Kinetic solvent isotope effects (KSIE) were measured for the hydrolyses of acetals of benzaldehydes in aqueous solutions covering the pH (pD) range of 1–6. For p-methoxybenzaldehyde diethyl acetal, k/k = 1.8–3.1, depending on the procedure used to calculate the KSIE and on the pH (pD) range used as the basis for k(k). It is shown that this variation is an experimental artifact, and is a characteristic of KSIE measurements in general. It is recommended that k be calculated from a least-squares fit of data to the equation kobs = k[L+], and that the KSIE be reported as k/k. The limitation remains, however, that the KSIE measured for a variety of substances over quite different pH (pD) ranges may not be comparable to more than ?20%. The source of these observations is discussed in terms of small changes in the activity coefficient ratios (a specific salt effect), including the solvent isotope effect on the activity coefficient ratio [eq. (3)]. 相似文献
Reactions of oxygen atoms with ethylene, propene, and 2-butene were studied at room temperature under discharge flow conditions by resonance fluorescence spectroscopy of O and H atoms at pressures of 0.08 to 12 torr. The measured total rate constants of these reactions are K = (7.8 ± 0.6)·10?13cm3s?1,K = (4.3 ± 0.4) ± 10?12 cm3 s?1, K = (1.4 ± 0.4) · 10?11 cm3 s?1. The branching ratios of H atom elimination channels were measured for reactions of O atoms with ethylene and propene. No H-atom elimination was found for the reaction of O-atoms with 2-butene. A redistribution of reaction O + C2 channels with pressure was found. A mechanism of the O + C2 reaction was proposed and the possibility of its application to other olefins is discussed. On the basis of mechanism the pressure dependence of the total rate constant for reaction O + C2 was predicted and experimentally confirmed in the pressure range 0.08–1.46 torr. 相似文献
Ultraviolet absorption spectra have been characterized for the acetyl-h3 and acetyl-d3 radicals, which were generated by the flash photolysis of the corresponding acetones. The spectra are broad and intense, with values of the extinction coefficient at the respective maxima estimated as: ?CH3CO(215) = (1.0 ± 0.1) × 104 L/mol·cm and ?CD3CO(207.5) = (1.0 ± 0.05) × 104 L/mol·cm. Rate constants for the reactions of mutual interaction were estimated as: k = 3.5 × 1010 L/mol·s and k = 3.4 × 1010 L/mol·s. Rate constants for the reactions of cross interaction were estimated as: k = 8.6 × 1010 L/mol·s and k = 5.2 × 1010 L/mol·s. The related values of the cross interaction ratios k/(kk)1/2 = 2.6 and k/(kk)1/2 = 1.6 do not differ significantly from the statistical value of 2. The participation of the radical displacement reactions was estimated in terms of the fractions k/k = 0.38 and k/k = 0.47. Corroborative spectra were obtained from the flash photolysis of methyl ethyl ketone and biacetyl, and the relative rates of the competing primary processes were estimated from the relative peak heights of the acetyl and methyl radicals in each system. 相似文献
Hexafluoroacetone (HFA) and O2 were photolyzed at 147.0 nm to investigate their use in chemical actinometry. The products, CO for the former and O3 in the latter case, were monitored. For accurate comparison, both of these substances were irradiated by a single light source with two identical reaction cells at 180° to each other. The light intensities I were measured under the same integrated as well as instantaneous photon flux based on ? and ?CO (quantum yield) as 2 and 1, respectively. Optimum conditions for maximum product yield were 5.0 torr HFA pressure and an O2 flow rate of 200 ml/min at 1 atm pressure for a 20-minute photolysis period. For light intensity variations between 1.09 × 1014 and 2.10 × 1015 photons absorbed/sec, the ratio I/IHFA was found to be unity. Calibration with the commonly used N2O actinometer for a ? value of 1.41 showed that I/IHFA and I/I are unity. Both HFA and O2 are suitable chemical actinometers at 147.0 nm with ?CO and ? of 1 and 2, respectively. The light intensity determination in the first case involves the measurement of only one product which is noncondensible at 77°K, whereas wet analysis for O3, the only product, in the second actinometer is necessary. Both of these determinations are quite simple and are preferable over product analysis in N2O actiometry, wherein N2 separation from other noncondensibles at 77°K is required. 相似文献
