Reaction OH + OH Studied over the 298–834 K Temperature and 1 - 100 bar Pressure Ranges (original) (raw)
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Reaction CH 3 + OH Studied over the 294–714 K Temperature and 1–100 bar Pressure Ranges
The Journal of Physical Chemistry A, 2012
Reaction of methyl radicals with hydroxyl radicals, CH 3 + 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( 1 D), produced in the photolysis of N 2 O at 193.3 nm, with H 2 O. Temporal profiles of CH 3 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 H 2 O/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, k 1 = (1.20 ± 0.20) × 10 −10 (T/300) −0.49 cm 3 molecule −1 s −1 .
Formation of vibrationally excited OH radicals in the O (1D) + H2S reaction
Chemical Physics Letters, 1993
The rate constant, macroscopic branching ratio and vibrational distribution for the reaction 0( 'D) + H&OH + SH were measured. Temporal dependencies of the OH radical concentration in different ro-vibronic states were monitored by the LIF technique after laser flash photolysis of ozone which generated 0( ID) atoms. The total rate constant was found to be (2.2k 0.5) X IO-"'cm-' molecule-' s-l. The relative fraction of channel producing OH and SH radicals is 0.26 k 0.04. The vibrational distributionofOHradicalsP(v=0):P(u=I):P(u~2):P(v~3):P(u=4):P(v=5)=0.19:0.21:0.26:0.2:0.11:0.03issimilarto that for the reaction of 0( 'D) atoms with H,, CH4 and NH3 which indicates the similar dynamics of these reactions.
Journal of Physical Chemistry A, 2005
The laser-induced fluorescence (LIF) excitation spectra of the 4-methylcyclohexoxy and d11-cyclohexoxy radicals have been measured for the first time. LIF intensity was used as a probe in direct kinetic studies of the reaction of O 2 with trans-4-methylcyclohexoxy and d11-cyclohexoxy radicals from 228 to 301 K. Measured rate constants near room temperature are uniformly higher than the Arrhenius fit to the lower-temperature data, which can be explained by the regeneration of cyclic alkoxy radicals from the product of their-scission and the effect of O 2 concentration on the extent of regeneration. The Arrhenius expressions obtained over more limited ranges were k O 2) (1.4-1 +8) × 10-13 exp[(-810 (400)/T] cm 3 molecule-1 s-1 for trans-4methylcyclohexoxy (228-292 K) and k O 2) (3.7-1 +4) × 10-14 exp)[(-760 (400) /T] cm 3 molecule-1 s-1 for d11-cyclohexoxy (228-267 K) independent of pressure in the range 50-90 Torr. The room-temperature rate constant for the reaction of trans-4-methylcyclohexoxy radical with O 2 (obtained from the Arrhenius fit) is consistent with the commonly recommended value, but the observed activation energy is ∼3 times larger than the recommended value of 0.4 kcal/mol and half the value previously found for the reaction of normal cyclohexoxy radical with O 2 .
The Journal of Physical Chemistry A, 1999
By using 193 nm laser photolysis and cavity ring-down spectroscopy to produce and monitor the propargyl radical (CH 2 CCH), the self-reaction and oxygen termolecular association rate coefficients for the propargyl radical were measured at 295 K between total pressures of 300 Pa and 13300 Pa (2.25 and 100 Torr) in Ar, He, and N 2 buffer gases. The rate coefficients obtained by simple second-order fits to the decay data were observed to vary with the photolytic precursors: allene, propargyl chloride, and propargyl bromide. By using a numerical fitting routine and more comprehensive mechanisms, a distinct rate coefficient for the selfreaction was determined, k ∞ (C 3 H 3 +C 3 H 3)) (4.3 (0.6) × 10-11 cm 3 molecule-1 s-1 at 295 K. This rate coefficient, which is a factor of 2.8 times slower than reported previously, was independent of total pressure and buffer choice over the entire pressure range. Other rate coefficients derived during the modeling included k(C 3 H 3 +H 665 Pa He)) (2.5 (1.1) × 10-10 cm 3 molecule-1 s-1 , k(C 3 H 3 +C 3 H 3 Cl 2)) (7 (4) × 10-11 cm 3 molecule-1 s-1 , and k(C 3 H 3 +C 3 H 3 Br 2)) (2.4 (2) × 10-11 cm 3 molecule-1 s-1. The association reaction C 3 H 3 + O 2 was found to lie in the falloff region between linear and saturated pressure dependence for each buffer gas (Ar, He, and N 2) between 300 Pa and 13300 Pa. A fit of these data derived the high-pressure limiting rate coefficient k ∞ (C 3 H 3 +O 2)) (2.3 (0.5) × 10-13 cm 3 molecule-1 s-1. Three measurements of the propargyl radical absorption cross-section obtained σ 332.5) (413 (60) × 10-20 cm 2 molecule-1 at 332.5 nm. Stated uncertainties are two standard deviations and include the uncertainty of the absorption cross section, where appropriate.
