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

Temperature-Dependent Rate Coefficients for the Reaction of CH₂OO with Hydrogen Sulfide

The reaction of the simplest Criegee intermediate CH₂OO with hydrogen sulfide was measured with transient UV absorption spectroscopy in a temperature-controlled flow reactor, and bimolecular rate coefficients were obtained from 278 to 318 K and from 100 to 500 Torr. The average rate coefficient at 298 K and 100 Torr was (1.7 ± 0.2) × 10^–13 cm³ s^–1. The reaction was found to be independent of pressure and exhibited a weak negative temperature dependence. <i>Ab initio</i> quantum chemistry calculations of the temperature-dependent reaction rate coefficient at the QCISD­(T)/CBS level are in reasonable agreement with the experiment. The reaction of CH₂OO with H₂S is 2–3 orders of magnitude faster than the reaction with H₂O monomer. Though rates of CH₂OO scavenging by water vapor under atmospheric conditions are primarily controlled by the reaction with water dimer, the H₂S loss pathway will be dominated by the reaction with monomer. The agreement between experiment and theory for the CH₂OO + H₂S reaction lends credence to theoretical descriptions of other Criegee intermediate reactions that cannot easily be probed experimentally

Strong Negative Temperature Dependence of the Simplest Criegee Intermediate CH₂OO Reaction with Water Dimer

The kinetics of the reaction of CH₂OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH₂OO decay rate is second order with respect to the H₂O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH₂OO reaction with water dimer can be described by an Arrhenius expression <i>k</i>(<i>T</i>) = <i>A</i> exp­(−<i>E</i>_a/<i>RT</i>) with an activation energy of −8.1 ± 0.6 kcal mol^–1 and <i>k</i>(298 K) = (7.4 ± 0.6) × 10^–12 cm³ s^–1. Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH₂OO by a factor of ∼2.5 at 278 K and decreases by a factor of ∼2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere

Strong Negative Temperature Dependence of the Simplest Criegee Intermediate CH₂OO Reaction with Water Dimer

The kinetics of the reaction of CH₂OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH₂OO decay rate is second order with respect to the H₂O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH₂OO reaction with water dimer can be described by an Arrhenius expression <i>k</i>(<i>T</i>) = <i>A</i> exp­(−<i>E</i>_a/<i>RT</i>) with an activation energy of −8.1 ± 0.6 kcal mol^–1 and <i>k</i>(298 K) = (7.4 ± 0.6) × 10^–12 cm³ s^–1. Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH₂OO by a factor of ∼2.5 at 278 K and decreases by a factor of ∼2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere