Kinetics of the photoaquation of Hexacyanoferrate(II) ion

Kinetics of the photoaquation of hexacyanoferrate(II) ion in aqueous solution were studied potentiometrically and spectrophotometrically. Supposing the simplest mechanism (see Fig. 3. in text), the photoaquation in alkaline medium can be well described. The value of the constants at pH = ll.0 are: ø = 0.8-1.0, k₆ = (3.0 ± 0.5) × 10^?8 s^?1 and k_?6 = 1.5 ± 0.2 mol^?1 dm₃ s^?1. To describe the photoaquation in neutral medium t was extended (k′ = 3.33 x 10² mol^?1 dm³s^?1). The quantum yield in acidic medium can be calculated by combination of ø values of different protonated complexes. The reversibility of photoaquation in alkaline medium is also explained by the scheme.     

Fluorescence quenching in molecular recognition by hydrogen bonding

Steady-state fluorescence and single photon timing have been used to study the effect of the presence of hydrogen bonding on the intermolecular quenching of pyrene covalently linked to a guanine-like receptor I by an aliphatic amine (N,N-dimethylpropylamine) covalently linked to cytosine derivative II. By comparing the fluorescence quenching of I by II with that of 10methylpyrene (1-MP) by triethylamine (TEA), as a model system in which no hydrogen bonding can occur, one could possibly analyze the effect of the hydrogen bonding between receptor and substrate as a quenching as it leads to a higher local concentration of donor and acceptor. While the quenching of I by II was observed with an apparent rate constant k_q of (1.78 ± 0.10) × 10⁹ M⁻¹ s⁻¹ and (8.72 ± 0.42) × 10⁸ M⁻¹ s⁻¹ in toluene and acetonitrile, respectively, no quenching could be observed in methanol. Upon excitation of 1-MP, no quenching by II could be detected in the same concentration range as used in the quenching of I. Quenching of I and of 1-MP by TEA (⩾ 10⁻² M) in toluene leads to exciplex formation with maxima centred at 540 and 514 nm, respectively. The rate constants of exciplex formation and dissociation of I with TEA were analyzed using a global compartmental analysis. The following values were obtained for the rate constants: k₀₁ = (9.70 ± 0.01) × 10⁶ s⁻¹, k₂₁ = (1.12 ± 0.003) × 10⁹ M⁻¹ s⁻¹, k₀₂ = (5.24 ± 0.01) × 10⁷ s⁻¹ and k₁₂ = (7.74 ± 0.08) × 10⁶ s⁻¹. Quenching of I by TEA in the presence of III, a hydrogen-bonding system without an alkyl amine substituent, leads to exciplex formation centred at 538 nm. The rate constant values for the exciplex formation and dissociation of I with TEA in the presence of III were: k₀₁ = (9.32 ± 0.08) × 10⁶ s⁻¹, k₂₁ = (9.32 ± 0.003) × 10⁸ M⁻¹ s⁻¹, k₀₂ = (6.16 ± 0.03) × 10⁷ s⁻¹ and k₁₂ = (21.90 ± 0.3) × 10⁶ s⁻¹. The apparent rate constants k_q for this system was (7.26 ± 0.56) × 10⁶ M⁻¹ s⁻¹. The observed decrease in the rate of exciplex formation of I with TEA in the presence of III could suggest that the guanine-like moiety in I forms hydrogen bonds with the cytosine-like moiety and this could decrease the electron affinity of I. The rate constant of exciplex dissociation increased, indicating that the exciplex is less stable in the presence of III. Because of the single exponential decay of I in the presence and absence of II and of the agreement between steady-state and transient fluorescence measurements, the information available for quantitative analysis of the association between I and II is limited.     

