Kinetics of the reactions of the IO radical with dimethyl sulfide,methanethiol, ethylene,and propylene

The reactions of IO radicals with CH₃SCH₃, CH₃SH, C₂H₄, and C₃H₆ have been studied using the discharge flow method with direct detection of IO radicals by mass spectrometry. The absolute rate constants obtained at 298 K are the following: IO + CH₃SCH₃ → products (1): k₁ = (1.5 ± 0.2) × 10^?14; IO + CH₃SH → products (2): k₂ = (6.6 ± 1.3) × 10^?16; IO + C₂H₄ →products (3): k₃ < 2 × 10^?16; IO + C₃H₆ → products (4): k₄ < 2 × 10^?16 (units are cm³ molecule^?1 s^?1). CH₃S(O)CH₃ and HOI were found as products of reactions (1) and (2), respectively. The present lower value of k₁ compared to our previous determination is discussed.     

Gas‐phase reactions of Cl atoms with propane,n‐butane,and isobutane

Using the relative kinetic method, rate coefficients have been determined for the gas‐phase reactions of chlorine atoms with propane, n‐butane, and isobutane at total pressure of 100 Torr and the temperature range of 295–469 K. The Cl₂ photolysis (λ = 420 nm) was used to generate Cl atoms in the presence of ethane as the reference compound. The experiments have been carried out using GC product analysis and the following rate constant expressions (in cm³ molecule^?1 s^?1) have been derived: (7.4 ± 0.2) × 10^?11 exp [‐(70 ± 11)/ T], Cl + C₃H₈ → HCl + CH₃CH₂CH₂; (5.1 ± 0.5) × 10^?11 exp [(104 ± 32)/ T], Cl + C₃H₈ → HCl + CH₃CHCH₃; (7.3 ± 0.2) × 10^?11 exp[?(68 ± 10)/ T], Cl + n‐C₄H₁₀ → HCl + CH₃ CH₂CH₂CH₂; (9.9 ± 2.2) × 10^?11 exp[(106 ± 75)/ T], Cl + n‐C₄H₁₀ → HCl + CH₃CH₂CHCH₃; (13.0 ± 1.8) × 10^?11 exp[?(104 ± 50)/ T], Cl + i‐C₄H₁₀ → HCl + CH₃CHCH₃CH₂; (2.9 ± 0.5) × 10^?11 exp[(155 ± 58)/ T], Cl + i‐C₄H₁₀ → HCl + CH₃CCH₃CH₃ (all error bars are ± 2σ precision). These studies provide a set of reaction rate constants allowing to determine the contribution of competing hydrogen abstractions from primary, secondary, or tertiary carbon atom in alkane molecule. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 651–658, 2002     

Deactivation of I(52P1/2) by CF3I,CH3I,C2H5I,and CH4

The technique of laser photolysis of alkyl and perfluoroalkyl iodides at 266 nm followed by time-resolved detection of the 1.3-μm emission from I^*(²P_1/2) has been used to measure the rate constants for deactivation of I^* by CH₃I, C₂H₅I, CF₃I, and CH₄. The recommended values are (2.76± 0.22) × 10^?13, (2.85 ± 0.40) × 10^?13, (3.5 ± 0.5) × 10^?17, and (7.52 ± 0.12) × 10^?14, respectively, in units of cm³ molecule^?1 S^?1.     

