Ingvar Wängberg

Rates of reaction between the nitrate radical and some unsaturated alcohols

Rate coefficients for nitrate radical gas-phase reactions with prop-2-en-l-ol (allyl alcohol), but-1-en-3-ol, and 2-methylbut-3-en-2-ol have been determined. Both absolute (fast flow discharge with diode laser detection of NO3) and relative (batch reactor and FTIR spectroscopy) rate techniques were used to measure the rate coefficients. The rate coefficients at 294 K are: (1.3 ± 0.2) × 10−14, (1.2 ± 0.3) × 10 −14, and (2.1 ± 0.3) × 10−14 cm3 molecule−1 s−1 for prop-2-en-1-ol, but-1-en-3-ol, and 2-methylbut-3-en-2-ol, respectively. The activation energy for reaction of NO3 with prop-2-en-1-ol was determined to 2.8 ± 2.5 kJ mol−1 in the temperature range between 273 and 363 K. The atmospheric importance of unsaturated alcohols and structure-reactivity considerations are also discussed.

Rates of reaction between the nitrate radical and some aliphatic esters

Rate coefficients for the reaction of NO3 with methyl formate, methyl acetate, methyl propionate, methyl butylrate, ethyl formate, ethyl acetate, ethyl propionate, propyl formate, propyl acetate and n-hexane have been determined at 296 ± 2 K. The rate coefficients, which should strictly be interpreted as upper limits, were found to be 0.36 ± 0.08, 0.7 ± 0.2, 3.3 ± 0.8, 4.8 ± 0.5, 1.7 ± 0.3, 1.3 ± 0.3, 3.3 ± 0.4, 5.4 ± 0.9, 5 ± 2 and 14.7 ± 3.0, respectively (in units of 10–17 cm3 molecule–1 s–1). The activation energy for the reaction between ethyl acetate and NO3 was determined to be 23 ± 8 kJ mol–1 between 273 and 373 K. The rate coefficients for aliphatic esters may be predicted from available group reactivity factors for alkanes provided that formate carbonyl hydrogen atoms are treated as primary hydrogen atoms.

UV absorption spectra and kinetics for alkyl and alkyl peroxy radicals originating from di-tert-butyl ether

Abstract Alkyl, (CH 3 ) 3 COC(CH 3 ) 2 CH 2 , and alkyl peroxy, (CH 3 ) 3 COC(CH 3 ) 2 CH 2 O 2 , radicals from di-tert-butyl ether (DTBE), have been studied in the gas phase at 296 K. A pulse radiolysis UV absorption technique was used to measure the spectra and kinetics. Absorption cross sections were quantified over the wavelength range 220–330 nm. At 240 nm, σ (CH 3 ) 3 COC(CH 3 ) 2 CH 2 = (3.6 ± 0.4) × 10 −18 cm 2 molecule −1 and at 260 nm, σ (CH 3 ) 3 COC(CH 3 ) 2 CH 2 O 2 = (3.8 ± 0.3) × 10 −18 cm 2 molecule −1 have been obtained. The observed rate constants for the self-reaction of (CH 3 ) 3 COC(CH 3 ) 2 CH 2 and (CH 3 ) 3 COC(CH 3 ) 2 CH 2 O 2 radicals were found to be (2.9 ± 0.2) × 10 −11 cm 3 molecule −1 s −1 and (2.7 ± 0.2) × 10 −12 cm 3 molecule −1 s −1 , respectively. For the reaction of the alkyl radical with O 2 in 1 atm pressure of SF 6 a rate constant of (7.2 ± 1.1) × 10 −13 cm 3 molecule −1 s −1 was found. The rate constants for the reaction of the alkyl peroxy radicals with NO and NO 2 were determined to be (1.8 ± 0.2) × 10 −12 and (9.9 ± 1.3) × 10 −12 cm 3 molecule −1 s −1 , respectively. As a part of the work the rate constants k (F + DTBE) and k (O + DTBE) were determined to be (2.6 ± 0.6) × 10 −10 and (4.0 ± 0.9) × 10 −12 cm 3 molecule −1 s −1 , respectively.

Mechanisms and products of the reactions of NO3 with cycloalkenes

The NO3 radical initiated oxidation of cyclopentene, cyclohexene and 1-methyl-cyclohexene has been studied. The products formed in an N2O5-NO2-N2-O2-cycloalkene-static reactor system, at 0.1 MPa and 296 K, were investigated using long path FTIR. The principal products were aldehydes formed via a ring opening process. The reactions also resulted in significant yields of three types of ring retaining nitrooxy-substituted compounds. The average yields of alkyl nitrates from, e.g., reactions with cycloalkene were 25.1% 2-oxo-cyclohexyl nitrate, 22.8% 2-hydroxy-cyclohexyl nitrate and 4.0% 1,2-cyclohexyl dinitrate. The mechanisms involved resembles those proposed for acyclic alkenes. In absence of NO, α-oxo and α-hydroxy-cycloalkyl nitrates are formed via self reactions of α-nitrooxy substituted cycloalkyl peroxy radicals. Estimated branching ratios for the reactants leading to ring retaining products in the presence and in the absence of NO are given and the possible relevance of these reactions for cycloalkenes under tropospheric conditions is discussed.