Kinetics of the reactions of OH radicals with CO and N2O
Chemical Physics Letters, 1976
Rate constants kt for the reaction of OH radicals with CO have been determined af 299 2 1 K over the pressure range 25-654 torr of argon using a flash photolysis-resonance fluorescence technique. The rate constant kt was observed to be independent of total pressure over the range studied with a value of kt = (1.54 + 0.16) X lo-l3 cm3 molcculc-' s-t. In addition, upper limits for the rate constants kz for the reaction of OH radicals with N20 of k2 < 2 X lo-l6 cm3 molecule-' 2' were obtained at both 298.0 and 442.8 K.
The Journal of Physical Chemistry A, 2007
Production of OH in the reaction of the neopentyl radical with O 2 has been measured by a laser photolysis/ cw absorption method for various pressures and oxygen concentrations at 673, 700, and 725 K. The MIT Reaction Mechanism Generator (RMG) was used to automatically generate a model for this system, and the predicted OH concentration profiles are compared to present and literature experimental results. Several reactions significantly affect the OH profile. The experimental data provide useful constraints on the rate coefficient for the formally direct chemical activation reaction of neopentyl radical with O 2 to form OH (CH 3) 3 CCH 2 + O 2 f OH + 3,3-dimethyloxetane (Rxn 1) At 673 K and 60 Torr, log k 1 (cm 3 molecule-1 s-1))-13.7 (0.5. Absolute absorbance measurements on OH and I indicate that the branching ratio for R + O 2 to OH is about 0.03 under these conditions. The data suggest that the ab initio neopentyl + O 2 potential energy surface of Sun and Bozzelli is accurate to within 2 kcal mol-1 .
The Journal of Physical Chemistry A, 1999
The reactions of ozone with alkenes have been shown recently to lead to the direct production of OH radicals in quantities that vary from 7 to 100% depending on the structure of the alkene. OH radicals are the most important oxidizing species in the lower atmosphere, and the OH-alkene reaction is a large source of new OH radicals, important in urban and rural air during both day and night. Evidence for OH formation comes both from low-pressure direct measurements and from tracer experiments at high pressure. With the goal of measuring OH formation yields with good precision, a small-ratio relative rate technique was developed. This method uses small amounts of fast-reacting aromatics and aliphatic ethers to trace OH formation yields. Here, we report OH formation yields for a series of terminal alkenes reacting with ozone. Measured OH yields were 0.29 (0.05, 0.24 (0.05, 0.18 (0.04, and 0.10 (0.03 for 1-butene, 1-pentene, 1-hexene, and 1-octene, respectively. For the methyl-substituted terminal alkenes methyl propene and 2-methyl-1-butene, OH yields were 0.72 (0.12 and 0.67 (0.12, respectively. The results are discussed both in terms of their atmospheric implications and the relationship between structure and OH formation.
Analytical Chemistry, 2013
Chemical dynamics of an ozone (O 3) pulsephotolytic reaction in aqueous solutions were studied with pump−probe transient far-ultraviolet (FUV) absorption spectroscopy. With a nanosecond pulse laser of 266 nm as pump light, transient spectra of O 3 aqueous solutions (78−480 μM, pH 2.5−11.3) were acquired in the time range from −50 to 50 μs in the wavelength region from 190 to 225 nm. The measured transient spectra were linearly decomposed into the molar absorption coefficients and the concentration−time profiles of constituted chemical components with a multivariate curve resolution method. From the dependences of the time-averaged concentrations for 20 μs of the constituted chemicals on the initial concentration of O 3 , it was found that the transient spectra involve the decomposition of O 3 and the formation of hydrogen peroxide (H 2 O 2) and a third component that is assigned to hydroxyl radical (OH) or perhydroxyl radical (HO 2). Furthermore, the pH dependence of the time-averaged concentration of the third components indicates that HO 2 is more probable than OH as the third component. The time-averaged concentration ratio of each chemical component to the initial O 3 concentration depends on the pH conditions from −0.95 to −0.60 for O 3 , 0.98 to 1.2 for H 2 O 2 , 0.002 to 0.29 for OH, and 0.012 to 0.069 for HO 2 .