Cm6 Molecule Research Articles

Depletion Kinetics of Chromium Atoms by Sulfur Dioxide

The gas-phase depletion kinetics of Cr(a7S3, a5S2, a5DJ) in the presence of SO2 are reported. Chromium atoms were produced by the 248 nm photodissociation of chromium carbonyl and were detected by laser-induced fluorescence. The ground state of Cr, a7S3, was found to react rapidly via a termolecular mechanism with SO2. At 297 K, the limiting low-pressure third-order rate constant is ko = (2.68 ± 0.41) × 10-28 cm6 molecule-2 s-1, and the limiting high-pressure second-order rate constant is k∞ = (2.73 ± 0.38) × 10-10 cm3 molecule-1 s-1; the uncertainties represent ±1σ in precision. ko was found to decrease with increasing temperature. The binding energy of CrSO2 is estimated at 53 kcal mol-1 from combining the kinetic results with unimolecular rate theory and density functional theory. Cr(a5S2) depleted with a rate constant of k = 6.32 × 10-10 cm3 molecule-1 s-1, and the a5DJ spin/orbit states depleted at the collision rate of 2.97 × 10-10 cm3 molecule-1 s-1; overall uncertainties are estimated at ≤35%. Both chemical and physical quenching are likely for the excited states. The present work completes a study on the depletion kinetics of group 6 transition metal atoms by SO2. Results are interpreted in terms of long-range attractive forces between Cr and SO2 and the orbital occupancies of the Cr atomic states.

Collisional Quenching and Energy Transfer of the z5DJo States of the Fe Atom

Room-temperature thermal rate constants for quenching and energy transfer have been determined for the individual fine-structure levels of the z5DJo term of the Fe atom in collisions with He, Ar, and several molecular gases (N2, O2, H2, and CH4). The z5DJo term is the upper level of a transition at 372 nm which is convenient for laser fluorescence detection of the Fe atom. Emission profiles from all excited levels were observed to be exponential, and total removal rate constants were derived. The rate constants for the molecular gases were significantly larger [in the range of (3−8) × 10-10 molecule-1 cm3 s-1] than for the inert gases [(0.3−1.3) × 10-10 molecule-1 cm3 s-1]. No evidence for collisional transfer to the neighboring b3H and a3D states was found, and upper limits for the collisional transfer rate constants are reported. Rate constants for intramultiplet transfer within the z5DJo state were determined for He and O2. The intramultiplet rate constants for He summed over all final fine-structure levels roughly equaled the total removal rate constants, while they were ∼10% of the total removal constants for O2.

Thermal Decomposition of Tin Tetrachloride Based on Cl- and Sn-Concentration Measurements

The kinetics of the thermal decomposition of tin tetrachloride has been studied experimentally and theoretically. Ab initio MO calculations showed that SnCl4 finally decomposed into Sn(3P) and four chlorine atoms through four subsequent Sn−Cl bond dissociation channels. Two sets of kinetic experiments were performed using a shock tube equipped with atomic resonance absorption spectroscopy (ARAS). Chlorine atoms were first measured over the temperature range of 1250−1700 K and the total density range of 1.7 × 1018 to 8.9 × 1018 molecules cm-3. The rate coefficient for the initial reaction step, SnCl4 (+M) → SnCl3(2A1) + Cl (+M) (eq 1a), was found to be in the falloff region fairly close to the low-pressure limit under the present conditions. The second-order rate coefficient based on the Cl-atom measurements was determined to be k1a2nd = 10-5.37±0.62 exp[−(285 ± 18) kJ mol-1/RT] cm3 molecule-1 s-1 (error limits at the 2 standard deviation level). The second group of experiments was carried out by detecting tin atoms over the temperature range of 2250−2950 K and at a total density of 3.2 × 1018 molecules cm-3. The second-order rate coefficients for the subsequent reaction steps: SnCl2(1A1) (+M) → SnCl(2Π) + Cl (+M) (eq 3a) and SnCl(2Π) (+M) → Sn(3P) + Cl (+M) (eq 4a) were obtained to be k3a2nd = 10-8.36±0.86 exp[−(310 ± 42) kJ mol-1/RT] cm3 molecule-1 s-1 and k4a2nd = 10-9.50±0.78 exp[−(265 ± 40) kJ mol-1/RT] cm3 molecule-1 s-1, respectively. The Rice−Ramsperger−Kassel−Marcus (RRKM) calculations including variational transition state theory were also applied for reactions 1a and 3a. Structural parameters and vibrational frequencies of the reactants and transition states required for the RRKM calculations were obtained from the ab initio MO calculations. Energy barriers of the reactions, E0's, which are the most sensitive parameters in the calculations, were adjusted until the RRKM rate coefficients matched the observed ones. These fittings yielded E0,1a = 326 kJ mol-1 for reaction 1a and E0,3a = 368 kJ mol-1 for reaction 3a, in good agreement with the Sn−Cl bond dissociation energies of SnCl4 and SnCl2, demonstrating that the experimental data for k1a and k3a were theoretically reasonable and acceptable.

