bluuebunny/crossref_metadata_2025_split huggingface dataset:
A direct investigation of the reaction CH3+OH: Overall rate constant and CH2 formation at T=298 K
Authors: Deters R. , Otting M. , Gg. Wagner H. , Temps F. , László B. , Dóbé S. , Bérces T. 
Published: July 2012

AbstractThe rate constant and the product distribution of the reaction CH3+OH r̊ products (1) were studied at room temperature over a range of pressures (0.7 mbar ≤p≤467 mbar) with M=
He as inert bath gas. At pressures of 45 to 467 mbar, towards the high pressure range of the reaction, experimental determinations were performed using the laser flash photolysis technique with transient UV absorption spectrometry (LFP/TAS) for direct monitoring of the CH3 and OH radicals. In this range, the overall rate constant for the reaction was found to be practically independent of pressure at k1 (298 K)=(4.4±0.8)x1013 cm3 mol−1 s−1. At pressures of 0.7 mbar to 4.0 mbar, the reaction was studied using the discharge flow (DF) technique with laser magnetic resonance (LMR) for the direct detection of the reactants CH3 and OH and the reaction product CH2. The measured overall rate constants at the low pressures were found to increase slightly with increasing pressure and approach the value at “high” pressure from the LFP/TAS experiments. From simulations of the measured concentration‐versus‐time profiles of the CH3, OH, and CH2 radicals, the branching ratio for the reaction channel CH3+OHr̊ CH2+H2O (1.1) was determined at p=1.33 mbar, T=298 K to be k1.1lk1=(0.89±0.09). Implications of the results are discussed considering the fall‐off curve for the reaction, the reaction mechanism and the product distribution, and the available thermochemical data.
A high temperature study of the reaction SiH4+H ⇋ SiH3+H2
Authors: Kunz A. , Roth P. 
Published: July 2012

AbstractThe reaction of silane with H atomsmagnified imagewas studied behind reflected shock waves at temperatures between 998 K and 1273 K and pressures around 1.5 bar. The thermal decomposition of a few ppm ethyl iodide (C2H5I) was used as a well known H‐atom source. The atomic resonance absorption spectroscopy (ARAS) was applied for time resolved and simultaneous measurements of H‐ and Si‐atom concentrations. The presence of an excess of SiH4 causes a fast consumption of H atoms according to reaction (R 5). The signals obtained were kinetically evaluated by computer simulations based on a simplified reaction mechanism. The rate coefficient for reaction (R 5) was found to be:
Formation kinetics and electrochemical behavior of an azobenzene self‐assembled monolayer on a gold electrode
Authors: Shao H. B. , Yu H. Z. , Cheng G. J. , Zhang H. L. , Liu Z. F. 
Published: July 2012

AbstractThe formation kinetics of the self‐assembled monolayer (SAM) of an azobenzenealkanethiol (HS‐CH2CH2NHC(O)‐ϕ‐N=N‐ϕ, denoted as AzoC2SH) on gold surface was studied using electrochemical techniques, while the variation of the voltammetric behavior of AzoC2SH monolayers prepared with different assembly time was investigated. A two‐step adsorption kinetics has been confirmed for the self‐assembly of AzoC2SH molecules on gold surface: the fast adsorption process and the following long‐term reorganization. The equilibrium constant (Keq) for the adsorption and the interaction factor between adsorbate‐adsorbate molecules (δ) were evaluated based on the Frumkin isotherm and determined to be (3.17±0.13)x106 M−1 and (‐0.34±0.04), respectively. We have also determined the adsorption rate constant (Kad) of (1.90±0.4)x103 M−1 s−1 on the basis of the Langmuir model. The value of kad is significantly larger than that of underivatized alkanethiols or ferrocenylalkanethiols, partially due to the strong attractive interaction between AzoC2SH molecules in the monolayer, which is evidenced by the negative interaction factor. Moreover, the electrochemical behavior shows a unique change when we prepare differently packed AzoC2SH monolayers by controlling the self‐assembling time. The apparent electron transfer rate constant (kapp) was found to show a remarkable decrease with the surface concentration of the adsorbate molecules in the monolayer increasing. Taking into account the molecular structural change associated with the reduction/oxidation of an azobenzene group, the decrease of kapp can be understood by considering that there is an increase of the structural inhibition effect when the packing density of the AzoC2SH monolayer on gold surface increases.
Physical characteristics of aluminium plating solutions containing quaternary aralkylammonium compounds
Authors: Simanavičius L. , Stakénas A. , Šarkis A. 
Published: July 2012

