- Vibrational excitation of H2O and HOD molecules produced by reactions of OH and OD with cyclo-C6H12, n-C4H10, neo-C5H12, HCI, DCI and NH3 as studied by infrared chemiluminescence
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The room-temperature reactions of OH(OD) radicals with cyclo-C6H12,n-C4H10, and neo-C5H12 have been investigated by observing the infrared chemiluminescence from the H2O(HOD) molecules generated in a fast-flow reactor. These hydrocarbon molecules are representative for abstraction from secondary and primary C-H bonds. The total vibrational energy released to H2O(HOD) was in the range of (fv)=0.55-0.65. The majority (80%-85%) of the vibrational energy is in the stretching modes and the main energy release is to the local mode associated with the new OH bond. The dynamics associated with the energy disposal to H2O(HOD) resemble the H+L-H dynamics for the analogous reactions of F atoms. The data from H2O and HOD are complementary because of the different collisional coupling between the energy levels of the v1. v2. and v modes: however, no specific isotope effect was found for the energy disposal to H2O versus HOD for reactions with the hydrocarbon molecules. In contrast, a very unusual isotope effect was found between the OH+HCl and OD+ HCl pairs The latter reaction gave the expected stretching mode excitation of HOD: however, the OH reaction gave H2O molecules with virtually no vibraitonal energy. This anomalous situation is partly associated with an inverse secondary kinetic-isotope effect, but the main isotope effect is on the dynamics of the energy disposal process itetf.
- Butkovskaya,Setser
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- Ultrafast dynamics of hydrogen bond exchange in aqueous ionic solutions
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The structural and dynamical properties of aqueous ionic solutions influence a wide range of natural and biological processes. In these solutions, water has the opportunity to form hydrogen bonds with other water molecules and anions. Knowing the time sca
- Park, Sungnam,Odelius, Michael,Gaffney, Kelly J.
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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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- Quantitative spectroscopic and theoretical study of the optical absorption spectra of H2O, HOD, and D2O in the 125-145 nm region
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Synchrotron radiation was used to determine the room temperature absorption spectra of water and its isotopomers D2O and HOD in absolute cross section units in the 125 to 145 nm wavelength region. The cross sections at a temperature of 300K were calculated by applying a Monte Carlo sampling over the initial rotational states of the molecules. Two more resonances were found at low energy in the case of HOD. The width of the resonances of 0.04 eV is the result of overlapping and narrower resonances in the spectra of molecules differing in rotational ground state.
- Cheng, Bing-Ming,Chung, Chao-Yu,Bahou, Mohammed,Lee, Yuan-Pern,Lee,Van Harrevelt, Rob,Van Hemert, Marc C.
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- Requirements for functional models of the iron hydrogenase active site: D2/H2O exchange activity in {(μ-SMe)(μ-pdt)[Fe(CO)2(PMe3)] 2+}[BF4-]
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Hydrogen uptake in hydrogenase enzymes can be assayed by H/D exchange reactivity in H2/D2O or H2/D2/H2O mixtures. Diiron(I) complexes that serve as structural models for the active site of iron hydrogenase are not active in such isotope scrambling but serve as precursors to FeIIFeII complexes that are functional models of [Fe]H2ase. Using the same experimental protocol as used previously for {(μ-H)(μ-pdt)[Fe(CO)2(PMe3)] 2+}, 1-H+ (Zhao et al. J. Am. Chem. Soc. 2001, 123, 9710), we now report the results of studies of {(μ-SMe)(μ-pdt)[Fe(CO)2(PMe3)]2 +}, 1-SMe+, toward H/D exchange. The 1-SMe+ complex can take up H2 and catalyze the H/D exchange reaction in D2/H2O mixtures under photolytic, CO-loss conditions. Unlike 1-H+, it does not catalyze H2/D2 scrambling under anhydrous conditions. The molecular structure of 1-SMe+ involves an elongated Fe...Fe separation, 3.11 A, relative to 2.58 A in 1-H+. It is proposed that the strong SMe- bridging ligand results in catalytic activity localized on a single FeII center, a scenario that is also a prominent possibility for the enzyme active site. The single requirement is an open site on FeII available for binding of D2 (or H2), followed by deprotonation by the external base H2O (or D2O).
- Georgakaki, Irene P.,Miller, Matthew L.,Darensbourg, Marcetta Y.