Atmospheric chemistry of di-tert-butyl ether: Rates and products of the reactions with chlorine atoms, hydroxyl radicals, and nitrate radicals

The rate constants for the gas-phase reactions of di-tert-butyl ether (DTBE) with chlorine atoms, hydroxyl radicals, and nitrate radicals have been determined in relative rate experiments using FTIR spectroscopy. Values of k(DTBE+CI) = (1.4 ± 0.2) × 10−10,k(DTBE+OH) = (3.7 ± 0.7) × 10−12, and k(DTBE+N03) = (2.8 ± 0.9) × 10−16 cm3 molecule−1 s−1 were obtained. Tert-butyl acetate was identified as the major product of both Cl atom and OH radical initiated oxidation of DTBE in air in the presence of NOx. The molar tert-butyl acetate yield was 0.85 ± 0.11 in the Cl atom experiments and 0.84 ± 0.11 in OH radical experiments. As part of this work the rate constant for reaction of Cl atoms with tert-butyl acetate at 295 K was determined to be (1.6 ± 0.3) × 10−11 cm3 molecule−1 s−1. The stated errors are two standard deviations (2σ).

Heterogeneous transformation of peroxyacetylnitrate

Abstract The heterogeneous decomposition of peroxyacetylnitrate (PAN) has been investigated using a flow reactor and infrared spectroscopic analysis. The decomposition rate in air due to glass surfaces follows the relation d[PAN]/d t = − S / V ([PAN] × 7 × 10 7 + [CH 3 C(O)OO] × 5 × 90 12 )exp(−9382/ T ) molecules cm −3 s −1 ( S / V =surface to volume ratio). The rate observed for NH 4 HSO 4 -covered surfaces is lower than in the glass case. The rate is high enough to affect many laboratory experiments but too slow to have any influence on PAN decomposition under ambient conditions.

Absolute rate coefficients for the reaction between nitrate radicals and some cyclic alkenes

Absolute rate coefficients for the reaction between NO3 and cyclopentene, cyclohexene and 1-methylcyclohexene have been determined to be (5.9 ± 1.1)× 10–13(295 K), (6.3 ± 1.3)× 10–13(294 K) and (1.5 ± 0.5)× 10–11(293 K)(units of cm3 molecule–1 s–1), respectively, using the fast-flow–discharge technique. The activation energies for the NO3+ cyclohexene and 1-methylcyclohexene reactions were 1 ± 4 and 0 ± 5 kJ mol–1. The simple cyclic alkenes react slightly faster than (Z)-but-2-ene which contains a similar active structural element. The rate coefficient for reaction with 1-methylcyclohexene is close to that for Δ3-carene or 2-carene but significantly greater than for α-pinene.

ATMOSPHERIC CHEMISTRY OF BIFUNCTIONAL CYCLOALKYL NITRATES

Abstract Reported here are some aspects of the atmospheric chemistry of the cycloalkyl nitrates trans-2-hydroxy-cyclopentyl-1-nitrate (HCPN), 2-oxo-cyclohexyl-1-nitrate (OCHN) and trans-1-methyl-cyclohexyl-1,2-dinitrate (MCHDN). Their UV absorption spectra have been measured and rate coefficients for their reaction with OH radicals have been determined at 302 K in 1000 mbar of synthetic air. The tropospheric lifetimes of the cycloalkyl nitrates have been estimated from calculations of their photolysis frequencies and first-order OH-radical loss rates. HCPN will not photolyse under atmospheric conditions and for OCHN and MCHDN 24 h globally-averaged photolysis lifetimes of 1.6 and 7.7 days, respectively, have been estimated. The globally-averaged 24 h lifetimes due to reaction with OH radicals are approximately 4 days for both HCHN and OCHN and 8 days for MCHDN. Therefore, for OCHN and MCHDN both photolysis and reaction with OH radicals will be significant atmospheric sinks. For HCHN wet deposition may also be important due to the increased solubility of this compound.

Laboratory Studies of Some Thermal Reactions Involving NOy Species

An investigation of the mechanism of heterogeneous PAN decomposition was made. It was found that the reactions of the acetylperoxy radical were influenced slightly by the presence of surfaces. The temperature dependence of the sidechannel of the reaction between nitrate radicals and nitrogen dioxide, producing nitric oxide, nitrogen dioxide and molecular oxygen, was investigated. Monte Carlo simulation calculations of the photolysis of nitrogen dioxide were performed. Reaction products and in some cases also kinetic information were determined for several alkenes reacting with nitrate radicals. Structurally related, simple carbonyl compounds were among the more abundant products together with nitrooxy-substituted ketones and alcohols. A systematic investigation of reaction kinetics and product distributions from nitrate radical reaction with aliphatic esters, ethers and alcohols was made. The rate coefficients for ethers and alcohols were found to be substantially higher than expected from simple correlations, e.g. for alkanes. This is interpreted as an effect of the oxygen atom, weakening adjacent hydrogen-carbon bonds. The oxygen atom also has an additional influence, possibly via stabilisation of the transition complex. The nitrate radical is quite specific for abstraction of the weakest bonded hydrogen atom and reaction products can often be deduced from this information combined with common knowledge of atmospheric gas-phase reactions.