The ethylperoxy radical spectrum and rate constant for mutual interaction measured by flash photolysis and kinetic spectroscopy

Intrinsic spectral and kinetic parameters have been measured for the ethylperoxy radical, which was formed in the gas phase by the flash photolysis of azoethane in the presence of an adequate excess of oxygen. Absolute values of the extinction coefficient ?(λ) were derived from complementary measurements of the yield of nitrogen and the absorbance of an equivalent concentration of ethylperoxy radicals. The absorption spectrum is broad, structureless and comparatively weak; ?(236) = 1.02 × 10³ liter mole^?1 cm^?1 at the maximum, and the oscillator strength is 3.4 × 10^?2. This spectrum resembles the spectrum of the methylperoxy radical closely in form, but it is less intense; the ratio of the values of oscillator strength is 0.5. The bimolecular reactions of mutual interaction of ethylperoxy radicals are not exclusively terminating, and ethoxy and hydroperoxy radicals are formed in kinetically significant quantities. A computer program was designed to simulate the rise and fall of the concentration of each radical species, and to perform the related kinetic analysis. This program predicted that a second-order plot of the decline of the absorbance of the ethylperoxy radical during the dark period would not show a significant departure from linearity, a conclusion which was confirmed by experiment. Accordingly, the gradient of each such plot yielded a value of k'/?(λ), where k' is the apparent value of the rate constant for the collective reactions of mutual interaction. This rate constant was evaluated from the product of corresponding values of k'/?(λ) and ?(λ); individual values are independent of the wavelength of measurement, and the mean value is k' = (6.6 ± 0.5) × 10⁷ liter mole^?1 sec^?1. Further kinetic analysis yielded the corresponding absolute value: k = (6.0 ± 0.6) × 10⁷ liter mole^?1 sec^?1. This value fits the pattern of a relationship between rate constant and structure shown by the methylperoxy, isopropylperoxy, and tert-butylperoxy radicals. Adequate sensitivity for the characterization of the spectrum of the ethylperoxy radical was achieved by the use of a pulsed xenon arc as the monitoring light source in conjunction with a dual beam detection system with twin cells and balanced photomultipliers, and the apparatus is described in detail.     

The kinetics of ClO formation in the flash photolysis of chlorine–oxygen mixtures

The production of ClOO and ClO radicals following the flash photolysis of chlorine + oxygen mixtures has been studied. For the mechanism the following kinetic parameters were measured: k₃K = 1.3 × 10¹⁰ l²/mol²·sec; k₂/k₃ = 17; and k₃/?(ClOO; 250 nm) = 9.7 × 10⁵ cm/sec. Then k₃ = 5.9 × 10⁹ l/mol·sec, k₂ = 1.0 × 10¹¹ l/mol·sec, and ?(ClOO; 250 nm) = 6.1 × 10³ l/mol·cm. From limits established for the equilibrium constant K, ΔH°_f (ClOO) = 94 ± 2 kJ/mol.     

A quantitative study of alkyl radical reactions by kinetic spectroscopy. IV. The flash photolysis of azopropanes

The flash photolysis of azo?n?propane and of azoisopropane has been studied by kinetic spectroscopy. Transient absorption spectra in theregion of 220–260 nm have been assigned to the n-propyl and isopropyl radicals. For the n-propyl radical, ?_max = 744 ± 39 l/mol cm at 245 nm and the rate constants for the mutual reactions were measured to be k_c = (1.0 ± 0.1) × 10¹⁰ l/mol sec (combination) and k_d = (1.9 ± 0.2) × 10⁹ l/mol sec (disproportionation). For the isopropyl radical, ?_max = 1280 ± 110 l/mol cm at 238 nm, with k_c = (7.7 ± 1.6) × 10⁹ l/mol sec and k_d = (5.0 ± 1.2) × 10⁹ l/mol sec The rate constant for the dissociation of the vibrationally excited triplet state of the azopropanes into radicals was measured from the variation in the quantum yield of radicals with pressure. For azo-n-propane k = (6.6 ± 1.3) × 10⁷ sec^?1, and for azoisopropane k = (1.6 ± 0.4) × 10⁸ sec^?1. Collisional deactivation of the vibrationally excited singlet and triplet states was found to occur on every collision for n-pentane; but nitrogen and argon were inefficient with a rate constant of 1.1 × 10¹⁰ l/mol sec. Spectra observed in the region of 220–260 and 370–400 nm areattributed to the cis isomers of the parent trans-azopropanes. These are formed, as permanent products, in increasing amounts as the pressure is increased.     

