Temperature dependence of the substrate selectivity in the hydroxylation of alkylbenzenes in the H2O2−H2SO4 system

The substrate selectivity in the hydroxylation of methylbenzenes in the H₂O₂−H₂SO₄ (70 wt.%) system was studied at 15–55 °C. The activation entropy correlates with the basicity of the arenes, while the substrate selectivity and activation enthalpy correspond both with the basicity and ionization potentials of ArH. We concluded that the structure of the reaction transition state is intermediate between a charge transfer complex and σ-complex. L. M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, 70 R. Lyuksemburg ul., Donetsk 340114, Ukraine. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 35, No. 1, pp. 39–43, January–February, 1999.     

Kinetics of decomposition of the liquid phase of the ternary system LiOH-H2O2-H2O in the presence of the solid phase of Li2O2 · H2O

The kinetics of decomposition of hydrogen peroxide in the liquid phase of the ternary system LiOH-H₂O₂-H₂O was studied in the presence the solid phase of Li₂O₂·H₂O and without it. The main kinetic parameters of the processes studied were determined.     

Comment on the possible role of the reaction H+H2O→H2+OH in the radiolysis of water at high temperatures

In a recent paper (Radiation Physics and Chemistry, 2005, vol. 74, pp. 210) it was suggested that the anomalous increase of molecular hydrogen radiolysis yields observed in high-temperature water is explained by a high activation energy for the reaction H+H₂O→H₂+OH. In this comment we present thermodynamic arguments to demonstrate that this reaction cannot be as fast as suggested. A best estimate for the rate constant is 2.2×10³ M⁻¹ s⁻¹ at 300 °C. Central to this argument is an estimate of the OH radical hydration free energy vs. temperature, ΔG_hyd(OH)=0.0278t−18.4 kJ/mole (t in °C, equidensity standard states), which is based on analogy with the hydration free energy of water and of hydrogen peroxide.     

An infrared study of the stretching modes of the water molecules in ZnF2·4H2O

The IR spectra of ZnF₂·4H₂O and its deuterated analogues are reported at ambient and liquid-nitrogen temperatures. The OH and OD stretching and bending vibrations of the water molecules are analysed in detail. The two types of water molecules give rise to different absorption peaks in the OH and OD stretching regions in samples that contain isotopically dilute HDO groups. The strongly hydrogen-bonded water molecules H₂O(1) and H₂O(4) show four broad OH and OD stretching modes at lower frequencies, while the weaker hydrogen-bonded ones H₂O(2) and H₂O(3) give rise to four narrow bands at higher frequencies. The ν_OD frequencies of isotopically dilute HDO groups correlate very well with the known R(H---F) and R(H---O) distances in the crystals and the assignment of these modes was done on this basis. It was also found that the ratio ν_OH/ν_OD decreases with decreasing values of R(H---O) or R(H---F) in ZnF₂·4H₂O.     

Synthesis of (VO)2P2O7 catalystsvia exfoliation-reduction of VOPO4·2H2O in butanol in the presence of ethanol

The exfoliation-reduction of VOPO₄·2H₂O in l-butanol oriso-butanol alone, and in a l-butanol/ethanol oriso-butanol/ethanol mixture, were conducted. Although all precursors were composed of a lamellar compound with intercalated alcohol molecules, VOHPO₄·0.5H₂O was formed when the exfoliation-reduction process was carried out in the mixed alcohol. All precursors transformed to a single phase of (VO)₂P₂O₇ under the reaction conditions forn-butane oxidation, but the crystallinity of (VO)₂P₂O₇ was different. The catalyst synthesized iniso-butanol/ethanol was well crystalline (VO)₂P₂O₇, and exhibited higher selectivity to maleic anhydride than that synthesized iniso-butanol alone for then-butane oxidation.     

