A nonempirical SCF molecular orbital study has been made on thiirane and the thiirane-forming addition of sulfur atoms to ethylene. All the lower lying triplet and singlet excited states of thiirane have a ring distorted equilibrium conformation in which the terminal methylene plane is orthogonal to the CCS plane and the considerable energy barrier with respect to rotation of the terminal methylene is responsible for the maintenance of the stereochemical information content of the addition reaction. Since all the vertical excited states lie at higher energies than the sum of the enthalpy change and activation energy of the reaction, a ring distorted triplet state activated complex is implicated which, in the C2H4 + S(3P) system, correlates with the third vertical triplet state of thiirane. The reaction product, the lowest nonvertical 3Σ(12σ → 13σ∗) excited state of thiirane, arises via the sequence: C2H4(1A1) + S(3P) → C2H4S[3B2(4π → 14σ∗)] → C2H4S[3A2(4π → 13σ∗)] → C2H4S[3Σ(12σ → 13σ∗)]. The ultimate fate of the triplet C2H4S is collision-induced intersystem crossing to the ground state. The C2H4 + S(1D2) system correlates with the electronic ground state of thiirane; therefore the addition is a simple concerted process.
ASJC Scopus subject areas
- Colloid and Surface Chemistry