The recombination of iodine atoms in the presence of nitric oxide

The recombination of iodine atoms following the flash photolysis of iodine in the presence of nitric oxide is interpreted through the mechanism with k₁ = 3.5 × 10⁹ l.²/mol²·sec; k₂ ≈ 1 × 10¹¹ l./mol·sec; k₃ = 2.1 × 10⁷ l./mol·sec at 298°K; E₃ = 11 kJ/ mol; and ΔH°₁ = 76 ± 6 kJ/mol. Lower and upper limits for the equilibrium constant are also established. The absorption spectrum of INO has been extended down to 223 nm and extinction coefficients for the region of 223–310 nm and 360–460 nm have been measured.     

Mutual combination of ClO radicals

The mutual combination reaction is proposed as the rate-limiting step in the removal of ClO radicals at moderate pressures. The third--order rate constants measured at room temperature were k₁(Ar) = 3.51 ± 0.14 × 10⁹ l²/mol²·ec; k₁(He) ≈ 2.8 × 10⁹ l²/mol²·sec, and k₁(O₂) ≈ 7.9 × 10⁹ l²/mol²·sec. There is also an independent second-order reaction for which k₃ ≈ 8 × 10⁶ l/mol·sec. A new absorption spectrum has been observed in the ultraviolet and attributed to Cl₂O₂. The extinction coefficient for Cl₂O₂ has been measured at six wavelengths, and, between 292 and 232 nm, it increases from 0.4 × 10³ to 2.9 × 10³ l/mol·cm. In the presence of the chlorine atom scavengers OClO or Cl₂O, Cl₂O₂ exists in equilibrium with ClO. The equilibrium constant Ke₁ = 3.1 ± 0.1 × 10⁶ l/mol at 298 K, and, with ΔS₁⁰ estimated to be ?133 ± 11 J/K·mol, ΔH₁⁰ = ?69 ± 3 kJ/mol and ΔH_f⁰(Cl₂O₂) = 136 ± 3 kJ/mol.     

INO thermodynamic properties and ultraviolet spectrum

The equilibrium I₂(g) + 2NO(g) = 2INO(g) has been studied at room temperature by ultraviolet absorption spectroscopy. The equilibrium constant has been measured as K_p = (2.7 ± 0.3) × 10^?6 atm^?1 at 298 K. Third-law calculations lead to ΔH°_f,298 (INO) = 120.0 ± 0.3 kJ/mol. The relative absorption spectrum of INO has been measured between 225 and 300 nm. Quantitative measurements gave ?(λ_max = 238 nm) = (1.79 ± 0.5) × 10⁴ L/mol·cm and ?(410 nm) = 234.7 ± 21 L/mol·cm.     

