Conformational Control during the Photolysis of Matrix-Isolated Chloroformylketene: Kinetic and Theoretical Studies of C₃O₂ Formation

Abstract

The conformers of chloroformylketene ClCOCHCO were generated by thermolysis of malonyl dichloride CH2(COCl)2 at 50 °C and isolated in rare gas matrices. Both conformers were charaterized by low-temperature infrared spectroscopy in argon and xenon matrices. The observed frequencies and intensities are in good agreement with the MP2/D95* ab initio calculations. The experimental spectra show that the s-trans conformer is more stable than the s-cis one. Consistent with this experimental result, high-level ab initio calculations predict that the two conformers have nearly identical energies. UV irradiations of chloroformylketene at λ ≥ 310 nm in rare gas matrices show interconversion of the conformers in the first step. In the second one, the s-cis conformer loses HCl to yield carbon suboxide. The reaction products (HCl and C3O2) are trapped in the same cage and form 1:1 complex, characterized by its stretching vibrational bands (2719.5 and 2232 cm-1 in argon). Modeling of the photochemical processes, using ab initio calculations at the MP2/3-21G* level, is performed in the ground state (S0) and the first singlet excited state (S1). The isomerization barrier in the S0 state (12 kcal·mol-1) is similar to the data obtained for other acylketenes. This barrier is lower in the S1 state (5 kcal·mol-1). The barrier for the reaction is higher in the S1 state (53.8 kcal·mol-1) than in S0 (30 kcal·mol-1), these theoretical results suggesting the reaction to take place in the ground state.

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