AbstractPhase equilibria in ambient temperature molten salt systems prepared from aluminium bromide and dimethylethylphenylammonium bromide or trimethylphenylammonium iodide are described. The experimental densities of some solutions of the mentioned mixtures in aromatic hydrocarbon and the compositions of these layering solutions are given depending on temperature.
EXAFS investigation of mixed zirconium‐titanium alkoxides
Authors: Reinöhl U. , Ertel T. S. , Hörner W. , Weber A. , Bertagnolli H. 
Published: July 2012

AbstractThe structures of zirconium alkoxides Zr(OR)4, with R=n‐propoxide and n‐butoxide, and of the corresponding mixed zirconium‐titanium alkoxides, used as precursors in the sol‐gel process, were investigated by EXAFS spectroscopy at the Zr K‐edge in solution of their parent alcohols.In all investigated systems the Zr‐atoms are surrounded by six O‐atoms. Additionally, a Zr‐Zr distance was found in the pure zirconium alkoxides, indicating the existence of dimers or oligomers. In the mixture of zirconium‐ and titanium alkoxides the hetero‐association is preferred, since a Zr‐Ti distance could be detected.
Dielectric study on ionic orientational disorder in the low‐temperature phases of ionic plastic crystal KNO2
Authors: Honda Hisashi , Onoda‐Yamamuro Noriko , Ishimaru Shin'Ichi , Ikeda Ryuichi , Yamamuro Osamu , Matsuo Takasuke 
Published: July 2012

AbstractThe complex dielectric permittivity of ionic plastic crystal KNO2 was measured in the frequency range 20‐106 Hz and the temperature range 22‐300 K. A small step‐like anomaly of the static dielectric permittivity appeared at the rhombohedral (phase II) to monoclinic (phase III) transition (Ttr2 = 264. 1 K). In phase III, the static dielectric permittivity decreased gradually on cooling over a wide temperature range 50‐264 K. These results indicate that the orientation of the NO−2 ion is still disordered at high temperatures in phase III and becomes ordered gradually with decreasing temperature. The dielectric dispersion associated with this motion occurred in the temperature range 70‐150 K. From these data and the previously reported 15N NMR data, we propose a model of the motion of NO2− in phase III in which the anion undergoes 180°‐flip motion about the axes perpendicular to the molecular C2 axis in an asymmetric double‐minimum potential.
A Raman spectroscopic study of hydration and water‐ligand replacement reaction in aqueous cadmium(II)‐sulfate solution: Inner‐sphere and outer‐sphere complexes
Authors: Rudolph Wolfram W. 
Published: July 2012

AbstractRaman and infrared spectra of aqueous CdSO4 and (NH4)2SO4 solutions have been recorded over a broad concentration (0. 040 to 3. 51 mol/kg) and temperature range (‐10 to 109°C). Whereas the v1SO2‐4 band profile is symmetrical in (NH4)2SO4 solutions, in CdSO4 solutions a shoulder appears on the high frequency side at 989 cm−1, which increases in intensity with increasing concentration and temperature. The relative molar scattering coefficient of the v1SO42−v band is the same for all forms of sulfate in (NH4)2SO4 and CdSO4 solutions and is independent of temperature up to 109°C. The high frequency shoulder is attributed to the formation of a 1:1 inner‐sphere complex [Cd2+(OH2)5OSO32− The v3SO42− antisymmetric stretching mode, normally forbidden in the isotropic scattering, contains a contribution in concentrated CdSO4 solutions. The bending modes, v2SO42− and v4SO42−, also forbidden in the isotropic scattering, show contributions at 472 and 640 cm−1, well apart from the frequencies normally observed in the anisotropic spectrum in (NH4)2SO4 solution. A polarized band at 240 cm−1 has been assigned to the Cd2+‐OSO32− ligand vibration. Further spectroscopic evidence for contact ion pair formation at 25°C is provided by i.r. spectroscopy. Higher associates or anionic complexes are not required to interpret the spectroscopic data, but it has been established that at room temperature the favourable complex is an outer‐sphere complex whereas above 109°C the inner‐sphere complex is becoming the favourable one (inner‐sphere complex formation is entropically driven). The Raman spectroscopic results confirm the stepwise reaction mechanism of sulfato‐complex formation in aqueous CdSO4 solution.The degree of association has been measured as a function of concentration and temperature. At concentrations above 1.0 mol · kg−1 the concentration quotient, QA equal to QU, has a relatively high and constant value of 0.14±0.02. The thermodynamic association constant, enthalpy and entropy for the outer‐sphere/inner‐sphere complex formation have been estimated to be: KII = 3.3±0.3, ΔH° =(6.2±0.2) kJ · mol−1 and ΔS° = (33±1)J · K−1 · mol−1.
Quantum cluster equilibrium theory of liquids part II: Temperature dependent chemical shifts, quadrupole coupling constants and vibrational frequencies in liquid ammonia
Authors: Ludwig R. , Weinhold F. , Farrar T. C. 
Published: July 2012