Kinetic flash spectroscopic study of the CH3O2?CH3O2 and CH3O2?SO2 reactions

Methylperoxy radicals were generated by the flash photolysis of azomethane–oxygen mixtures. The observed broadband spectrum of the CH₃O₂ radical is similar, but not identical to those reported previously. The CH₃O₂ decay followed second-order kinetics at high CH₃O₂ concentrations with k₄' = (2.5 ± 0.3) × 10⁸ liter/mol·sec (23 ± 2°C); 2CH₃O₂ → products (4). Because of the potential loss of CH₃O₂ through the reactions with HO₂ and CH₃O radicals subsequently formed in this system, simulations suggest that the true k₄ is in the range: 2.5 × 10⁸ ≥ k₄ ≥ 2.3 × 10⁸ liter/mol·sec. Deviations from linearity of the plot of the reciprocal of the CH₃O₂ absorbance versus time were seen at long times and were attributed to the reaction (5) with an apparent rate constant k₅' ? (1.6 ± 0.4) × 10⁵ liter/mol·sec; CH₃O₂ + Me₂N₂ → product (5). The CH₃O₂–SO₂ reaction, CH₃O₂ + SO₂ → products (16), was studied by observing CH₃O₂ decay in flashed mixtures of Me₂N₂, O₂, and SO₂. The results gave the apparent second-order rate constant k₁₆' ? (6.4 ± 1.4) × 10⁶ liter/mol·sec. It appears likely that each occurrence of reaction (5) and (16) is followed by the loss of an additional CH₃O₂ radical and that k₅ ? k₅'/2 and k₁₆ ? k₁₆'/2. Our findings suggest that a significant fraction of the SO₂ oxidation in a sunlight-irradiated NO_x?RH-polluted atmosphere, may occur by reaction with CH₃O₂ as well as from the HO and HO₂ reactions.     

Kinetics of iodination of hydrogen sulfide by iodine and the heat of formation of the SH radical

The kinetics of the gas-phase thermal iodination of hydrogen sulfide by I₂ to yield HSI and HI has been investigated in the temperature range 555–595 K. The reaction was found to proceed through an I atom and radical chain mechanism. Analysis of the kinetic data yields log k (l/mol·sec) = (11.1 ± 0.18) – (20.5 ± 0.44)/θ, where θ = 2.303 RT, in kcal/mol. Combining this result with the assumption E_?1 = 1 ± 1 kcal/mol and known values for the heat of formation of H₂S, I₂, and HI, ΔH_f,298⁰(SH) = 33.6 ± 1.1 kcal/mol is obtained. Then one can calculate the dissociation energy of the HS? H bond as 90.5 ± 1.1 kcal/mol with the well-known values for ΔH_f,298⁰ of H and H₂S.     

Absolute rate constants for the reactions between amino and alkyl radicals at 298 K

The absolute rate constants for the reactions of NH₂ radicals with ethyl, isopropyl, and t-butyl radicals have been measured at 298 K, using a flash photolysis–laser resonance absorption method. Radicals were generated by flashing ammonia in the presence of an olefin. A new measurement of the NH₂ extinction coefficient and oscillator strength at 597.73 nm was performed. The decay curves were simulated by adjusting the rate constants of both the reaction of NH₂ with the alkyl radical and the mutual interactions of alkyl radicals. The results are k(NH₂ + alkyl) = 2.5 (±0.5), 2.0 (±0.4), and 2.5 (±0.5) × 10¹⁰ M^?1·s^?1 for ethyl, isopropyl, and t-butyl radicals, respectively. The best simulations were obtained when taking k(alkyl + alkyl) = 1.2, 0.6, and 0.65 × 10¹⁰M^?1·s^?1 for ethyl, isopropyl, and t-butyl radicals, respectively, in good agreement with literature values.     

An absolute measurement of the rate constant for ethyl radical combination

An absolute value of k_r of ethyl radicals at 860 ± 17°K of 4.5 × 10⁹ M^?1·sec^?1 was determined under VLPP conditions, where the value of k_r/k_r^∞ should be about 1/2. Thus k_r^∞(M^?1·sec^?1) ～ 10¹⁰ at 860°K. An error of as much as a factor of 2 in k_r would be surprising, but possible. The value of 10¹⁰M^?1·sec^?1 seems to be a factor of from 2 to 5 too high to be compatible with extensive data on the reverse reaction and the accepted thermochemistry. Changes in the heat of formation and entropy of the ethyl radical can change the situation somewhat, but even these changes when applied to the work of Hiatt and Benson [3] indicate that ethyl combination should be ～ 10^9.3 M^?1·sec^?1. More work is necessary if a better value is desired.     

