Rate coefficients at 297 K for proton transfer reactions with H2O. Comparisons with classical theories and exothermicity

Rate coefficients for proton transfer reactions of the type XH⁺ + H₂O → H₃O⁺ + X where X = H₂, CH₄, CO, N₂, CO₂ and N₂O and the type H₂O + X^? → XH + OH^? where X = H, NH₂ and C₂H₅NH have been measured at 297 K using the flowing afterglow technique. The results compare favourably with the predictions of the average-dipole-orientation theory. A trend is observed with exothermicity on a plot of (k_exp/k_ADO)_{298 K} versus ?ΔH_{298 K}⁰. The question is raised whether the relatively low probability observed for slightly exothermic proton transfer reactions is a consequence of reaction mechanism or results from the presence of a small activation energy barrier.     

SIFT studies of the reactions of H3O+, NO+ and O+2 with a series of volatile carboxylic acids and esters

We report the results of a selected ion flow tube (SIFT) study of the reactions of H₃O⁺, NO⁺ and O⁺₂ with some nine carboxylic acids and eight esters. We assume that all the exothermic proton transfer reactions of H₃O⁺ with all the acid and esters molecules occur at the collisional rate, i.e. the rate coefficients, k, are equal to k_c; then it is seen that k values for most of the NO⁺ and O⁺₂ reactions also are equal to or close to k_c. The major ionic products of the H₃O⁺ reactions with both the acids and esters are the protonated parent molecules, MH⁺, but minor channels are also evident, these being the result of H₂O elimination from the excited (MH⁺)1 in some of the acid reactions and an alcohol molecule elimination (CH₃OH or C₂H₅OH) in some of the ester reactions. The NO⁺ reactions with the acids and esters result in both ion-molecule association producing NO⁺M in parallel with hydroxide ion (OH⁻) transfer with some of the acids, and parallel methoxide ion (CH₃O⁻) and ethoxide ion (C₂H₅O⁻) transfer as appropriate with some of the esters. The O⁺₂ reactions proceed by dissociative charge transfer with the production of two or more ionic fragments of the parent molecules, the different isomeric forms of both the acid and the ester molecules resulting in different product ions.     

Electrochemical reactions with protonations at equilibrium: Part X. The kinetics of the p-benzoquinone/hydroquinone couple on a platinum electrode

The kinetics of the p-benzoquinone/hydroquinone Q/QH₂ couple on a platinum electrode are analysed on the basis of the theory presented earlier (E. Laviron, J. Electroanal. Chem., 146 (1983) 15) for the nine-member square scheme when the protonations are assumed to be at equilibrium, using experimental data from the literature. The square scheme is of the NN type. The Tafel plots and the variations of the experimental apparent rate constants between pH 0 and 7 are in good agreement with the theoretical predictions. The heterogeneous rate constants found for the elemental electrochemical steps are as follow: Q Q^?, k_h3=1/6×10^?3 cm s^?1; QH^.QH^?, k_h5=0.11 cm s^?1; QH⁺QH^., k_h2?160 cm s^?1; k_h4 for the reaction QH₂^+.QH₂ is in the range 0.5–4 cm s^?1. Between pH 0 and 7, the reaction sequence during the reduction is, for the most part, successively H⁺e^?H⁺e^?, e^?H⁺H⁺e^?, and e^?H⁺e^?H⁺ (reverse sequence during the oxidation).     

Temperature dependence of the reaction rate constants for O(3P) atoms with C2H4, C3H6 and NO(M = N2O), determined by a modulation technique

Rate constants for the reaction of O(³P) atoms with C₃H₄, C₃H₆ and NO(M = N₂O) have been measured over the temperature range 300–392°K using a modulation-phase shift technique. The Arrhenius expressions obtained are:C₂H₄, k₂ = 3.37 × 10⁹ exp[?(1270 ± 200)/RT]liter mole^?1 sec^?1,C₃H₆, k₂ = 2.08 × 10⁹ exp[?(0 ± 300)/RT]liter mole^?1 sec^?1,NO(M = N₂O), k₁ = 9.6 × 10⁹ exp[(900 ± 200/RT]liter² mole^?2 sec^?1.These temperature dependencies of k₂ are in good agreement with recent flash photolysis-resonance flourescence measurements, although lower than previous literature values.     