Rate coefficients for the reactions of chlorine atoms with methanol and acetaldehyde

Rate coefficients have been measured for the reactions of Cl atoms with methanol (k₁) and acetaldehyde (k₂) using both absolute (laser photolysis with resonance fluorescence) and relative rate methods at 295 ± 2 K. The measured rate coefficients were (units of 10⁻¹¹ cm³ molecule⁻¹ s⁻¹): absolute method, k₁ = (5.1 ± 0.4), k₂ = (7.3 ± 0.7); relative method k₁ = (5.6 ± 0.6), k₂ = (8.4 ± 1.0). Based on a critical evaluation of the literature data, the following rate coefficients are recommended: k₁ = (5.4 ± 0.9) × 10⁻¹¹ and k₂ = (7.8 ± 1.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ (95% confidence limits). The results significantly improve the confidence in the database for reactions of Cl atoms with these oxygenated organics. Rate coefficients were also measured for the reactions of Cl₂ with CH₂OH, k₅ = (2.9 ± 0.6) × 10⁻¹¹ and CH₃CO, k₆ = (4.3 ± 1.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, by observing the regeneration of Cl atoms in the absence of O₂. Based on these results and those from a previous relative rate study, the rate coefficient for CH₃CO + O₂ at the high pressure limit is estimated to be (5.7 ± 1.9) × 10⁻¹² cm³ molecule⁻¹ s⁻¹. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 776–784, 1999     

Cavity ring‐down spectroscopic study of the kinetics of the reactions of FCO radicals with O2 and NO at 295 K

Cavity ring‐down UV absorption spectroscopy was used to study the kinetics of the recombination reaction of FCO radicals and the reactions with O₂ and NO in 4.0–15.5 Torr total pressure of N₂ diluent at 295 K. k(FCO + FCO) is (1.8 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. The pressure dependence of the reactions with O₂ and NO in air at 295 K is described using a broadening factor of Fc = 0.6 and the following low (k₀) and high (k_∞) pressure limit rate constants: k₀(FCO + O₂) = (8.6 ± 0.4) × 10⁻³¹ cm⁶ molecule⁻¹ s⁻¹, k_∞(FCO + O₂) = (1.2 ± 0.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, k₀(FCO + NO) = (2.4 ± 0.2) × 10⁻³⁰ cm⁶ molecule⁻¹ s⁻¹, and k_∞ (FCO + NO) = (1.0 ± 0.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹. The uncertainties are two standard deviations. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 130–135, 2001     

Rate coefficients for the gas‐phase reaction of OH radicals with selected dihydroxybenzenes and benzoquinones

Rate coefficients have been determined for the gas‐phase reaction of the hydroxyl (OH) radical with the aromatic dihydroxy compounds 1,2‐dihydroxybenzene, 1,2‐dihydroxy‐3‐methylbenzene and 1,2‐dihydroxy‐4‐methylbenzene as well as the two benzoquinone derivatives 1,4‐benzoquinone and methyl‐1,4‐benzoquinone. The measurements were performed in a large‐volume photoreactor at (300 ± 5) K in 760 Torr of synthetic air using the relative kinetic technique. The rate coefficients obtained using isoprene, 1,3‐butadiene, and E‐2‐butene as reference hydrocarbons are k_OH(1,2‐dihydroxybenzene) = (1.04 ± 0.21) × 10⁻¹⁰ cm³ s⁻¹, k_OH(1,2‐dihydroxy‐3‐methylbenzene) = (2.05 ± 0.43) × 10⁻¹⁰ cm³ s⁻¹, k_OH(1,2‐dihydroxy‐4‐methylbenzene) = (1.56 ± 0.33) × 10⁻¹⁰ cm³ s⁻¹, k_OH(1,4‐benzoquinone) = (4.6 ± 0.9) × 10⁻¹² cm³ s⁻¹, k_OH(methyl‐1,4‐benzoquinone) = (2.35 ± 0.47) × 10⁻¹¹ cm³ s⁻¹. This study represents the first determination of OH radical reaction‐rate coefficients for these compounds. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 696–702, 2000     

A laser flash photolysis kinetic study of reactions of the Cl2− radical anion with oxygenated hydrocarbons in aqueous solution

