Ian M. Campbell

Rate constants for reactions of hydroxyl radicals with ester vapours at 292 K

Abstract The variations of yields of CO2 from the gas phase H2O2 + NO2 + CO chain reaction system with added ester vapours have given the rate constants for reactions with OH radicals: methyl acetate (1.1 ± 0.3) × 103, ethy acetate (1.16 ± 0.15) × 109, methyl propionate (1.7 ± 0.6) × 108 and ethyl propionate (1.06 ± 0.15) × 109, all in dm3 mol−1 s−1, at 292 K. These results suggest that hydrogen atom abstraction takes place at the alkoxy end of the molecule.

Gas-phase reactions of hydroxyl radicals with aldehydes in flowing H2O2+ NO2+ CO mixtures

The chain reaction in gaseous H2O2+ NO2+ CO mixtures, previously used as a thermal source of OH radicals in static systems, has been adapted for use in a flow system operating at total pressures of 39.9 kPa and 298.2 K. The reaction mechanism is H2O2+ NO2→ OH + HNO3(1), OH + CO → CO2+ H (2), H + NO2→ OH + NO (3), OH + NO2(+M)→ HNO3(+M). (4), Addition of a substrate S reactive towards OH induces the competitive reaction OH + S → products. (S) From the variation in the yield of CO2(monitored by gas chromatography) as a function of [S] the rate constant ratio kS/k2 may be derived. It is shown that such ratios can be successfully measured for systems (in this case a series of aldehydes) which would be difficult to study in the corresponding static system.The aldehyde rate constants, kS, are expressed relative to kS(acetaldehyde), and taking kS(acetaldehyde)= 0.94 × 1010 dm3 mol–1 s–1 values of 10–10kS/dm3 mol–1 s–1 of 1.08 ± 0.14, 1.52 ± 0.19, 1.05 ± 0.12, 0.83 ± 0.10, 1.11 ± 0.12 and 0.51 ± 0.05 are obtained for, respectively, propionaldehyde, n-butyraldehyde, i-butyraldehyde, n-valeraldehyde, i-valeraldehyde and trimethylacetaldehyde.Comparison with other measurements is made and the variations in these values are discussed.

Rate constants for the reactions of the hydroxyl radical with indane, indene and styrene

Values of second-order rate constants for the reactions [graphic omitted], have been measured using a discharge flow system operating at 295 K over the pressure range 65–1000 Pa. The OH concentration was monitored by resonance fluorescence and helium was used as the carrier gas. For indane the predominant mechanism is abstraction with a rate constant of (5.6 ± 0.8)× 109dm3 mol–1s–1. For both indene and styrene addition is the major reaction channel, giving rise to a pressure-dependent rate constant.

Gas-phase reactions of hydroxyl radicals with alkyl nitrite vapours in H2O2+ NO2+ CO mixtures

The yields of CO2 from the chain reaction in H2O2+ NO2+ CO + alkyl nitrite mixtures, in which OH is the chain carrier and alkyl nitrites induce a chain termination step, have been used to deduce rate constants (ks) for OH attack on alkyl nitrites (RONO) in the vapour phase at ambient temperatures. Values of ks/109 dm3 mol–1 s–1 as a function of R were determined as follows: 0.71 ± 0.12 (CH3), 1.15 ± 0.23 (C2H5), 1.56 ± 0.32 (n-C3H7), 3.41 ± 1.48 (n-C4H9), 3.89 ± 0.58 (sec-C4H9), 3.47 ± 0.52 (i-C4H9), 0.91 ± 0.15 (t-C4H9), all based on ks=(1.63 ± 0.16)× 109 dm3 mol–1 s–1 for OH + n-butane. The small increase in ks from R = CH3 to t-C4H9 is considered to support a recent postulate that both H-abstraction and NO-abstraction pathways are operative, at least for R = CH3.Under typical, sunlit urban atmosphere conditions it is deduced that OH attack on alkyl nitrites is a minor removal process compared to their photodissociative destruction.

Rate constants for the reactions of hydroxyl radicals with propane and ethane

The kinetics of the reaction OH + C3H6→ H2O + C2H7(1) have been studied in a discharge-flow system under first-order conditions. The OH radicals were generated by the reaction of H atoms with NO2 and the concentration of OH, monitored by resonance fluorescence, was followed as a function of reaction time in a large excess of the alkane. A value of k1=(7.2±1.1)× 108 dm3 mol–1 s–1 at 295 K was obtained.As a check on the technique the rate constant for the reaction OH + C2H6→ H2O + C2H5(2) was determined in a similar fashion. The value obtained, k2=(1.61±0.24)× 108 dm3 mol–1 s–1, is in excellent agreement with other literature values.

