Christopher Anastasi

UV Spectrum and kinetics of hydroxymethyl radicals

Abstract Hydroxymethyl radicals were produced by pulse radiolysis of gas mixtures with varying mole ratios of Ar, SF6, HCl and CH3OH to initiate the reactions (1) X+CH3OH→HX+CH2OH and (2) X+CH3OH→HX+CH3O, where X=OH, F and Cl. The ultraviolet absorption spectrum of CH2OH is composed of several vibronic progressions in the range 200–285.3 nm. Band heads of 3s and 3p Rydberg transitions have been identified by comparison with the results of recent ab initio configuration interaction studies of CH2OH. Variations in the yields and kinetics of CH2OH were studied by monitoring the transient absorption at the 0-0 band of the 3s Rydberg transition at 285.3 nm. Branching ratios for reactions (1) and (2) with X=F and OH were determined as well as rate constants for a number of elementary reactions, including the self-reaction of CH2OH and the reaction with O2.

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.

Atmospheric lifetimes of selected fluorinated ether compounds

Abstract Atmospheric lifetimes have been estimated for a selection of ethers, the latter representing a class of compounds being considered as replacements for chlorofluorocarbons. The estimates are based on laboratory measurements of rate constants for the reaction of the OH radical with the ethers, and a comparison with the behaviour of methyl chloroform in the atmosphere. The lifetimes for the ethers ranged from a few hours to half a year, significantly lower than those of chlorofluorocarbons and other replacements being considered.

Reaction of CH3 radicals with OH at room temperature and pressure

The kinetics of the reaction CH3+ OH(+M)→ CH3OH(+M) have been studied by pulse radiolysis combined with transient ultraviolet absorption spectrophotometry. The radical source reactions (2) F + CH4→ HF + CH3 and (3) F + H2O → HF + OH were initiated by pulse radiolysis of Ar/SF6/CH4/H2O mixtures at a total pressure of 1 atm and room temperature. The kinetics of methyl radicals were studied by monitoring the transient absorption signals at 216.4 nm. In the absence of water vapour the observed decay was simple second order in accordance with the self-reaction (4) CH3+ CH3(+M)→ C2H6(+M). The initial yield of F-atoms was derived from the observed maximum absorbance of CH3 produced via reaction (2) using a consensus value of σ(CH3). The relative yields of CH3 and OH were controlled by varying the [H2O]/[CH4] concentration ratio and the ratio of the rate constants k2/k3= 3.2 ± 0.2 was derived from the observed variation in the yield of methyl radicals as a function of the [H2O]/[CH4] concentration ratio. The decay kinetics of CH3 were studied as a function of the relative radical yields, G(OH)/G(CH3). The kinetic features were analysed by computer modelling of reactions (2)–(4) combined with the reaction (1) CH3+ OH(+M)→ CH3OH(+M), (5) OH + OH(+M)→ H2O2(+M) and (6) OH + OH → O + H2O. A representative set of experimental decay curves could be fitted within the signal-to-noise ratio after adjustment of the cross-combination rate constant to a value of k1=(9.4 ± 1.3)× 10–11 cm3 molecule–1 s–1.

Rate constants for the reactions of CH3 radicals with C2H5, i-C3H7 and t-C4H9 radicals

Molecular modulation spectrometry has been used to measure the overall cross-reaction rate constants for the reactions of CH3 radicals with C2H5, i-C3H7 and t-C4H9 radicals at 308 K. The values of the rate constants obtained are (in cm3 mol–1 s–1): CH3+ C2H5, k=(28.5 ± 2.2)× 1012; CH3+ i-C3H7, k=(24.3 ± 2.3)× 1012; CH3+ t-C4H9, k=(12.8 ± 1.1)× 1012. Mutual reaction rate constants and absorption cross-sections for these radicals have also been measured in the same system. The values obtained are (k in cm3 mol–1 s–1 and σ in cm2 molecule–1): CH3, k=(20.8 = 0.8)× 1012 and σ=(30.8 ± 0.8)× 10–18; C2H5, k=(10.2 ± 0.9)× 1012 and σ=(1.89 ± 0.10)× 10–18; C3H7, k=(6.78 ± 0.58)× 1012 and σ=(3.89 ± 0.19)× 10–18; t-C4H9, k=(4.96 ± 0.20)× 1012 and σ=(7.47 ± 0.24)× 10–18. Combining these results with the appropriate disproportionation-combination ratios gives values for the corresponding combination and disproportionation rate constants, and cross-reaction and cross-combination ratios have been evaluated. An evaluation of the data for i-C3H7+ i-C3H7 leads to k=(6.69 ± 2.14)× 1012 cm3 mol–1 s–1 being recommended as the best value for the overall rate constant in the temperature range 300–400 K.

