Unimolecular Decomposition Rate of the Criegee Intermediate (CH₃)₂COO Measured Directly with UV Absorption Spectroscopy

The unimolecular decomposition of (CH₃)₂COO and (CD₃)₂COO was measured by direct detection of the Criegee intermediate at temperatures from 283 to 323 K using time-resolved UV absorption spectroscopy. The unimolecular rate coefficient <i>k</i>_d for (CH₃)₂COO shows a strong temperature dependence, increasing from 269 ± 82 s^–1 at 283 K to 916 ± 56 s^–1 at 323 K with an Arrhenius activation energy of ∼6 kcal mol^–1. The bimolecular rate coefficient for the reaction of (CH₃)₂COO with SO₂, <i>k</i>_SO₂, was also determined in the temperature range 283 to 303 K. Our temperature-dependent values for <i>k</i>_d and <i>k</i>_SO₂ are consistent with previously reported relative rate coefficients <i>k</i>_d/<i>k</i>_SO₂ of (CH₃)₂COO formed from ozonolysis of tetramethyl ethylene. Quantum chemical calculations of <i>k</i>_d for (CH₃)₂COO are consistent with the experiment, and the combination of experiment and theory for (CD₃)₂COO indicates that tunneling plays a significant role in (CH₃)₂COO unimolecular decomposition. The fast rates of unimolecular decomposition for (CH₃)₂COO measured here, in light of the relatively slow rate for the reaction of (CH₃)₂COO with water previously reported, suggest that thermal decomposition may compete with the reactions with water and with SO₂ for atmospheric removal of the dimethyl-substituted Criegee intermediate