AbstractTemperature dependent NMR chemical shifts for hydrogen and nitrogen, NMR quadrupole coupling constants for deuterium and nitrogen as well as all vibrational frequencies for liquid ammonia are calculated by the ab initio quantum cluster equilibrium (QCE) method and compared with experimental measurements. All properties were obtained from equilibrium populations of ammonia clusters calculated between the melting (196 K) and the boiling (239 K) point. The calculated vibrational frequencies and chemical shifts are in good agreement with experimental NMR and Raman measurements. The theoretical quadrupole coupling constants for liquid ammonia lie midway between the available experimental data for the solid and the gas phase.
A model of the electronic properties of activated carbon
Authors: Kastening Bertel 
Published: July 2012

AbstractActivated carbon may be regarded as consisting of graphite nanocrystals forming the walls of the pores. These crystallites contain three graphene layers having sizes of 2‐4 nm perpendicular to the “c‐axis”. A model has been developed of the electronic properties of the material. The model is based on this nano‐structure and on experimental results concerning the variation of the conductivity, the thermoelectric power and the ESR signal of activated carbon, immersed in electrolyte solutions, on changing the electrode potential. According to the model, the graphene layers (“domains”) are charged with one, and preferentially two, electrons (or defect electrons) or uncharged, the π‐electron system being delocalized throughout a domain. The various types of domains are in thermodynamic equilibria with one another. Charge transport is regarded as transport of charged domains, resembling the “extra” conduction of protons in H‐bonding solvents like water, while uncharged domains (like neutral water clusters) do not contribute to the conductivity. Correspondence with electronic energy band models is obtained by constructing electronic levels from the energetic data. Close structural and mechanistic similarities to conducting polymers suggest the applicability of the model also to this material.
Radiative and nonradiative energy transfer between CO and pyrene involving higher excited states: A Spectroscopic Analysis
Authors: Gudipati Murthy S. , Kalb Martin 
Published: July 2012

AbstractA detailed spectroscopic analysis of radiative and nonradiative electronic energy transfer (EET) between CO and pyrene involving the higher electronically excited states is presented. Spectroscopic investigations on CO and pyrene that are codeposited in Ar matrices reveal that when CO is excited between 160 and 120 nm, into the A1π(ν')← X1S̀+ (ν″ = 0) vibronic transitions, fluorescence of pyrene occurs in addition to the radiative transitions in CO itself. A part of the EET from CO to pyrene is due to the radiative EET, which is confirmed from the modulation of the Cameron emission bands of CO between 200 and 250 nm by the excitation spectrum of pyrene in the same wavelength region. An evidence for nonradiative EET of Förster‐type is also obtained by investigating sandwich matrices with layers of Ar containing only pyrene and pyrene as well as CO. Present results are in line with those observed between CO and anthracene [Chem. Phys. Lett. 268, 169 (1997)].
Computer modeling of formation of soot precursors in the oxidation of methane
Authors: Németh A. , Héberger K. 
Published: July 2012

AbstractA previously developed method was extended for computer modeling of chemical processes involving large number of species. Improvements introduced for the combinatorial generation of stoichiometrically possible reaction steps include hierarchical structuring of the chemical system under study, replacing invariant atomic groups by symbols, as well as the combined application of two complexity constraints for the reduction of the set of reactions.Modeling revealed that while in the oxidation of methane the formation of ethynyl benzene (phenylacetylene), 1‐naphtyl radical, diethynyl benzene and ethynyl naphtalene by acetylene addition to the precursor radicals can be accepted to proceed in a single elementary step, this is not valid for anthracyl radical. Formation of acenaphtylene, 12‐biphenyl radical, 3‐phenantryl radical, phenantrene, and pyrene needs further investigation in this respect. Furthermore, formation of benzene, phenyl, and biphenyl by recombination of propargyl or substituted propargyl radicals in a single elementary reaction was not supported by modeling. Results suggest the possibility of not yet identified intermediates participating in these processes.
The origin and nature of chemiluminescence in the dibenzoylperoxide‐dimethylaniline reaction
Authors: Vasvári Gábor , Hofmann Tibor 
Published: July 2012

