| Abstract
| - Irradiation (λ > 254 nm) of matrix-isolated 2,5-diiodothiophene (10) gives rise to IR bands assigned toethynylthioketene (6). Diethynyl sulfide (3), which would form in the process of retro-Bergman cyclizationof the incipient 2,5-didehydrothiophene (4), is not detected. Under the same irradiation conditions, matrix-isolated diethynyl sulfide (3) rearranges to thioketene 6 and butatrienethione (5), the global minimum onC4H2S potential energy surface. The photochemical formation of thioketene 6 from either diyl 4 or sulfide3 may be interpreted in line with a recent computational prediction on the thermal ring opening of diyl4, which favors C−S bond cleavage, leading to 6, over C−C bond cleavage, leading to 3. Photolysis ofmatrix-isolated 3,4-thiophenedicarboxylic acid anhydride (11) enables the observation of the photoequilibration of three low-energy C4H2S isomers, butatrienethione (5), ethynylthioketene (6), and butadiynylthiol(7), via apparent [1,3]-hydrogen shifts.
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