Kinetic Study of the OH + Isoprene and OH + Ethylene Reactions between 2 and 6 Torr and over the Temperature Range 300−423 K

The rate constants for the OH + isoprene and OH + ethylene reactions have been measured in He with 10% O2 (P = 2−6 Torr) and over the temperature range 300−423 K using a discharge-flow system coupled with laser-induced fluorescence. The measured rate constants for the OH + ethylene reaction are in good agreement with previously reported values. The termolecular rate constant (k0) at the low-pressure limit for the OH + ethylene reaction was determined to be (2.62 ± 0.25) × 10-29 cm6 molecule-2 s-1 at 300 K where the uncertainty represents 2 standard errors. An Arrhenius expression of k0 = (4.9 ± 0.2) × 10-31 exp[(1210 ± 130)/T] cm6 molecule-2 s-1 was obtained from a weighted linear least-squares fit of the k0 data versus temperature. The observed negative activation energy (-Ea/R) is larger than the currently recommended value. Unlike the OH + ethylene reaction, the rate constant for the OH + isoprene reaction is independent of pressure between 2 and 6 Torr at 300 K. The measured rate constant of (1.10 ± 0.04) × 10-10 cm3 molecule-1 s-1 at 300 K and 2 Torr agrees well with those measured at higher pressures. However, the rate constant for the OH + isoprene reaction begins to show a pressure dependence at temperatures of 343 K and higher. At 343 K, the termolecular rate constant was observed to be (10.50 ± 1.57) × 10-27 cm6 molecule-2 s-1. An Arrhenius expression of k0 = (9.3 ± 5.4) × 10-29 exp[(1560 ± 230)/T] cm6 molecule-2 s-1 was obtained from a weighted linear least-squares fit of the k0 data versus temperature. The negative activation energy for the OH + isoprene reaction is similar to that observed for the OH + ethylene reaction.

Mechanism and Kinetics of the Reactions of NO2 or HNO3 with Alumina as a Mineral Dust Model Compound

The reaction of alumina as a model substance for mineral aerosols with NO2 or HNO3 was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The formation of nitrate on the Al2O3 surface was observed in both cases. In addition, during the initial phase of the NO2 reaction, intermediate nitrite formation was observed. The DRIFTS data provide insight into the reaction mechanism, which involves reaction of surface OH groups, the formation of a {AlOOH···NO2} adduct, and the formation of acidic OH groups. The reaction order in NO2 of 1.86 ± 0.1 was determined from a quantitative kinetic evaluation of a series of experiments with NO2 concentrations in the range of 1013 to 1015 molecules cm-3. The reactive uptake coefficient, γ, was determined from the infrared absorbance, which was calibrated by ion chromatography, and from the Al2O3 Brunauer−Emmett-Teller (BET) surface area. γ depended linearly on the NO2 concentration and varied from γ = 7.3 × 10-10 to 1.3 × 10-8 for [NO2] = 2.5 × 1013 to 8.5 × 1014 molecules cm-3. Estimations of the atmospheric impact showed that at these above conditions (γ = 10-9) nitrate formation on mineral aerosol from the NO2 reaction would be negligible.

