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Structural Causes of Singlet/triplet Preferences of Norrish Type II Reactions in Carbonyls
preprintsubmitted on 11.09.2020, 07:40 and posted on 11.09.2020, 10:24 by Keiran Rowell, Scott Kable, Meredith J. T. Jordan
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 S1 and T1 thresholds agree well with triplet quenching experiments. However, many linear-response methods deliver poor S1 energetics, which is explained by a S1/S0 conical intersection in close proximity to the S1 transition state. Multiconfigurational CASSCF calculations confirm a conical intersection features across all carbonyl classes.
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 T1 dominates only in species where the S1 thresholds are high — typically ketones. The α, β-unsaturated carbonyls cannot cleave the α–β bond, causing them to photoisomerise. A concerted S0 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 T1 threshold.