Phillip D. Lightfoot

The rate constant for the HO2+HO2 reaction at elevated temperatures

Abstract The HO 2 +HO 2 reaction has been studied at atmospheric pressure between 298 and 777 K, using a new high-temperature flash photolysis/UV kinetic spectroscopy apparatus. An upward curvature of the Arrhenius plot is observed at temperatures above 600 K, supporting a recent suggestion that - in addition to the well-established low-temperature association complex mechanism - a direct abstraction reaction with a positive activation energy operates at combustion temperatures. Arrhenius parameters for this direct reaction are estimated.

Absolute rate constants for the gas-phase reactions of silylene with silane, disilane and the methylsilanes

Absolute rate constants for reactions of silylene have been determined by time-resolved measurements of its decay at room temperature, following formation by pulsed-laser photolysis of phenylsilane in the presence of various added silanes. For SiH4 and Si2H6 the rate coefficients are pressure dependent and the former reaction is successfully modelled using RRKM theory. High-pressure (or pressure-independent) rate constants (in 10–10 cm3 molecule–1 s–1) are: SiH4, ca. 4.0; Si2H6, ca. 6.5; MeSiH3, 3.66 ± 0.22; Me2SiH2, 3.31 ± 0.26; and Me3SiH, 2.47 ± 0.14. These results are compared with other determinations and the rate constants for the analogous reactions of SiMe2. A model for the insertion reaction is proposed in which the nucleophilic stage of the process plays an important role.

Flash photolysis study of the CH3O2+ CH3O2 and CH3O2+ HO2 reactions between 600 and 719 K: unimolecular decomposition of methylhydroperoxide

The reactions: CH3O2+ CH3O2→ 2CH3O + O2(1a), → CH3OH + HCHO + O2(1b) and CH3O2+ HO2→ CH3OOH + O2(2) have been studied between 600 and 719 K and at atmospheric pressure, using the flash photolysis/UV absorption method. The peroxy radicals were generated via the photolysis of molecular oxygen around 200 nm in the presence of CH4(for CH3O2) and/or CH3OH (for HO2). Results for k1 and k2 are in good agreement with earlier lower-temperature work in this laboratory. Reanalysis of all results from this laboratory to date, using recently reported temperature-dependent absorption cross-sections for CH3O2 and HO2 gives, for the temperature range 248–700 K: k1/cm3 molecule–1 s–1=(1.0 ± 0.1)× 10–13 exp[(416 ± 32)K/T], k2/cm3 molecule–1 s–1=(2.9 ± 0.3)× 10–13 exp[(862 ± 44)K/T] Above 600 K, the unimolecular decomposition of methylhydroperoxide becomes important: CH3OOH + M → CH3O + OH + M (3) Under most conditions, CH3O and OH are converted rapidly to HO2 and CH3O2 respectively, effectively reversing reaction (2). The occurrence of reaction (3) alters the form and increases the timescale of the radical decay, as the only remaining termination reaction for hydroperoxy radicals is their self-reaction. An Arrhenius fit gives: k3/s–1= 10(14.8±0.7) exp[–(177 ± 9)kJ mol–1/RT] Errors are 1σ.

The UV absorption spectrum of SiH3

Abstract Flash photolysis of CCl 4 /SiH 4 /N 2 mixtures in the far-UV gives rise to strong transient absorptions in the region 205–250 nm which are attributed to the first experimental observation of the (A 2 A 1 ← X 2 A 1 ) transition of the SiH 3 radical.

The photodissociation of phenylsilane at 193 nm

Abstract Molecular hydrogen is observed to be one of the major primary products in the 193 nm photodissociation of phenylsilane. A two-channel dissociation mechanism is proposed, yielding PhSiH+H 2 and SiH 2 +PhH with the former predominant. The implications of this observation for experiments which utilise phenylsilane as a precursor for SiH 2 radicals are discussed.