The mechanism of the photochemical reactions of SO2 with isobutane excited at 3130 Å

The mechanism of the reactions of electronically excited SO₂ with isobutane has been studied through the measurement of the initial quantum yields of product formation in 3130 Å irradiated gaseous binary mixtures of SO₂ and isobutane and ternary mixtures of SO₂, isobutane, C₆H₆ or CO₂. Under low-pressure conditions (P < 10 torr) the kinetic treatment of the present data shows that only one singlet and one triplet state, presumably the ¹B₁ and ³B₁ states, are involved in the photoreaction mechanism. The data give k_2a = 8.4 × 10⁹; SO₂(¹B₁) + isobutane → products (2a); k_5a ? k₅ = 8.7 × 10⁸ l./mol·sec; SO₂(³B₁) + isobutane → products (5a) SO₂(³B₁) + isobutane → (SO₂) + isobutane (5b) k_1a/k₁ = 0.145 ± 0.037; SO₂(¹B₁) + SO₂ → SO₂(³B₁) + SO₂ (1a) SO₂(¹B₁) + SO₂ → (2SO₂) (1b) k_2b/k₂ = 0.273 ± 0.018; SO₂(¹B₁) + isobutane → SO₂(³B₁) + isobutane (2b); SO₂(¹B₁) + isobutane → (SO₂) + isobutane (2c) error limits are ± 2 σ. The contribution from the excited SO₂(¹B₁) molecules to the quantum yields of the photolyses of SO₂–isobutane mixtures is not negligible. Under high-pressure conditions (P > 10 torr) the low-pressure mechanism coupled with the saturation effect on the phosphorescence lifetimes of SO₂(³B₁) molecules cannot alone rationalize the quantum yields. The evaluation suggests that some nonradiative intermediate state (X) is involved in the formation of “extra” triplet molecules. This ill-defined state decays largely nonradiatively to SO₂ in experiments at low pressures, X → SO₂ (12). In the presence of C₆H₆ the low-pressure data give k₇ = (8.5 ± 1.8) × 10¹⁰, and the high-pressure data give k₇ = (8.3 ± 0.6) × 10¹⁰ and (9.9 ± 0.9) × 10¹⁰l./mol·sec; SO₂(³B₁) + C₆H₆ → nonradiative products (7). These estimates are in good agreement with values directly measured from low-pressure lifetime studies, (8.1 ± 0.7) × 10¹⁰ and (8.8 ± 0.8) × 10¹⁰l./mol·sec.     

The oxidation of methyl radicals at room temperature

Using the technique of molecular modulation spectrometry, we have measured directly the rate constants of several reactions involved in the oxidation of methyl radicals at room temperature: k₁ is in the fall-off pressure regime at our experimental pressures (20–760 torr) where the order lies between second and third and we obtain an estimate for the second-orderlimit of (1.2 ± 0.6) × 10^?12 cm³/molec · sec, together with third-order rate constants of (3.1 ± 0.8) × 10^?31 cm⁶/molec² · sec with N₂ as third body and (1.5 ± 0.8) × 10^?30 with neopentane; we cannot differentiate between k_2a and k_2c and we conclude k_2a + (k_2c) = (3.05 ± 0.8) × 10^?13 cm³/molec · sec and k_2b = (1.6 ± 0.4) × 10^?13 cm³/molec · sec; k₃ = (6.0 ± 1.0) × 10^?11 cm³/molec · sec.     

Kinetic Studies on the Reaction of Sodium Hexacyanoferrate（ll） Complex with Peroxydisulfate Anion

The oxidation of Na₄Fe(CN)₆ complex by S₂O anion was found to follow an outer‐sphere electron transfer mechanism. We firstly carried out the reaction at pH=1. The specific rate constants of the reaction, k_ox, are (8.1±0.07)×10^?2 and (4.3±0.1)×10^?2 mol^?1·L·s^?1 at μ=1.0 mol·L^?1 NaClO₄, T=298 K for pH=1 (0.1 mol·L^?1 HCl0₄) and 8, respectively. The activation parameters, obtained by measuring the rate constants of oxidation 283–303 K, were ΔH^≠=(69.0±5.6) kJ·mol^?1, ΔS^≠=(?0.34±0.041)×10² J·mol^?1·K^?1 at pH=l and ΔH^≠=(41.3±5.5) kJ·mol^?1, ΔS^≠=(?1.27±0.33)×10² J·mol^?1·K^?1 at pH=8, respectively. The cyclic voltammetry of Fe(CN) shows that the oxidation is a one‐electron reversible redox process with E_1/2 values of 0.55 and 0.46 V vs. normal hydrogen electrode at μ=1.0 mol·L^?1 LiClO₄, for pH=1 and pH=8 (Tris). respectively. The kinetic results were discussed on the basis of Marcus theory.     