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- Isotope effects in liquid water by infrared spectroscopy
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The heavy and light liquid water (H2O-D2O) mixtures were studied by Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy. The deformation bands of liquid water clearly indicate the presence of the three typ
- Max, Jean-Joseph,Chapados, Camille
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- The dynamics of the OH + HD gas-phase reaction: Absolute reaction cross section and H/D atom product branching ratio
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The dynamics of the OH + HD reaction were studied in the gas-phase using the laser photolysis/vacuum-UV laser-induced fluorescence pump-and-probe technique. Translationally energetic OH(2II) radicals with an average reagent translational energy of Ec.m. = 0.24 eV in the (OH-HD)-center-of-mass system were generated by the laser photolysis of H2O2 at 248 nm. Doppler profiles of nascent D and H atoms produced in reactive collisions of OH with room-temperature HD molecules were detected under single-collision conditions by VUV-LIF at the Lyman-α transition. For the OH + HD reaction an absolute reaction cross section of σrR(0.24 eV) = (0.14 ± 0.05) A2 was determined by means of a calibration method using OH + D2 as a reference reaction. The branching ratio for the OH + HD → H + HOD (D + H2O) product channels was measured to be ΓH/D = (1.2 ± 0.2).
- Brownsword,Hillenkamp,Schmiechen,Volpp,Wolfrum
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- Micropolarity and Hydrogen-Bond Donor Ability of Environmentally Friendly Anionic Reverse Micelles Explored by UV/Vis Absorption of a Molecular Probe and FTIR Spectroscopy
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In the present work we show how two biocompatible solvents, methyl laurate (ML) and isopropyl myristate (IPM), can be used as a less toxic alternative to replace the nonpolar component in a sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) reverse micelles (RMs) formulation. In this sense, the micropolarity and the hydrogen-bond ability of the interface were monitored through the use of the solvatochromism of a molecular probe (1-methyl-8-oxyquinolinium betaine, QB) and Fourier transform infrared spectroscopy (FTIR). Our results demonstrate that the micropolarity sensed by QB in ML RMs is lower than in IPM RMs. Additionally, the water molecules form stronger H-bond interactions with the polar head of AOT in ML than in IPM. By FTIR was revealed that more water molecules interact with the interface in ML/AOT RMs. On the other hand, for AOT RMs generated in IPM, the weaker water–surfactant interaction allows the water molecules to establish hydrogen bonds with each other trending to bulk water more easily than in ML RMs, a consequence of the dissimilar penetration of nonpolar solvents into the interfacial region. The penetration process is strongly controlled by the polarity and viscosity of the external solvents. All of these results allow us to characterize these biocompatible systems, providing information about interfacial properties and how they can be altered by changing the external solvent. The ability of the nontoxic solvent to penetrate or not into the AOT interface produces a new interface with attractive properties.
- Girardi, Valeria R.,Silber, Juana J.,Falcone, Ruben Darío,Correa, N. Mariano
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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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- 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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- Dynamics of OH and OD radical reactions with HI and GeH4 as studied by infrared chemiluminescence of the H2O and HDO products
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The infrared chemiluminescence of vibrationally excited H2O and HDO from the highly exothermic reactions of OH and OD radicals with HI and GeH4 was observed in the 2200-5500 cm-1 range. The experiments utilized a fast-flow reactor with 0.3-1 Torr of Ar carrier gas at 300 K; the OH(OD) radicals were produced via the H(D)+NO2 reaction and the H or D atoms were generated by a discharge in a H2(D2)Mr mixture. The H2O and HOD vibrational distributions were determined by computer simulation of the emission spectra in the 2200-3900 cm-1 range. The total vibrational energy released to H2O and HOD molecules is, respectively, 〈fυ〉 = 0.36 and 0.41 from HI and 〈fυ〉 = 0.46 and 0.51 from GeH4. These values are significantly smaller than for the reactions of OH and OD with HBr, 〈fυ〉 = 0.61 and 0.65. The populations of the O-H stretching vibration of HOD and the collisionally coupled υ1 and υ3 stretching modes of H2O decrease with increasing vibrational energy. In contrast, the vibrational distribution from the HBr reaction is inverted. The bending mode distributions in all stretching states of H2O and HOD extend to the thermodynamic limit of each reaction. A surprisal analysis was made for H2O(HOD) distributions from the title reactions and compared with that for OH(OD)+HBr. The surprisal analysis tends to confirm that the dynamics for the HI and GeH4 reactions differ from the HBr reaction. The HI reaction may proceed mainly via addition-migration, while the GeH4 reaction may involve both direct abstraction and addition-migration. A rate constant for the OH+GeH4→H2O+GeH3 reaction was evaluated by comparing the H2O emission intensities with that of the OH+HBr→H2O+Br reaction, kGeH4/kHBr = 6.5 ± 0.9. Secondary kinetic-isotope effects, kOH/kOD = 1.4 ± 0.1, 1.0±0.2, and 1.3±0.2, were determined for reactions of OH and OD with GeH4, HI, and HBr, respectively, by comparing the relative H2O and HOD emission intensities.