Studies of the reaction of Br + propylene to produce HBr and allyl radical were made using VLPR (Very Low Pressure Reactor) over the range 263–363 K. Apparent bimolecular rate constants k were found to vary in an inverse manner with the initial concentration of bromine atoms introduced into the reactor. Plots of k against [Br] give straight lines whose intercepts were taken to be the true bimolecular, metathesis rate constant k1. The reaction scheme is where k2 ? k1 and k?1 [HBr] is negligibly small under our conditions. Arrhenius parameters for k1 were assigned for linear and bent transition states and shown to give excellent fits to the observed intercepts. where θ = 2.303 RT (kcal mol?1). The dependence of k on [Br] is accounted for in terms of the reactivity of Br* (2P1/2) produced in the microwave discharge. The activation energy for the metathesis reaction of Br* with propylene is shown to be very small. 相似文献
The mechanism of the photolysis of formaldehyde was studied in experiments at 3130 Å and in the pressure range of 1–12 torr at 25°C. The experiments were designed to establish the quantum yields of the primary decomposition steps (1) and (2), CH2O + hν → H + HCO (1): CH2O + hν → H2 + CO (2), through the effects of added isobutene, trimethylsilane, and nitric oxide on ΦCO and Φ. The ratio ΦCO/Φ was found to be 1.01 ± 0.09(2σ) and (Φ + ΦCO)/2 = 1.10 ± 0.08 over the range of pressures and a 12-fold change in incident light intensity. Isobutene and nitric oxide additions reduced Φ to about the same limiting value, 0.32 ± 0.03 and 0.34 ± 0.04, respectively, but these added gases differed in their effects on ΦCO. With isobutene addition ΦCO/Φ reached a limiting value of 2.3; with NO addition ΦCO exceeded unity. The addition of small amounts of Me3SiH reduced Φ to 1.02 ± 0.08 and lowered ΦCO to 0.7. These findings were rationalized in terms of a mechanism in which the “nonscavengeable,” molecular hydrogen is formed in reaction (2) with ?2 = 0.32 ± 0.03, while the “free radical” hydrogen is formed in reaction (1) with ?1 = 0.68 ± 0.03. In the pure formaldehyde system these reactions are followed by (3)–(5): H + CH2O → H2 + HCO (3); 2HCO → CH2O + CO (4); 2HCO → H2 + 2CO (5). The data suggest k4/k5 ? 5.8. Isobutene reduced Φ by the reaction H + iso-C4H8 → C4H9 (20), and the results give k20/k3 ? 43 ± 4, in good agreement with the ratio of the reported values of the individual constants k3 and k20. 相似文献
In the radiolysis of water vapor containing small concentrations of cyclohexane, the principal products which account for about 98% of all end products are found to be hydrogen, cyclohexene, and bicyclohexyl. Cyclohexene and bicyclohexyl yields were determined over a range of temperatures (70–200°C), total pressures (50–2400 torr), and total doses (0.15–2.0 Mrad). The disproportionation–combination ratio k/k for c-C6H11 radicals could be determined as 0.56 ± 0.01 from the ratio of cyclohexene to bicyclohexyl yield. By using c-C6D12, the ratio k/k for c-C6D11 radicals is found to be 0.38 ± 0.01. Comparison of the reactivity pattern of C6H11 and C6D11 radicals leads to (k)/(k)/(k/k) = 1.47 ± 0.02. The corresponding values for the reactions of c-C6H11 with c-C6D11 were also determined. 相似文献
The extinction coefficients and the decay kinetics of I and (SCN) have been characterized over the 15–90°C-temperature range. The extinction coefficients of I at 385 and 725 nm were determined to be 10,000 and 2560M?1 cm?1, respectively, based on the extinction coefficient of (SCN) at 475 nm being equal to 7600M?1 cm?1. At these three wavelengths, all extinction coefficients were constant over the temperature range studied. The rate of decay of both I and (SCN) was found to be a function of I? and SCN? concentration, respectively, as well as temperature. 相似文献
Kinetics of the complex formation of chromium(III) with alanine in aqueous medium has been studied at 45, 50, and 55°C, pH 3.3–4.4, and μ = 1 M (KNO3). Under pseudo first-order conditions the observed rate constant (kobs) was found to follow the rate equation: Values of the rate parameters (kan, k, KIP, and K) were calculated. Activation parameters for anation rate constants, ΔH≠(kan) = 25 ± 1 kJ mol?1, ΔH≠(k) = 91 ± 3 kJ mol?1, and ΔS≠(kan) = ?244 ± 3 JK?1 mol?1, ΔS≠(k) = ?30 ± 10 JK?1 mol?1 are indicative of an (Ia) mechanism for kan and (Id) mechanism for k routes (‥substrate Cr(H2O) is involved in the k route whereas Cr(H2O)5OH2+ is involved in k′ route). Thermodynamic parameters for ion-pair formation constants are found to be ΔH°(KIP) = 12 ± 1 kJ mol?1, ΔH°(K) = ?13 ± 3 kJ mol?1 and ΔS°(KIP) = 47 ± 2 JK?1 mol?1, and ΔS°(K) = 20 ± 9 JK?1 mol?1. 相似文献
