Photolysis thresholds are calculated
for the Norrish Type II (NTII) intramolecular γ-hydrogen abstraction reaction in 22 structurally informative
carbonyl species. The B2GP-PLYP excited state <i>S</i><sub>1</sub> and <i>T</i><sub>1</sub> thresholds agree
well with triplet quenching experiments.
However, many linear-response methods deliver poor <i>S</i><sub>1</sub> energetics, which
is explained by a <i>S</i><sub>1</sub>/<i>S</i><sub>0</sub> conical intersection in close proximity to the <i>S</i><sub>1
</sub>transition state. Multiconfigurational
CASSCF calculations confirm a conical
intersection features across all carbonyl
classes. <div><br></div><div>Structure–activity relationships are
determined that could be used in atmospheric carbonyl photochemsitry modelling. This is exemplified for butanal,
whose NTII quantum yields are too
low when used as a ‘surrogate’ for
larger carbonyls, since butanal lacks the
γ-substitution that stabilises the 1,4-
biradical. Reaction on <i>T</i><sub>1</sub> dominates
only in species where the <i>S</i><sub>1</sub> thresholds
are high — typically ketones. The α, β-unsaturated carbonyls cannot cleave
the α–β bond, causing them to photoisomerise. A concerted <i>S</i><sub>0</sub> NTII mechanism is calculated to be viable and
may explain the recent detection of
NTII photoproducts in the photolysis
of pentan-2-one below the <i>T</i><sub>1</sub> threshold.</div>
Antioxidant Activity of 1’-Hydroxyethylnaphthazarins and their Derivatives