- Butkovskaya,Setser
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- Kinetic Study of the Reaction of OH with H2 and D2 from 250 to 1050 K
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Absolute rate coefficients for the reactions OH + H2 -> H2O + H (k1) and OH + D2 -> DHO + D (k2) have been measured between 250 and 1050 K by using the flash photolysis-resonance fluorescence technique.The adaptation of the flash pho
- Ravishankara, A.R.,Nicovich, J. M.,Thompson, R. L.,Tully, F. P.
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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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- 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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- Excitation function and reaction threshold studies of isotope exchange reactions: H + D2 → D + HD and H + D2O → D + HOD
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Laser photolysis of H2S at 248 nm and at 193 nm was used to generate nonequilibrium distributions of translationally energetic hydrogen atoms at high dilution in a flowing moderator gas (N2 or Ar). The pulsed laser photolysis/laser-induced fluorescence pump-and-probe method allowed the measurement of the line shapes of the moderated H atom Doppler profiles as well as the concentrations of D atoms produced in the reactive collisions between the H atoms and D2 or D2O reagents. H and D atoms were detected with sub-Doppler resolution via (2p2P ← 1s2S) laser-induced fluorescence. The measured H atom Doppler profiles were used to describe the evolution of the initially generated nascent nonequilibrium H atom speed distribution toward its equilibrium Maxwell-Blotzmann form. In this way the excitation function and reaction threshold for the reactions H + D2 → HD + D and H + D2O → HOD + D could be determined for the first time from the measured nonequilibrium D atom formation rates and single-collision absolute reaction cross sections measured at higher collision energies.
- Brownsword, Richard A.,Hillenkamp, Matthias,Laurent, Thomas,Volpp, Hans-Robert,Wolfrum, Jürgen,Vatsa, Rajesh K.,Yoo, Hee-Soo
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- On the structure of the water trimer. A matrix isolation study
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The infrared spectra of several isotopomers of the water trimer have been studied in argon and krypton matrices.The results show that the trimer is cyclic, with three equivalent water molecules.
- Engdahl, Anders,Nelander, Bengt
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- Fast reactions of atom substitution from polyatomic molecules and solid salts by thermally equilibrated atomic reactants
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A new class of fast thermal gas-phase reactions of the direct substitution of atoms in polyatomic molecules by atomic reactants is discovered. The Arrhenius parameters are determined for the reactions of atomic deuterium with a number of hydrogen-containing compounds, occurring via the direct substitution of hydrogen atoms and via the abstraction of hydrogen atoms. Fast substitution of alkali metal atoms from the crystals of their salts for the reaction chain carriers of hydrogen flames is found. The importance of reactions of these types in chain combustion is demonstrated. The kinetic isotope effects of hydrogen atom abstraction from hydrocarbon molecules by hydrogen and deuterium atoms are studied. A method for the kinetic studies of free atoms and radicals is developed, which takes into account the role of longitudinal diffusion in the jet and does not require the knowledge of the concentrations of atoms and radicals or the rate constants of other reactions.
- Azatyan
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- The state-to-state photodissociation dynamics of HOD(A?)