Kinetic study of reactions of C2H5O2 with NO at 298 K and 0.55 – 2 torr

The kinetics of C₂H₅O₂ and C₂H₅O₂ radicals with NO have been studied at 298 K using the discharge flow technique coupled to laser induced fluorescence (LIF) and mass spectrometry analysis. The temporal profiles of C₂H₅O were monitored by LIF. The rate constant for C₂H₅O + NO → Products (2), measured in the presence of helium, has been found to be pressure dependent: k₂ = (1.25±0.04) × 10^?11, (1.66±0.06) × 10^?11, (1.81±0.06) × 10^?11 at P (He) = 0.55, 1 and 2 torr, respectively (units are cm³ molecule^?1 s^?1). The Lindemann-Hinshelwood analysis of these rate constant data and previous high pressure measurements indicates competition between association and disproportionation channels: C₂H₅O + NO + M → C₂H₅ONO + M (2a), C₂H₅O + NO → CH₃CHO + HNO (2b). The following calculated average values were obtained for the low and high pressure limits of k_2a and for k_2b : k = (2.6±1.0) × 10^?28 cm⁶ molecule^?2 s^?1, k = (3.1±0.8) × 10^?11 cm³ molecule^?1 s^?1 and k_2b ca. 8 × 10^?12 cm³ molecule^?1 s^?1. The present value of k, obtained with He as the third body, is significantly lower than the value (2.0±1.0) × 10^?27 cm⁶ molecule^?2 s^?1 recommended in air. The rate constant for the reaction C₂H₅O₂ + NO → C₂H₅O + NO₂ (3) has been measured at 1 torr of He from the simulation of experimental C₂H₅O profiles. The value obtained for k₃ = (8.2±1.6) × 10^?12 cm³ molecule^?1 s^?1 is in good agreement with previous studies using complementary methods. © 1995 John Wiley & Sons, Inc.     

Relaxation of DF(v = 1) by various polyatomic molecules

By means of the technique of laser-induced fluorescence, the room-temperature vibrational relaxation of DF(v = 1) has been studied in the presence of several polyatomic chaperones. The rate coefficients obtained [in units of (μ;sec·torr)^?1] are CH₄, 0.22; C₂H₆, 0.61; C₄H₁₀, 1.26; C₂H₂, 4.0 × 10^?2; C₂H₂F₂, 1.86 × 10^?2; C₂H₄, 0.175; CH₃F, 0.36; CF₃H, 1.95 × 10^?2; CF₄, 1.0 × 10^?3; CBrF₃, 5.6 × 10^?4; NF₃, 5.1 × 10^?4; SO₂, 1.27 × 10^?2; and BF₃, 7.1 × 10^?3. Results are also reported for vibrational relaxation rate coefficients for HF(v = 1) in the presence of the following chaperones: CH₄, 2.6 × 10^?2; C₂H₆, 5.9 × 10^?2; C₃H₈, 8.4 × 10^?2; and C₄H₁₀, 0.128. A comparison of DF and HF results indicates that for deactivation by C_nH_n+2, rate coefficients for DF are approximately an order of magnitude larger than for HF. The deactivation rate coefficient of DF(v = 1) by CH₄ was found to decrease with increasing temperature between 300 and 740°K.     

Gas phase thermal diffusivities by a thermal lens technique

A simplified design of thermal lens apparatus is presented in which a chopped cw argon laser beam produces a transient thermal lens in a cylindrical gas cell. The axial intensity variation of a cw helium-neon laser probing this lens is analysed to yield the thermal diffusivities and thus the thermal conductivity coefficients of Kr, CO₂, CH₄, C₂H₆, C₃H₈, C₃H₆ and C₄H₁₀ as 9.4 × 10^?3 ± 4%, 1.6 × 10^?2 ± 3%, 2.98 × 10^?2 ± 4%, 2.03 × 10^?2 ± 4%, 2.05 × 10^?2 ± 7%, 1.6 × 10^?2 ± 8% and 1.9 × 10^?2 ± 8% respectively in W m^?1 K^?1 at 300 K. The method is rapid, requiring only that the sample be transparent at both laser frequencies used. A simplified mathematical analysis is shown to be adequate for this system. For the conditions specified, self-lensing of the argon laser beam is shown to be compensated by using an effective laser beam diameter.     