Reactions of the IO· radical resulting in hydrogen atom abstraction from halogen-containing molecules

A jet-stream kinetic technique and the resonance fluorescence method applied to detection of iodine atoms were used to measure the rate constants of the reactions of the IO^· radical with the halohydrocarbons CHFCl-CF₂Cl (k = (3.2 ± 0.9) × 10^?16 cm³ molecule s^?1) and CH₂ClF (k = (9.4 ± 1.3) × 10^?16 cm³ molecule s^?1), the hydrogen-containing haloethers CF₃-O-CH₃ (k = (6.4 ± 0.9) × 10^?16 cm³ molecule s^?1) and CF₃CH₂-O-CHF₂ (k = (1.2 ± 0.6) × 10^?15 cm³ molecule s^?1), and hydrogen iodide (k = (1.3 ± 0.9) × 10^?12 cm³ molecule s^?1) at 323 K.     

Pyrolytic production of Se (3P) from carbon diselenide. II. Rate of CSe2 decay

Pyrolytic decay of carbon diselenide was monitored by ultraviolet absorption spectroscopy in reflected shock waves in the temperature range of 1600–2600°K. The temperature dependence of the absorption coefficient of CSe₂ at 2308 Å was determined and was used to provide kinetic information along with a deconvolution procedure which accounted for and removed systematic distortions of the fast time-resolved absorbance profile. For temperatures of 1600–2600°K and argon densities of 1.5–7.0 × 10^?5 mol/cm³ dilute (1.0–9.0 × 10^?9 mol/cm³) CSe₂ pyrolyzed with measured first-order decay rates in the range of log₁₀ k₁ (sec^?1) = 3.0?5.7; at midrange (2100°K and 4.3 × 10^?5 mol/cm³ in Ar) k₁ ≈ 3 × 10⁴ sec^?1. The decay probably occurs via a unimolecular low-pressure process, first order in both CSe₂ and Ar, for which k₂ ± 10⁹ cm³/mol·sec at 2100°K. The deconvoluted data yield Arrhenius activation energies of 53.2 kcal/mol under second-order treatment, but the activation energy is less reliable than the general magnitude of the rate constant. A comparison of CSe₂ with other molecules which are isoelectronic in their valence shells (CO₂, CS₂, OCS, and N₂O) is made.     

Quenching of biacetyl (3Au) molecules by near-resonant and off-resonant collisional partners

Laser-induced time-resolved phosphorescence has been used to evaluate the quenching of gaseous biacetyl (³A_u) molecules by various molecules at 25°C. The quenching of biacetyl (³A_u) molecules by biacetyl itself was not detectable under our experimental conditions, and a pressure-independent lifetime of 1.70 ± 0.08 msec was found. The bimolecular rate constants (units of l/mol·sec) for quenching of the ³A_u molecules by cis-2-pentene, trans-2-pentene, cis-1,3-pentadiene, trans-1,3-pentadiene, and oxygen were found to be (3.3 ± 1.9) × 10³, (4.0 ± 0.2) × 10⁴, (3.9 ± 0.1) × 10⁸, (1.3 ± 0.1) × 10⁸, and (5.2 ± 0.4) × 10⁸, respectively.     

Determination of Kinetic Parameters of Ethyl Methacrylate Polymerization Initiated by the Redox Pair SO2-tert-Butyl Hydroperoxide

Abstract

In the bulk polymerization of ethyl methacrylate with the redox initiator pair sulfur dioxide-tert-butyl hydroperoxide (SO₂-TBHP), the kinetic parameters were determined by the dead-end polymerization technique using the dilatometric method. Polymerization was conducted with various initiator pair compositions in the temperature interval of 12–35°C. An activation energy of 14.1 kJ/mol for [SO₂]/[TBHP] = 0.44 was determined for this temperature range. The values of k ² _p/k _t obtained in this study were in the interval 1.34 × 10^?4 to 1.11 × 10^?3 L/mol·s. The f/k _d ratios for the redox pair at different temperatures and for different initiator ratios were also calculated. The f/k _d ratios of the initiator pair changed between 15.1 and 187.6 seconds. The wide variations in these kinetic parameters were explained on the basis of competitive reactions between the redox pair and their reaction products.     

