Branching Ratios and Rate Constants for Decomposition and Isomerization of β-Hydroxyalkoxy Radicals Formed from OH Radical-Initiated Reactions of C-C 2-Methyl-1-Alkenes in the Presence of NO.

A series of C-C 2-methyl-1-alkenes were reacted with OH radicals in the presence of NO in a Teflon environmental chamber, and molar yields of the 2-ketone products were measured using gas chromatography. Yields were corrected for secondary reactions with OH radicals and for gas-wall partitioning of the 2-methyl-1-alkene and 2-ketone, with the latter correction being determined from measurements of gas-wall partitioning of 2-ketone standards. Molar yields of 2-ketones decreased with increasing 2-methyl-1-alkene carbon number from a maximum of 0.82 for C to a minimum of 0.34 ± 0.02 for C-C, which after normalization for the fraction of reaction that occurred by OH radical addition to the C═C double bond (with the rest occurring by H atom abstraction) were 0.86 and 0.39 ± 0.01. These yields were combined with branching ratios determined previously for site-specific OH radical addition to the C═C double bond and for formation of β-hydroxynitrates to determine branching ratios for decomposition and isomerization of β-hydroxyalkoxy radicals. Branching ratios for decomposition decreased with increasing 2-methyl-1-alkene carbon number from a maximum of 0.97 for C to a minimum of 0.49 ± 0.01 for C-C, while the corresponding values for isomerization increased from 0.03 to 0.51 ± 0.01. The results were used to estimate absolute rate constants and activation energies for decomposition and isomerization and were also combined with previously measured yields of β-hydroxynitrates, dihydroxynitrates, trihydroxynitrates, and H atom abstraction products to obtain yields of ∼75% for the C-C reaction products, with the remainder likely being mostly dihydroxycarbonyls and trihydroxycarbonyls.