Tropospheric chemistry of aromatic compounds: evidence for a rate-determining addition of NO_3 and OH to substituted toluenes

摘要

The OH- and the NO_3-initiated oxidation of aromatic hydrocarbons are major sinks for these species, observed in high concentrations in urban areas. This demands a better knowledge of mechanisms and reaction products. The plot of log k (six compounds) for para-substituted toluenes vs Hammett's p for the reaction with OH has a p=-2.3+-0.2 r~2=0.96. This is in line with the known rate-determining addition of OH involving a polar transition state. The analogous plot with NO_3(nine compounds) gives p=-4.3+-0.6; r~2=0.87. This negative value suggests again a polar transition state very similar to that occurring with OH and indicates a rate-determining addition for this reaction. Unrestricted Hartree-Forch (UHF)-Austin Model 1 (AM1) calculations suggest that the isomeric cyclohexadienyl radicals formed in the addition of OH or NO_3 to toluene and xylenes have similar stability, with the exception of the "ipso" adduct, which is less stable. The adducts formed by addition of oxygen or NO_2 to the cyclohexadienyl radicals are mixtures of substituted cis- and trans 1,2-dihydrobenzenes. AM1 calculations show that these isomers have very similar stability. Hence, the ring cleavage/rearomatization ratio in the reaction of OH with aromatic compounds is probably due to the stereochemical requirements of the 32 reaction leading to rearomatization. This mechanism is in fact possible only for the cis isomer.