Kinetics of the Reaction of 2-Chloro-quinoxaline with Hydroxide Ion in ACN–H2O and DMSO–H2O Binary Solvent Mixtures

The kinetics of alkaline hydrolysis of 2-chloroquinoxaline (QCl) with hydroxide ion was investigated spectrophotometrically at different percentages of aqueous–organic solvent mixtures with acetonitrile (10–60% v/v) and with dimethylesulphoxide (10–80%) over the temperature range from 25 to 45 °C. The reaction was performed under pseudo first order conditions with respect to 2-chloroquinoxaline (QCl). An increase in the percentage of organic solvent (v/v) has different effects on the reaction rate constants, presumably due to hydrogen bond donor and acceptor differences of the media and other solvatochromic parameters. The data were discussed in terms of the Kamelt-Taft parameter and E _T(30). A nonlinear relation between the logarithm of the rate constant and reciprocal of the dielectric constant suggests the presence of selective solvation by the polar water molecules. Activation parameters ΔH ^#, ΔS ^# and ΔG ^# were determined and discussed.     

An analysis of the vibrational frequencies and amplitudes of the H5O+2 ion in H4SiW12O40·6H2O (TSA·6H2O) and H3PW12O40·6H2O (TPA·6H2O)

The infrared, Raman and inelastic neutron scattering (INS) spectra of TSA·6H₂O and TPA·6H₂O are in agreement with those expected for the presence of H₅O⁺₂ ions. Force fields for different assignment schemes are compared with the observed vibrational frequencies and the INS spectral profile. All but two schemes are eliminated. Whilst low-resolution INS spectroscopy cannot distinguish between these two schemes, the orientations of the vibrational ellipsoids for one scheme are in better agreement with those reported from low-temperature crystallographic studies of the H₅O⁺₂ ion.     

Intercalation of primary diamines in the lamellar niobate HNb3O8 · H2O

The intercalation of alkyl diamines in the protonic oxide HNb₃O₈ · H₂O is quantitative for the diamines H₂N(CH₂)_nNH₂ with n ranging from 2 to 10. All the intercalated oxides [H₃N(CH₂)_nNH₃]_0.5Nb₃O₈ · yH₂O are hydrated at room temperature; they can be easily and reversibly dehydrated to the oxides [H₃N(CH₂)_nNH₃]_0.5Nb₃O₈. The structural behavior of those compounds is compared to that of the alkyldiammonium titanoniobates [H₃N(CH₂)_nNH₃]_0.5TiNbO₅. An interpretation of their structural properties is given which takes into account the tendency of amines to assume an orientation transverse to oxide layers, the conformation of the amine chains, and the tendency to form dense organic layers.     

Reply to comment on the possible role of the reaction H+H2O→H2+OH in the radiolysis of water at high temperatures

In reply to “Comment on the possible role of reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures” (Bartels, 2009 Comment on the possible role of the reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures. Radiat. Phys. Chem. 78, 191–194) we present an alternative thermodynamic estimation of the reaction rate constant k. Based on the non-symmetric standard state convention we have calculated that the Gibbs energy of reaction Δ_rG=57.26 kJ mol^?1 and the reaction rate constant k=7.23×10^?5 M^?1 s^?1 at ambient temperature. Re-analysis of the thermodynamic estimation (Bartels, 2009 Comment on the possible role of the reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures. Radiat. Phys. Chem. 78, 191–194) showed that the upper limit for the rate constant at 573 K is k=1.75×10⁴ M^?1 s^?1 compared to the value predicted by the diffusion-kinetic modelling (3.18±1.25)×10⁴ M^?1 s^?1 (Swiatla-Wojcik, D., Buxton, G.V., 2005. On the possible role of the reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures. Radiat. Phys. Chem. 74(3–4), 210–219). The presented thermodynamic evaluation of k(573) is based on the assumption that k can be calculated from Δ_rG and the rate constant of the reverse reaction which, as discussed, are both uncertain at high temperatures.     

Cathodic reduction of superconducting oxocuprates YBa2Cu3O7− x at high current densities

The cathodic reduction of YBa₂Cu₃O₇ in aqueous electrolytes at ambient temperature turns out to be strongly dependent upon current density with respect to the reaction mechanism. At low current density, topotactic electron/proton transfer is the dominant process, while at higher current densities, two competing reactions appear, i.e. the topotactic conversion and an irreversible reaction leading to products amorphous in terms of X-ray diffraction. Received: 21 April 1999 / Accepted: 7 June 1999     

Distillation of LiCl from the LiCl–Li2O molten salt of the electrolytic reduction process

