Structural Causes of Singlet/triplet Preferences of Norrish Type II Reactions in Carbonyls
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>