A laser photolysis–long path laser absorption (LP‐LPLA) experiment has been used to determine the rate constants for H‐atom abstraction reactions of the dichloride radical anion (Cl₂⁻) in aqueous solution. From direct measurements of the decay of Cl₂⁻ in the presence of different reactants at pH = 4 and I = 0.1 M the following rate constants at T = 298 K were derived: methanol, (5.1 ± 0.3)·10⁴ M⁻¹ s⁻¹; ethanol, (1.2 ± 0.2)·10⁵ M⁻¹ s⁻¹; 1‐propanol, (1.01 ± 0.07)·10⁵ M⁻¹ s⁻¹; 2‐propanol, (1.9 ± 0.3)·10⁵ M⁻¹ s⁻¹; tert.‐butanol, (2.6 ± 0.5)·10⁴ M⁻¹ s⁻¹; formaldehyde, (3.6 ± 0.5)·10⁴ M⁻¹ s⁻¹; diethylether, (4.0 ± 0.2)·10⁵ M⁻¹ s⁻¹; methyl‐tert.‐butylether, (7 ± 1)·10⁴ M⁻¹ s⁻¹; tetrahydrofuran, (4.8 ± 0.6)·10⁵ M⁻¹ s⁻¹; acetone, (1.41 ± 0.09)·10³ M⁻¹ s⁻¹. For the reactions of Cl₂⁻ with formic acid and acetic acid rate constants of (8.0 ± 1.4)·10⁴ M⁻¹ s⁻¹ (pH = 0, I = 1.1 M and T = 298 K) and (1.5 ± 0.8) · 10³ M⁻¹ s⁻¹ (pH = 0.42, I = 0.48 M and T = 298 K), respectively, were derived. A correlation between the rate constants at T = 298 K for all oxygenated hydrocarbons and the bond dissociation energy (BDE) of the weakest C‐H‐bond of log k_2nd = (32.9 ± 8.9) − (0.073 ± 0.022)·BDE/kJ mol⁻¹ is derived. From temperature‐dependent measurements the following Arrhenius expressions were derived: k (Cl₂⁻ + HCOOH) = (2.00 ± 0.05)·10¹⁰·exp(−(4500 ± 200) K/T) M⁻¹ s⁻¹, E_a = (37 ± 2) kJ mol⁻¹ k (Cl₂⁻ + CH₃COOH) = (2.7 ± 0.5)·10¹⁰·exp(−(4900 ± 1300) K/T) M⁻¹ s⁻¹, E_a = (41 ± 11) kJ mol⁻¹ k (Cl₂⁻ + CH₃OH) = (5.1 ± 0.9)·10¹²·exp(−(5500 ± 1500) K/T) M⁻¹ s⁻¹, E_a = (46 ± 13) kJ mol⁻¹ k (Cl₂⁻ + CH₂(OH)₂) = (7.9 ± 0.7)·10¹⁰·exp(−(4400 ± 700) K/T) M⁻¹ s⁻¹, E_a = (36 ± 5) kJ mol⁻¹ Finally, in measurements at different ionic strengths (I) a decrease of the rate constant with increasing I has been observed in the reactions of Cl₂⁻ with methanol and hydrated formaldehyde. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 169–181, 1999     

Kinetics of the reaction of CF3 with O2 over the temperature range 233–373 K

The rate constant of the reaction CF₃ + O₂ + M → CF₃O₂ + M has been measured as a function of temperature in the fall-off region between 1 and 10 Torr, by laser photolysis and time-resolved mass spectrometry. The values of the fall-off parameters measured at room temperature are in good agreement with previous values and the temperature dependence of k₁₍₀₎ is reasonably well accounted for by the theoretical analysis proposed by Troe and co-workers. With M = N₂k₁₍₀₎ = (1.9 ± 0.2) × 10⁻²⁹ (T/298)^{(−4.7 ± 0.4)} cm⁶ molecule⁻² s⁻¹. Reliable values of k_1(∞) cannot be derived from these low pressure measurements. The rate expression proposed, k_1(∞) = (9 ± 2) X 10⁻¹² (T/298)^(0±1) cm³ molecule⁻¹ s⁻¹, is consistent with a “broadening parameter” of the form F_c = exp(−T/395) and previous values reported at room temperature.     