Detailed modeling of the temperature and pressure dependence of the reaction H+O 2 (+M)→HO 2 (+M)

New ab initio calculations of the potential energy surface of the recombination reaction H+O2 (+M) →HO2 (+M), together with statistical unimolecular rate theory employing extensive classical trajectory calculations on this potential, allowed the characterization of thermally averaged and energy E- and angular momentum J-specific rate constants of the H+O2HO2 recombination/dissociation system. On the basis of these results, falloff curves of the reaction over wide temperature and pressure ranges were constructed that are consistent with the limited available experimental data and that allow for extrapolations into experimentally unexplored ranges of conditions. The calculations go beyond earlier work in a variety of ways: (1) a potential with improved precision in the kinetically relevant range of interfragment distances was used, (2) the dynamics was shown to be neither adiabatic nor sudden such that neither simplified SACM- nor RRKM-type approaches apply, (3) the falloff curves were dominated by rotational and weak collision broadening, illustrating the necessity of using E- and J-specific rate constants k(E,J) in the calculations, (4) centrifugal barriers E0(J) were revised by considering the full dimensionality of the problem, and (5) vibrational anharmonicities were approximated in an improved way by including stretchbend coupling contributions. The paper provides analytical representations of the pressure and temperature dependence of the rate coefficients over wide ranges of conditions. In the simplest form, limiting low-pressure rate constants krec,0/[N2]=(5.4±0.5)× 10−32 (T/300 K)−1.4 cm6 molecule−2 s−1 and krec,0/[Ar]=(2.2±0.3)×10−32 (T/300 K)−1.2 cm6 molecule−1 s−1 as well as limiting high-pressure rate constants krec,∞=(9.5±1.0)×10−11 (T/300 K)+0.44 cm3 molecule−1 s−1 are recommended over the range 300–2000 K. Falloff curves are constructed with Fcent≈0.5±0.1 independent of the temperature over the range 300–2000 K.

Experimental and theoretical study of the temperature and pressure dependences of the recombination reactions O+NO2(+M)→ NO3(+M) and NO2+NO3(+M)→N2O5(+M)

The recombination reactions O + NO2(+M) → NO3(+M) and NO2 + NO3(+M) → N2O5(+M) were studied at temperatures of 300 and 400 K, and at pressures of the bath gas M = N2 between 1 and 900 bar. Oxygen atoms were generated by laser flash photolysis of N2O at 193 nm, NO3 radicals were monitored by light absorption at 578 nm. The measured fall-off curve of the reaction O + NO2(+M) → NO3(+M) could be well represented by limiting low pressure rate constants k3,0 = (1.3 ± 0.3) × 10−31 (T/300 K)−1.5 [N2] cm6 molecule−2 s−1, limiting high pressure rate constants k3,∞ = (2.3 ± 0.2) × 10−11 (T/300 K)0.24 cm molecule−1 s−1, and fall-off broadening factors of Fc = 0.71 exp(−T/1700 K). The derived results are consistent with earlier relative rate measurements. Theoretical modeling of k3,0, Fc and k3,∞ led to consistency with the experimental data. There is strong evidence that, besides the electronic ground state NO3(2A′), the first excited electronic state NO3(2E′) also contributes to the observed recombination reaction. The measured fall-off curve of the reaction NO2 + NO3(+M) → N2O5(+M) was represented by limiting low pressure rate constants k5,0 = 3.6 × 10−30(T/300 K)−5.0 [N2] cm6 molecule−2 s−1, limiting high pressure rate constants k5,∞ = (1.9 ± 0.3) × 10−12(T/300 K)0.2 cm3 molecule−1 s−1 and Fc = 0.38 exp(−T/4900 K). A theoretical analysis of these values is also presented.