Direct measurements of the peroxy—hydroperoxy radical isomerisation, a key step in hydrocarbon combustion

Abstract The rate coefficient for an alkyl peroxy (RO 2 ) → alkyl hydroperoxy radical isomerisation reaction has been measured directly for the first time (with R = neo-pentyl). The alkyl radical was generated by the 248 nm pulsed photolysis of neo-pentyl iodide and the OH product of the peroxy radical reaction sequence was detected by laser-induced fluorescence. The time constant for the growth of OH varies with [O 2 ], as the rate determining step in the reaction sequence changes. This variation permits the rate coefficients for peroxy radical isomerisation ( k 3 ) and decomposition ( k −2 ) to be determined. At 700 K, k 3 =1.24 × 10 3 s −1 and k −2 =9.1 × 10 3 s −1 . The value obtained for k 3 is more than an order of magnitude smaller than that obtained by Baldwin, Hisham and Walker using indirect methods. It is demonstrated that the present experimental results are compatible with those of Baldwin, Hisham and Walker and the discrepancy arises from their use of an inaccurate estimate of the equilibrium constant for formation of the peroxy radical.

Ultraviolet absorption spectra of the CH2Cl and CHCl2 radicals and the kinetics of their self-recombination reactions from 273 to 686 K

The UV absorption spectra of the chloromethyl (CH2Cl) and dichloromethyl (CHCl2) radicals have been determined between 197.5 and 230 nm, together with the absolute rate constants for their association reactions: CH2Cl + CH2Cl → Products (1a) CHCl2+ CHCl2→ Products (1b) as a function of temperature from 273 to 686 K and between 29 and 760 Torr N2 total pressure. The transient decays of the radicals were monitored by time-resolved UV absorption following the flash photolysis of Cl2 mixed with the parent molecules (CH3Cl or CH2Cl2). At a resolution of 2 nm, no vibrational structure was detected in either spectrum which both appear as strong broad electronic bands with maxima around 200 nm (CH2Cl) and 215 nm (CHCl2). At these wavelengths the absolute absorption cross-sections were measured as (1.45 ± 0.16)× 10–17 and (1.37 ± 0.24)× 10–17 cm2 molecule–1, respectively, relative to that of CH3O2 at 240 nm (4.55 × 10–18 cm2 molecule–1). The experimental results, supported by RRKM calculations, demonstrate that the measured rate constants for removal of the radicals correspond to the high-pressure limiting rate constants for recombination, both of which exhibit a negative temperature dependence, represented by the following expressions: CH2Cl: k1a=(2.8 ± 0.3)× 10–11(T/298)–(0.85 ± 0.14) cm3 molecule–1 s–1. CHCl2: k1b=(9.3 ± 1.7)× 10–12(T/298)–(0.74 ± 0.10) cm3 molecule–1 s–1. Errors are 1 σ. The present results are compared to existing data on the self-recombination reactions of the CH3 and CCl3 radicals; the negative temperature dependences of the self-association rate constants for the series CH3, CH2Cl, CHCl2 and CCl3 are shown to be consistent within the framework of a recently developed variational transition-state theory method.

Flash photolysis study of the spectra and self-reactions of neopentylperoxy and t-butylperoxy radicals

The self-reaction of neopentylperoxy radicals, neo-C5H11O2(NPTO2): NPTO2+ NPTO2→ 2t-C4H9CH2O + O2(1a), → t-C4H9CHO + t-C4H9CH2OH + O2(1b), → t-C4H9CH2OOCH2C4H9-t + O2(1c), has been studied from 248 to 373 K and from 50 to 760 Torr total pressure. The neopentylperoxy radicals fromed via channel (1a) react, under most experimental conditions, by unimolecular decomposition: t-C4H9CH2O + M → t-C4H9+ HCHO + M. (2). The t-butyl radicals so formed are rapidly converted into t-butylperoxy radicals under the conditions employed in this work; these radicals are unreactive on the timescale of the NPTO2 decay and enable the branching ratio for reaction (1) to be determined via their UV absorption. The overall rate constant for reaction (1) displays a strong negative temperature dependence, being well described by k1/cm3 molecule–1 s–1= 3.02 × 10–19(T/298)9.46 exp(4260/T) over the temperature range studied here. The non-terminating channel (1a) becomes increasingly important with increasing temperature, with β=(197 ± 67)exp[-(1658 ± 98)/T], where β is the ratio of those radicals which react via the non-terminating channel (1a) to those which react via the terminating channels (1b) and (1c). By measuring the reduction in the fraction of NPYO2 radicals converted to t-butylperoxy radicals with increasing oxygen concentration, rate constants for reaction (2) were determined, giving E2/kJ mol–1= 42.7 ± 2.1. The UV spectra of NPTO2 and t-C4H9O2 have been determined relative to that of CH3O2; both are similar in shape and magnitude to those of other alkylperoxy radicals, displaying maxima around 240 nm, with σ240 nm(NPTO2)/ cm2 molecule–1=(6.2 ± 1.1)× 10–18 and σ240 nm(t-C4H9O2)/cm2 molecule–1=(4.7 ± 0.8)× 10–18. The self-reaction of t-butylperoxy radicals: 2t-C4H9O2→ 2t-C4H9O + O2(3), was also briefly studied, resulting in k3/cm3 molecule–1 s–1≈ 1.0 × 10–11 exp(–3894/T). Errors are 1σ.

