Eric S. C. Kwok

Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structure-reactivity relationship: An update

Abstract The structure-reactivity approach proposed by Atkinson (1986, Chem. Rev. 86 , 69-201) and extended by Atkinson (1987, Int. J. Chem. Kinet. 19 , 799-828) for the calculation of rate constants for the gas-phase reactions of the OH radical with organic compounds has been re-investigated using the presently available database. Substituent group factors for several new groups are derived, including those for fluorinated eithers. Using a large fraction of the available database to derive the parameters needed to calculate the OH radical reaction rate constants, the 298 K rate constants of ∼90% of approximately 485 organic compounds are predicted to within a factor of 2 of the experimental values. Disagreements between calculated and experimental rate constants most commonly occur for halogen-containing compounds, and in particular for haloalkanes, haloalkenes and halogenated ethers. Disagreements also arise for ethers, especially for polyethers and cycloethers. The present estimation technique is reasonably reliable when used within the database used in its derivation, but extrapolation to organic compounds outside of this database results in a lack of assurance of its reliability, and its use for organic compounds which belong to classes other than those used in its development is discouraged.

Reactions of Gas-Phase Phenanthrene under Simulated Atmospheric Conditions

Rate constants for the gas-phase reactions of phenanthrene with OH radicals, NO 3 radicals, and O 3 have been determined at 296±2 K and atmospheric pressure of air. The rate constants obtained (in cm 3 molecule -1 s -1 units) were (1.27±0.23)×10 -11 for the OH radical reaction, (1.2±0.4)×10 -13 for the NO 3 radical reaction under atmospheric conditions, and (4.0±1.0)×10 -19 for the O 3 reaction. These rate constants indicate that the OH radical and NO 3 radical reactions will be the dominant atmospheric loss processes for phenanthrene and that the overall atmospheric lifetime of gas-phase phenanthrene will be ≤1 day

Reactions of alkoxyl radicals in the atmosphere

The products of the gas-phase reactions of the OH radical with n-pentane and [2H12] pentane have been studied at 296 ± 2 K and 740 Torr total pressure of N2–O2 diluent gas in the presence of NO. Direct air sampling, atmospheric-pressure ionisation tandem mass spectrometry showed the formation of the previously predicted C5-hydroxycarbonyl product, together with pentanones. The effect of varying the O2 concentration on the C5-hydroxycarbonyl and pentanones was consistent with the hydroxycarbonyl being the product formed after isomerisation of the pentan-2-oxyl radical. Gas chromatographic analyses of pentan-2-one and pentan-3-one and pentan-2-yl and pentan-3-yl nitrates allowed rate constants for the decomposition and isomerisation reactions of the pentan-2-oxyl and pentan-3-oxyl radicals, relative to their reactions with O2, to be determined.

Products of the gas-phase reactions of o-,m- and p-xylene with the OH radical in thepresence and absence ofNOx

The products of the gas-phase reactions of the OH radical with o-, m- and p-xylene, in the presence and absence of NO x , have been investigated using direct air sampling, atmospheric-pressure ionisation tandem mass spectrometry (API–MS). Partially and fully deuteriated xylenes were also studied. In addition to ion peaks due to tolualdehydes and dimethylphenols, ion peaks observed from the xylene, [ 2 H 6 ]xylene and [ 2 H 10 ]xylene reactions are attributed to the [M+H] + ions of the C 4 -unsaturated dicarbonyl HC(O)CHCHCHO, C 5 -unsaturated dicarbonyls such as CH 3 C(O)CHCHCHO and its isomers, C 6 -unsaturated dicarbonyls such as CH 3 C(O)CHCHC(O)CH 3 and its isomers, C 8 -di-unsaturated dicarbonyls such as CH 3 C(O)C(CH 3 )CHCHCHCHO and its isomers, C 8 -unsaturated epoxy-dicarbonyls and, possibly, C 8 -epoxy-cyclohexenones, and their partially and fully deuteriated analogues. Possible reaction mechanisms leading to the observed products are presented.

