Reactions of the Alkoxy Radicals Formed Following OH-Addition to α-Pinene and β-Pinene. C−C Bond Scission Reactions

The atmospheric degradation pathways of the atmospherically important terpenes α-pinene and β-pinene are studied using density functional theory. We employ the correlation functional of Lee, Yang, and Parr and the three-parameter HF exchange functional of Becke (B3LYP) together with the 6-31G(d) basis set. The C−C bond scission reactions of the β-hydroxyalkoxy radicals that are formed after OH addition to α-pinene and β-pinene are investigated. Both of the alkoxy radicals formed from the α-pinene−OH adduct possess a single favored C−C scission pathway with an extremely low barrier (∼3 kcal/mol) leading to the formation of pinonaldehyde. Neither of these pathways produces formaldehyde, and preliminary computational results offer some support for suggestions that 1,5 or 1,6 H-shift (isomerization) reactions of alkoxy radicals contribute to formaldehyde production. In the case of the alkoxy radical formed following OH addition to the methylene group of β-pinene, there exists two C−C scission reactions with nearly identical barrier heights (∼7.5 kcal/mol); one leads to known products (nopinone and formaldehyde) but the ultimate products of the competing reaction are unknown. The single C−C scission pathway of the other alkoxy radical from β-pinene possesses a very low (∼4 kcal/mol) barrier. The kinetically favored C−C scission reactions of all four alkoxy radicals appear to be far faster than expected rates of reaction with O2. The rearrangement of the α-pinene−OH adduct, a key step in the proposed mechanism of formation of acetone from α-pinene, is determined to possess a barrier of 11.6 kcal/mol. This value is consistent with another computational result and is broadly consistent with the modest acetone yields observed in product yield studies.