The reactions of FeO with O3, H2, H2O, O2 and CO2

The FeO(X 5Δ) radical plays a central role in the atmospheric chemistry of meteor-ablated iron and in the iron-catalysed inhibition of flames. This paper reports a kinetic study of a series of FeO reactions, using the pulsed two-photon dissociation at 248 or 193 nm of ferrocene vapour in the presence of NO2 or O3, respectively, followed by time-resolved laser-induced fluorescence spectroscopy of FeO at 582.0 nm [FeO(D′ 5Δ4–X 5Δ4)]. For the recombination reactions of FeO with O2, CO2 and H2O, ab initio quantum calculations on FeO3, FeCO3 and Fe(OH)2 were coupled with RRKM theory to show that the first two reactions were close to their low pressure limits under the experimental conditions, yielding k(FeO+O2+N2, 196–519 K)=(3.86±0.07)×10−30 (T/300 K)+0.50±0.06 and k(FeO+CO2+N2, 233–475 K)=(3.09±0.22)×10−31 (T/300 K)−1.19±0.24 cm6 molecule−2 s−1, at the 95% confidence level. By contrast, the much faster FeO+H2O (+N2) reaction was found to be in the fall-off region over the experimental pressure range; an RRKM fit to the results yields: log10(krec,0)=−31.05+4.438log10T−1.218log102T, krec,∞=5.35×10−10 exp(−611/T), Fc=0.28, with an uncertainty of 15% over the experimental range of 298–527 K and 3.1–8.3 Torr. The corresponding reactions with He as the third body were slower by factors of 2–4. The reaction between FeO and O3 is fast with a small T-dependence: k(FeO+O3→FeO2+O2 , 194–341 K)=(2.94±0.43)×10−10 exp[(−1.45±0.29) kJ mol−1/RT] cm3 molecule−1 s−1. No reaction was observed between FeO and H2, indicating an upper limit of k(FeO+H2→Fe+H2O, 320 K)⩽7×10−14 cm3 molecule−1 s−1. Finally, the atmospheric implications of these results are discussed.

Low-Pressure Study of the Reactions of Br Atoms with Alkenes. 3. Reactions with 2-Methyl-2-butene, 2,3-Dimethyl-2-butene, and 1-Hexene

The kinetics of the reactions of Br atoms with 2-methyl-2-butene (C5H10), 2,3-dimethyl-2-butene (C6H12), and 1-hexene have been studied by the mass spectrometry discharge-flow method at 0.5−2.0 Torr of He over the temperature range T = 233−320 K. The occurrence of both hydrogen atom abstraction and addition channels has been observed for all the reactions. The Arrhenius expressions k1a = (5.2 ± 2.5) × 10-16 exp{(1910 ± 250)/T} and k2a = (8.4 ± 6.3) × 10-16 exp{(1950 ± 290)/T} cm3 molecule-1 s-1 have been obtained (at 1 Torr total pressure of He) for the addition reactions Br + C5H10 ↔ C5H10Br (1a,−1a) and Br + C6H12 ↔ C6H12Br (2a,−2a), respectively. Considering k7 and k8 as the rate constants for the reactions of Br2 with C5H10Br and C6H12Br, respectively, the following expressions have been determined: k1ak7/ k-1a = (1.4 ± 0.8) × 10-35 exp{(5250 ± 300)/T} and k2ak8/k-2a = (6.2 ± 6.8) × 10-35 exp{(5190 ± 510)/T} cm6 molecule-2 s-1. Kinetic data have been also obtained for the H atom abstraction channels: k1b = (2.1 ± 0.2) × 10-11 exp{−(655 ± 40)/T}, k2b = (2.4 ± 0.6) × 10-11 exp{−(75 ± 130)/T}, and k3b = (3.25 ± 0.4) × 10-12 exp{−(425 ± 70)/T} cm3 molecule-1 s-1, for the reactions of Br with 2-methyl-2-butene, 2,3-dimethyl-2-butene, and 1-hexene, respectively. From this kinetic study, the enthalpies of reactions 1a and 2a and the heats of formation of the adduct radicals C5H10Br and C6H12Br have been estimated: ΔH°1a = −(10.0 ± 1.6), ΔH°2a = −(9.8 ± 2.1), ΔH°f(C5H10Br) = 6.8 ± 1.6, and ΔH°f(C6H12Br) = 0.1 ± 2.5 (in kcal mol-1).

