Modified transition state theory and negative apparent activation energies of simple metathesis reactions: Application to the reaction CH₃ + HBr + CH₄ + Br

Article on modified transition state theory and negative apparent activation energies of simple metathesis reactions and application to the reaction CH₃ + HBr + CH₄ + Br

Reaction OH + OH Studied over the 298–834 K Temperature and 1 - 100 bar Pressure Ranges

Self-reaction of hydroxyl radicals, OH + OH → H₂O + O (1a) and OH + OH → H₂O₂ (1b), was studied using pulsed laser photolysis coupled to transient UV–vis absorption spectroscopy over the 298–834 K temperature and 1–100 bar pressure ranges (bath gas He). A heatable high-pressure flow reactor was employed. Hydroxyl radicals were prepared using reaction of electronically excited oxygen atoms, O­(¹D), produced in photolysis of N₂O at 193 nm, with H₂O. The temporal behavior of OH radicals was monitored via transient absorption of light from a dc discharge in H₂O/Ar low-pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light was determined by accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study combined with the literature data indicate that the rate constant of reaction 1a, associated with the pressure independent component, decreases with temperature within the temperature range 298–414 K and increases above 555 K. The pressure dependent rate constant for (1b) was parametrized using the Troe expression as <i>k</i>_1b,inf = (2.4 ± 0.6) × 10^–11(<i>T</i>/300)^−0.5 cm³ molecule^–1 s^–1, <i>k</i>_1b,0 = [He] (9.0 ± 2.2) × 10^–31(<i>T</i>/300)^−3.5±0.5 cm³ molecule^–1 s^–1, <i>F</i>_c = 0.37

Kinetics of the Reaction of CH₃O₂ Radicals with OH Studied over the 292–526 K Temperature Range

Reaction of methyl peroxy radicals with hydroxyl radicals, CH₃O₂ + OH → CH₃O + HO₂ (1a) and CH₃O₂ + OH → CH₂OO + H₂O (1b) was studied using pulsed laser photolysis coupled to transient UV–vis absorption spectroscopy over the 292–526 K temperature range and pressure 1 bar (bath gas He). Hydroxyl radicals were generated in the reaction of electronically excited oxygen atoms O­(¹D), produced in the photolysis of N₂O at 193.3 nm, with H₂O. Methyl peroxy radicals were generated in the reaction of methyl radicals, CH₃, produced in the photolysis of acetone at 193.3 nm, and subsequent reaction of CH₃ with O₂. Temporal profiles of OH were monitored via transient absorption of light from a DC discharge H₂O/Ar low-pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light was determined by accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The overall rate constant of the reaction is <i>k</i>_1a+1b = (8.4 ± 1.7) × 10^–11(<i>T</i>/298 K)^−0.81 cm³ molecule^–1 s^–1 (292–526 K). The branching ratio of channel 1b at 298 K is less than 5%

Reaction CH₃ + OH Studied over the 294–714 K Temperature and 1–100 bar Pressure Ranges

Reaction of methyl radicals with hydroxyl radicals, CH₃ + OH → products (1) was studied using pulsed laser photolysis coupled to transient UV–vis absorption spectroscopy over the 294–714 K temperature and 1–100 bar pressure ranges (bath gas He). Methyl radicals were produced by photolysis of acetone at 193.3 nm. Hydroxyl radicals were generated in reaction of electronically excited oxygen atoms O­(¹D), produced in the photolysis of N₂O at 193.3 nm, with H₂O. Temporal profiles of CH₃ were recorded via absorption at 216.4 nm using xenon arc lamp and a spectrograph; OH radicals were monitored via transient absorption of light from a dc discharge H₂O/Ar low pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light inside the reactor was determined by an accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study indicate that the rate constant of reaction 1 is pressure independent within the studied pressure and temperature ranges and has slight negative temperature dependence, <i>k</i>₁ = (1.20 ± 0.20) × 10^–10(<i>T</i>/300)^−0.49 cm³ molecule^–1 s^–1