Electrolytic reduction of the uranium oxide in LiCl–Li₂O molten salt for the treatment of spent nuclear fuel requires the separation of the residual salt from the reduced metal product, which contains about 20 wt% salt. In order to separate the residual salt and reuse it in the electrolytic reduction, a vacuum distillation process was developed. Lab-scale distillation equipment was designed and installed in an argon atmosphere glove box. The equipment consisted of an evaporator in which the reduced metal product was contained and exposed to a high temperature and reduced pressure; a receiver; and a vertically oriented condenser that operated at a temperature below the melting point of lithium chloride. We performed experiments with LiCl–Li₂O salt to evaluate the evaporation rate of LiCl salt and varied the operating temperature to discern its effect on the behavior of salt evaporation. Complete removal of the LiCl salt from the evaporator was accomplished by reducing the internal pressure to <100 mTorr and heating to 900 °C. We achieved evaporation efficiency as high as 100 %.     

Nitration Reaction Catalyzed by Horseradish Peroxidase in the Presence of H2O2 and NaNO2

The enzymatic nitration of phenol and m-cresol catalyzed by horseradish peroxidase was studied in the presence of H2O2 and NaNO2. The results showed that the nitration products of phenol were 2-nitro and 4-nitrophenols. There was also a small amount of by-products of hydroquinone and catechol. The influences of various reaction parameters, including pH, organic solvent type, and concentrations of NaNO2 and H2O2, on the nitration products were investigated. The yields of 4-nitrophenol and 2-nitrophenol were 14% and 12%, respectively. The nitration products of m-cresol were 4-nitro-m-cresol and 6-nitro-m-cresol, and the yields of 4-nitro-m-cresol and 6-nitro-m-cresol were 19% and 30%, respectively.     

Study of the solubility of components in the system Mg(ClO3)2-2NH2C2H4OH · H3C6H5O7-H2O

The solubility of components in the system Mg(ClO₃)₂-2NH₂C₂H₄OH · H₃C₆H₅O₇-H₂O was studied from the complete freezing temperature ?59.4°C to 20.0°C. A polythermal solubility diagram was constructed, in which the crystallization fields were determined for ice, Mg(ClO₃)₂ · 16H₂O, Mg(ClO₃)₂ · 12H₂O, Mg(ClO₃)₂ · 6H₂O, 2NH₂C₂H₄OH · H₃C₆H₅O₇ · H₂O, 2NH₂C₂H₄OH · H₃C₆H₅O₇, and two new compounds, [(HOC(CH₂COOH)₂COO)₂Mg · 2H₂O] and [HOC(CH₂COO)₂MgCOOH · 2H₂O], which were identified by chemical and physicochemical analysis methods.     

Selective reduction of nitroarenes to N-arylhydroxylamines by use of Zn in a CO2–H2O system, promoted by ultrasound

The promoting effect of ultrasound on the selective reduction of nitroarenes to N-arylhydroxylamines by use of Zn in an environmentally benign CO₂–H₂O system has been demonstrated. The yield of N-phenylhydroxylamine reaches 95 % when the reaction is carried out with a Zn-to-nitrobenzene molar ratio of 2.2 under ultrasound (40 kHz) at 25 °C and normal pressure of CO₂ for 60 min. Application of ultrasound to the reaction has the advantages of higher yield of N-arylhydroxylamines, shorter reaction time, and consumption of less Zn.     

Ab initio molecular dynamics simulations of the oxygen reduction reaction on a Pt(111) surface in the presence of hydrated hydronium (H3O)(+)(H2O)2: direct or series pathway?

Car-Parrinello molecular dynamics simulations have been performed to investigate the oxygen reduction reaction (ORR) on a Pt(111) surface at 350 K. By progressive loading of (H3O)(+)(H2O)(2,3) + e- into a simulation cell containing a Pt slab and O2 for the first reduction step, and either products or intermediate species for the subsequent reduction steps, the detailed mechanisms of the ORR are well illustrated via monitoring MD trajectories and analyzing Kohn-Sham electronic energies. A proton transfer is found to be involved in the first reduction step; depending on the initial proton-oxygen distance, on the degree of proton hydration, and on the surface charge, such transfer may take place either earlier or later than the O2 chemisorption, in all cases forming an adsorbed end-on complex H-O-O*. Decomposition of H-O-O* takes place with a rather small barrier, after a short lifetime of approximately 0.15 ps, yielding coadsorbed oxygen and hydroxyl (O + HO*). Formation of the one-end adsorbed hydrogen peroxide, HOO*H, is observed via the reduction of H-O-O*, which suggests that the ORR may also proceed via HOO*H, i.e., a series pathway. However, HOO*H readily dissociates homolytically into two coadsorbed hydroxyls (HO* + HO*) rather than forming a dual adsorbed HOOH. Along the direct pathway, the reduction of H-O* + O* yields two possible products, O* + H2O* and HO* + HO*. Of the three intermediates from the second electron-transfer step, HOO*H from the series pathway has the highest energy, followed by O* + H2O* and HO* + HO* from the direct pathway. It is therefore theoretically validated that the O2 reduction on a Pt surface may proceed via a parallel pathway, the direct and series occurring simultaneously, with the direct as the dominant step.     