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HOD, rotationally state selected in the third and fourth OH stretching overtone (|04〉 |05〉) levels, has been photodissociated via the A? state at λ ? 288 nm. In accord with previous studies H + OD is the dominant dissociation channel, with the yield of OH photofragments being below the detection limit in both sets of experiments. Model Franck-Condon calculations suggest that the OD photofragment rotational state distributions, determined by OD(A-X) laser induced fluorescence (LIF), reflect primarily the rotational and zero-point bending motions in the selected rovibrational state of HOD(X?) However, in contrast to the state selected photodissociation of H2O, the OD rotational distributions from HOD also reveal the influence of a small exit channel torque, which is enhanced in the photodissociation of the deuterated molecule by the shift in the OD centre-of-mass relative to that in OH. A modified Franck-Condon model, which accounts approximately for this modest impulsive torque, qualitatively reproduces the observed behaviour.
- Brouard,Langford
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- A Kinetic Study of OH Radical Reactions with Methane and Perdeuterated Methane
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We measured absolute rate coefficients for the reactions of the hydroxyl raical with methane (k1) and methane-d4 (k2) using the laser photolysis/laser-induced fluorescence technique.We characterized k1 and k2 over the temperature range 293-800 K at pressures between 400 and 750 Torr of helium.We find excellent agreement between our results and the recent determinations of k1 at lower temperatures by Vaghjiani and Ravishankara.The measured rate coefficients, in the units cm3 molecule-1 s-1, fit well to the three-parameter expressions k1(T)=9.65*10-20T2.58 exp(-1082/T) and k2(T)=8.70*10-22T3.23 exp(-1334/T).The kinetic isotope effect for abstraction of the H and D atoms varies from 6.75 at 293 K to 1.96 at 800 K.We compare our results to recently reported calculations by Melissas and Truhlar.
- Dunlop, James R.,Tully, Frank P.
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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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- Kinetics of hydroxyl radical reactions with isotopically labeled hydrogen
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The rate coefficients for the reactions of hydroxyl radical (OH) with H2 (k1), HD (k2), and D2 (k3) were measured between ~230 and ~420 K to be k1 = 7.21 × 10-20T2.69
- Talukdar, Ranajit K.,Gierczak, Tomasz,Goldfarb, Leah,Rudich, Yinon,Madhava Rao,Ravishankara
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- Diffusion of HDO in pure and acid-doped ice films
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In these experiments, a few bilayers of D2O were vapor-deposited on a pure crystalline H2O ice film or an ice film doped with a small amount of HCl. Upon deposition, H/D isotopic exchange quickly converted the D2O layer into an HDO-rich mixture layer. Infrared absorption spectroscopy followed the changes of the HDO from the initial HDO mixture layer to HDO isolated in the H2O ice film. This was possible because isolated HDO in H2O ice has a unique, sharp peak in the O-D stretch region that can be distinguished from the broad peak due to the initial HDO mixture layer. The absorbance of isolated HDO displayed first-order kinetics and was attributed to diffusion of HDO from the HDO-rich mixture layer into the underlying H2O ice film. While negligible diffusion was observed for pure ice films and for ice films with HCl concentrations up to 1 × 10 -4 mole fraction, diffusion of HDO occurred for higher concentrations of (2-20) × 10-4 mole fraction HCl with a concentration- independent rate constant. The diffusion under these conditions followed Arrhenius behavior for T = 135-145 K yielding Ea = 25 ± 5 kJ/mol. The mechanism for the HDO diffusion involves either (i) molecular self-diffusion or (ii) long-range H/D diffusion by a series of multiple proton hop and orientational turn steps. While these spectroscopic results compare favorably with recent studies of molecular self-diffusion in low-temperature ice films, the diffusion results from all the ice film studies at low temperatures (ca. T 220 K). A comparison and discussion of the various diffusion studies are included in this report.
- Oxley, Susan P.,Zahn, Caitlin M.,Pursell, Christopher J.