Kinetics and mechanism associated with the reactions of hydroxyl radicals and of chlorine atoms with 1‐propanol under near‐tropospheric conditions between 273 and 343 K

Rate constants for the reactions of OH radicals and Cl atoms with 1‐propanol (1‐C₃H₇OH) have been determined over the temperature range 273–343 K by the use of a relative rate technique. The value of k(Cl + 1‐C₃H₇OH) = (1.69 ± 0.19) × 10^?12 cm³ molecule^?1 s^?1 at 298 K and shows a small increase of 10% between 273 and 342 K. The value of k(OH + 1‐C₃H₇OH) increases by 14% between 273 and 343 K with a value of (5.50 ± 0.55) × 10^?12 cm³ molecule^?1 s^?1 at 298 K, and further when combined with a single independent experimentally determined value at 753 K gives k(OH + 1‐C₃H₇OH) = 4.69 × 10^?17T^1.8 exp(422/T) cm³ molecule^?1 s^?1, which fits each data point to better than 2%. Two well‐established structure–activity relationships for H abstraction by OH radicals give accurate predictions of the rate constant for OH + 1‐C₃H₇OH, provided the β‐CH₂ group is given an increased reactivity of a factor of about 2 over that for the structurally equivalent CH₂ group in alkanes at 298 K. A quantitative product analysis was carried out at 298 K for the Cl‐initiated photooxidation of 1‐C₃H₇OH, using both FTIR and gas chromatography. HCHO, CH₃CHO, and C₂H₅CHO were the only major organic primary products observed, although HCOOH was found in much smaller amounts as a secondary product. A key characteristic of the analysis was that the initial values of the product ratio [CH₃CHO]/[C₂H₅CHO] were effectively constant for NO pressures between 0.15 and 0.3 Torr, but fell by about 35% as the pressure fell to 0.0375 Torr. From a detailed consideration of the mechanism for the oxidation, it is suggested that C₂H₅CHO, CH₃CHO (+HCHO), and 3 molecules of HCHO are formed uniquely from CH₃CH₂CHOH, CH₃CHCH₂OH, and CH₂CH₂CH₂OH radicals, respectively. On this basis, use of the product yields gives the branching ratios of 56, 30, and 14% for Cl atom reaction at the α‐, β‐, and γ‐C? H positions in 1‐C₃H₇OH at 298 K. Given the very low temperature coefficients involved, little change will occur over tropospheric temperature ranges. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 110–121, 2002     

The rate of the reactions of C2O radicals with H and H2

The rate constants for the reactions C₂O + H → products (1) and C₂O + H₂ → products (2) have been determined at room temperature by means of laser-induced fluorescence detection of C₂O radicals, generated either by the KrF excimer laser photolysis Of C₃O₂, or by the reaction of C₃O₂ with O atoms. Values of k₁ = (3.7 ± 1.0) × 10^?11 cm³ s^?1 and k₂ = (7 ± 3) × 10^?13 cm³ s^?1 were obtained.     

High-temperature pyrolysis of acetaldehyde

High-temperature (>1000°K) pyrolysis of acetaldehyde (～1% in an atmosphere of pure nitrogen) was examined in a turbulent flow reactor which permits accurate determination of the spatial distribution of the stable species. Results show that the products in order of decreasing importance are CO, CH₄, H₂, C₂H₆, and C₂H₄. Rates of formation were consistent with the Rice–Herzfeld mechanism by including reactions to explain C₂H₄ formation and the possible presence of ketene. A steady-state treatment of the complete mechanism indicates that the overall reaction order decreases from \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{3}{2} $\end{document} to 1, which is supported by the new experimental data. Using earlier low-temperature results, the rate constant for the reaction CH₃CHO → CH₃ + CHO (1) was found as k₁=10^15.85±0.21 exp (?81,775±1000/RT) sec^?1. Also, data for the ratio of rate constants for reactions CH₃CHO + CH₃ → CH₄ + CH₃CO (4) and 2CH₃ → C₂H₆(6) were fitted to the empirical expression k₄/k₆^1/2=10^?13.89±0.03T^6.1 exp(?1720±70/RT) (cm³/mole·sec)^1/2 and causes for the curvature are discussed. The noncatalytic effect of oxygen on acetaldehyde pyrolysis at high temperature is explained.     