On the infrared spectra of libratory modes of H2O molecules in SrX2 · 6H2 O (X = Cl,Br)

A detailed study of the libratory modes of H₂O molecules in the i.r. spectra of SrX₂ · 6H₂O (X = Cl, Br) is reported. The rocking, wagging and twisting libratory modes of (H₂O)_b (bridging type bonding) and (H₂O)_t (terminal type bonding) molecules are assigned at 705, 552, 658 and 460, 400, 438 cm⁻¹ and at 687, 532, 625 and 448, 370, 405 cm⁻¹ in the respective spectra. Using a semi-empirical relation reported by the authors in an earlier communication, the barrier height for (H₂O)_b is estimated to be 22.16 and 20.04 kcal/mole and that for (H₂O)_t to be 10.04 and 8.64 kcal/mole in the respective salts. The value of the force constants K_H and K_H′ for H₂O molecules are also reported.     

Transport numbers of H+ and O2- in the electrochemical system (H2 + H2O),Me/BaCe0.9Nd0.1O3-α/Me,(H2 + H2O)

In the temperature range 873–1123 K, transport numbers of oxygen ions and protons are determined in the system (H₂ + H₂O), Me/BaCe_0.9Nd_0.1O_3-α/Me,(H₂ + H₂O), where Me = Ag, Au, Pt, Ni, by the emf and current methods. The determined transport numbers are independent of the determination method, the electrode material, the current direction (anodic and cathodic polarization of the electrode), polarizability of electrodes, and the partial water (hydrogen) pressure in the gas phase. This unambiguously suggests that the transport numbers refer to the solid electrolyte, and not the electrochemical system as a whole. It also follows that partial currents of the hydrogen ionization and the oxygen ion discharge are determined by the transport numbers of protons and oxygen ions in the electrolyte. At a constant temperature, their ratio is affected by neither the electrode potential nor the gas phase composition, i.e., both electrode reactions have a common limiting step (or steps). Deceased.     

OH Dynamics in a Nanosecond Pulsed Plasma Filament in Atmospheric Pressure He–H2O upon the Addition of O2

The influence of the addition of O₂ on the OH production in a He + 0.1 % H₂O discharge is investigated using laser induced fluorescence. The plasma properties $(T_{\rm g},\;n_{\rm e})$ are reported and used to explain the observed time and spatially resolved OH density, which is absolutely calibrated using Rayleigh scattering. Compared to the case when only H₂O is added, an increase in the measured OH density is observed in the far afterglow. A zero-dimensional chemical kinetic model is constructed, which allows to determine the reactions responsible for the OH production in the far afterglow. When O₂ is admixed, the key reaction $\hbox{O} + \hbox{OH} \longrightarrow \hbox{O}_{2} + \hbox{H}$ causes quenching of OH and production of increased densities of H, HO₂ and H₂O₂, which subsequently leads to additional OH production in the late afterglow.     

Parameters of Li2O2 · H2O crystallization from the LiOH-H2O2-H2O ternary system

The decomposition kinetics of peroxide products contained in the liquid phase of the LiOH-H₂O₂-H₂O ternary system were studied, and the applicability of the solubility method to studying this system was demonstrated for hydrogen peroxide concentrations in the liquid phase from 2 to 6 wt % and temperatures of 21–33°C. The stabilizing influence of solid Li₂O₂ · H₂O on hydrogen peroxide decomposition was demonstrated. The temperature and concentration boundaries of existence were determined for the Li₂O₂ · H₂O phase, whose identity was verified by chemical analysis and qualitative X-ray powder diffraction analysis.