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- Coordination-Induced N-H Bond Weakening in a Molybdenum Pyrrolidine Complex: Isotopic Labeling Provides Insight into the Pathway for H2Evolution
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The synthesis and characterization of a cationic molybdenum pyrrolidine complex are described that exhibits significant coordination-induced N-H bond weakening. The N-H bond dissociation free energy (BDFE) of the coordinated pyrrolidine in [(PhTpy)(PPh2Me)2Mo(NH(pyrr))][BArF24] ([1-NH(pyrr)]+PhTpy = 4′-Ph-2,2′,6′,2″-terpyridine, NH(pyrr) = pyrrolidine, ArF24 = [C6H3-3,5-(CF3)2]4) was determined to be between 41 and 51 kcal mol-1 by thermochemical analysis and supported by a density functional theory (DFT) computed value of 48 kcal mol-1. The complex [1-NH(pyrr)]+ underwent proton-coupled electron transfer (PCET) to 2,4,6-tri-tert-butylphenoxyl radical, as well as spontaneous H2 evolution upon gentle heating to furnish the corresponding molybdenum pyrrolidide complex [(PhTpy)(PPh2Me)2Mo(N(pyrr))][BArF24] ([1-N(pyrr)]+). Thermolysis of the deuterated isotopologue [1-ND(pyrr)]+ still produced H2 with concomitant incorporation of the isotopic label into the pyrrolidide ligand in the product [(1-N(pyrr-dn)]+ (n = 0-2), consistent with an H2 evolution pathway involving intramolecular H-H bond formation followed by an intermolecular product-forming PCET step. These observations provide the context for understanding H2 evolution in the nonclassical ammine complex [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] ([1-NH3]+) and are supported by DFT-computed reaction thermochemistry. Overall, these studies offer rare insight into the H2 formation pathway in nonclassical amine complexes with N-H BDFEs below the thermodynamic threshold for H2 evolution and inform the development of well-defined, thermodynamically potent PCET reagents.
- Bezdek, Máté J.,Pelczer, István,Chirik, Paul J.
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supporting information
p. 3050 - 3059
(2020/09/02)
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- The Nature of Hydrogen Adsorption on Platinum in the Aqueous Phase
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The thermodynamic state of H2 adsorbed on Pt in the aqueous phase was determined by kinetic analysis of H2 reacting with D2O to HDO, HD, and D2, and by DFT-based ab initio molecular dynamics simulations of H2 adsorption on Pt(111), Pt(110), and Pt nanoparticles. Dissociative adsorption of H2 on Pt is significantly weakened in the aqueous phase compared to adsorption at gas–solid interfaces. Water destabilizes the adsorbed H atoms, decreasing the heat of adsorption by 19–22 kJ (Formula presented.) while inducing an additional entropy loss of 50–70 J (Formula presented.) K?1. Upon dissociative adsorption of H2, the average distance of water from the Pt surface increases and the liquid adopts a structure that is more ordered than before close to the Pt surface, which limits the translation mobility of the adsorbed H atoms. The presence of hydrated hydronium ions next to the Pt surface further lowers the H?Pt bond strength.
- Yang, Guoju,Akhade, Sneha A.,Chen, Xi,Liu, Yue,Lee, Mal-Soon,Glezakou, Vassiliki-Alexandra,Rousseau, Roger,Lercher, Johannes A.
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supporting information
p. 3527 - 3532
(2019/02/05)
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- C-H bond activation by rhodium(I) phenoxide and acetate complexes: Mechanism of H-D exchange between arenes and water
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New rhodium(I) complexes (PNP)Rh(X) (PNP = 2,6-bis(di-tert- butylphosphinomethyl)pyridine) (X = OTf(1), OAc (3), OH (8), OCH 2CF3 (9), OC6H5 (10), OC 6H4NO2 (11)) have been prepared. Hydroxide complex 8 and trifluoroethoxide complex 9 undergo stoichiometric activation of benzene-d6 to form the phenyl complex (PNP)Rh(C6D 5). Acetate and aryloxide complexes 3, 10, and 11 are active catalysts for H-D exchange between arenes and water. Control experiments indicate that the rhodium complexes are the active catalysts and that the observed exchange is not catalyzed by adventitious acid. Mechanistic studies of the H-D exchange reaction support a pathway involving dissociation of aryloxide or acetate ligand. The reaction is accelerated by added alcohol and, for the acetate complex, inhibited by added sodium acetate.
- Hanson, Susan Kloek,Heinekey, D. Michael,Goldberg, Karen I.