Rate constants for the elementary reactions between CN radicals and CH4, C2H6, C2H4, C3H6, and C2H2 in the range: 295 ⩽ T/K ⩽ 700

Pulsed laser photolysis, time-resolved laser-induced fluorescence experiments have been carried out on the reactions of CN radicals with CH₄, C₂H₆, C₂H₄, C₃H₆, and C₂H₂. They have yielded rate constants for these five reactions at temperatures between 295 and 700 K. The data for the reactions with methane and ethane have been combined with other recent results and fitted to modified Arrhenius expressions, k(T) = A′(298) (T/298)ⁿ exp(?θ/T), yielding: for CH₄, A′(298) = 7.0 × 10^?13 cm³ molecule^?1 s^?1, n = 2.3, and θ = ?16 K; and for C₂H₆, A′(298) = 5.6 × 10^?12 cm³ molecule^?1 s^?1, n = 1.8, and θ = ?500 K. The rate constants for the reactions with C₂H₄, C₃H₆, and C₂H₂ all decrease monotonically with temperature and have been fitted to expressions of the form, k(T) = k(298) (T/298)ⁿ with k(298) = 2.5 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.24 for CN + C₂H₄; k(298) = 3.4 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.19 for CN + C₃H₆; and k(298) = 2.9 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.53 for CN + C₂H₂. These reactions almost certainly proceed via addition-elimination yielding an unsaturated cyanide and an H-atom. Our kinetic results for reactions of CN are compared with those for reactions of the same hydrocarbons with other simple free radical species. © John Wiley & Sons, Inc.     

Kinetics into the steady state. I. study of the reaction of oxygen atoms with methyl radicals

The exponential relaxation of CH₃, produced by the reaction O + C₂H₄ → CH₃ + HCO, to its steady-state concentration was quantitatively monitored after the reactants were mixed. The relaxation profiles yield the rate constant of the reaction O + CH₃ → H₂CO + H equal to (1.85 ± 0.28) × 10^-10 cm³/molecule-sec at 300°K. Ancillary experiments yielded values for the rate constant for the reaction of O atoms with C₂H₄ at 300°K, the average of which is 7.7 × 10^-12 cm³/molecule-sec. The experimental technique, which employs a fast-flow reactor coupled to a photoionization mass spectrometer, is described in detail and its potential discussed.     

Kinetics and mechanism of atmospheric reactions of partially fluorinated alcohols

Gas-phase reactions typical of the Earth’s atmosphere have been studied for a number of partially fluorinated alcohols (PFAs). The rate constants of the reactions of CF₃CH₂OH, CH₂FCH₂OH, and CHF₂CH₂OH with fluorine atoms have been determined by the relative measurement method. The rate constant for CF₃CH₂OH has been measured in the temperature range 258–358 K (k = (3.4 ± 2.0) × 10¹³exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(1.5 ± 1.3) kJ/mol). The rate constants for CH₂FCH₂OH and CHF₂CH₂OH have been determined at room temperature to be (8.3 ± 2.9) × 10¹³ (T = 295 K) and (6.4 ± 0.6) × 10¹³ (T = 296 K) cm³ mol^?1 s^?1, respectively. The rate constants of the reactions between dioxygen and primary radicals resulting from PFA + F reactions have been determined by the relative measurement method. The reaction between O₂ and the radicals of the general formula C₂H₂F₃O (CF₃CH₂? and CF₃?HOH) have been investigated in the temperature range 258–358 K to obtain k = (3.8 ± 2.0) × 10⁸exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(10.2 ± 1.5) kJ/mol. For the reaction between O₂ and the radicals of the general formula C₂H₄FO (? HFCH₂O, CH₂F?HOH, and CH₂FCH₂?) at T = 258–358 K, k = (1.3 ± 0.6) × 10¹¹exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(5.3 ± 1.4) kJ/mol. The rate constant of the reaction between O₂ and the radicals with the general formula C₂H₃F₂O (?F₂CH₂O, CHF₂?HOH, and CHF₂CH₂?) at T = 300 K is k = 1.32 × 10¹¹ cm³ mol^?1 s^?1. For the reaction between NO and the primary radicals with the general formula C₂H₂F₃O (CF₃CH₂? and CF₃?HOH), which result from the reaction CF₃CH₂OH + F, the rate constant at 298 K is k = 9.7 × 10⁹ cm³ mol^?1 s^?1. The experiments were carried out in a flow reactor, and the reaction mixture was analyzed mass-spectrometrically. A mechanism based on the results of our studies and on the literature data has been suggested for the atmospheric degradation of PFAs.     

