title: Unimolecular Decomposition Rate of the Criegee Intermediate (CH3)2COO Measured Directly with UV Absorption Spectroscopy.
abstract: 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 kd 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, kSO2, was also determined in the temperature range 283 to 303 K. Our temperature-dependent values for kd and kSO2 are consistent with previously reported relative rate coefficients kd/kSO2 of (CH3)2COO formed from ozonolysis of tetramethyl ethylene. Quantum chemical calculations of kd 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.
authors.affiliation: Department of Chemistry, University of California, Berkeley , Berkeley, California 94720, United States.|Institute of Atomic and Molecular Sciences, Academia Sinica , Taipei 10617, Taiwan.
authors.forename: Mica C
authors.initials: MC
authors.lastname: Smith
authors.affiliation: Institute of Atomic and Molecular Sciences, Academia Sinica , Taipei 10617, Taiwan.|Department of Chemistry, National Taiwan University , Taipei 10617, Taiwan.
authors.forename: Wen
authors.initials: W
authors.lastname: Chao
authors.affiliation: Institute of Atomic and Molecular Sciences, Academia Sinica , Taipei 10617, Taiwan.
authors.forename: Kaito
authors.initials: K
authors.lastname: Takahashi
authors.affiliation: Department of Chemistry, University of California, Berkeley , Berkeley, California 94720, United States.|Department of Earth and Planetary Science, University of California, Berkeley , Berkeley, California 94720, United States.
authors.forename: Kristie A
authors.initials: KA
authors.lastname: Boering
authors.affiliation: Institute of Atomic and Molecular Sciences, Academia Sinica , Taipei 10617, Taiwan.|Department of Chemistry, National Taiwan University , Taipei 10617, Taiwan.
authors.forename: Jim Jr-Min
authors.initials: JJ
authors.lastname: Lin