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MNDOC‐CI calculations for organic photoreactions. I. The α‐cleavage reaction of carbonyl compounds

1990; Wiley; Volume: 3; Issue: 2 Linguagem: Inglês

10.1002/poc.610030204

1099-1395

Manfred Reinsch, Martin Klessinger,

Radical Photochemical Reactions

Abstract MNDOC‐CI results are reported for the photochemical α‐cleavage reaction of carbonyl compounds. Potential energy surfaces for some excited singlet and triplet states and cross‐sections along the bitopic reaction path show that the activation energy of the α‐cleavage reaction is determined by the relative energetic position of the lowest two triplet states. The reactivity of aliphatic ketones depends only on the degree of branching at the α‐carbon, because the triplet barrier is due to an (avoided) crossing of two surfaces corresponding to an nπ* and ππ* excitation respectively, which correlates with the stability of the leaving alkyl radical. For conjugated carbonyl compounds, on the other hand, the position of the crossing point is independent of the 3 nπ* excitation energy, but if the 3 ππ* surface is below the 3 nπ* surface all the way along the reaction coordinate, the reaction is determined by the barrier on the 3 ππ* surface and any additional stabilization of the 3 ππ* state increases the barrier.