Investigation of the Heterogeneous Reactivity of HCl, HBr, and HI on Ice Surfaces

The interactions of hydrogen halide gases (HX = HCl, HBr, and HI) with thin ice films representative of atmospheric aerosols have been studied using a Knudsen cell reactor coupled to a Fourier transform infrared−reflection absorption (FTIR−RAS) spectroscopic probe. The gas-phase uptake and reaction products resulting from the exposure of hydrogen halides to ice surfaces over a wide range of temperatures (110−210 K), hydrogen halide partial pressures (5−1000 × 10-7 Torr), and ice film thicknesses (10−100 nm) are reported. Studies of HCl and HBr showed efficient reactions on crystalline and amorphous microporous ice films at 110 K to form H3O+ until reaching coverages ranging from (5−20) × 1015 molecules cm-2, after which the rate of reaction dramatically decreased. The uptake of HCl on hexagonal crystalline ice at temperatures representative of the lower stratosphere and upper troposphere (180−210 K) was found to depend strongly on HCl partial pressure. Over the temperature range studied, exposure of ice to HCl partial pressures below the HCl equilibrium partial pressure at the liquid/ice coexistence point resulted in uptake limited to (3.5 ± 1.6) × 1015 molecules cm-2. In contrast, exposure to HCl pressures larger than the HCl equilibrium partial pressure resulted in unlimited uptake. HBr and HI were efficiently and continuously taken up by ice surfaces (γ ≥ 0.02) over a range of atmospherically relevant temperatures (180−210 K). Although crystalline hydrates of HX:H2O are stable over the temperature range examined, the incorporation of hydrogen halides into ice always resulted in the formation of amorphous HX:H2O product layers with the exception of HBr uptake at high flow rates (flow rate ≥ 3.1 × 1015 molecules s-1) which resulted in the formation of a mixture of crystalline hydrates.

The Gas-Phase Kinetics of Reactions of Alkali Metal Atoms with Nitric Oxide

Time-resolved atomic resonance absorption spectroscopy was employed, with flash-photolysis generation of atomic potassium and sodium at 734 K, to investigate the kinetics of K + NO + Ar and Na + NO + Ar. The low-pressure limit recombination rate constants are (1.0 ± 0.2) × 10-31 and (1.9 ± 0.5) × 10-32 cm6 molecule-2 s-1, respectively. Mainly ionic, triplet state adducts are predicted. RRKM modeling yielded K−NO and Na−NO bond dissociation enthalpies of 104 and 66 kJ mol-1, which compare well with UQCISD(T)/6-311+ G(2d) ab initio values of 81 and 68 kJ mol-1, respectively. For Li + NO +Ar, the predicted binding energy is 134 kJ mol-1, which yields a predicted rate constant of 1.4 × 10-31 cm6 molecule-2 s-1.

Shock-Tube Studies on the Reactions of CF2(XA1) with O(3P) and H Atoms

The reactions of CF2(X1A1) radicals with O(3P) and H atoms have been studied by using a shock tube/atomic resonance absorption spectroscopy technique over the temperature ranges 2000−2430 and 1450−1860 K and the total density range 6.1 × 1018 to 1.2 × 1019 molecules cm-3. Nitrous oxide and ethyl iodide were used as precursors of O(3P) and H atoms, respectively. Electronically ground state CF2(X1A1) radicals were produced through the thermal decomposition of chlorodifluoromethane. The rate coefficients for the reactions CF2(X1A1) + O(3P) and CF2(X1A1) + H were obtained from the decay profiles of O and H atom concentrations as k(CF2+O) = 10-10.39±0.07 and k(CF2+H) = 10-10.18±0.21 exp[−(19.0 ± 6.7) kJ mol-1/RT] cm3 molecule-1 s-1 (error limits at the two standard deviation level). Neither rate coefficient had any pressure dependence under the present experimental conditions. The G2-level ab initio molecular orbital calculation was also performed to examine the product channels for the CF2(X1A1) + O(3P) and CF2(X1A1) + H reactions. The theoretical calculation showed that the most energetically favorable pathways for CF2(X1A1) + O(3P) and CF2(X1A1) + H systems were the channels producing FCO + F and CF + HF, respectively. The G2 energy of the transition state for the channel CF2(X1A1) + O(3P) → FCO + F was 116 kJ mol-1 lower than that of the reactants CF2(X1A1) + O(3P), while the energy of the three-centered transition state for the channel CF2(X1A1) + H → CF + HF is 45 kJ mol-1 higher than that of the reactants CF2(X1A1) + H. These results could qualitatively explain the difference of the temperature dependence observed between k(CF2+O) and k(CF2+H).

