Isotope effect in the formation of hydrogen peroxide by the sonolysis of light and heavy water

The kinetics of the formation of hydrogen peroxide by the sonolysis of light and heavy water in argon and oxygen atmospheres was investigated. The sonochemical reaction has a zero order with respect to hydrogen peroxide (H₂O₂, D₂O₂, or DHO₂). The measurement of the kinetic isotope effect (α), defined as the ratio of the reaction rates in H₂O and D₂O, carried out under argon gave a value of 2.2±0.3. The observed isotope effect decreases with an increase in the concentration of light water in H₂O−D₂O mixtures. No isotope effect is displayed in the oxygen atmosphere (α=1.05±0.10). The isotope effect is determined presumably by the mechanism of sonochemical decomposition of water molecules, which includes the H₂O−Ar^* and D₂O−Ar^* energy exchange (where Ar^* are argon atoms in the³P_2.0 excited state) in the nonequilibrium plasma generated by a shock wave, arising upon a cavitation collapse. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 645–649, April, 2000.     

Hydrolyse acide du p-nitrophényl-diazométhane dans les mélanges H2O?D2O: analyse des effets isotopiques

The velocity of the hydrogen ion catalysed hydrolysis of p-nitrophenyl-diazo-methane (I) has been measured in H₂O? D₂O mixtures, giving an isotopic α_i = 0.49. The product isotope effect r = 5.1, determined from product analyses, combined with the (overall) solvent isotope effect k_H/k_D = 2.81, yields the primary kinetic isotope effect (k_H/k_D)^I = 3.8, and the secondary kinetic isotope effect (k_H/k_D)^II = 0.75. The CICH₂COOH-catalysed hydrolysis of I in H₂O? D₂O mixtures gave a straight-line plot of k_n/k_H versus the atomic fraction n of deuterium. With four carboxylic acids, as catalysts, values of about 4.3 for the kinetic (overall) isotope effects were observed.     

Solvent isotope effects upon the kinetics of some simple electrode reactions

The effects of replacing H₂O with D₂O solvent upon the electrochemical kinetics of simple transition-metal redox couples containing aquo, ammine or ethylenediamine ligands have been investigated at mercury electrodes as a means of exploring the possible contribution of ligand-aqueous solvent interactions to the activation barrier to outer-sphere electron transfer. The general interpretation of solvent isotope effects upon electrode kinetics is discussed; it is concluded that double-layer corrected isotopic rate ratios (k^H/k^D)^E determined at a constant electrode potential vs. an aqueous reference electrode, as well as those determined at the respective standard potentials in H₂O and D₂O (k_S^H/k_S^D), have particular significance since the solvent liquid-junction potential can be arranged to be essentially zero. For aquo redox couples, values of (k_S^H/k_S^D) were observed that are substantially greater than unity and appear to be at least partly due to a greater solvent-reorganization barrier in D₂O arising from ligand-solvent hydrogen bonding. For ammine and ethylenediamine complexes values of (k^H/k^D)^E substantially greater than, and smaller than, unity were observed upon the separate deuteration of the ligands and the surrounding solvent respectively. Comparison of isotope rate ratios for corresponding electrochemical and homogeneous outer-sphere reactions involving cationic ammine and aquo complexes yields values of (k^H/k^D) for the former processes that are typically markedly larger than those predicted by the Marcus model from the homogeneous rate ratios. These discrepancies appear to arise from differences in the solvent environments in the transition states for electrochemical and homogeneous reactions.     

Overall kinetics of photosystem II: pH Dependence and deuterium isotope effect

The overall kinetics of photosystem II was scanned by means of a double flash technique. Oxygen evolution by pea thylakoids provided with an artificial electron acceptor was measured under a regime of double flashes of variant intervals. The pH optimum in H₂O lies near p¹H 7.2 (p¹H meaning the pH in H₂O), with a first-order rate constant of 800 s⁻¹ at 20°C. In deuterium oxide (D₂O), a plateau of maximum reaction rate was found between p²H 6.6 and 7.8 (p²H meaning the “pH” in D₂O), the highest rate constant being 550 s⁻¹. The apparent kinetic deuterium isotope effect is therefore 1.45. Outside this plateau region, there seems to exist another isotope effect of 1.2 to 1.3. These effects are small but may nevertheless reflect the fact that more than one step of the photosystem II reaction sequence is involved in the splitting of a bond to hydrogen. However, the effects may also be solvent effects or located on the acceptor side of the photosystem.     