An FTIR spectroscopic study of the reactions Br + CH3CHO → HBr + CH3CO and CH3C(O)OO + NO2 ⇌ CH3C(O)OONO2 (PAN)

Based on an FTIR-product study of the photolysis of mixtures containing Br₂? CH₃CHO and Br₂? CH₃CHO? HCHO in 700 torr of N₂, the rate constant for the reaction Br + CH₃CHO → HBr + CH₃CO was determined to be 3.7 × 10^?12 cm³ molecule^?1 s^?1. In addition, the selective photochemical generation of Br at λ > 400 nm in mixtures containing Br₂? CH₃CHO? ¹⁴NO₂ (or ¹⁵NO₂)? O₂ was shown to serve as a quantitative preparation method for the corresponding nitrogen-isotope labeled CH₃C(O)OONO₂ (PAN). From the dark-decay rates of ¹⁵N-labeled PAN in large excess ¹⁴NO₂, the rate constant for the unimolecular reaction CH₃C(O)OO¹⁵NO₂ → CH₃C(O)OO + ¹⁵NO₂ was measured to be 3.3 (±0.2) × 10^?4 s^?1 at 297 ± 0.5 K.     

Kinetics of phenyl radical reactions with propane,n‐butane,n‐hexane,and n‐octane: Reactivity of C6H5 toward the secondary C?H bond of alkanes

The kinetics of C₆H₅ reactions with n‐C_nH_2n+2 (n = 3, 4, 6, 8) have been studied by the pulsed laser photolysis/mass spectrometric method using C₆H₅COCH₃ as the phenyl precursor at temperatures between 494 and 1051 K. The rate constants were determined by kinetic modeling of the absolute yields of C₆H₆ at each temperature. Another major product C₆H₅CH₃ formed by the recombination of C₆H₅ and CH₃ could also be quantitatively modeled using the known rate constant for the reaction. A weighted least‐squares analysis of the four sets of data gave k (C₃H₈) = (1.96 ± 0.15) × 10¹¹ exp[?(1938 ± 56)/T], and k (n‐C₄H₁₀) = (2.65 ± 0.23) × 10¹¹ exp[?(1950 ± 55)/T] k (n‐C₆H₁₄) = (4.56 ± 0.21) × 10¹¹ exp[?(1735 ± 55)/T], and k (n?C₈H₁₈) = (4.31 ± 0.39) × 10¹¹ exp[?(1415 ± 65)T] cm³ mol^?1 s^?1 for the temperature range studied. For the butane and hexane reactions, we have also applied the CRDS technique to extend our temperature range down to 297 K; the results obtained by the decay of C₆H₅ with CRDS agree fully with those determined by absolute product yield measurements with PLP/MS. Weighted least‐squares analyses of these two sets of data gave rise to k (n?C₄H₁₀) = (2.70 ± 0.15) × 10¹¹ exp[?(1880 ± 127)/T] and k (n?C₆H₁₄) = (4.81 ± 0.30) × 10¹¹ exp[?(1780 ± 133)/T] cm³ mol^?1 s^?1 for the temperature range 297‐‐1046 K. From the absolute rate constants for the two larger molecular reactions (C₆H₅ + n‐C₆H₁₄ and n‐C₈H₁₈), we derived the rate constant for H‐abstraction from a secondary C? H bond, k_s?CH = (4.19 ± 0.24) × 10¹⁰ exp[?(1770 ± 48)/T] cm³ mol^?1 s^?1. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 49–56, 2004     

Kinetics and mechanism for the reactions of H atoms with CH3SH and C2H5SH

A kinetic study of the reactions of H atoms with CH₃SH and C₂H₅SH has been carried out at 298 K by the discharge flow technique with EPR and mass spectrometric analysis of the species. The pressure was 1 torr. It was found: k₁ = (2.20 ± 0.20) × 10^?12 for the reaction H + CH₃SH (1) and k₂ = (2.40 ± 0.16) × 10^?12 for the reaction H + C₂H₅SH (2). Units are cm³ molecule^?1 s^?1. A mass spectrometric analysis of the reaction products and a computer simulation of the reacting systems have shown that reaction (1) proceeds through two mechanisms leading to the formation of CH₃S + H₂ (1a) and CH₃ + H₂S (1b).     

