Global potential energy surface and product pair-correlated distributions for the F(2P) + SiH4 reaction. Comparison with experiments.

In this paper we study the gas-phase hydrogen abstraction reaction between fluorine atoms and silane in a three-step process: potential energy surface, kinetics and dynamics. Firstly, we developed for the...

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>₁ and <i>T</i>₁ thresholds agree well with triplet quenching experiments. However, many linear-response methods deliver poor <i>S</i>₁ energetics, which is explained by a <i>S</i>₁/<i>S</i>₀ conical intersection in close proximity to the <i>S</i>₁transition 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>₁ dominates only in species where the <i>S</i>₁ 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>₀ 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>₁ threshold.</div>

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>₁ and <i>T</i>₁ thresholds agree well with triplet quenching experiments. However, many linear-response methods deliver poor <i>S</i>₁ energetics, which is explained by a <i>S</i>₁/<i>S</i>₀ conical intersection in close proximity to the <i>S</i>₁transition 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>₁ dominates only in species where the <i>S</i>₁ 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>₀ 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>₁ threshold.</div>