Primary and secondary kinetic isotope effects in E2 elimination reactions

Primary and secondary deuterium kinetic isotope effects have been measured for elimination of LCl (L = H or D) from some substituted 1,2-diaryl-1-chloroethanes. Although changes in these effects are in agreement with theoretical predictions, the high values of (E_H-E_D)_β and the significantly low A_H/A_D may suggest that either proton tunnelling or an internal return mechanism is complicating this E2 elimination.     

Deuterium exchange in the systems of H2O+/H2O and H3O+/H2O

The reactions of H₂O⁺, H₃O⁺, D₂O⁺, and D₃O⁺ with neutral H₂O and D₂O were studied by tandem mass spectrometry. The H₂O⁺ and D₂O⁺ ion reactions exhibited multiple channels, including charge transfer, proton transfer (or hydrogen atom abstraction), and isotopic exchange. The H₃O⁺ and D₃O⁺ ion reactions exhibited only isotope exchange. The variation in the abundances of all ions involved in the reactions was measured over a neutral pressure range from 0 to 2 × 10⁻⁵ Torr. A reaction scheme was chosen, which consisted of a sequence of charge transfer, proton transfer, and isotopic exchange reactions. Exact solutions to two groups of simultaneous differential equations were determined; one group started with the reaction of ionized water, and the other group started with the reactions of protonated water. A nonlinear least-squares regression technique was used to determine the rate coefficients of the individual reactions in the schemes from the ion abundance data. Branching ratios and relative rate coefficients were also determined in this manner.A delta chi-squared analysis of the results of the model fitted to the experimental data indicated that the kinetic information about the primary isotopic exchange processes is statistically the most significant. The errors in the derived values of the kinetic information of subsequent channels increased rapidly. Data from previously published selected ion flow tube (SIFT) study were analyzed in the same manner. Rigorous statistical analysis showed that the statistical isotope scrambling model was unable to explain either the SIFT or the tandem mass spectrometry data. This study shows that statistical analysis can be utilized to assess the validity of possible models in explaining experimentally observed kinetic behaviors.     

The kinetics of isotope exchange reactions: Use of initial rates to measure isotope effects on carbon acid ionization

Multistep hydrogen isotope exchange reactions, such as the íonization of a carbon acid via a carbanion intermediate in a protic solvent, when conducted using an isotopic tracer to monitor the exchange, have the unusual feature that their rate-determining steps always refer to the transfer of the tracer isotope and never to the isotope present in macroscopic amounts. This property of these reactions is discussed and rationalized using a free energy versus reaction coordinate diagram. It is further shown that this property does not invalidate a commonly used method of measuring kinetic isotope effects on carbon acid ionization in which rates of incorporation of tritium tracers into RH and RD substrates are compared, despite the fact that tritium transfer is rate determining in both exchanges, but it is valid only if initial rate measurements are used. When the comparison is made in a protio solvent, e.g., H₂O, the portion of the initial reaction which may be used depends strongly on the magnitude of the isotope effect. It ranges from less than 1% tritium incorporation for large isotope effects to 10% or more for isotope effects near unity. On the other hand, when a deuterated solvent, e.g., D₂O, is used, the range of validity of the method for large isotope effects is extended dramatically.     

Use of 13C isotope shifts for assignment of deuterium labelling sites in 1,3-diphenylpropan-1-one

¹³C NMR spectroscopy was used for the unequivocal analysis of the mixture of products of RuCl₂[P(C₆H₅)₃]₃-catalysed transfer deuteriation of benzylideneacetophenone by (a) DCOONa/H₂O, (b) HCOONa/D₂O and (c) DCOONa/D₂O. Signal assignments in the ¹³C spectra were obtained mainly from the deuterium-induced ¹³C isotope shifts. The geminal ¹³C? ²H shift of the β-carbon of deuteriated 1,3-diphenylpropan-1-one is almost twice that for the α-carbon.     