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p. 1454 - 1463
(2009/01/31)
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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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- 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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- 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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- The four isotopomer reactions of NH(a) and ND(a) with NH3(X) and ND3(X)
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The reactions NH(a) + NH3 (X) → products (1) ND(a) + NH3 (X) → products (2) NH(a) + ND3 (X) → products (3) ND(a) + ND3 (X) → products (4) were studied in a quasi-static reaction cell at room temperature and pressures of 10 and 20 mbar with He as the main carrier gas. The electronically excited reactants NH(a) and ND(a) were generated by laser-flash photolysis of HN3 and DN3, respectively, at λ = 308 nm and detected by laser-induced fluorescence (LIF). Also the ground state species NH(X) and ND(X) as products were detected by LIF. From the measured concentration-time profiles of NH(a) and ND(a) under pseudo-first order conditions, the following rate constants were obtained: k1, = (9.1 ± 0.9) × 1013 cm3 mol-1 s-1 k2 = (9.6 ± 1.0) × 1013 cm3 mol-1 s-1 k3 = (8.0 ± 1.0) × 1013 cm3 mol-1 s-1 k4 = (7.2 ± 0.8) × 1013 cm3 mol-1 s-1. The major products are the corresponding NHi-D2-i(X) radicals (i = 0, 1, 2), whereas quenching processes such as NH(a) + ND3 → NH(X) + ND3 are of minor importance (1%). The isotope exchange NH(a) + ND3 → ND(X) + NHD2 is negligible, and the corresponding channel on the singlet surface NH(a) + ND3(X) → ND(a) + NHD2 (X) contributes with 1% to the overall NH(a) depletion in that reaction. The experimental findings are discussed in terms of a chemical activation mechanism by means of statistical rate theory.
- Adam,Hack,Olzmann
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p. 439 - 455
(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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- Interaction of water with GeCl4, SnCl4, and AsCl 3
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The interaction of water with GeCl4, SnCl4, and AsCl3 was studied by IR spectroscopy. The results demonstrate that these chlorides contain molecular water in monomeric form. At water concentrations above 10-2 mol/l, GeCl4 also contains H3O+ ions. The mechanisms of GeCl4 and AsCl3 hydrolysis were studied over a wide range of water concentrations.
- Efremov,Potolokov,Nikolashin,Fedorov
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p. 837 - 846
(2008/10/08)
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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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- 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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- Pressure Dependence of the OH-Stretch Raman Spectra of Supercooled Water
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Raman spectra of the OH-stretching region of liquid H2O and 10percent H in D2O have been obtained between -50 and +100 deg C at pressures up to 240 MPa, extending into the deeply supercooled region.Application of pressure made it possible to study liquid water at temperatures 20 K lower than reported so far.The low-frequency shoulder of the H2O-spectra grows rapidly with falling temperature.The relative intensity of the collective band increases monotonically to the lowest temperature studied.Supercooled water shows a number of thermodynamic and transport anomalies, which become apparent from the unusual p-, T-dependence of the compressibility and the molecular mobility in the pressure range between 0 and 200 MPa.The Raman spectra, however, do not change significantly in this pressure region.At the lowest temperatures they are practically independent of pressure.This finding suggests that the intensity of the collective Raman band originates from short range correlations of the OH-bond vibrations, that are not identical to the more extended density fluctuations responsible for the well known anomalies of supercooled water. - Keywords: Glasses / High Pressure / Liquids / Spectroscopy, Raman
- Karger, N.,Luedemann, H.-D.,Sceats, M. G.
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p. 1104 - 1109
(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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- 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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- Hydrogen isotope exchange between D2 and H2O catalyzed by platinum plate
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The reaction rate of hydrogen isotope exchange between D2 and H2O catalyzed by platinum plate is studied.The exchange reaction is described with the kinetic model which is the modification of that for the exchange reaction catalyzed by alumina-supported platinum catalyst.For the comparison of experimental results with this model relative amount of the number of sites for hydrogen adsorption was estimated from the initial rate of hydrogen isotope exchange between H2 and D2 on the same surface.The results show that the kinetic model is applicable for the plate catalyst if the number of the sites for hydrogen absorption, which is very sensitive to the surface state of the catalyst, was estimated not from the macroscopic surface area but from our scheme. Key words: hydrogen isotope exchange reaction, platinum plate as catalyst.
