Photolysis of the dichlorocyclobutenedione in rare gas at 10 K. Infrared spectral analysis and ab initio calculations of vibrational frequencies. First identification of two new species (dichloro-substituted bisketene and dichloropropadienone). Kinetics and reaction mechanism

The photochemistry of 3,4-dichlorocyclobutene-1,2-dione (A) embedded in rare gas matrices and irradiated by a filtered broad band source (lambda > 335 nm) has been studied by FTIR spectroscopy. The new IR absorption bands were assigned to the photoproducts by ab initio calculations of theoretical vibrational spectra, at the MP2/6-31G* level. A new highly reactive intermediate bisketene, 2,3-dichloro-1,3-butadiene-1,4-dione (O=C=C-Cl)(2) (B), was formed; the experimental infrared band activities of the symmetric and antisymmetric vibrational modes of B in the C=O spectral range corroborate its calculated twisted structure. B is decomposed by further irradiation and leads to the formation of carbon monoxide (D), dichlorocyclopropenone (E), and dichloropropadienone (Cl2C=C=C=O) (F), by two concurrent pathways (branching ratios: 93% for E, 7% for F in an argon matrix). Photolysis of E and F by the full light of a mercury lamp (lambda > 230 nm) led to the infrared characterization of dichloroacetylene (G), formed via a simple primary dissociation process. A study of the integrated absorbances versus time permitted us to characterize the kinetic behavior of the process and to establish the reaction mechanism.