Experimental and theoretical study of the F, Cl and Br reactions with formaldehyde and acetaldehyde

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 Cl and Br atoms were studied at 298 +/- 2 K and 1013 +/- 10 hPa using long-path FTIR detection. For formaldehyde the only products observed were HCl, HBr and CO; for acetaldehyde the product distribution suggests one dominant channel: CH3CHO +X --> CH3CO+HX. The kinetic isotope effects at 298 K were determined by the relative rate method as: k(Cl+HCHO)/k(Cl+DCDO)=1.302 +/- 0.014, k(Cl+H)(CHO)(13)/k(Cl+DCDO)=1.217 +/- 0.025, k(Br+HCHO)/k(Br+DCDO)=7.5 +/- 0.4 and k(Br+H)(CHO)(13)/k(Br+DCDO)=6.8 +/- 0.4, k(Cl+CH3CHO)/k(Cl+CH3CDO)=1.343 +/- 0.023, k(Cl+CH3CHO)/k(Cl+CD3CDO)=1.323 +/- 0.018, k(Cl+CD3CHO)/k(Cl+CH3CDO)=1.345 +/- 0.015, k(Cl+CD3CHO)/k(Cl+CD3CDO)=1.394 +/- 0.021, k(Br+CH3CHO)/k(Br+CH3CDO)=3.98 +/- 0.26, k(Br+CH3CHO)/k(Br+CD3CDO)=3.79 +/- 0.29, k(Br+CD3CHO)/k(Br+CH3CDO)=4.02 +/- 0.10 and k(Br+CD3CHO)/k(Br+CD3CDO)=3.96 +/- 0.20. Quoted errors represent 3 sigma from the statistical analyses and do not include possible systematic errors. The reactions of F, Cl and Br atoms with formaldehyde and acetaldehyde were studied by quantum chemical methods on the MP2 level of theory using the cc-pVDZ basis sets. The calculations indicate the existence of a weak adduct in which the halogen atoms are bonded to the aldehydic oxygen. Transition states of the reactions X+HCHO --> HX+CHO and X+CH3CHO --> HX +CH3CO (X=F, Cl, Br) were located. Reaction rate coefficients and kinetic isotope effects, calculated from conventional transition state theory are compared to experimental data and the deviations are tentatively attributed to adduct formation.