Kinetics and mechanisms of the reactions of chlorine atoms with ethane,propane, and n-butane

Absolute (flash photolysis) and relative (FTIR-smog chamber and GC) rate techniques were used to study the gas-phase reactions of Cl atoms with C₂H₆ (k₁), C₃H₈ (k₃), and n-C₄H₁₀ (k₂). At 297 ± 1 K the results from the two relative rate techniques can be combined to give k₂/k₁ = (3.76 ± 0.20) and k₃/k₁ = (2.42 ± 0.10). Experiments performed at 298–540 K give k₂/k₁ = (2.0 ± 0.1)exp((183 ± 20)/T). At 296 K the reaction of Cl atoms with C₃H₈ produces yields of 43 ± 3% 1-propyl and 57 ± 3% 2-propyl radicals, while the reaction of Cl atoms with n-C₄H₁₀ produces 29 ± 2% 1-butyl and 71 ± 2% 2-butyl radicals. At 298 K and 10–700 torr of N₂ diluent, 1- and 2-butyl radicals were found to react with Cl₂ with rate coefficients which are 3.1 ± 0.2 and 2.8 ± 0.1 times greater than the corresponding reactions with O₂. A flash-photolysis technique was used to measure k₁ = (5.75 ± 0.45) × 10⁻¹¹ and k₂ = (2.15 ± 0.15) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 298 K, giving a rate coefficient ratio k₂/k₁ = 3.74 ± 0.40, in excellent agreement with the relative rate studies. The present results are used to put other, relative rate measurements of the reactions of chlorine atoms with alkanes on an absolute basis. It is found that the rate of hydrogen abstraction from a methyl group is not influenced by neighboring groups. The results are used to refine empirical approaches to predicting the reactivity of Cl atoms towards hydrocarbons. Finally, relative rate methods were used to measure rate coefficients at 298 K for the reaction of Cl atoms with 1- and 2-chloropropane and 1- and 2-chlorobutane of (4.8 ± 0.3) × 10⁻¹¹, (2.0 ± 0.1) × 10⁻¹⁰, (1.1 ± 0.2) × 10⁻¹⁰, and (7.0 ± 0.8) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, respectively. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 43–55, 1997.     

Flash photolysis of SiBr4: the UV spectrum of SiBr2

In the flash photolysis of SiBr₄ both the absorption and the emission spectra corresponding to the B̃²Σ−X̃²Π transition of SiBr have been observed. A broad, structureless absorption band has also been detected in the 340–400 nm region which could be assigned to the hitherto unreported à ¹B₁−x̃ ¹A₁ transition of SiBr₂. The decay of both absorption spectra followed first-order kinetics yielding the pseudo-first-order rate constants: k(SiBr)=2.6 × 10⁴s⁻¹ and k(SiBr₂) = 8.9 × 10³⁻¹. Assuming that the principal reactions consuming these intermediates are SiBr+SiBr₄→Si₂Br₅ and SiBr₂+SiBr₄→ Si₂Br₆, the second-order rate constants have the values k(SiBr)= 9.7×10⁹ M⁻¹s⁻¹ and k(SiBr₂)= 3.3×10⁸M⁻¹s⁻¹.     

Absolute rate constants of the reactions of OH with cyclohexane and ethane at 296 ± 2 K by the discharge flow method

We present absolute measurements of the rate constant for the reactions of OH with cyclohexane: k₁=(8.6±0.8) × 10⁻¹² cm³ molecule⁻¹s⁻¹ and with ethane: k₃= (2.74±0.3) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, both measured at room temperature by discharge flow resonance fluorescence. Our result for k₁ is above the average of two previously published measurements, but is in agreement with the preferred values of two recent reviews, as deduced from either relative measurements or theoretical correlations.     

Theoretical and experimental studies of the diketene system: Product branching decomposition rate constants and energetics of isomers

The kinetics and mechanism for the thermal decomposition of diketene have been studied in the temperature range 510–603 K using highly diluted mixtures with Ar as a diluent. The concentrations of diketene, ketene, and CO₂ were measured by FTIR spectrometry using calibrated standard mixtures. Two reaction channels were identified. The rate constants for the formation of ketene (k₁) and CO₂ (k₂) have been determined and compared with the values predicted by the Rice–Ramsperger–Kassel–Marcus (RRKM) theory for the branching reaction. The first-order rate constants, k₁ (s⁻¹) = 10^{15.74 ± 0.72} exp(−49.29 (kcal mol⁻¹) (±1.84)/RT) and k₂ (s⁻¹) = 10^{14.65 ± 0.87} exp(−49.01 (kcal mol⁻¹) (±2.22)/RT); the bulk of experimental data agree well with predicted results. The heats of formation of ketene, diketene, cyclobuta-1,3-dione, and cyclobuta-1,2-dione at 298 K computed from the G2M scheme are −11.1, −45.3, −43.6, and −40.3 kcal mol⁻¹, respectively. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 580–590, 2007     

