Gas-phase chemiluminescence study of chemically activated tetramethyl-1,2-dioxetane formed from the reaction of O₂(¹Δ_g) with 2,3-dimethyl-2-butene

Chemically activated tetramethyl-1,2-dioxetane has been prepared by the reaction of O₂(¹Δ_g) with 2,3-dimethyl-2-butene at temperatures of 450 to 775 K and a pressure of 0.25 Torr. The observed product of the reaction was excited ¹nπ* (S₁) acetone, which was identified by chemiluminescence spectra of the acetone (S₁ → S₀) transition. Neither acetone (T_i) nor any other excited states were observed under the above conditions. The temperature dependence of the chemiluminescence gave an activation energy for the cycloaddition reaction of 8610 ± 200 (1σ) cal/mol. The quantum yield for acetone (S₁) was 4 × 10^-3 per reactive collision; its (1σ) error is estimated as ± a factor of 3. Chemiluminescence spectra taken at O₂(³Σ_g^-) partial pressures greater than 2 Torr showed formaldehyde (S₁ → S₀) bands. This is attributed to the well-known hydrocarbon "cool flame" mechanism, due to the presence of methyl radical formed by the thermal decomposition, in two steps, of 2,3-dimethyl-3-hydroperoxy-1-butene, another product path for the title reaction. This is the first report on the chemically activated decomposition of tetramethyl-1,2-dioxetane.