Atmospheric Chemistry of Tetrachloroethene (Cl2CCCl2): Products of Chlorine Atom Initiated Oxidation

The products following Cl atom initiated oxidation of C2Cl4 at 700−760 Torr of air and 230−299 K in the presence and absence of NOx were investigated using three different FTIR smog chamber techniques. There was no measurable effect of temperature on the product yields. CCl3C(O)Cl and COCl2 were formed with molar yields of 68 ± 6% and 77 ± 12% in the presence of NOx and 87 ± 11% and 32 ± 4% in the absence of NOx. These results give branching ratios for the CCl3C(O)Cl and COCl2 forming channels of 0.64 and 0.36 in the presence of NOx and 0.84 and 0.16 in the absence of NOx. Contrary to a recent report by Hasson and Smith (J. Phys. Chem. A, 1999, 103, 2031), variation of the initial C2Cl4 by a factor of 300 over the range (0.016−5.6) × 1014 molecule cm-3 had no discernible effect (<10%) on the product distributions. The different product distribution observed in the presence of NOx may reflect the formation and subsequent decomposition of chemically activated C2Cl5O radicals, formed in the exothermic reaction of C2Cl5O2 with NO. The kinetics of the reaction of Cl atoms with C2Cl4 were measured in 2.0−700 Torr of air at 296 K. The results are in good agreement with the previous study by Nicovich et al. (J. Phys. Chem. 1996, 100, 680). The combined data can be described using Fc = 0.6, ko = (1.8 ± 0.3) × 10-28 cm6 molecule-2 s-1 and k∞ = (4.0 ± 0.4) × 10-11 cm3 molecule-1 s-1. Results are discussed with respect to the atmospheric chemistry of C2Cl4.

Production of HCl in the OH + ClO Reaction: Laboratory Measurements and Statistical Rate Theory Calculations

The branching ratio for the OH + ClO reaction has been measured using the turbulent flow technique with high-pressure chemical ionization mass spectrometry for the detection of reactants and products. In our earlier study, OD was used instead of OH due to the large HCl background produced by the ClO source (J. Chem. Soc., Faraday Trans. 1997, 93, 2665). Improvements to our experimental technique have significantly reduced this HCl background, thus making it possible to observe directly the production of very small concentrations of HCl (∼109 molecule cm-3) from the minor channel of the OH + ClO reaction. At room temperature and ∼100 Torr pressure, the rate constant for this minor channel was determined to be (9.5 ± 1.6) × 10-13 cm3 molecule-1 s-1 with a two standard deviation error limit, which remained unchanged when the pressure was increased to 200 Torr. The temperature dependence of the rate constant for this minor channel was also investigated between 207 and 298 K, and the data were fit to the following Arrhenius expression: (3.2 ± 0.8) × 10-13 exp[(325 ± 60)/T] cm3 molecule-1 s-1. The branching ratio for the HCl channel was determined to be 0.07 ± 0.03 at all pressures and temperatures investigated in this study. Statistical rate theory calculations were also performed on the OH + ClO reaction system and are in good agreement with the experimental results.

Low-Pressure Study of the Reactions of Br Atoms with Alkenes. 2. Reactions with Ethene and trans-2-Butene

