Excited states of cyclic-conjugated anions: a theoretical study of the photodecarboxylation of cycloheptatriene and cyclopentadiene carboxylate anions

Abstract

Correlation diagrams and semiempirical calculations suggest that the same carbanion intermediates are involved in both the photodecarboxylation reaction of cycloheptatrienyl and cyclopentadienyl carboxylate anions and the photodeprotonation reaction of cycloheptatriene and cyclopentadiene; therefore, the same ‘4n rule’ applies to both processes, which can proceed only on the HOMO→LUMO excited potential energy surface of A″ symmetry. This is confirmed by ab initio CASSCF calculations which show that the second excited singlet state of cycloheptatriene carboxylic acid is of A″ symmetry, and that photodecarboxylation proceeds without barriers only on this A″ potential energy surface. This validates the proposition that the difference in excited-state acidity of suberene and the parent compound cycloheptatriene is due to a different ordering of the excited states.