= Solution
At night, <nitrate radical> addition and <ozone> attack are relevant oxidations of <isoprene>; direct daytime photochemistry is absent, though small OH sources can still exist. The nitrate-containing products identify the NO$_3$-addition route. Add NO$_3$ to the terminal <carbon> next to the methyl-substituted alkene <carbon>, producing a nitrooxy substituent and an allylic radical. Resonance moves the radical to the far terminal <carbon> while shifting the double bond. <Oxygen> trapping gives a <peroxy radical> with the <carbon> skeleton
$$
\mathrm{O_2NOCH_2-C(CH_3)=CH-CH_2OO^{\bullet}}.
$$
A nitrate-forming termination branch, for example the organic-nitrate branch of reaction with NO, replaces the terminal peroxyl group by a <nitrate ester>, giving $\mathrm{O_2NOCH_2-C(CH_3)=CH-CH_2ONO_2}$. An alternative branch forms an <alkoxy radical>, such as peroxy-radical reaction with NO$_3$ producing RO$^{\bullet}$, NO$_2$ and O$_2$. Reaction of the terminal CH$_2$O$^{\bullet}$ group with O$_2$ then gives an aldehyde plus HO$_2$, producing $\mathrm{O_2NOCH_2-C(CH_3)=CH-CHO}$. These are covalent O–NO$_2$ ester groups, not C–NO$_2$ nitro groups. The delocalized radical can be trapped after conformational change, so this mechanism does not require one universal alkene geometry for both product channels. Product structures alone do not fix a unique branching ratio.
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-3-isoprene-products.png]
{title=Two nitrate-containing product channels from an isoprene allylic radical}
{description=Original molecular drawings of the dinitrate and nitrooxy aldehyde carbon skeletons; branching yields and a unique stereochemical ratio are not inferred from the structures alone.}
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