A kinetic study of the reaction of fluorine atoms with CH3F,CH3Cl,CH3Br,CF2H2, CO,CF3H,CF3CHCl2, CF3CH2F,CHF2CHF2, CF2ClCH3, CHF2CH3, and CF3CF2H at 295 ± 2 K

A variety of relative and absolute techniques have been used to measure the reactivity of fluorine atoms with a series of halogenated organic compounds and CO. The following rate constants were derived, in units of cm³ molecule^?1 s^?1: CH₃F, (3.7 ± 0.8) × 10^?11, CH₃Cl, (3.3 ± 0.7) × 10^?11; CH₃Br, (3.0 ± 0.7) × 10^?11; CF₂H₂, (4.3 ± 0.9) × 10^?12; CO, (5.5 ± 1.0) × 10^?13 (in 700 torr total pressure of N₂ diluent); CF₃H, (1.4 ± 0.4) × 10^?13; CF₃CCl₂H (HCFC-123), (1.2 ± 0.4) × 10^?12; CF₃CFH₂ (HFC-134a), (1.3 ± 0.3) × 10^?12, CHF₂CHF₂ (HFC-134), (1.0 ± 0.3) × 10^?12; CF₂ClCH₃ (HCFC-42b), (3.9 ± 0.9) × 10^?12, CF₂HCH₃ (HFC-152a), (1.7 ± 0.4) × 10^?11; and CF₃CF₂H (HFC-125), (3.5 ± 0.8) × 10^?13. Quoted errors are statistical uncertainties (2σ). For rate constants derived using relative rate techniques, an additional uncertainty has been added to account for potential systematic errors in the reference rate constants used. Experiments were performed at 295 ± 2 K. Results are discussed with respect to the previous literature data and to the interpretation of laboratory studies of the atmospheric chemistry of HCFCs and HFCs. © 1993 John Wiley & Sons, Inc.     

A shock tube study of methyl-methyl reactions between 1200 and 2400 K

The methyl-methyl reaction was studied in a shock tube using uv narrowline laser absorption to measure time-varying concentration profiles of CH₃. Methyl radicals were rapidly formed initially by pyrolysis of various precursors, azomethane, ethane, or methyl iodide, dilute in argon. The contributions of the various product channels, C₂H₆, C₂H₅ + H, C₂H₄ + H₂, and CH₂ + CH₄, were examined by varying reactant mixtures and temperature. The measured rate coefficients for recombination to C₂H₆ between 1200 and 1800 K are accurately fit using the unimolecular rate coefficients reported by Wagner and Wardlaw (1988). The rate coefficient for the C₂H₅ + H channel was found to be 2.4 (±0.5) × 10¹³ exp(?6480/T) [cm³/mol-s] between 1570 and 1780 K, and is in agreement with the value reported by Frank and Braun-Unkhoff (1988). No evidence of a contribution by the C₂H₄ + H₂ channel was found in ethane/methane/argon mixtures, although methyl profiles in these mixtures should be particularly sensitive to this channel. An upper limit of approximately 10¹¹ [cm³/mol-s] over the range 1700 to 2200 K was inferred for the rate coefficient of the C₂H₄ + H₂ channel. Between 1800 and 2200 K, methyl radicals are also rapidly removed by CH₃ + H ? ¹CH₂ + H₂. In this temperature range, the reverse reaction was found to have a rate coefficient of 1.3 (±0.3) × 10¹⁴ [cm³/mol-s], which is 1.8 times the room-temperature value. © 1995 John Wiley & Sons, Inc.     