- Miyamoto, Shin-Ichi,Sakka, Tetsuo,Iwasaki, Matae
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p. 857 - 861
(2007/10/02)
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- H/D Isotope Exchange in the D2-H2O System under the Influence of Ultrasound
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Water was irradiated by 300-kHz ultrasound under atmospheres of argon, deuterium, and mixtures of the two gases, and the isotopic composition of the gas was determined.The yields of H2 and HD increase with increasing deuterium content of the gas mixture, reach maxima at 35percent D2 (H2) and 60percent D2 (HD), and fall off at higher D2 concentrations.The yield of the D2 consumption peaks at 50percent D2.The absolute yields are greater by almost one order of magnitude than those of ordinary sonolytic reactions such as H2O2 formation.The results are explained by reactions of the H and OH radicals generated in the sonolysis of water.In the absence of deuterium, a large percentage of these radicals recombine.D2 at low concentrations scavenges mainly OH radicals, the principal products being H2 and HDO.At higher D2 concentration, D2 also scavenges H atoms, the principal products now being HD and HDO.The decrease in all yields at higher D2 concentrations is explained by the lower temperatures reached in the gas bubbles, which are formed by cavitation.
- Fischer, Christian-H.,Hart, Edwin J.,Henglein, Arnim
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p. 222 - 224
(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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- FTIR spectra of water-hydrogen fluoride complexes in solid argon. Evidence for inversion doubling in the HF librational modes of H2O- - HF
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Co-condensation of dilute Ar/HF and Ar/H2O samples at 12 K produces a number of sharp new infrared absorptions.The major species, which exhibits a strong 3554.7 cm-1 band and two quartets beginning at 753.1 and 635.6 cm-1, is identified as H2O- - HF.Isotopic substitution in the base submolecule changes the splitting in the latter multiplets and provides evidence for inversion doubling of the H-F librational modes in the H2O- -HF complex.The reverse complex HF- -HOH, identified at 3915.5 cm-1, exhibits a stronger interaction when HOH is replaced by DOD.Two H-F stretching fundamentals, which show small D2O shifts, increased markedly on sample warming and are assigned to the 1:2 complex H2O- -HF- -HF with an open structure.
- Andrews, Lester,Johnson, Gary L.
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p. 3670 - 3677
(2007/10/02)
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- Studies of the binary reactions of H3O+*(H2O)0,1,2 ions and their deuterated analogues with D2O, H20,and NH3
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The rate coefficients and product ion distributions for the binary of H3O+*(H2O)0,1,2 and D3O+*(D2O)0,1,2 ions with D2O and H2O, respectively, and with NH3 have been studied at 300 K using a selected ion flow tube (SIFT) apparatus.The ions were created in a flowing afterglow ion source and after mass filtering were injected at low energy into the SIFT.All the reactions proceeded at or near the gas kinetic limit.In the D2o and H2O thermoneutral isotopic exchange reactions, the distribution of H and D amongst the product ions and neutrals was seen to be purelystatistical.This implies that these reactions proceed via the formation of an intermediate long-lived association ion in which total randomization of the H and D atoms takes place prior to unimolecular decomposition.No appreciable isotopic exchange occurred in the exothermic NH3 reactions which apparently proceeded via the simpler mechanisms of D+ (or H+) or D3O+ (or H3O+) transfer.The differing mechanisms for the H2O and D2O reactions compared to the NH3 reactions are rationalized in terms of the thermicities of the reactions and the lifetimes of the respective intermediate ions.
- Smith, D.,Adams, N. G.,Henchman, M. J.
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p. 4951 - 4957
(2007/10/02)
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- Matrix isolation and laser diagnostic studies of catalytic oxidation of H2 and D2 on platinum
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The reaction of hydrogen (or deuterium) and oxygen one pure polycrystalline platinum surfaces has been investigated in the 10-5 to 10-6 Torr range using the matrix isolation and laser-induced fluorescence techniques.Water,formed with a 1.1 +/- 0.2 kcal/mole activation energy, was the onyl product detected by infrared spectroscopy at catalyst temperatures between 150 and 700 deg C.Laser-induced fluorescence experiments were used to observe the production of HO radicals both in argon matrices and in the gas phase.HO production was found to have an activation energy of 31 +/- 1 kcal/mole and could only be detected escaping from the surface at catalyst temperatures above 500 deg C.The dependence of H2O and HO radical production on the partial pressures of H2 and O2 has been made.Reaction mechanisms are discussed in relation to other studies of the oxidation of hydrogen on single crystal and polycrystalline platinum surfaces.
- Tevault, D. E.,Talley, L. D.,Lin, M. C.
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p. 3314 - 3319
(2007/10/02)
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