Photochemistry of Thietane Excited to Its Second Excited Electronic Singlet State

Article abstract of DOI:10.1021/j150618a021

Thietane, excited to its S2 state, undergoes fragmentation to ethylene and thioformaldehyde by one channel and a competing reaction, unique to the S2 state, is decomposition to cyclopropane and sulfur atoms.Previous work on the S2 state of thietane has been extended to include a more detailed examination of the spectra and energy partitioning in the cyclopropane formimg reaction; and, by photolizing cis- and trans-3-ethyl-2-propylthietane, we have followed the stereochemical course of both reaction channels.The products of both reactions largely retain the stereochemistry of the reactant.Energy partitioning indicates that S(³P) is the atomic fragment when cyclopropane is produced.A mechanism which accomodates the experimental results assumes that, once excited to its ¹B2 electronic state, intersystem crossing to the ³B2 state competes with C-S bond rupture that forms the 1,4-diradical intermediate which yields the ring cleavage products.The ³B2 state decomposes to cyclopropane and S(³P) by a mechanism that likely involves a singlet trimethylene diradical as an intermediate.