The reactions of some interstellar ions with benzene,cyclopropane and cyclohexane

The reactions of the cyclic molecules C₆H₆ (benzene), c-C₃H₆ (cyclopropane) and c-C₆H₁₂ (cyclohexane) with ArH⁺ (ArD⁺), H₃⁺, N₂H⁺, CH₅⁺, HCO⁺, OCSH⁺, C₂H₃⁺, CS₂H⁺ and H₃O⁺ have been studied at 300 K using a SIFT apparatus. All the reactions except those of C₂H₃⁺ proceed via proton transfer and all are fast except the H₃O⁺ and CS₂H⁺ reactions with c-C₆H₁₂ which are endothermic and which establish that the proton affinity of c-C₆H₁₂ is 160 ± 1 kcal mol⁻¹, which is considerably lower than the published value. In the c-C₃H₆ and the c-C₆H₁₂ reactions multiple products are observed and hence “breakdown curves” for the protonated molecules are constructed and the appearance energies of the various ion products are consistent with available thermochemical data. The reactions of C₂H₃⁺ with these cyclic molecules are atypical within this series of reactions in that they appear to proceed largely via hydride ion transfer. The implications of the results of this study to interstellar chemistry are alluded to.     

Structure of aminonitrones and electronic effect of substituents on their acid-base properties

A series of para-substituted aromatic aminonitrones p-RC₆H₄C(NH₂)=N⁺(Me)O^– (R = NMe₂, H, Br, Cl, CF₃) have been prepared. Acidity constants of the conjugate acids RC₆H₄C(NH₂)N⁺(Me)OH at 25°C in a EtOH–H₂O mixture (5: 95) have been determined by potentiometric titration. A linear correlation between log (k_R/k_H) and σ_para values has been revealed, and a ρ²⁹⁸(σ_para) parameter has been determined as of 0.635.     

Inner-sphere oxidation of iron(II) by tris(malonato)cobaltate(III)

The kinetics of oxidation of Fe²⁺ by [Co(C₃H₂O₄)₃]^3? in acidic solutions at 605 nm showed a simple first-order dependence in each reactant concentration. The second-order rate constant dependence on [H⁺] is in accordance with eqn (i) k₂ = k′₂ + k₃[H⁺] (i) where k′₂ and k₃ have values of 73.4 ± 14.0 M ^?1 s^?1 and 353 ± 41 M^?2 s^?1, respectively, at 1.0 M ionic strength (NaClO₄) and 25°C. At 310 nm the formation and decomposition of an intermediate, believed to be [FeC₃H₂O₄]⁺, was observed. The increase in the rate of oxidation with increasing [H⁺] was interpreted in terms of a “one-ended” dissociation mechanism which facilitates chelation of Fe₂₊ by the carbonyl oxygens of malonate in the transition state.     

The effects of isotopic substitution and competitive adsorption on the oxidation of aldehydes and alcohols at gold electrodes

The nature of the mechanism of the anodic oxidation of aldehydes in aqueous base on gold electrodes has been probed using isotopic substitution and competitive adsorption studies. A primary kinetic isotope effect of k_H/k_D=3?4 was observed upon substitution of deuterium for protium on the formyl group of benzaldehyde and cyclohexanecarboxaldehyde on gold in aqueous base using the techniques of cyclic voltammetry, rotating disc electrode voltammetry and chronoamperometry. Similar results are reported for the same aldehydes on a silver electrode, and also for the anodic oxidation of 2-propanol and 2-propanol-2-d on gold under similar conditions. Inhibition of the anodic cyclic voltammetric peak for benzaldehyde on gold by a variety of adsorbed species including CN^?, I^?, Br^?, (C₂H₅)₄N⁺ and diethylenetriamine is also described. These observations are used to propose a mechanism for the low potential oxidations of aldehydes and alcohols on gold involving a rate limiting dissociative adsorption step with cleavage of the α-carbon-hydrogen bond.     

Further studies of the N2+ + N2 → N4+ association reaction

Data are presented which strongly suggest that stabilisation of the excited intermediate (N₄⁺)* complex in the reaction (1) N₂⁺ + 2N₂ (rate coefficient k₁) occurs via N₂ switching whereas for (2) N₂⁺ + N₂ + He (rate coefficient k₂) it occurs via superelastic He collisions. This explains the differing temperature variations of k₁ and k₂ previously obtained for these reactions. Drift tube data are also presented which show how k₁ varies with N₂⁺/N₂ centre-of-mass energy as compared with thermal energy.     