Gas-phase atmospheric chemistry of dibenzo-p-dioxin and dibenzofuran.

Rate constants for the gas-phase reactions of dibenzofuran and dibenzo-p-dioxin with the OH radical and O 3 have been determined at 297±2 K and atmospheric pressure of air. Using a relative rate method, rate constants for the OH radical reactions were 3.9×10 -12 cm 3 molecule -1 s -1 for dibenzofuran and 1.48×10 -11 cm 3 molecule -1 s -1 for dibenzo-p-dioxin. Upper limits to the rate constants for the O 3 reactions of <8×10 -20 and <5×10 -20 cm 3 molecule -1 s -1 were obtained for dibenzofuran and dibenzo-p-dioxin, respectively

Kinetics and products of the gas-phase reactions of (CH3)4Si, (CH3)3SiCH2OH, (CH3)3SiOSi(CH3)3 and (CD3)3SiOSi(CD3)3 with Cl atoms and OH radicals

Direct air-sampling atmospheric-pressure ionisation tandem mass spectrometry and FTIR spectroscopy were used to analyse the products of the OH radical- and Cl atom-initiated reactions of hexamethyldisiloxane, [2H18]hexamethyldisiloxane, tetramethylsilane and trimethylsilylmethanol at room temperature and atmospheric pressure. The data obtained indicate the initial formation of (CH3)3SiOSi(CH3)2OCHO and (CD3)3SiOSi(CD3)2OCDO from hexamethyldisiloxane and [2H18]hexamethyldisiloxane, respectively, and (CH3)3SiOCHO from both tetramethylsilane and trimethylsilylmethanol.

Accelerator mass spectrometry for assaying irreversible covalent modification of an enzyme by acetoacetic ester

Protein modification (sometimes known as crosslinking) often requires two or more steps to affix a small molecule irreversibly. Two-step reductive alkylation of the enzyme rabbit muscle aldolase with ethyl 3- 14 C-acetoacetate and sodium cyanoborohydride attaches less radioactivity than with cyanoborohydride omitted. The 14 C level incorporated into aldolase corresponds to only about 15‐30 modified protein molecules per million. Accelerator mass spectrometry (AMS) provides the only technique currently available for investigating the shorter chains from CNBr-cleavage of modified aldolase. Examination of individual fragments reveals that reductive alkylation of the active site lysine in the presence of cyanoborohydride (1BH3CN) is negligible when compared with the extent of covalent modification in the absence of cyanoborohydride (2BH3CN). Labeling by ethyl acetoacetate cannot result from simple acetoacetylation, because dialysis with hydroxylamine does not wash it out. The amount of 14 C incorporated from ethyl 3- 14 C-acetoacetate without cyanoborohydride is roughly proportional to the number of tyrosine residues in each CNBr-fragment, and we surmise that ethyl acetoacetate attaches irreversibly via a reaction specific to that amino acid. Cyanoborohydride inhibits this reaction, but appears to diminish the susceptibility of the active site tyrosine (which is close to a lysine in the tertiary structure of aldolase) less than other tyrosine residues. (Int J Mass Spectrom 179/180 (1998) 185‐193)

Kinetics of the Gas-Phase Reactions of Dibenzothiophene with OH Radicals, NO3 Radicals, and O3

Abstract The kinetics of the gas-phase reactions of OH radicals, NO3 radicals and O3 with dibenzothiophene have been studied at 297 ± 2 K and atmospheric pressure of air. An upper limit to the rate constant for the O3 reaction of >6 × 10−19 cm3 molecule−1 s−1 was obtained, and using a relative rate method a rate constant for the OH radical reaction of (8.1 ± 2.6) × 10−12 cm3 molecule−1 s−1 was measured. The measured rate constant for the NO3 radical reaction increased with increasing NO2 concentration, with a rate constant at low NO2 concentrations representative of the ambient atmosphere of > 7 × 10−14 cm3 molecule−1 s−1 being measured. Nitrodibenzothiophenes were observed as minor products of the OH radical and NO3 radical reactions in the presence of NO2.