The activation energy parameters for the reaction of PdX (X=Cl?, Br?) in aqueous halide acid solution with thiourea (tu) and selenourea (seu) have been determined. High rates of reaction parallel low enthalpies and appreciable negative entropy of activation. The rate law in each case simplifies to kobs=k[L] where L=tu or seu, and only ligand-dependent rate constants are observed at 25°C. The ligand-dependent rate constants for the first identifiable step in the PdCl + X system is (9.1±0.1) × 103M?1 sec?1 and (4.5±0.1) × 104M?1 sec?1 for X=tu and seu, respectively, while for the PdBr + X system it is (2.0±0.1) × 104M?1 sec?1 and (9.0±0.1) × 104M?1 sec?1 for X=tu and seu, respectively. 相似文献
The hexacyanoferrate(III)-thallium(I) reaction in aqueous acetic acid containing large concentrations of hydrochloric acid is considerably accelerated both by hydrogen and chloride ions as well as increasing acetic acid in the medium. The experimental results obey the rate law (1) where β1 to β6 are the cumulative stability constants of the species TlCl, TlCl, TlCl, HFe(CN), H2Fe(CN) and H3Fe(CN)6 respectively and ka and kb are the rate constants associated with the mono- and di-protonated oxidant species. The main active species are H2Fe(CN) and TlCl. 相似文献
The absolute rate constants have been measured for several gas-phase chlorine atom-molecule reactions at 25°C by resonance fluorescence. These reactions and their corresponding rate constants in units of cm3 mole?1 sec?1 are: The effects of varying the substrate pressure, total pressure, light intensity and chlorine-atom source on the value of the bimolecular rate constants have been investigated for all these reactions. Conditions under which no competing side reaction occurs were established and the reported rate constants were measured under these conditions. For reactions (2), (5), (6), (7), and 8, there is a discrepancy of a factor of two between the rate constants measured in this work and values in the literature; it is suggested that this is due to an error in the previously measured value of k/k upon which the relative measurements in the literature ultimately depend. 相似文献
Rate constants for the reaction O(3P) + SO2 + M have been determined over the temperature range of 299°–440°K, using a flash photolysis–NO2 chemiluminescence technique. For M?Ar, the Arrhenius expression was obtained. At room temperature k2Ar = (1.05 ± 0.21) × 10?33 cm6/molec2·sec. In addition, the rate constants k2 = (1.37 + 0.27) × 10?33 cm6/molec2·sec, k2 = (9.5 ± 3.0) ± 10?33 cm6/molec2·sec, k3 = (1.1 ± 0.2) ± 10?31 cm6/molec2·sec, and k3 = (2.6 ? 0.9) ± 10?31 cm6/molec2·sec were obtained at room temperature where k3M is the rate constant for the reaction O + NO + M → NO2 + M. The rate data are compared and discussed with literature values. 相似文献
A kinetic spectrophotometric investigation of the reaction of the hydrogen peroxide anion with methyl p-nitrophenyl sulfate in methanol solvent resulted in the evaluation of the pKa of HOOH in methanol at 25°C as 15.8 ± 0.2. Since normal kinetic procedures for the determination of the equilibrium constant K for the process CH3O? + H2O2 ? CH3OH + HO were found to be associated with high uncertainty, another procedure was devised to establish the magnitude of K. This method is based on an analysis of the changing slopes of plots of pseudo-first-order rate constants against the total base concentration as the stoichiometric amount of hydrogen peroxide is varied. The method is applicable to any system in which anionic nucleophiles generated in situ compete with solvent anions. Such a corroboration of kinetically determined equilibrium constants is believed essential. The kinetic data allow the specific rate constant kHOO-for the reaction of methyl p-nitrophenyl sulfate with hydrogen peroxide anions to be evaluated and yield the rate constant ratio k/k = 8.8 ± 2.2. This confirms the existence of an α effect at saturated carbon in this system. 相似文献
The kinetics of the permanganate oxidation of formic acid in aqueous perchloric acid has been studied. The results indicate that this reaction is autocatalyzed by both manganese(II) ion (formed as a reaction product) and colloidal manganese dioxide (formed as an intermediate). The apparent rate constants corresponding to the noncatalytic and autocatalytic reaction pathways are given, respectively, by the following equations The activation energies associated with the true rate constants, ??, ??, ??, ??, ??, and ?? are 37.2, 62.5, 70.9, 52.5, 40.8, and 59.9 kJ mol?1, respectively. The percentage of the total reaction corresponding to each pathway is given for typical experimental conditions. Mechanisms in agreement with the kinetic data are proposed for the six different reaction pathways observed. 相似文献