Direct measurements of the neopentyl peroxy-hydroperoxy radical isomerisation over the temperature range 660-750 K

The rate constant for the isomerisation reaction neo-C 5 H 11 O 2 →C 5 H 10 OOH ( k 3 ) has been determined directly over the temperature range 660–750 K. neo-C 5 H 11 I was photolysed at 248 nm using a KrF laser in the presence of O 2 and He. The alkyl radical generated in the photolysis reacts with O 2 to form the peroxy radical which then isomerises to the hydroperoxy radical. Subsequent rapid reactions lead to the generation of OH, which was detected by laser induced fluorescence as a function of time. At high [O 2 ] the time constant, λ + , for the build up of OH tends to − k 3 . As [O 2 ] decreases, earlier reactions in the peroxy radical chain become important and analysis of the [O 2 ] dependence of λ + allows both k 3 and k 2 , the rate constant for the peroxy radical decomposition, to be determined. Data analysis shows that the results are fully compatible with the steady-state measurements of Baldwin et al except that values for k 3 a factor of over ten lower than their values are obtained. The discrepancy is shown to be due to errors in the equilibrium constant, K 2 , they used for the (R2) reaction. C 5 H 11 + O 2 ⇌ C 5 H 11 O 2 An Arrhenius analysis gives ( k 3 / s 1 ) = 10 12.2 − ˙ 0.77 exp { − ( 1.48 ± 0.12 ) × 10 4 K / T } The measurements of k −2 were combined with literature data for k 2 and calculated values of ΔS 2 8 to give ΔH 2 8 (298)=142±6kJ mol −1 for the neo-C 5 H 11 + O 2 ⇌ C 5 H 11 O 2 equilibrium, in satisfactory agreement with group additivity values.

UV absorption spectrum and self-reaction of cyclohexylperoxy radicals

The kinetics and mechanism of the self-reaction of cyclohexylperoxy radicals: 2c-C6H11O2→ 2c-C6H11O + O2(1a), → c-C6H10O + c-C6H11OH + O2(1b) have been studied using both time-resolved and end-product analysis techniques. Determination of the product yields from the photooxidation of Cl2–c-C6H12–O2–N2 mixtures using FTIR spectrometry demonstrates that the branching ratio for the radical-producing channel (1a) is 0.29 ± 0.02 at 295 K. Furthermore, the dependence of the product yields on oxygen partial pressure shows that ring-opening of the cyclohexyloxy radical formed in channel (1a): c-C6H11O + M → CH2(CH2)4CHO + M (4) competes with the reaction with oxygen: c-C6H11O + O2→ c-C6H10O + HO2(2) under atmospheric conditions. Flash photolysis–UV absorption experiments were used to obtain the UV spectrum of the cyclohexylperoxy radical and the kinetics of reaction (1). The spectrum of c-C6H11O2 is similar to those of other alkylperoxy radicals, with a maximum cross-section of (4.95 ± 0.51)× 1018 cm2 molecule–1 at 250 nm, measured relative to a value of 4.55 × 10–18 cm2 molecule–1 for CH3O2 at 240 nm. Reaction (1) is slow compared to the self-reactions of primary alkylperoxy radicals, but is significantly faster than that of isopropylperoxy radicals at room temperature. Experiments as a function of temperature from 253 to 373 K give: kobs(2.0 ± 0.4)× 10–13 exp[–(487 ± 64)K/T] cm3 molecule–1 s–1 for reaction (1). The room-temperature branching ratio measurement enables a value of 2.84 × 10–14 cm3 molecule–1 s–1 to be assigned to k1 at 298 K. The above errors are 1σ and represent experimental uncertainty only; assuming a 10% uncertainty in the CH3O2 calibration cross-section, absolute uncertainties in the values of the cyclohexylperoxy cross-sections and kobs are 16% and 17%, respectively.