AbstractIt has been established that the dibenzoylperoxide (POOP)+dimethylaniline (DMA) reaction is suitable to the slow production of free radicals at ambient temperature and in the absence of oxygen. The process is accompanied by a well measurable chemiluminescence (CL) without added sensitizer. The reaction carried out in acetonitrile is mainly ionic with the formation of radicals in between. We assume the existence of a 2:1 DMA/POOP complex as a possible source of radicals. The kinetics of CL is determined by the formation and decomposition rate of this complex. Calculated rate constant values: k(f) = 3 × 10−3 M−1 s−1 and k(d) = 1 × 10−3 s−1. Based on the spectrum of the CL light and taking into consideration the analytical results we assume that the energy of CL is delivered almost totally by the cage termination product (ΔH(r) =‐385 kJ/mol) formed in excited state and this energy is transferred to the triphenylmethane dye‐salt products (Malachite Green, Crystal Violet) which are the probable emitters of the CL light. Hydroperoxide is not formed in the reaction.
Hydrogen transfer: Experiment and theory
Authors: Limbach Hans‐Heinrich , Manz Jörn 
Published: July 2012

AbstractThis overview paper provides a short introduction into the topics discussed at the International Autumn Bunsen Discussion meeting 1997 on “Hydrogen Transfer Theory and Experiment” which took place in Berlin during 10–13 September 1997 under the auspices of the Deutsche Bunsengesellschaft für Physikalische Chemie and the Freie Universität Berlin.
Characterization of proton transfer in HO2−and HF2+: An atoms in molecules study
Authors: Hamilton L. P. 
Published: July 2012

AbstractProton transfer is examined in HO2−and HF2+. These species are isoelectronic and a direct comparison of their electron densities is possible. Electron densities are calculated at the QCISD/6‐311++G(2d,2p) level for the nonlinear equilibrium geometry and the C2vsaddle point and linear saddle point geometries. Bader's theory of atoms in molecules is applied to partition the electron density into its atomic components and atomic and molecular properties are calculated. These quantities are used to characterize the proton dynamics as being like internal rotation.
Proton dynamics and the tautomerization potential in benzoic acid crystals
Authors: Neumann M. A. , Craciun S. , Corval A. , Johnson M. R. , Horsewill A. J. , Benderskii V. A. , Trommsdorff H. P. 
Published: July 2012

AbstractTwo‐proton exchange along the two hydrogen bonds mediates the tautomerization in benzoic acid (BA) dimers. Optical spectroscopy and quasi‐elastic neutron scattering (QENS) have been employed to characterize the proton dynamics in doped and pure BA crystals. The proton motion in BA is governed by a multidimensional potential energy surface (PES), and recent theoretical methods, based on a perturbative instanton approach, to describe tunneling in such PES are presented. This PES is also modulated by the interaction with the solid state environment as manifest by the energy difference between the otherwise equivalent tautomers. The value of this energy difference in pure crystals is an important parameter in the data analysis of NMR and QENS. Both methods give mutually consistent values that differ significantly from earlier determinations via infrared and 13C NMR as well as a recent evaluation using neutron diffraction data. The energy difference between tautomers is altered for dimers in the vicinity of impurity molecules. This is the basis for the optical spectroscopic methods, which enable a direct and accurate determination of the level structure and tautomerization dynamics of these coupled dimers in the limit of very low temperatures, where coherent tunneling is also observed in some cases. Measurements with new impurity molecules make it possible to monitor simultaneously at least 10 different tautomer configurations and prove that the influence of the probe molecules on the proton dynamics is small. The transition to thermally activated barrier crossing at higher temperatures is accessed via the width of the QENS line that is determined by the inverse of the proton correlation time. The quantitative data analysis of the scattered intensity as a function of temperature and scattering angle yields the energy difference, A, between the two wells (A/kB=90+20 K), the length (0.686 Å), and direction of the proton jump vector. These measurements complement NMR investigations presented in the preceding paper.
Observation of thermal tautomerism of thermochromic salicylideneaniline derivatives in the solid state by 15N CPMAS NMR down to cryogenic temperatures
Authors: Benedict Claudia , Langer Uwe , Limbach Hans‐Heinrich , Ogata Hironori , Takeda Sadamu 
Published: July 2012