Relative rates of reaction of hydroxyl radicals with O(3P) atoms and CO molecules

The rates of decay of O(3P) atoms in H2/CO/N2 mixtures in a discharge flow system have been measured, using O + CO chemiluminescence. The mechanism is: O + H2 → OH + H (1), O + OH → O2 + H (2), CO + OH → CO2 + H (3). At 425 K, k2/k3 = 260 ± 20; literature values of k3 combine to yield k2 = (2.65 ± 0.52) × 1010 dm3 mol−1 s−1.

The rate constants for the reaction of the hydroxyl radical with benzene

Values of the pressure-dependent second-order rate constant for the reaction OH + C6H6+ M → C6H6OH + M have been measured using a discharge-flow system operating at 295 K over a pressure range of 60–1200 Pa. The OH concentration was monitored by resonance fluorescence and helium was used as the carrier gas. This is the first application of discharge-flow methods to this reaction. The results are in general agreement with those from other techniques. Extrapolation to obtain the low-pressure limiting value of the rate constant is difficult because of the scatter in the data at higher pressures. A range of values is obtained leading to a recommended value of (1 ± 0.8)× 1013 dm6 mol–2 s–1 for k°.

Mechanism and kinetics of the chain reaction in H2O2+ NO2+ CO systems

The mechanism and kinetics of the chain reaction in H2O2+ NO2+ CO mixtures at total pressures ≈ 10 kPa and 292 K have been investigated by measuring the reproducible final yields of CO2 for conditions of initial [H2O2] corresponding to ⩽ 20% of the saturated vapour pressure and excess [NO2]. The reduction of yields of CO2 upon addition of n-butane results from the additional termination step OH + n-C4H10→ H2O + C4H9(8) competing against the propagation step CO + OH → CO2+ H. (2) Literature values of k8 and k2 are applied to indicate unity stoichiometry for the heterogeneous initiation step H2O2+ NO2→ OH + HNO3(1) for Pyrex glass or boric acid-coated surfaces showing maximal CO2 production efficiencies. The variations in the yield of CO2 with initial [NO2] or added [NO] yield rate constants for the termination reactions OH + NO2(+M)→ HNO3(+M)(4), OH + NO(+M)→ HNO2(+M)(5)k4(M = CO)=(2.5 ± 0.4)× 109 dm3 mol–1 s–1 at 13.3 kPa, k5(M = CO)=(8.2 ± 1.2)× 108 dm3 mol–1 s–1 at 13.3 kPa and k4(M ≈ N2)=(1.2 ± 0.3)× 109 dm3 mol–1 s–1 at 7.32 kPa; these indicate that the reaction proceeds under conditions of efficient diffusive mixing rather than boundary layer formation, by comparison with corresponding literature values.A mechanism is advanced to explain the variation of CO2 yields as a function of small initial [NO2] for low [H2O2].

Formation of organic nitro-compounds in flowing H2O2+ NO2+ N2+ organic vapour systems. Part 2.—H2O2+ NO2+ N2+ alkane system

The principal products from the surface-initiated reactions in flowing mixtures of H2O2, NO2, N2 and RH, where RH = ethane, propane, n-butane and n-pentane, have been identified as the nitroalkane, alkyl nitrite and alkyl nitrate. The product yields have been measured; in the case of propane the variation of the yields with total gas pressure has also been studied.Values have been obtained for the relative rates of primary and secondary H-atom abstraction from each alkane by OH and for the rate-constant ratios k3/k4 and k5/k6 at 298 K: R + NO2(+ M)→ RNO2(+ M)(3) R + NO2(+ M)→ RONO*(+ M)(4) RONO*+ M → RONO + M (5) RONO*→ RO + NO. (6).The trends in the product yields with variation of pressure and change of R indicate that RO radicals are produced via reactions (4)–(6) rather than by a direct single-step reaction of R with NO2.

Formation of organic nitro-compounds in flowing H2O2+ NO2+ N2+ organic vapour systems. Part 3.—Effects of O2 addition on H2O2+ NO2+ N2+ alkane systems

The effects of oxygen on the product distribution from the surface-initiated reactions in flowing mixtures of H2O2, NO2, N2 and RH, where RH = ethane, propane, n-butane and n-pentane, at 298 K have been studied. In the absence of O2, the principal products are the corresponding nitroalkane, alkyl nitrite and alkyl nitrate. In the presence of sufficiently large concentrations of O2, the predominant product is the alkyl nitrate and the only other products of significance, in some cases, are the corresponding carbonyl compounds.The variation of the product yields with [O2]/[NO2] gives values for the rate-constant ratios k8/(k3+k4) for reaction at both primary and secondary radical sites: R + NO2(+ M)→ RNO2(+ M)(3), R + NO2(+ M)→ RONO*(+ M)(4), R + O2(+ M)→ RO2(+ M). (8) Possible mechanisms by which the products are formed are discussed.