D2O–H2O solvent isotope effects on the volumetric properties of aqueous 1,3-dimethyl-2-imidazolidinone between (278.15 and 318.15) K

Densities of dilute solutions of 1,3-dimethyl-2-imiazolidinone in H₂O and D₂O, with the solute mole-fractions ranging up to 0.01, have been measured with an error of 1.5 · 10⁻⁵ g · cm⁻³ at (278.15, 288.15, 298.15, 308.15, 313.15, and 318.15) K and atmospheric pressure using a vibrating-tube densimeter. The partial molar volumes of the dissolved DMI (down to the infinite dilution) and solvent (H₂O or D₂O) as well as the excess molar volumes of the isotopically distinguishable solutions have been calculated. The effects of the solvent isotope substitution, solute concentration and temperature on the volume changes caused by DMI hydration have been considered. The obvious relationship between the D₂O–H₂O solvent isotope effects on the partial molar volume and enthalpy of solution of DMI has been discovered.     

Kinetic study of the isotope exchange reactions of malonic acid and its derivatives in various acidic D2O media using 1H NMR spectroscopy. Implication in the Belousov‐Zhabotinsky reaction

The kinetics of the isotope exchange reactions of RCH(COOH)₂ (RH, D, Me, Et, Bu, and Ph) in D₂O solution were studied by using ¹H NMR spectroscopy. It was observed that the rate of isotope exchange reaction was inhibited by the presence of 1 M of DNO₃, DCl, DBr or D₂SO₄ and catalyzed by the presence of 4 M of D₂SO₄. No inhibition effect was observed in the case of D₃PO₄. The effect of inorganic acids follows the order of D₃PO₄>D₂SO₄ ≫ (DNO₃, DCl, DBr). The conjugated base (RCH(COOD)(COO⁻)) of RCH(COOD)₂ plays an important role in the isotope exchange reaction. The presence of deuterium ion suppresses the generation of RCH(COOD)(COO⁻) ion from RCH(COOD)₂ and inhibits the rate of isotope exchange. In general, the order of reactivity of RCH(COOH)₂ toward isotope exchange with deuterium atom is RPh>(H, Br)>Me>(Et, Bu). © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 455–461, 1999     

Comparison of temperature,pressure and isotope effects on the proton jump between the hydroxide and oxonium ion

The limiting molar conductances ° of potassium deuteroxide KOD in D₂O and potassium hydroxide KOH in H₂O were determined at 5 and 45°C as a function of pressure to clarify the difference in the temperature, pressure and isotope effects on the proton jump between an OD^– (OH^–) and a D₃O⁺ (H₃O⁺) ion. The excess conductances of the OD^– ion in D₂O and the OH^– ion in H₂O, _E ⁰ (OD^-) and _E ⁰ (OH^-), increase with increasing temperature and pressure as in the case of the excess deuteron and proton conductances, _E ⁰ (D⁺) and _E ⁰ (H⁺). However, the temperature effect on the excess conductance is larger for the OD^–(OH^–) ion than for the D₃O⁺ (H₃O⁺) ion but the pressure effect is much smaller for the OD^– (OH^–) ion than for the D₃O⁺ (H₃O⁺) ion. These findings are correlated with larger activation energies and less negative activation volumes found for the OD^– (OH^–) ion than for the D₃O⁺ (H₃O⁺) ion. Concerning the isotope effect, the value of _E ⁰ (OH^-)/ _E ⁰ (OD^-) deviates considerably from at each temperature and pressure in contrast with that of _E ⁰ (H⁺)/ _E ⁰ (D⁺), although both of them decrease with increasing temperature and pressure. These results are discussed mainly in terms of the difference in repulsive force between the OD^– (OH^–) or the D₃O⁺ (H₃O⁺) ion and the adjacent water molecule, the difference in strength of hydrogen bonds in D₂O and H₂O, and their variations with temperature, pressure, and isotope.     