Studies on the oxidation mechanism of H2S based on direct examination of the key reactions

By conducting an excimer laser photolysis (193 and 248 nm) behind shock waves, three elementary reactions important in the oxidation of H₂S have been examined, where, H, O, and S atoms have been monitored by the atomic resonance absorption spectrometry. For HS + O₂ → products (1), the rate constants evaluated by numerical simulations are summarized as: k₁ = 3.1 × 10⁻¹¹exp|-75 kJ mol⁻¹/RT| cm³molecule⁻¹s⁻¹ (T = 1400-1850 K) with an uncertainty factor of about 2. Direct measurements of the rate constants for S + O2 → SO + O (2), and SO + O₂ → SO₂ + O (3) yield k₂ = (2.5 ± 0.6) × 10⁻¹¹ exp|-(15.3 ± 2.5) kJ mol⁻¹/RT| cm³molecule⁻¹s⁻¹ (T = 980-1610 K) and, k₃ = (1.7 ± 0.9) × 10⁻¹² exp|-(34 ± 11) kJ mol⁻¹/RT| cm³molecule⁻¹s⁻¹ (T = 1130-1640 K), respectively. By summarizing these data together with the recent experimental results on the H(SINGLE BOND)S(SINGLE BOND)O reaction systems, a new kinetic model for the H₂S oxidation process is constructed. It is found that this simple reaction scheme is consistent with the experimental result on the induction time of SO₂ formation obtained by Bradley and Dobson. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 57–66, 1997.     

Temperature-dependent kinetics studies of the reactions of C2H5O2 and n-C3H7O2 radicals with NO

The rate coefficients for the gas-phase reactions of C₂H₅O₂ and n-C₃H₇O₂ radicals with NO have been measured over the temperature range of (201–403) K using chemical ionization mass spectrometric detection of the peroxy radical. The alkyl peroxy radicals were generated by reacting alkyl radicals with O₂, where the alkyl radicals were produced through the pyrolysis of a larger alkyl nitrite. In some cases C₂H₅ radicals were generated through the dissociation of iodoethane in a low-power radio frequency discharge. The discharge source was also tested for the i-C₃H₇O₂ + NO reaction, yielding k_{298 K} = (9.1 ± 1.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, in excellent agreement with our previous determination. The temperature dependent rate coefficients were found to be k(T) = (2.6 ± 0.4) × 10⁻¹² exp{(380 ± 70)/T} cm³ molecule⁻¹ s⁻¹ and k(T) = (2.9 ± 0.5) × 10⁻¹² exp{(350 ± 60)/T} cm³ molecule⁻¹ s⁻¹ for the reactions of C₂H₅O₂ and n-C₃H₇O₂ radicals with NO, respectively. The rate coefficients at 298 K derived from these Arrhenius expressions are k = (9.3 ± 1.6) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ for C₂H₅O₂ radicals and k = (9.4 ± 1.6) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ for n-C₃H₇O₂ radicals. © 1996 John Wiley & Sons, Inc.     

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.     

Kinetics of the gas‐phase reactions of Cl atom with selected C2–C5 unsaturated hydrocarbons at 283 < T < 323 K