Determination of hydrogen ion by ion chromatography (IC) with sulfonated cation-exchange resin as the stationary phase and aqueous EDTA (ethylenediamine-N,N,N′,N′-tetraacetic acid) solution as the mobile phase

An ion chromatographic (IC) method has been developed for determination of hydrogen ion (H⁺). It is based on the use of sulfonated cation-exchange resin as stationary phase, aqueous ethylenediamine-N,N,N′,N′-tetraacetic acid (dipotassium salt, EDTA-2K, written as K₂H₂Y) solution as mobile phase, and conductivity for detection. H⁺ was separated mainly by cation-exchange, but its elution was accelerated by the presence of EDTA. The order of elution for the model cations was H⁺ > Li⁺ > Na⁺ > NH₄ ⁺ > Ca²⁺ > > Mg²⁺. A sharp and highly symmetrical peak was obtained for H⁺ and this was attributed to the capacity of H₂Y₂ ^2– to receive and bind H⁺. H⁺ was detected conductiometrically and detector response (reduction in conductivity as a result of H⁺+H₂Y^2–→H₃Y^–) was linearly proportional to the concentration of H⁺ in the sample. The detection limit for H⁺ with this IC system was better than 4.7 μmol L^–1. A significant advantage of this method was the ability to separate and determine, in one step, H⁺ and other cations. The successful determination of H⁺ and other cation species in real acid-rain samples demonstrated the usefulness of this method.     

Kinetics and mechanism of the iodine oxidation of hydroxylamine

Studies of the stoichiometry and kinetics of the reaction between hydroxylamine and iodine, previously studied in media below pH 3, have been extended to pH 5.5. The stoichiometry over the pH range 3.4–5.5 is 2NH₂OH + 2I₂ = N₂O + 4I^? + H₂O + 4H⁺. Since the reaction is first-order in [I₂] + [I₃^?], the specific rate law, k₀, is k₀ = (k₁ + k₂/[H⁺]) {[NH₃OH⁺]₀/(1 + K_p[H⁺])} {1/(1 + K_I[I^?])}, where [NH₃OH⁺]₀ is total initial hydroxylamine concentration, and k₁, k₂, K_p, and K_I are (6.5 ± 0.6) × 10⁵ M^?1 s^?1, (5.0 ± 0.5) s^?1, 1 × 10⁶ M^?1, and 725 M^?1, respectively. A mechanism taking into account unprotonated hydroxylamine (NH₂OH) and molecular iodine (I₂) as reactive species, with intermediates NH₂OI₂^?, HNO, NH₂O, and I₂^?, is proposed.     

Die Kristallstruktur des Hydrazinium-monofluorids

The hydrazinium monofluoride N₂H₅F crystallizes with orthorhombic symmetry, space group P2₁2₁2₁,a=4.592 Å,b=8.217 Å,c=12.341 Å, and 8 formula units. The structure was determined by the permutation method in projection, and by the three-dimensionalPatterson function, refined by Full-Matrix-Least-Squares (R=0.056). The structure consists of N₂H₅ ⁺- and F^?-ions, bonded with hydrogen bonds N?H ... N and N?H ... F. Each N₂H₅ ⁺ ion is surrounded by four F^?-ions and two nitrogen atoms of two different N₂H₅ ⁺ ions. Each F^? ion is connected with four different N₂H₅ ⁺ ions.     

Kinetics of O(1D) interactions with the atmospheric gases N2, N2O,H2O,H2, CO2, and O3

Rate coefficients for collisional removal of O(¹D) by six atmospheric gases have been measured by monitoring the appearance of O(³P) following photolytic production of O(¹D). The measured values, k_M±2σ, in units of 10^?11 cm^?3 molecule ^?1 s^?1 are k_O3 = 22.8±2.3, k_N2 = 2.52 ± 0.25, k_CO2 = 10.4 ± 1.0,k_H2O 195± 2.0, k_N2O = 11.7 ± 1.2, and k_H2, = 11.8±1.2.     

Pulse radiolysis studies of intermolecular charge transfers involving tryptophan and three-electron-bonded intermediates derived from methionine

The oxidation processes of the radiation-generated, three-electron-bonded intermediates AcMet₂ [S??S]⁺ and AcMet [S??Br] were investigated by pulse radiolysis via their reactions with tryptophan (TrpH). These intermediates were derived from N-acetyl-methionine amide (N-AcMetNH₂) and N-acetyl-methionine methyl ester (N-AcMetOMe). The bimolecular rate constant k of the reaction between each intermediate and l-tryptophan (TrpH) was measured. For N-AcMetNH₂, k for the reaction of AcMet₂ [S??S]⁺ with TrpH were 3.4?×?10⁸ and 2.2?×?10⁸?dm³?mol^?1?s^?1 at pH?=?1 and 4.5, respectively. For N-AcMetOMe, k for the reaction of AcMet₂ [S??S]⁺ with TrpH were 4.0?×?10⁸ and 2.8?×?10⁸?dm³?mol^?1?s^?1 at pH 1 and 4.5, respectively. The rate constants for the intermolecular transformation of Met [S??Br] into TrpH⁺ or Trp were also estimated. For N-AcMetNH₂, k for the reaction of AcMet₂ [S??Br] with TrpH were 2.6?×?10⁸ and 3.3?×?10⁸?dm³?mol^?1?s^?1 at pH 1 and 4.5, respectively. Related mechanisms were discussed.     

