Experimental and theoretical studies of gas phase NO3 and OH radical reactions with formaldehyde, acetaldehyde and their isotopomers

Abstract

The vapour phase reactions of formaldehyde, formaldehyde-d₂, ¹³C-formaldehyde, acetaldehyde, acetaldehyde-1-d₁, acetaldehyde-2,2,2-d₃, and acetaldehyde-d₄ with NO₃ and OH radicals were studied at 298 ± 2 K and 1013 ± 10 mbar using long-path FTIR detection. The values of the kinetic isotope effects at 298 K, as determined by the relative rate method, were: k_NO3+HCHO/k_NO3+DCDO = 2.97 ± 0.14, k_NO3+HCHO/k_NO3+H¹³CHO = 0.97 ± 0.02, k_OH+HCHO/k_OH+DCDO = 1.62 ± 0.08, and k_OH+H¹³CHO/k_OH+DCDO = 1.64 ± 0.12, k_OH+HCHO/k_OH+H¹³CHO = 0.97 ± 0.11, k_NO3+CH3CHO/k_NO3+CD3CHO = 1.19 ± 0.11, k_NO3+CH3CHO/k_NO3+CD3CDO = 2.51 ± 0.09, k_OH+CH3CHO/k_OH+CH3CDO = 1.42 ± 0.10, k_OH+CH3CHO/k_OH+CD3CHO = 1.13 ± 0.04, and k_OH+CH3CHO/k_OH+CD3CDO = 1.65 ± 0.08. Quoted errors represent 3σ from the statistical analyses. These errors do not include possible systematic errors.

The reactions of NO₃ and OH radicals with formaldehyde and acetaldehyde were studied by quantum chemical methods on the MP2 and CCSD(T) levels of theory using the aug-cc-pVDZ and aug-cc-pVTZ basis sets which include diffuse functions. The calculations indicate the existence of weak adducts in which the radicals are bonded to the aldehydic oxygen. Transition states of the reactions X + HCHO → products, and X + CH₃CHO → products (X = OH, NO₃) were located. Energy level diagrams for reactants, intermediates and products in the OH reactions are presented and discussed in relation to the observed product distribution. Reaction rate coefficients and kinetic isotope effects calculated from different models of the reaction rate theory are compared to experimental data.