomas J.
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p. 7209 - 7218
(2007/10/03)
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- Dioxygen complexes from the reactions of [Cp*RuH2(PP)]+ (PP = dppm, dppe) with air
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Exposure of [Cp*RuH2(dppm)]BF4 in acetone or methanol to air produced a mixture of [Cp*Ru(O2)(dppm)]BF4 and [Cp*Ru(O2)(Ph2PCH2P(O)Ph 2)]BF4. Reaction of Cp*RuCl(dppm) with NaBPh4 in methanol in air produced the dioxygen complex [Cp*Ru(O2)(dppm)]BPh4, which has been characterized by X-ray diffraction. Reaction of Cp*RuCl(dppm) with H2O2 in the presence of NaBPh4 in air produced [Cp*Ru(O2)(Ph2PCH2P(O)Ph 2)]BPh4 and Ph2P(O)CH2P(O)PPh2. Reaction of [Cp*RuH2(dppe)]BF4 in acetone or methanol with air also produced a mixture of [Cp*Ru(O2)(dppe)]BF4 and [Cp*Ru(O2)(Ph2PCH2CH 2P(O)Ph2)]BF4.
- Jia, Guochen,Ng, Weng Sang,Chu, Hei Shing,Wong, Wing-Tak,Yu, Nai-Teng,Williams, Ian D.
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p. 3597 - 3602
(2008/10/08)
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- Mechanism for the reaction of hydroxyl radicals with dimethyl disulfide
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Strong infrared chemiluminescence from the reactions of OH and OD radicals with CH3SSCH3 was observed in a discharge flow reactor viewed by a Fourier transform spectrometer. The recorded spectra were identical to the H2O and HDO plus D2O emission spectra from the OH+CH3SH and OD+CH3SD reactions, respectively. These observations strongly suggest that the primary reaction in the OH and OD+CH3SSCH3 system generates CH3SH and CH3SD molecules with the observed emission arising from the OH+CH3SH and OD+CH3SD secondary reactions.
- Butkovskaya,Setser
-
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- Gas-phase reactions of ClONO2 with Cl-(D2O)n = 0-3 and NO2-
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The reactions of ClONO2 with the Cl-(D2O)n≤3 and NO2- ions were studied in a flow-tube apparatus at several temperatures in the range 170-298 K and at a helium buffer gas pressure of 0.28 Torr. Rate constants for these reactions were determined, establishing that all these ions react quite efficiently with ClONO2. The product ions observed in reaction sequences are reported, and reaction mechanisms are proposed to account for the findings. In the case of Cl- and NO2- the main product ion is NO3-, while the hydrates Cl-(D2O)n≤1-3 lead to production of the hydrated species, NO3-(D2O)m≤n-1. The main features of the observed secondary reactions can be described in terms of adduct formation and ligand switching. Limits on the bond energy of (NO3ClNO3)-, 14.6 kcal/mol≤D(NO3--ClONO2)≤26 kcal/mol, were determined. Finally, implications of these studies to atmospheric chemistry are discussed.
- Wincel,Mereand,Castleman Jr.
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p. 8248 - 8254
(2007/10/03)
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- Gas Phase Reactions of Cl2O with NO(1-)x=2,3*(D2O)n=0-2 and Cl(1-)*(D2O)n=0-2
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The results of laboratory measurements are reported for the reactions of Cl2O with NO2(1-)*(D2O)n=0-2, NO3(1-)*(D2O)n=0-2, and Cl(1-)*(D2O)n=0-2.These were studied using a flow-tube apparatus at several temperatures within the range 168-298 K and at a helium buffer gas pressure of 0.30+/-0.02 Torr.Rate constants for primary reactions and product ions observed in various reaction sequences are reported, along with a discussion of possible reaction mechanisms.The findings contribute to a further mechanistic understanding of ionic reactions as well as other heterogeneous ones involving chlorine-containing molecules.Interestingly, evidence for a water-catalyzed reaction between Cl2O and Cl(1-)/NO3(1-) was obtained.Also, the present findings suggest that several ionic reactions may lead to the formation of ClONO2 as a neutral product.Finally, limits on the electron affinity of Cl2O are determined to be 2.52 eV EA(Cl2O) 3.61 eV.
- Vincel, H.,Mereand, E.,Castleman, A. W.
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p. 15678 - 15685
(2007/10/02)
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- Temperature Dependence of g Values for H2O and D2O irradiated with Low Linear Energy Transfer Radiation
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The g values for the primary species formed in the γ-radiolysis of light and heavy water have been measured as a function of temperature up to 300 deg C.With the exception of g(H2) and g(D2), all the g values are consistent with the generally accepted diffusion-kinetic model of spurs, i.e., with an increase in temperature, the g values of the free radicals increased while those of peroxide decreased.The g values for H2 and D2 increased with temperature which suggests that they are formed by other mechanisms in addition to radical-radical reactions in the spur.
- Elliot, A. John,Chenier, Monique P.,Ouellette, Denis C.
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p. 1193 - 1198
(2007/10/02)
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- Kinetic Study of the Reaction OD + DNO3 -> D2O + NO3
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Rate constants for the reaction of OD radicals with DNO3 have been determined in the temperature range 269-446 K by a flash photolysis-resonance absorption technique.OD radicals were generated by photolysis of DNO3 at 222 nm with a KrCl excimer laser and
- Singleton, Donald L.,Paraskevopoulos, George,Irwin, Robert S.
