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

The vapour phase reactions of formaldehyde, formaldehyde-d(2), C-13-formaldehyde, acetaldehyde, acetaldehyde-1-d(1), acetaldehyde-2,2,2-d(3), and acetaldehyde-d(4) 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 3sigma 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.