The reactions of Br atoms with ethene and trans-2-butene have been studied by the mass spectrometry discharge-flow method, at a total pressure of helium 0.5−2.0 Torr in the temperature range 233−320 K. The expression = (3.0 ± 1.2) × 10-33 exp{(730 ± 220)/T} cm6 molecule-2 s-1 has been obtained for the low-pressure limit of the rate constant for the addition reaction Br + C2H4 + M ↔ C2H4Br + M (1,−1). The expressions k1k5/k-1 = (3.1 ± 0.6) × 10-36 exp{(3540 ± 280)/T} cm6 molecule-2 s-1 and k1k6a/k-1 = (1.8 ± 0.9) × 10-35 exp{(3390 ± 270)/T} cm6 molecule-2 s-1 have been obtained for the combinations of the rate constants of reactions (1,−1) and reactions Br2 + C2H4Br → Br + C2H4Br2 (5) and Br + C2H4Br → C2H4Br2 (6a). For the reaction of Br atoms with trans-2-butene, both the addition channel, Br + C4H8 ↔ C4H8Br (2a,−2a), and the H atom abstraction channel, Br + C4H8 → C4H7 + HBr (2b) have been observed and the following kinetic data were obtained: = (2.1 ± 0.8) × 10-32 exp{(1300 ± 90)/T} cm6 molecule-2 s-1, k2ak9/k-2a = (3.8 ± 1.7) × 10-36 exp{(4630 ± 230)/T} cm6 molecule-2 s-1 (where k9 is the rate constant of the reaction C4H8Br + Br2 → C4H8Br2 + Br) and k2b = (5.8 ± 1.7) × 10-12 exp{−(960 ± 160)/T} cm3 molecule-1 s-1. Thermochemical data around room temperature could be derived from these kinetic measurements; the enthalpies of reactions 1 and 2a are = −(6.8 ± 1.6) kcal mol-1 and = −(8.8 ± 1.5) kcal mol-1, and the heats of formation of the adducts C2H4Br and C4H8Br are = 32.4 ± 1.6 kcal mol-1 and = 20.5 ± 1.8 kcal mol-1.

Detection of the Methyl Radical on Neptune

We report the first detection of the methyl radical (CH3) in the upper atmosphere of Neptune. Observations with the Short-Wavelength Spectrometer of the Infrared Space Observatory (ISO) satellite at a resolving power of 2200 revealed several emission features from the ν2 Q-branch of CH3 around 16.50 μm. The column density of methyl radicals above the 0.2 mbar level is 1.6+1.2−0.9 × 1013 molecules cm-2. Results are compared with predictions of photochemical models. The CH3 abundance is mostly sensitive to the eddy mixing profile and to the poorly known methyl recombination rate at low pressure. Using Slagle et al.'s expression for this rate, the present observations imply an eddy mixing coefficient at the methane homopause in the range of 2-9 × 106 cm2 s-1. Recombination rates higher than used in current photochemical models would lead to larger eddy mixing coefficients.

Association Reactions of Manganese, Iron, and Ruthenium with Nitric Oxide

The gas-phase association reactions of ground-state manganese, iron, and ruthenium atoms with nitric oxide are reported. The transition metal atoms were produced by the 248 nm photodissociation of 2-methylcyclopentadienylmanganese tricarbonyl, ferrocene, and ruthenocene. Detection of these transition metal atoms was by laser-induced fluorescence. Manganese was found to be the least reactive with nitric oxide of the transition metals in this work. The limiting low-pressure third-order rate constant, ko, of Mn + NO + Ar can be expressed as (9.5 ± 2.5) × 10-33 exp[−1.5 ± 0.4 kcal mol-1/(RT)] cm6 molecule-2 s-1. ko for Fe + NO + N2 at 296 K is (2.3 ± 0.5) × 10-32 cm6 molecule-2 s-1; this reaction is independent of temperature over the range 296−622 K. Ruthenium was found to be the most reactive toward NO. In Ar buffer, ko = (7.3 ± 1.0) × 10-30 cm6 molecule-2 s-1 and the limiting high-pressure rate constant k∞ = (3.8 ± 0.8) × 10-11 cm3 molecule-1 s-1 for a fixed broadening factor of Fc = 0.6. The uncertainties here represent precision. Recent density functional theory calculations on transition metal mononitrosyls were combined with RRKM calculations to estimate the binding energies of the Mn−NO and Fe−NO adducts.