Kinetic isotope effect in the reaction of oh radical with acetone-D6

The discharge-flow method with resonance fluorescence detection of OH radicals was applied to obtain the rate constant value of k _D = 1.95 ± 0.14 (1σ) 10¹⁰ cm³ mol^-1s^-1 at 298 K. Combination with k _H from our previous study gives the kinetic isotope effect of k _H / k _D = 5.33 ± 0.41. OH + CH₃C(O)CH₃ → Products (H) OH + CD₃C(O)CD₃ → Products(D) This revised version was published online in June 2006 with corrections to the Cover Date.     

Water as a solute in nitromethane: Effect of H2O–D2O isotope substitution on the solution volumetric properties between 278.15 K and 318.15 K

Densities of solutions of H₂O and D₂O in nitromethane, with the solute mole fractions ranging up to 0.03, were measured with an error of 1.5 · 10⁻⁵ cm³ · mol⁻¹ at (278.15, 288.15, 298.15, 308.15, and 318.15) K using a vibrating-tube densimeter. Apparent and partial volumes and isobaric expansibilities (down to the infinite dilution) of water isotopologues were calculated. The temperature-dependent behavior of D₂O–H₂O solute isotope effects on the molar quantities studied were described taking into account the structure- and interaction-related peculiarities of the dissolving medium in question.     

Kinetic isotope effects for oxidation reaction of olefins by p-quinones in water-acetonitrile solutions of cationic and chloride anionic palladium complexes

Kinetic isotope effects for oxidation reactions of ethylene and cyclohexene in solutions of cationic palladium(ii) complexes in MeCN-H₂O(D₂O) systems, were measured. It was established that the ratio of the initial reaction rates ${{R_0^{H_2 O} } \mathord{\left/ {\vphantom {{R_0^{H_2 O} } {R_0^{D_2 O} }}} \right. \kern-0em} {R_0^{D_2 O} }} $ is equal to 1 for both reactions with the use of cationic complexes of the type Pd(MeCN) _x (H₂O)_4?x ²⁺, which differs from oxidation reactions catalyzed by chloride palladium complexes in the same solutions, where the ratio ${{R_0^{H_2 O} } \mathord{\left/ {\vphantom {{R_0^{H_2 O} } {R_0^{D_2 O} }}} \right. \kern-0em} {R_0^{D_2 O} }} $ = 5.0±0.16 and 4.73±0.14 at H⁺ molar fraction of 0.48 and 0.16, respectively (H⁺ molar fraction was calculated based on the sum of [H⁺] and [D⁺]).     

Anomalously large kinetic isotope effect

Activated diffusion of water between macromolecules in swollen cellulose is accompanied by anomalously high kinetic isotope effects of oxygen. The separation factor of heavy-oxygen water (H₂ ¹⁸O /H₂ ¹⁶O) is thousands of permilles instead of tens of permilles according to modern Absolute Rate Theory. This anomalous separation under usual conditions is disguised by the opposing process of very fast equalization to equilibrium through water-filled cellulose pores. This process is quicker by approximately 3 orders of magnitude than diffusion through the cellulose body. As a consequence, this opposition-directed equalization virtually eliminates the results of isotope separation. To reveal this anomaly it is necessary to suppress equalization, which was the primary problem for both discovery of this anomaly and its investigation. The method of investigating the anomalous separation in cellulose was developed with suppression of this negative influence. Discussion of the theoretical nature of the anomalous kinetic isotope effect is presented. This theoretical study would probably permit the discovery and use for isotope separation of the anomalously high isotope effect for other chemical elements, in particular, for those heavier than oxygen.      

