David A. Parkes

Study of the spectra and recombination kinetics of alkyl radicals by molecular modulation spectrometry. Part 2.—The recombination of ethyl, isopropyl and t-butyl radicals at room temperature and t-butyl radicals between 250 and 450 K

Using the technique of molecular modulation spectrometry, we have found transient ultraviolet absorptions which we assign to ethyl, isopropyl and t-butyl radicals. The spectra are broad continua with maxima near 250, 233 and 230 nm, respectively. From the rate constants of their mutual reactions, measured by monitoring the radicals in the ultraviolet, and the disproportionation/recombination ratios found from separate experiments, the rates of recombination are measured at room temperature to be: ethyl, (1.3 ± 0.3)× 10–11 cm3 molecule–1 s–1; isopropyl, (8.3 ± 2.0)× 10–12 cm3 molecule–1 s–1; t-butyl, (4.0 ± 1.0)× 10–12 cm3 molecule–1 s–1. The t-butyl rate falls by a factor of 2 as the temperature is increased to 423 K. These results are compared with other recent gas- and liquid-phase determinations, and we conclude that the currently accepted thermochemistry of larger alkyl radicals and our results are incompatible.

Flash photolysis study of the spectra of CH3O2 and C(CH3)3O2 radicals and the kinetics of their mutual reactions and with NO

The ultraviolet spectra of the alkyl peroxy radicals, CH3O2 and C(CH3)3O2, have been obtained using flash photolysis with photoelectric recording. They are in good agreement with those found using molecular modulation spectroscopy. The overall rates of the “mutual” reactions: 2CH3O2→ products (3), and 2C(CH3)3O2→ products (4) also agree very well with rates found previously.Two other reactions of importance in low temperature oxidation have been studied. A lower limit of 10–12 cm3 molecule–1 s–1 was found for the rate constants of the overall reactions of CH3O2 and of C(CH3)3O2 with NO. The rate constant for the reaction between OH and t-butyl hydro-peroxide was found to be (3.0 ± 0.8)× 10–12 cm3 molecule–1 s–1 at room temperature.

Study of the spectra and recombination kinetics of alkyl radicals by molecular modulation spectrometry. Part 1.—The spectrometer and a study of methyl recombination between 250 and 450 K and perdeutero methyl recombination at room temperature

Using the technique of molecular modulation spectrometry, the rate constant for methyl radical recombination has been measured to be (4.0 ± 0.9)× 10–11 cm3 molecule–1 s–1 between 250 and 450 K. The rate constant for perdeuteromethyl radicals is identical (within 20 %) at room temperature. With 0.6 nm resolution the effective peak absorption cross-section for methyl radicals at 216 nm is (3.2 ± 0.6)× 10–17 cm2, consistent with an absolute cross-section of (4.2 ± 0.8)× 10–17 cm2. With a resolution of 1.0 nm the observed maximum cross-section for CD3 absorption, 3.0 × 10–17, is consistent with an absolute cross-section of (1.0 ± 0.2)× 10–16 cm2.

Self-reactions of isopropylperoxy radicals in the gas phase

The photo-oxidation of 2,2′-azopropane has been studied in order to determine the overall rate constant for the second-order removal of isopropylperoxy radicals. Arrhenius parameters of log10(Aobs/dm3 mol–1 s–1)= 9.15 ± 0.03 and Eobs= 18.65 ± 0.50 kJ mol–1 have been determined over a temperature range 300–373 K. These values are compared with those obtained for the self-reactions of primary and tertiary alkylperoxyradicals, and the corresponding reactions in solution.

Reactions of the O– negative ion with hydrogen and the lower hydrocarbons

The reactions between O–, formed by dissociative attachment to oxygen, and H2, D2, CH4, C2H6, C2H4 and C2H2 have been studied using a drift tube and mass filter. Gas densities ranged over a factor of five about 1017 molecule cm–3 and reduced fields were of the order of 3 × 10–16 V cm2 molecule–1. The reaction with the alkanes gave a single pair of products R + OH–, but the remainder produced both negative ions and free electrons. The following rate constants were measured and with the minor exception of OH– production from C2H4 were found to be substantially independent of the reduced field. [graphic omitted] The reaction with C2H4 also produced OH–, C2H–, C2OH– and C2H3O– at rates of 0.5 to 10, 5, 1.5 and 2 % respectively of the combined rate of the two major channels.The upper limits for the rates of the reactions of O–3 with the same molecules were two orders of magnitude lower.

Oxygen negative ion reactions with carbon dioxide and carbon monoxide. Part 1

A drift tube and mass filter have been used to measure the rates of some O– negative ion molecule reactions, thought to be important in the radiolysis of carbon dioxide. Measurements of the clustering reaction O–+ CO2+ M → CO–3+ M in carbon dioxide give a third-order rate constant which falls with increasing pressure. This suggests an intermediate CO–3 ion with a lifetime of approximately 10–8 s. The limiting, low-pressure, rate constant is (1.1 ± 0.1)× 10–27 cm6 molecule–2 s–1 and, in the high-pressure limit, it is (2.7 ± 0.3)× 10–10 cm3 molecule–1 s–1. The rate also falls slowly with increasing reduced field. In O2 the rate constant is a factor of 3.5 lower, but it is difficult to measure the pressure dependence as accurately because CO–3 is also produced by the reaction: O–3+ CO2→ CO–3+ O2k=(5.5 ± 0.5)× 10–10 cm3 molecule–1 s–1. The rate constant measured in O2 for the associative detachment reaction O–+ CO → CO2+ e is (7.3 ± 0.7)× 10–10 cm3 molecule–1 s–1. Similar experiments in carbon dioxide are complicated by changes in the electron energy distribution as CO is added, but an upper limit of significantly less than 10–13 cm3 molecule–1 s–1 is suggested for the competing reaction: CO–3+ CO → 2CO2+ e. The reaction of O–3 with CO is very slow.

