Photolysis of matrix-isolated allenylketene: An experimental and theoretical study of the allenylcarbene reactivity

The photolysis (lambda greater than or equal to 230 nm) of 3,4-pentadienoyl chloride 1 isolated in argon matrix at 10 K as well as the follow-up react ions were studied by FTIR spectroscopy. The experimental data point out tha t the first step in the photochemical reaction is the elimination of HCl an d the formation of an intermediate product identified as allenyl ketene 2. Further irradiation determines the decarbonylation of 2 leading to a very r eactive carbenic species, allenylcarbene 3, which undergoes subsequent reac tions. The final products identified were vinylacetylene 4 and acetylene 7. The assignment of the intermediates and final products as well as the reac tion mechanism is supported by high-level theoretical calculations, LSDA/6- 31G** and CASSCF/6-31G*.