Kinetics of OH radical reactions with a series of ethers

The rate constants for the reactions of OH with dimethyl ether (k₁), diethyl ether (k₂), di-n-propyl ether (k₃), di-isopropyl ether (k₄), and di-n-butyl ether (k₅) have been measured over the temperature range 230–372 K using the pulsed laser photolysis-laser induced fluorescence (PLP-LIF) technique. The temperature dependence of k₁,k₄, can be expressed in the Arrhenius plots form: k₁ = (6.30 ± 0.10) × 10^?12 exp[?(234 ± 34)/T] and k₄ = (4.13 ± 0.10) × 10^?12 exp[(274 ± 26)/T]. The Arrhenius plots for k₂,k₃, and k₅, were curved and they were fitted to the three parameter expressions: k₂ = (1.02 ± 0.08) × 10^?17 T² exp[(797 ± 24)/T], k₃ = (1.84 ± 0.23) × 10^?17T² exp[(767 ± 34)/T], and k₅ = (6.29 ± 0.74) × 10^?18T² exp[(1164 ± 34)/T]. The values at 298 K are (2.82 ± 0.21) × 10^?12, (1.36 ± 0.11) × 10^?11,(2.17 ± 0.16) × 10^?11, (1.02 ± 0.10) × 10^?11, and (2.69 ± 0.22) × 10^?11 for k₁, k₂, k₃, k₄, and k₅, respectively, (in cm³ molecule^?1 s^?1). These results are compared to the literature data. © 1995 John Wiley & Sons, Inc.     

Kinetic study of the oxidation reaction of 4-methylphenyl radical

A kinetic study of the reaction of the 4-methylphenyl radical (4-C₆H₄CH₃) with the oxygen molecule was conducted using experimental and theoretical approaches. The absorption spectrum for the λ = 266 nm photolysis of the 4-C₆H₄CH₃X (X = Cl, Br)/N₂/O₂ mixture was measured in the wavelength range of λ = 503-512 nm using N₂ as the buffer gas at a total pressure of 40 Torr using a cavity ring-down spectroscopy apparatus coupled with a pulsed laser photolysis system. Based on the absorbance of the product of the 4-C₆H₄CH₃ + O₂ reaction at λ = 504 nm, the reaction rate coefficient for the 4-C₆H₄CH₃ + O₂ reaction was determined to be k = (1.21 ± 0.10) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ and k = (1.18 ± 0.21) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ using 4-C₆H₄CH₃Cl and 4-C₆H₄CH₃Br, respectively, as the radical precursor. And there was no pressure dependence in the total pressure range of 10-90 Torr varying partial pressure of N₂ buffer gas at T = 296 ± 5 K. The geometries, vibration frequencies, and potential energy surfaces of the reactants, major products, and transition states in the 4-C₆H₄CH₃ + O₂ reaction were determined using the CBS-QB3 method. The k value at the high-pressure limit was calculated to be 1.26 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ using the variational transition-state theory. The calculated value of k was consistent with the experimental value, which indicated that the 4-C₆H₄CH₃ + O₂ reaction reaches the high-pressure limit at 10 Torr. Therefore, the oxidation of the 4-C₆H₄CH₃ radical is almost 10 times faster than that of the benzyl radical, which has the same chemical formula, at the high-pressure limit.     

Reactions of isopropylperoxy radicals with NO and NO2

The rate constants for the reactions have been measured directly by flash photolysis and kinetic spectroscopy. At room temperature, k₃ = (3.4 ± 0.1) × 10⁹ L/mol·s, independent of pressure in the range of 55–400 torr, and k₆ = (2.1 ± 0.2) × 10⁹ L/mol·s.