AbstractThe temperature dependent solid state 15N CPMAS NMR spectra of 15N enriched salicylideneaniline derivatives, N‐salicylideneaniline (SA), N,Nî‐bissalicylidene‐p‐phenylenediamine (BSP) and N,Nî‐di(2‐hydroxy‐1‐naphthylidene)‐p‐phenylenediamine (DNP) were measured in order to study possible proton transfer processes in the NHO hydrogen bonds. For DNP a remarkable change of the 15N chemical shifts of more than 50 ppm was observed between 26 and 388 K by using the CPMAS NMR technique down to cryogenic temperatures. The result clearly indicates very small enthalpy differences among the four tautormers associated with the combination of two possible forms, OH and NH form, of the two NHO hydrogen bonds in a DNP molecule.
Coherent and incoherent dihydrogen dynamics in a ruthenium trihydride complex with the tris(pyrollyl)phosphine ligand
Authors: Gründemann Stephan , Limbach Hans‐Heinrich , Rodriguez Venancio , Donnadieu Bruno , Sabo‐Etienne Sylviane , Chaudret Bruno 
Published: July 2012

AbstractThe ruthenium IV trihydride complex Cp*RuH3(Ppy3) (1) accommodating the π‐accepting tris(pyrollyl)phosphine ligand has been synthesized by standard methods. This complex exhibits average exchange couplings J of the coherent dihydrogen exchange and average rate constants k of the incoherent dihydrogen exchange which are significantly larger than the corresponding triphenylphosphine and tricyclohexylphosphine analogs. No kinetic HH/HD isotope effects on the incoherent exchange are observed within the margin of error. Using a simple kinetic model involving incoherent vibrational excitation it is shown that in principle both the coherent and the incoherent processes should be related. However, whereas the energy of activation of k corresponds to the barrier of rotation, the energy of activation of the coherent dihydrogen exchange J is much smaller i.e. determined by states well below the barrier. Nevertheless, it is found that chemical modification which reduces the energy of activation of J also reduces the energy of activation of k which demonstrates the relation between both kinds of processes.
On the unexpected isotope effect on the exchange coupling constant in partially deuterated transition‐metal trihydride complexes
Authors: Gelabert Ricard , Moreno Miquel , Lluch José M. 
Published: July 2012

AbstractIn this paper we attempt to explain the unusual isotope effect displayed by the 1H‐NMR scalar H‐H coupling of the hydrides in complexes [CpIrL(H)3]+ (L=various phosphine and phosphite ligands) when one of the equivalent protons is turned into a deuteron (D.M. Heinekey, J.M. Millar, T.F. Koetzle, N.G. Payne, and K.W. Zilm, J. Am. Chem. Soc. 112, 909 (1990)]. It is found that in the case of the perprotio species the system can be described as a three‐coupled well system, but when a deuterium is introduced the system is better described as a double‐coupled well. Finally, it is shown that the coupling in the case of a double‐well system is greater than in an equivalent triple‐well system, in this way giving a plausible explanation to the experimental facts reported.
Indirect characterisation of hydrogenation intermediates using PASADENA NMR spectroscopy — evolution of zero‐quantum coherence in AB spin systems
Authors: Hübler P. , Natterer J. , Bargon J. 
Published: July 2012

AbstractAn indirect method is presented for the detection and characterisation of intermediates that occur during homogeneously catalysed hydrogenations. In combination with parahydrogen as a reactand, this method takes advantage of NMR spectroscopy and zero‐quantum coherence that becomes modified under the Hamiltonian of an intermediate. The method allows the investigation of highly reactive intermediates that are inaccessible for conventional analytical methods. A theoretical framework is presented based on a density matrix description. It is shown that PASADENA (Parahydrogen and Synthesis Allow Dramatically Enhanced Nuclear Alignment) NMR spectra of strongly coupled spin systems (i.e., AB systems) contain structural information of intermediates, even if they cannot be detected directly.