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

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

Rate Constants and Hydrogen Isotope Substitution Effects in the CH₃ + HCl and CH₃ + Cl₂

Article on rate constants and hydrogen isotope substitution effects in the CH₃ + HCl and CH₃ + Cl₂ reactions

Reaction CH₃ + CH₃ → C₂H₆ Studied over the 292–714 K Temperature and 1–100 bar Pressure Ranges

Reaction of recombination of methyl radicals, CH₃ + CH₃ → C₂H₆ (1) was studied using pulsed laser photolysis coupled to transient UV–vis absorption spectroscopy over the 292–714 K temperature and 1–100 bar pressure ranges (bath gas He), very close to the high-pressure limit. Methyl radicals were produced by photolysis of acetone at 193.3 nm or in the reaction of electronically excited oxygen atoms O­(¹D), produced in the photolysis of N₂O at 193.3 nm, with CH₄, and subsequent reaction of OH with CH₄. Temporal profiles of CH₃ were recorded via absorption at 216.36 and 216.56 nm using a xenon arc lamp and a spectrograph. The absolute intensity of the photolysis light inside the reactor was determined by an accurate <i>in situ</i> actinometry based on the ozone formation in photolysis of N₂O/O₂/N₂ mixtures. The rate constant of reaction 1 in the high-pressure limit has a negative temperature dependence: <i>k</i>_1,inf = (5.66 ± 0.43) × 10^–11(<i>T</i>/298 K)^−0.37 cm³ molecule^–1 s^–1 (292–714 K)

Kinetics of the Gas-Phase Reaction of Hydroxyl Radicals with Dimethyl Methylphosphonate (DMMP) over an Extended Temperature Range (273&ndash;837 K)

The kinetics of the reaction of hydroxyl radical (OH) with dimethyl methylphosphonate (DMMP, (CH3O)2CH3PO) (reaction 1) OH + DMMP &rarr; products (1) was studied at the bath gas (He) pressure of 1 bar over the 295&ndash;837 K temperature range. Hydroxyl radicals were produced in the fast reaction of electronically excited oxygen atoms O(1D) with H2O. The time-resolved kinetic profiles of hydroxyl radicals were recorded via UV absorption at around 308 nm using a DC discharge H2O/Ar lamp. The reaction rate constant exhibits a pronounced V-shaped temperature dependence, negative in the low temperature range, 295&ndash;530 K (the rate constant decreases with temperature), and positive in the elevated temperature range, 530&ndash;837 K (the rate constant increases with temperature), with a turning point at 530 &plusmn; 10 K. The rate constant could not be adequately fitted with a standard 3-parameter modified Arrhenius expression. The data were fitted with a 5-parameter expression as: k1 = 2.19 &times; 10&minus;14(T/298)2.43exp(15.02 kJ mol&minus;1/RT) + 1.71 &times; 10&minus;10exp(&minus;26.51 kJ mol&minus;1/RT) cm3molecule&minus;1s&minus;1 (295&ndash;837 K). In addition, a theoretically predicted pressure dependence for such reactions was experimentally observed for the first time