Reactions of the copper dimer,Cu2, in the gas phase

Reactions of Cu₂ with several small molecules have been studied in the gas phase, under thermalized conditions at room temperature, in a fast-flow reactor. They fall into one of two categories. Cu₂ does not react with O₂, N₂O, N₂, H₂, and CH₄ at pressures up to 6 torr. This implies bimolecular rate constants of less than 5 × 10^?15 cm³ s^?1 at 6 torr He. Cu₂ reacts with CO, NH₃, C₂H₄, and C₃H₆ in a manner characteristic of association reactions. Second-order rate constants for all four of these reagents are dependent on total pressure. The reactions with CO, NH₃, and C₂H₄ are in their low pressure limit at up to 6 torr He buffer gas pressure. The reaction with C₃H₆ begins to show fall-off behavior at pressures above 3 torr. Limiting low-pressure, third-order rate constants are 0.66 ± 0.10, 8.8 ± 1.2, 9.3 ± 1.4, and 85 ± 15 × 10^?30 cm⁶ s^?1 in He for CO, NH₃, C₂H₄, and C₃H₆, respectively. Modeling studies of these rate constants imply that the association complexes are bound by at least 20 kcal mol^?1 in the case of C₂H₄ and C₃H₆ and at least 25 kcal mol^?1 in the other cases. The implications of these results for Cu-ligand bonding are developed in comparison with existing work on the interactions of these ligands with Cu atoms, larger clusters, and surfaces. © 1994 John Wiley & Sons, Inc.     

Minor processes in the photolysis of azomethane at low pressure: An upper limit for the rate of the unimolecular elimination of H2 from vibrationally excited ethane

The photolysis of azomethane in the near UV has been studied at room temperature and pressures from 10 mtorr to 10 torr. The main products, C₂H₆ and N₂, accounted for more than 99% of the reaction. Minor hydrocarbon products observed were (with quantum yields) C₃H₈ (3.5 × 10^?3), C₂H₄ (3.2 × 10^?4), CH₄ (3 × 10^?3), and n-C₄H₁₀ (trace). Quantum yields of H₂ of 4 × 10^?5 and 2 × 10^?5 were measured at azomethane pressures of 0.1 and 1.0 torr, respectively. The minor hydrocarbon products can be accounted for by reactions of CH₃ and C₂H₅ radicals following hydrogen abstraction from azomethane by CH₃. The H₂ product observed represents an upper limit for the H₂ elimination from vibrationally excited C₂H₆ formed by CH₃ combination in the system, corresponding to a rate of elimination ca. 5 × 10^?5 times the competing rate of dissociation to 2CH₃. Assuming a frequency factor of 10¹³ s^?1 for the H₂ elimination, a lower limit of about 90 kcal mol^?1 was estimated for the energy barrier.     

FTIR spectroscopic study of the Cl- and Br-atom initiated oxidation of ethene

The reaction mechanism of the halogen (Cl and Br)-atom initiated oxidation of C₂H₄ was studied using the long path FTIR spectroscopic method in 700 torr of air at 296 ± 2 K. Among the major halogen-containing products were X? CH₂CHO, X? CH₂CH₂OH, and X? CH₂CH₂OOH (X = Cl or Br) which were shown to be formed via the self-reaction of the X? CH₂CH₂OO radicals, i.e., 2X? CH₂CH₂OO → 2X? CH₂CH₂O + O₂; (a) 2X? CH₂CH₂OO → X? CH₂CHO + X? CH₂CH₂OH + O₂ and (b) followed by X? CH₂CH₂O + O₂ → X? CH₂CHO + HO₂ and X? CH₂CH₂OO + HO₂ → X? CH₂CH₂OOH + O₂. From the observed yields of X? CH₂CHO and X? CH₂CH₂OH the branching ratios for reactions (a) and (b) were determined to be k_a/k_b = 1.35 ± 0.07(2σ) for both X = Cl and Br. In addition, the O₂-dependence of the rate constant for the Br + C₂H₄ reaction was determined by the relative rate technique as a function of O₂ partial pressure from 140 to 700 torr at 700 torr total pressure of N₂/O₂ diluent. Rate constants for the reactions of Cl-atoms with Cl-CH₂CHO and Br-atoms with Br-CH₂CHO were also determined to be [4.3 ± 0.2(2sigma;)] × 10^?11 and less than or equal to [1.83 ± 0.11(2σ)] × 10^?13 cm³ molecule^?1 s^?1, respectively.