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p. 694 - 697
(2007/10/02)
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- Kinetics and Mechanism of the Reactions of O(3P) with SiH4, CH3SiH3, (CH3)2SiH2, and (CH3)3SiH
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The reactions of O(3P) atoms with the silanes Me4-nSiHn (n = 1-4) have been investigated at room temperature in a discharge flow system with mass spectrometric detection and also in stationary photolysis experiments.Analysis of the end products provided conclusive evidence that the only primary process occuring in each case was the abstraction of hydrogen from the Si-H bond by the O atom leading to the formation of the OH and silyl radicals.The values of the rate constants obtained are k/10-13 cm3 s-1): k(O + SiH4) = 3.5, k(O + SiD4) = 1.4, k(O + MeSiH3) = 8.9; k(O + Me2SiH2) = 18.0, k(O + Me3SiH) = 30.6, and k(O + Me3SiD) = 16.0.The marked increase in rate constant with methylation is unexpected in view of the known similarity of the Si-H bond dissociation energy in SiH4 and the methylsilanes.A possible explanation is offered in terms of a reaction model involving partial charge transfer from Si to the attacking O, followed by proton transfer.
- Horie, O.,Taege, R.,Reimann, B.,Arthur, N. L.,Potzinger, P.
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p. 4393 - 4400
(2007/10/02)
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- Effects of Hydration on Reactions of O-(H2O)n (n = 0-1). 2. Reactions with H2 and D2
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The rate constants for the reactions of O-(H2O)n (n=0-2) with H2 and D2 have been measured as a function of temperature.In addition, the dependences of the rate constants on average center-of-mass kinetic energy (cm>) and the branching ratios for n=0 have been measured at several temperatures.For n=0, the reactions with H2 and D2 are 48percent and 50percent effficient, respectively, and depend only weakly on temperature and cm>.Both associative detachment and a channel that produces OH- are observed.The minor (cm>.One H2O ligand reduces the rate constants on the order of a factor of 50.The reaction pathways for n=1 are approximately 90percent associative detachment and 10percent production of OH-(H2O).The rate constants for n=1 are found to increase with increasing temperature.The second H2O ligand reduces the rates to below our detectable limit.No dependence on rotational energy was found in either the rate constant or the branching ratio for n=0.
- Viggiano, A. A.,Morris, Robert A.,Deakyne, Carol A.,Dale, F.,Paulson, John F.
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p. 3644 - 3647
(2007/10/02)
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- Deuterium isotope fractionation within protonated water clusters in the gas phase
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Ion product distributions have been analyzed for the collision-activated loss of H2O, HOD, or D2O from the water clusters (L2O)nL+ (L = H, D; n = 2-4). The ionic products of collision-induced dissociation (CID) are observed to be depleted in deuterium with respect to the statistical product distributions predicted for complete randomization of H and D. The measured isotope distributions in the CID product ions are independent of collision energy within experimental error, suggesting that the observed depletion of deuterium is not the result of a kinetic effect in the unimolecular decomposition, but rather a reflection of the individual cluster structures. An equilibrium isotope effect model is proposed wherein the deuterium in the cluster preferentially migrates to the peripheral positions and localizes on the neutral water molecules in the solvent shell, rather than occupying sites in the cluster ion core, such as those on the core hydronium (lyonium) ion. Deuterium enrichment in the neutral water component of each cluster ion results in enhanced loss of deuterated neutral water upon collisional activation. The present isotope fractionation results are compared with literature data for bimolecular gas-phase H/D-exchange reactions and with condensed-phase isotope fractionation data. The observation of isotope fractionation as an equilibrium effect in stabilized gas-phase water cluster ions suggests that isotope fractionation in the bimolecular reactions between (H2O)nH+ ions and D2O results primarily from a nonstatistical distribution of hydrogen and deuterium in the transient reaction intermediates.
- Graul, Susan T.,Brickhouse, Mark D.,Squires, Robert R.
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p. 631 - 639
(2007/10/02)
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- Reactions of OH and OD with H2O2 and D2O2
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The reactions OH + H2O2 --> products (k1) and OD + D2O2 --> products (k3) were studied in the temperature range 273-410 K.The obtained Arrhenius expressions are as follows: k1 = (2.76 +/- 0.80)E-12 exp((-110 +/- 60)/T) and
- Vaghjiani, Ghanshyam L.,Ravishankara, A. R.,Cohen, N.
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p. 7833 - 7837
(2007/10/02)
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- Kinetics of OH (v = 0 and 1) and OD (v = 0 and 1) Studied by Time-Resolved Laser-Induced Fluorescence
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Coupled with pulsed photochemical generation of OH (and OD), laser-induced fluorescence has been used to study the kinetics of OH (v = 0 and 1) and OD (v = 0 and 1) with HCl, DCl and CO. - Keywords: Chemical Kinetics / Energy Trasfer / Isotope Effects / Photochemistry / Radicals
- Smith, Ian W. M.,Williams, Martin D.
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p. 319 - 320
(2007/10/02)
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