The reaction of Cl atoms with a series of C₂–C₅ unsaturated hydrocarbons has been investigated at atmospheric pressure of 760 Torr over the temperature range 283–323 K in air and N₂ diluents. The decay of the hydrocarbons was followed using a gas chromatograph with a flame ionization detector (GC‐FID), and the kinetic constants were determined using a relative rate technique with n‐hexane as a reference compound. The Cl atoms were generated by UV photolysis (λ ≥ 300 nm) of Cl₂ molecules. The following absolute rate constants (in units of 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, with errors representing ±2σ) for the reaction at 295 ± 2 K have been derived from the relative rate constants combined to the value 34.5 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ for the Cl + n‐hexane reaction: ethene (9.3 ± 0.6), propyne (22.1 ± 0.3), propene (27.6 ± 0.6), 1‐butene (35.2 ± 0.7), and 1‐pentene (48.3 ± 0.8). The temperature dependence of the reactions can be expressed as simple Arrhenius expressions (in units of 10⁻¹¹ cm³ molecule⁻¹ s⁻¹): k_ethene = (0.39 ± 0.22) × 10⁻¹¹ exp{(226 ± 42)/T}, k_propyne = (4.1 ± 2.5) × 10⁻¹¹ exp{(118 ± 45)/T}, k_propene = (1.6 ± 1.8) × 10⁻¹¹ exp{(203 ± 79)/T}, k_1‐butene = (1.1 ± 1.3) × 10⁻¹¹ exp{(245 ± 90)/T}, and k_1‐pentene = (4.0 ± 2.2) × 10⁻¹¹ exp{(423 ± 68)/T}. The applicability of our results to tropospheric chemistry is discussed. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 478–484, 2000     

Determination of the rate constants for the gas-phase reactions of methyl butenol with OH radicals,ozone, NO3 radicals,and Cl atoms

Using a relative rate method, rate constants for the gas-phase reactions of 2-methyl-3-buten-2-ol (MBO) with OH radicals, ozone, NO₃ radicals, and Cl atoms have been investigated using FTIR. The measured values for MBO at 298±2 K and 740±5 torr total pressure are: k_OH=(3.9±1.2)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹, k_O3=(8.6±2.9)×10⁻¹⁸ cm³ molecule⁻¹ s⁻¹, k=(8.6±2.9)×10⁻¹⁵ cm³ molecule⁻¹ s⁻¹, and k_Cl=(4.7±1.0)×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. Atmospheric lifetimes have been estimated with respect to the reactions with OH, O₃, NO₃, and Cl. The atmospheric relevance of this compound as a precursor for acetone is, also, briefly discussed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 589–594, 1998     

Temperature dependence of the rate coefficients for the reaction of chlorine atoms with chloromethanes

Rate coefficients for the reaction of Cl atoms with CH₃Cl (k₁), CH₂Cl₂ (k₂), and CHCl₃ (k₃) have been determined over the temperature range 222–298 K using standard relative rate techniques. These data, when combined with evaluated data from previous studies, lead to the following Arrhenius expressions (all in units of cm³ molecule⁻¹ s⁻¹): k₁ = (2.8 ± 0.3) × 10⁻¹¹ exp(−1200 ± 150/T); k₂ = (1.5 ± 0.2) × 10⁻¹¹ exp(−1100 ± 150/T); k₃ = (0.48 ± 0.05) × 10⁻¹¹ exp(−1050 ± 150/T). Values for k₁ are in substantial agreement with previous measurements. However, while the room temperature values for k₂ and k₃ agree with most previous data, the activation energies for these rate coefficients are substantially lower than previously recommended values. In addition, the mechanism of the oxidation of CH₂Cl₂ has been studied. The dominant fate of the CHCl₂O radical is decomposition via Cl‐atom elimination, even at the lowest temperatures studied in this work (218 K). However, a small fraction of the CHCl₂O radicals are shown to react with O₂ at low temperatures. Using an estimated value for the rate coefficient of the reaction of CHCl₂O with O₂ (1 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹), the decomposition rate coefficient for CHCl₂O is found to be about 4 × 10⁶ s⁻¹ at 218 K, with the barrier to its decomposition estimated at 6 kcal/mole. As part of this work, the rate coefficient for Cl atoms with HCOCl was also been determined, k₇ = 1.4 × 10⁻¹¹ exp(−885/T) cm³ molecule⁻¹ s⁻¹, in agreement with previous determinations. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 515–524, 1999     

Relative Rate Studies of the Reactions of Atomic Chlorine with Acetone and Cyclic Ketones

Rate coefficients have been measured for Cl atom reactions under ambient conditions with acetone and four cyclic ketones. Cl was generated by UV photolysis of Cl₂, and other species were monitored by FT‐IR spectroscopy. The measurements yield k(Cl + acetone) = (2.0 ± 0.7) × 10⁻¹², k(Cl + cyclobutanone) = (10.1 ± 0.8) × 10⁻¹¹, k(Cl + cycloheptanone) = (24.0 ± 2.3) × 10⁻¹¹, k(Cl + 2‐methyl cyclopentanone) = (15.2 ± 1.2) × 10⁻¹¹, and k(Cl + 2‐methyl cyclohexanone) = (11.2 ± 1.0) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, where the uncertainties represent 95% confidence limits. These results are discussed in the context of structure‐activity relationships. We also present a prediction for Cl + cyclopropanone based on ab initio properties of the transition state.     