Chlorine Atom Initiated Oxidation of Chlorinated Ethenes: Results for 1,1-Dichloroethene (H2CCCl2), 1,2-Dichloroethene (HClCCClH), Trichloroethene (HClCCCl2), and Tetrachloroethene (Cl2CCCl2)

Experiments have been carried out on the oxidation of the four chlorinated ethenes H2CCCl2, HClCCClH, HClCCCl2, and Cl2CCCl2. Reaction was initiated by continuous photolysis of Cl2, and product yields were measured using Fourier transform infrared (FTIR) spectroscopy. In all cases, experiments were performed at different concentrations of the chlorinated ethenes (down to 3 × 1013 molecule cm-3), Cl2 and O2. In the cases of H2CCCl2 and HClCCCl2, the product yields were invariant with these changes, consistent with them being determined by competitive unimolecular dissociations of the chlorinated alkoxy radical; that is, H2ClCCCl2O (HCl2CCCl2O) (+M) → H2ClCCOCl (HCl2CCOCl) + Cl (+M) and H2ClCCCl2O (HCl2CCCl2O) (+M) → COCl2 + CH2Cl (CHCl2) (+M), and the subsequent formation of HCOCl and/or COCl2 from CH2Cl and CHCl2. In the case of HCl2CCHClO formed from HClCCClH, H atom abstraction by O2 is competitive with CCl and CC bond cleavage, so the yields depended on the concentration of O2. With C2Cl4, the only pro...

Atmospheric Chemistry of Cyclohexane: UV Spectra of c-C6H11• and (c-C6H11)O2• Radicals, Kinetics of the Reactions of (c-C6H11)O2• Radicals with NO and NO2, and the Fate of the Alkoxy Radical (c-C6H11)O•

A pulse radiolysis technique was used to measure the UV absorption spectra of c-C6H11• and (c-C6H11)O2• radicals over the ranges 230−290 and 220−300 nm, σ(c-C6H11•)250 nm = (7.0 ± 0.8) × 10-18 and σ((c-C6H11)O2•)250 nm = (5.7 ± 0.6) × 10-18 cm2 molecule-1. The rate constant for the self-reaction of c-C6H11• radicals was k3 = (3.0 ± 0.4) × 10-11 cm3 molecule-1 s-1. The addition reaction of c-C6H11• radicals with O2 proceeds with a rate constant k2 = (1.3 ± 0.2) × 10-11 cm3 molecule-1 s-1. Rate constants for reactions of (c-C6H11)O2• radicals with NO and NO2 were k4 = (6.7 ± 0.9) × 10-12 and k5 = (9.5 ± 1.5) × 10-12 cm3 molecule-1 s-1, respectively. FTIR−smog chamber techniques were used to record the IR spectrum of the peroxynitrate (c-C6H11)O2NO2, determine that the reaction between (c-C6H11)O2• radicals and NO produces a (16 ± 4)% yield of the nitrate (c-C6H11)ONO2, and study the atmospheric fate of cyclohexoxy radicals. Decomposition via C−C bond scission and reaction with O2 are competing fates of the cyclohexoxy radical. In 700−750 Torr total pressure at 296 ± 2K, the rate constant ratio kdecomp/kO2 = (8.1 ± 1.5) × 1018 molecule cm-3. At 296 K in 1 atm of air, 61% of cyclohexoxy radicals decompose and 39% react with O2. These results are discussed with respect to the literature data concerning the atmospheric chemistry of cyclohexane and analogous compounds.

Peroxyacetyl Nitrate in Atlanta, Georgia: Comparison and Analysis of Ambient Data for Suburban and Downtown Locations

ABSTRACT Peroxyacetyl nitrate (PAN) concentrations were measured at downtown and suburban locations in Atlanta, GA, in July and August 1992 as part of the SOS-SORP/ONA (Southern Oxidants Study–Southern Oxidants Research Program on Ozone Non-Attainment). PAN concentrations were generally higher at the downtown location than at the suburban location, but on days when the O3 concentration exceeded 80 ppbv, PAN concentrations were similar at both locations. On days when O3 did not exceed 80 ppbv, suburban PAN concentrations were much lower than downtown concentrations and resembled those reported for rural areas in the eastern United States. Regression analysis of PAN and O3 on NOx and total non-methane hydrocarbons (TNMHC) showed PAN to be most strongly dependent on morning NOx concentrations, while O3 was most dependent on morning TNMHC concentrations. NOx, PAN, and meteorological data from the suburban site were used in a one-dimensional transport model to estimate the accumulation rate of PAN to be ~1.5 x 106 molecules cm-3 sec-1. A simple kinetic model estimated peroxyacetyl radical concentrations to be ~0.5 pptv at the suburban location.