Thermal and state-selected rate constant calculations for O(3p) + H2 → OH + H and isotopic analogs

We present a new parametrization (based on ab initio calculations) of the bending potentials for the two lowest potential energy surfaces of the reaction O(³P) + H₂, and we use it for rate constant calculations by variational transition-state theory with multidimensional semiclassical tunneling corrections. We present results for the temperature range 250–2400 K for both the rate constants and the intermolecular kinetic isotope effects for the reactions of O(³P) with D₂ and HD. In general, the calculated rate constants for the thermal reactions are in excellent agreement with available experiments. We also calculate the enhancement effect for exciting H₂ to the first excited vibrational state. The calculations also provide information on which aspects of the potential energy surfaces are important for determining the predicted rate constants.     

Nonadiabatic relaxation dynamics of water anion cluster and its isotope effects by ring‐polymer molecular dynamics simulation

We have applied a recently developed hybrid quantum ring‐polymer molecular dynamics method to the nonadiabatic p → s relaxation dynamics in water anion clusters to understand the isotope effects observed in previous experiments. The average relaxation times for (H₂O)₅₀^? and (D₂O)₅₀^? were calculated at 120 and 207 fs, respectively, and are comparable to the experimental results. Therefore, we conclude that nuclear quantum effects play an essential role in understanding the observed isotope effects for water anion cluster nonadiabatic dynamics. The nonadiabatic relaxation mechanisms are also discussed in detail. © 2014 Wiley Periodicals, Inc.     

Iron(II) sulfate oxidation with oxygen on a Pt/C catalyst: A kinetic study

The kinetics of iron(II) sulfate oxidation with molecular oxygen on the 2% Pt/Sibunit catalyst was studied by a volumetric method at atmospheric pressure, T = 303 K, pH 0.33–2.4, [FeSO₄] = 0.06?0.48 mol/l, and [Fe₂(SO₄)₃] = 0?0.36 mol/l in the absence of diffusion limitations. Relationships were established between the reaction rate and the concentrations of Fe²⁺, Fe³⁺, H⁺, and Cl^? ions in the reaction solution. The kinetic isotope effect caused by the replacement of H₂O with D₂O and of H⁺ with D⁺ was measured. The dependence of Fe²⁺ and Fe³⁺ adsorption on the catalyst pretreatment conditions was studied. A reaction scheme is suggested, which includes oxygen adsorption, the formation of a Fe(II) complex with surface oxygen, and the one-electron reduction of oxygen. The last step can proceed via two pathways, namely, electron transfer with H⁺ addition and hydrogen atom transfer from the coordination sphere of the iron(II) aqua complex. A kinetic equation providing a satisfactory fit to experimental data is set up. Numerical values are determined for the rate constants of the individual steps of the scheme suggested.     

Kinetic isotope effect during acid hydrogen exchange in σ-benzyl-π-cyclopentadienyliron dicarbonyl

Conclusions The kinetic isotope effect K_D/K_T was measured during acid hydrogen exchange in -C₅H₅-(CO)₂Fe--CH₂C₆H₅. The obtained K_D/K_T value testifies in support of the generality of the mechanisms of hydrogen exchange in the benzene ring of this complex and in other derivatives of the benzene series.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 221–222, January, 1979.     

Infrared spectra (700–200 cm−1) and6Li/7Li and H2O/D2O isotope effects for four isotopically substituted lithium formate monohydrates

The infrared spectra, in the 700–200 cm^–1 region, have been reported for⁶LiHCO₂ · H₂O,⁶LiHCO₂ · D₂O,⁷LiHCO₂ · H₂O and⁷LiHCO₂ · D₂O and the observed fundamental bands have been discussed taking into account the⁶Li/⁷Li and H₂O/D₂O isotope wavenumber shifts on the fundamental vibrations.

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Infrarotspektren (700–200 cm^–1) und⁶Li/⁷Li- und H₂O/D₂O-Isotopeneffekte für vier isotopensubstituierte Lithiumformat-monohydrate

Zusammenfassung Die Infrarotspektren in der Region von 700–200 cm^–1 werden für⁶LiHCO₂ · H₂O,⁶LiHCO₂ · D₂O,⁷LiHCO₂ · H₂O und⁷LiHCO₂ · D₂O angegeben und die beobachteten Grundschwingungen zusammen mit den isotopischen Verschiebungen der Wellenzahlen diskutiert.