Influence of the outer-sphere cation nature on the crystal structures of salts with a cis-[Rh(NH3)2(NO2)4]− anion

Complex rhodium(III) salts of the composition cis-M[Rh(NH₃)₂(NO₂)₄]·nH₂O, where M = K⁺, Cs⁺, Ag⁺, (CH₃)₄N⁺, are synthesized and described. Their molecular and crystal structures are determined by single crystal X-ray diffraction.     

Molecular fragmentations: Part IX. Structures and energies of mass spectral fragments derived from xanthan hydride, 5-amino-1,2,4-dithiazolin-3-thione

Structures and energies have been calculated, in the MNDO approximation, for xanthan hydride (C₂H₂N₂S₃) and its molecular cation, and for the mass spectral fragment ions H₂NCNCS⁺, HNCNCS⁺, CS₂⁺, H₂N₂CS⁺ (two isomers), HN₂CS⁺, S₂⁺, H₂NCS⁺ (three isomers), HNCS⁺ (two isomers), H₃N₂C₂⁺ (four isomers), CS⁺ and HNCS⁺² (two isomers), together with the corresponding neutral fragments.     

Crystal structure of hexaaqua(isocyanurato-O)calcium isocyanurate monohydrate

Crystals of [Ca(C₃H₂N₃O₃)(H₂O)₆]⁺ · (C₃H₂N₃O₃)^? · H₂O (I) are restudied by X-ray crystallography. The previous X-ray study is shown to be incorrect. The crystal structure of I (space group $P\bar 1$ , a = 6.503 Å, b = 10.830 Å, c = 11.824 Å, α = 103.58°, β = 92.64°, γ = 98.82°, Z = 2) is solved by a direct method and refined by the full-matrix least-squares method in the anisotropic approximation to R = 0.037 for 2792 independent reflections (CAD4 automated diffractometer, λMoK _α radiation). In the [Ca(C₃H₂N₃O₃)(H₂O)₆]⁺ complex cation, the coordination polyhedron of the Ca atom is a strongly distorted octahedron with one additional vertex. The crystal of I contains a developed three-dimensional system of interionic (intermolecular) hydrogen bonds involving all H atoms.     

Proton transfer reactions from H3+ ions to N2, O2, and CO molecules

The rate constants for proton transfer from H₃⁺ ions to N₂, O₂, and CO have been measured as function of hydrogen buffer gas partial pressure. The rate constant for proton transfer from H₃⁺ to N₂ shows a very large pressure dependence, increasing from 1.0 × 10^?9 cm³/s at low H₂ partial pressures to 1.7 × 10^?9 cm³/s at high H₂ partial pressures. The rate constants for proton transfer from H₃⁺ to O₂ and CO are constant with partial pressure of H₂; giving values of 6.4 × 10^?10 cm³/s and 1.7 × 10^?9 cm³/s, respectively. The roles of excess vibrational energy in H₃⁺ ions and of equilibrium between forward and back reaction are discussed. Back reaction is observed only for the reaction of H₃⁺ ions with O₂, and an equilibrium constant of K = 2.0 ± 0.4 at 298 K has been determined. From these data the proton affinity of O₂ is deduced to be 0.47 ± 0.11 kcal/mole higher than that of H₂.     

The oxidation of carbohydrazide by the 12-tungstocobaltate(III) ion in acidic medium: Kinetics and mechanism

The redox reaction between the 12-tungstocobaltate(III) ion and carbohydrazide is first order with respect to both the oxidant and the substrate. The observed pseudo first-order rate constant, k_obs, is retarded by increasing the concentrations of H⁺ and alkali metal ion (Li⁺, Na⁺ and K⁺). There is a linear correlation between the k_obs and the concentrations of carbohydrazide and H⁺ ion, but the plots of k_obs against the concentrations of the alkali metal ions is non-linear. However, the same data is applicable to the Davies equation for the effect of the ionic strength on the k_obs.     

Quelques precisions sur les effets catalytiques des micelles cationiques hydroxylees

The alkaline hydrolysis of p-nitrophenyl acetate and of CH₃CO₂(CH₂)₂N⁺(CH₃)₂C₁₆H₃₃, Br^? was studied in the presence of micelles C₁₆H₃₃N⁺(CH₃)₂CH₂CH₂OH, Br^? and CTAB, C₁₆H₃₃N⁺(CH₃)₃,Br^?. A pathway involving an intermediate is suggested for the hydrolysis of the ester. Hydrolysis rate of the intermediate in the presence of micelles is the same as hydrolysis rate for the ester in the absence of micelles. Consequently, hydrolysis of p-nitrophenyl acetate is not catalysed by one type of micelle, while it is enhanced by another type of micelle.