Oxidation of dimethyl ether: Absolute rate constants for the self reaction of CH3OCH2 radicals,the reaction of CH3OCH2 radicals with O2, and the thermal decomposition of CH3OCH2 radicals

The rate constant for the reaction of CH₃OCH₂ radicals with O₂ (reaction (1)) and the self reaction of CH₃OCH₂ radicals (reaction (5)) were measured using pulse radiolysis coupled with time resolved UV absorption spectroscopy. k₁ was studied at 296K over the pressure range 0.025–1 bar and in the temperature range 296–473K at 18 bar total pressure. Reaction (1) is known to proceed through the following mechanism: CH₃OCH₂ + O₂ ↔ CH₃OCH₂O₂^# → CH₂OCH₂O₂H^# → 2HCHO + OH (k_prod) CH₃OCH₂ + O₂ ↔ CH₃OCH₂O₂^# + M → CH₃OCH₂O₂ + M (k_RO2) k = k_RO2 + k_prod, where k_RO2 is the rate constant for peroxy radical production and k_prod is the rate constant for formaldehyde production. The k₁ values obtained at 296K together with the available literature values for k₁ determined at low pressures were fitted using a modified Lindemann mechanism and the following parameters were obtained: k_RO2,0 = (9.4 ± 4.2) × 10⁻³⁰ cm⁶ molecule⁻² s⁻¹, k_RO2,∞ = (1.14 ± 0.04) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, and k_prod,0 = (6.0 ± 0.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, where k_RO2,0 and k_RO2,∞ are the overall termolecular and bimolecular rate constants for formation of CH₃OCH₂O₂ radicals and k_prod,0 represents the bimolecular rate constant for the reaction of CH₃OCH₂ radicals with O₂ to yield formaldehyde in the limit of low pressure. k_RO2,∞ = (1.07 ± 0.08) × 10⁻¹¹ exp(−(46 ± 27)/T) cm³ molecule⁻¹ s⁻¹ was determined at 18 bar total pressure over the temperature range 296–473K. At 1 bar total pressure and 296K, k₅ = (4.1 ± 0.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ and at 18 bar total pressure over the temperature range 296–523K, k₅ = (4.7 ± 0.6) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. As a part of this study the decay rate of CH₃OCH₂ radicals was used to study the thermal decomposition of CH₃OCH₂ radicals in the temperature range 573–666K at 18 bar total pressure. The observed decay rates of CH₃OCH₂ radicals were consistent with the literature value of k₂ = 1.6 × 10¹³exp(−12800/T)s⁻¹. The results are discussed in the context of dimethyl ether as an alternative diesel fuel. © 1997 John Wiley & Sons, Inc.     

Kinetics and mechanism of the Cl + CO reaction in air

Long-path FTIR spectroscopy was used to study the kinetics and mechanism of the reaction of Cl atoms with CO in air. The relative rate constants at 298 K and 760 torr for the forward direction of the reaction of Cl with ¹³CO and the reaction of Cl¹³CO with O₂ were k₁ = (3.4 ± 0.8) × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ and k₂ = (4.3 ± 3.2) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, respectively (all uncertainty limits are 2σ). The rate constant for the net loss of ¹³CO due to reaction with Cl in 1 atm of air at 298 K was k_Cl+COobs = (3.0 ± 0.6) × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹. The only observed carbon-containing product of the Cl + ¹²CO reaction was ¹²CO₂, with a yield of 109 ± 18%. Our results are in good agreement with extrapolations from previous studies. The reaction mechanism and the implications for laboratory studies and tropospheric chemistry are discussed. © 1996 John Wiley & Sons, Inc.