Products and Mechanism of the Reaction of 1‑Pentadecene with
NO<sub>3</sub> Radicals and the Effect of a −ONO<sub>2</sub> Group on Alkoxy Radical
Decomposition
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Abstract
The linear C<sub>15</sub> alkene,
1-pentadecene, was reacted with NO<sub>3</sub> radicals in a Teflon
environmental chamber and yields of secondary organic aerosol (SOA)
and particulate β-hydroxynitrates, β-carbonylnitrates,
and organic peroxides (β-nitrooxyhydroperoxides + dinitrooxyperoxides)
were quantified using a variety of methods. Reaction occurs almost
solely by addition of NO<sub>3</sub> to the CC double bond
and measured yields of β-hydroxynitrate isomers indicate that
92% of addition occurs at the terminal carbon. Molar yields of reaction
products determined from measurements, a proposed reaction mechanism,
and mass-balance considerations were 0.065 for β-hydroxynitrates
(0.060 and 0.005 for 1-nitrooxy-2-hydroxypentadecane and 1-hydroxy-2-nitrooxypentadecane
isomers), 0.102 for β-carbonylnitrates, 0.017 for organic peroxides,
0.232 for β-nitrooxyalkoxy radical isomerization products, and
0.584 for tetradecanal and formaldehyde, the volatile C<sub>14</sub> and C<sub>1</sub> products of β-nitrooxyalkoxy radical decomposition.
Branching ratios for decomposition and isomerization of β-nitrooxyalkoxy
radicals were 0.716 and 0.284 and should be similar for other linear
1-alkenes ≥ C<sub>6</sub> whose alkyl chains are long enough
to allow for isomerization to occur. These branching ratios have not
been measured previously, and they differ significantly from those
estimated using structure–activity relationships, which predict
>99% isomerization. It appears that the presence of a −ONO<sub>2</sub> group adjacent to an alkoxy radical site greatly enhances
the rate of decomposition relative to isomerization, which is otherwise
negligible, and that the effect is similar to that of a −OH
group. The results provide insight into the effects of molecular structure
on mechanisms of oxidation of volatile organic compounds and should
be useful for improving structure–activity relationships that
are widely used to predict the fate of these compounds in the atmosphere
and for modeling SOA formation and aging