Rate coefficients and Arrhenius parameters for the reaction of the NO3 radical with acetaldehyde and acetaldehyde-1d

Rate coefficients for the reaction between NO3 and acetaldehyde have been d etermined by the absolute rate fast-flow-discharge technique and by the rel ative rate method. The flow-tube experiments were carried out under pseudo- first-order conditions in NO3 over the temperature range 263-363 K using He as a carrier gas. The data suggests an activation energy of E-a/R=1950 +/- 290 K and k(296)=(9.1 +/-0.8)x10(-15) cm(3) molecule(-1) s(-1) (3 sigma er rors) as an upper limit. Mixing ca. 20% of oxygen in the He carrier gas res ulted in E-a/R=2020 +/- 260 K and k(296)=(2.5 +/-0.5)x10(-15) cm(3) molecul e(-1) s(-1). The relative rate experiments, performed in a static reactor e mploying long path FTIR detection, gave k(298)=(2.62 +/-0.29)x10(-15) cm(3) molecule(-1) s(-1), and showed a moderate kinetic isotope effect, k(CH3CHO +NO3)/k(CH3CDO+NO2)=2.37 +/-0.08 at 298 K. The differences in the reaction rate coefficients obtained by the two methods are analysed and discussed in terms of secondary reactions involving NO3 in the flow-tube. Model studies indicate that acetyl and peroxyacetyl radicals react with NO3 with rate co efficients of 2.5x10(-11) and 1.5x10(-13) molecule cm(-3) s(-1) at 296 K an d 5 mbar, respectively. The reaction was also studied by quantum mechanical methods and the transition states for the abstraction of aldehydic and met hylic hydrogen atoms were located. Their relative energies, calculated on t he MP2/cc-pVDZ//CCSDT/cc-pVDZ level, conform to the reaction proceeding ent irely through H-ald-abstraction at room temperature.