Ultraviolet absorption spectra and kinetics of CH3S and CH2SH radicals

Abstract The ultraviolet absorption spectra of CH 3 S and CH 2 SH radicals have been measured between 215 and 380 nm using the pulse-radiolysis/kinetic-abso method. One absorption band between 250 and 300 nm and one around 215 nm have been tentatively assigned to the CH 2 SH and CH 3 S radicals, respectively. This spectrum has been used to measure the self-reaction rates of these radicals. Rate constants of 4 × 10 −11 and 7 × 10 −11 cm 3 molecule −1 s −1 have been measured at 298 K for CH 3 S and CH 2 SH recombination, respectively. The possible reaction pathways are discussed.

Reaction kinetics in acetyl chemistry over a wide range of temperature and pressure

The molecular modulation spectrometer has been used to study the complex chemical kinetics involved in acetyl radical chemistry. This has involved direct monitoring of both acetyl and methyl radicals in the same experiment and over a variety of temperatures (263 ⩽T/K ⩽ 343) and total gas concentration (0.3 ⩽[M]/1019 molecule cm–3⩽ 2.7) conditions. These measurements have been complemented by a non-linear least-squares analysis of the experimental data and simple product studies. Rate data on four reactions and the absorption cross-section of the acetyl radical at 223 nm have been determined in this way. Unimolecular rate theory, based on Kassel integrals, has been applied to the pressure-dependent formation and decay of the radical to extract limiting values for the rate constants at T= 303 and 343 K.

Reactions of oxygenated radicals in the gas phase. Part 10.—Self-reactions of ethylperoxy radicals

The photo-oxidation of azoethane has been studied by molecular modulation spectroscopy in order to determine the overall rate constant for the second-order removal of ethylperoxy radicals between 303 and 457 K. The principal molecular products of the reaction between 302 and 373 K are acetaldehyde, ethanol and ethyl hydroperoxide. From the reaction rate data and analytical results, the following results have been obtained for the reactions 2C2H5O2˙→ CH2CHO + C2H5OH + O2. (3a) 2C2H5O2˙→ 2C2H5O˙+ O2. (3b)k3b/k3a increases with temperature from 1.75 ± 0.05 at 302 K to 2.45 ± 0.15 at 373 K. Arrhenius expressions for reactions (3a) and (3b) have been estimated.

Ab initio configuration interaction study of the Rydberg states of the hydroxymethyl radical CH2OH

Abstract The ultraviolet absorption spectrum of the hydroxy methyl radical has been studied by ab initio configuration interaction methods using a large Gaussian-type basis set. The first four vertical electronic transition energies have been calculated to be at 287, 243, 221 and 219 nm. Dipole moments are presented for all the states together with the oscillator strengths for the transitions. A detailed analysis of the orbitals shows that the transitions are of Rydberg type which can be classified as π→3s, π→3p x , π→3p y and π→3p z .

Rate constants for: OH + NO2 (+ N2) → HNO3 (+ N2) between 220 and 358 K

Abstract Rate constants have been determined for the reaction OH + NO 2 (+ N 2 ) → HNO 3 (+ N 2 ), using time-resolved resonance absorption to follow the removal of OH radicals produced by flash photolysis of HNO 3 . The measurements cover the ranges: 220 ⩽ T ⩽ 358 K and 3.2 × 10 17 ⩽ [N 2 ] ⩽ 4.0 × 10 18 molecule cm −3 .