Reactions of oxygenated radicals in the gas phase. Part 6.—Reactions of isopropylperoxy and isopropoxy radicals

The principal products of the photo-oxidation of trans-2,2′-azopropane are acetone, isopropanol, isopropyl hydroperoxide and cis-2,2′-azopropane. The reaction mechanism has been simulated in detail. From the analytical results recorded in this paper and results from the self-reaction of isopropylperoxy radicals, the following rate data have been obtained for these reactions at 302 K. 2(CH3)2CHO2·→(CH3)2CHOH +(CH3)2CO + O2(3a), 2(CH3)2CHO2·→ 2(CH3)2CHO2·+ O2, (3b)(CH3)2CHO·+ O2→(CH3)2CO + HO2·, (5)(CH3)2CHO2H +(CH3)2CHO·→(CH3)2CHOH +(CH3)2CHO2·; (8), k3a= 2.15 ± 0.10 × 105 dm3 mol–1 s–1; k3b= 2.99 ± 0.20 × 105 dm3 mol–1 s–1; k8/k5= 166 ± 5. Further, a value of kd/kc= 0.60 ± 0.01 at 302 K has been found: 2(CH3)2ĊH → CH3CH2CH3+ CH3CHCH2(d), 2(CH3)2ĊH →(CH3)2CH—CH(CH3)2(c)

Recombination of tertiary butyl peroxy radicals. Part 1.—Product yields between 298 and 373 K

Overall product distributions resulting from the recombination of t-butyl peroxy radicals have been studied over the temperature range 298–373 K. The results indicate that over this range there is a switch from the terminating channels (forming alcohol and aldehyde/ketone) towards non-terminating channels (forming two alkoxy radicals) for the two further recombination processes that follow the initial combination of t-butyl peroxy radicals: CH3O2˙+ t-BuO2˙→ products, 2CH3O2˙→ products.There is also direct evidence for the presence of a terminating channel to form di-t-butyl peroxide. This reaction proceeds at a rate of ca. 0.14 of the non-terminating recombination rate at 298 K, but this fraction falls to 0.025 at 333 K and the reaction is not evident at 373 K.Our results demonstrate the importance of abstraction reactions involving alkoxy radicals (t-butoxy and methoxy) and one of the principal recombination products, t-butyl hydroperoxide. Rate constant ratios involving these processes have been derived from the product distributions and from additional studies in which t-butyl hydroperoxide was added. Rate constants of ca. 10–13 cm3 molecule–1 s–1 for these abstraction processes are consistent with our results.

Electron attachment and negative ion-molecule reactions in nitrous oxide

The negative ions formed in N2O and N2O and N2O + O2 mixtures have been studied in the gas phase using a drift tube and mass filter. Gas pressures were in the Torr range, and reduced fields were varied between 10–17 and 10–15 V cm2 molecule–1. The observed ion spectrum was found to be governed by the following reaction, with their associated thermal rate constants: e + N2O → N2+ O–, k1= 4 × 10–15 cm3 molecule–1 s–1, O–+ N2O → NO–+ NO, k3=(1.95 ± 0.06)× 10–10 cm3 molecule–1 s–1, NO–+ N2O → NO–2+ N2, k4=(2.8 ± 0.2)× 10–14 cm3 molecule–1 s–1, NO–+ 2N2O → N3O–2+ N2O, k5=(8.5 ± 1.5)× 10–30 cm6 molecule–2 s–1, O–+ 2N2O → N2O–2+ N2O, k6=(4.2 ± 0.5)× 10–29 cm6 molecule–2 s–1, NO–+ N2O → N2O + NO + e, k11=(6.0 ± 1.0)× 10–12 cm3 molecule–1 s–1, O–2+ N2O → O–3+ N2, k10 < 10–12 cm3 molecule–1 s–1. The rates of reaction, (5), (6) and (11) were weak functions of reduced field. In no experiment was any evidence found for the existence of a long-lived N2O– ion.

Negative ion reactions in nitrous oxide + carbon dioxide mixtures

The rate constants have been measured for the following reactions in N2O + CO2 mixtures at pressures in the Torr region, using a drift tube and mass filter: NO–+ 2CO2→ CO2·NO–+ CO2, k9= 7.5 × 10–29 cm6 molecule–2 s–1, NO–+ CO2→ NO + CO2+ e, k10= 1.0 × 10–11 cm3 molecule–1 s–1, NO–+ CO2+ N2O → CO–3+ N2+ NO, k12= 1.0 × 10–27 cm6 molecule–2 s–1, → N3O–2+ CO2, or → CO2·NO–+ NO2O, k13= 1.5 × 10–28 cm6 molecule–2 s–1. The results obtained here confirm the importance of collisional detachment from NO– in N2O negative ion chemistry.