Kinetic Study of CH 3 + HBr and CH 3 + Br Reactions by Laser Photolysis−Transient Absorption over 1−100 Bar Pressure Range (original) (raw)
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International Journal of Chemical Kinetics, 2012
The gas-phase kinetics of CHBr 2 + NO 2 and CH 3 CHBr + NO 2 reactions have been studied in direct time resolved measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals were generated by pulsed laser photolysis of bromoform and 1,1-dibromoethane at 248 nm. The subsequent decays of the radical concentrations were monitored as a function of [NO 2 ] under pseudo-first-order conditions. The rate coefficients of both reactions are independent of bath gas (He) pressure and display negative temperature dependence under the conditions of 2-6 Torr pressure (He) and 250-480 K. The obtained bimolecular rate coefficients are k(CHBr 2 + NO 2 ) = (9.8 ± 0.4) × 10 −12 (T /300 K) −1.65±0.18 cm 3 s −1 (288-483 K) and k(CH 3 CHBr + NO 2 ) = (2.27 ± 0.06) × 10 −11 (T /300 K) −1.28±0.11 cm 3 s −1 (250-483 K), with the uncertainties given as one standard error. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are ±25%. The reaction products identified were CBr 2 O for the CHBr 2 + NO 2 reaction and CHBrO and CH 3 CHO with minor amounts of CH 3 for the CH 3 CHBr + NO 2 reaction, respectively.
Journal of the American Chemical Society, 1993
The photodecomposition of vicinal dibromides at 266 nm produces bromine atoms with a quantum yield of -2.0. This results from an efficient primary photocleavage of a C-Br bond, followed by rapid elimination of a second bromine atom from radicals of the type RCH-CH2Br. This cleavage occurs with a lifetime of <20 ns at room temperature. Bromine atoms react with bromine ions with a rate constant of 1.6 X 1010 M-1 s-1 to yield Brz*-, an easily detectable and long-lived radical ion. This reaction can be used as a probe in order to determine absolute rate constants for other reactions of bromine atoms. For example, the rate constants for methanol, 2-propanol, and triethylamine are 9.3 X 105, 4.1 X 107, and 2.9 X 1010 M-1 s-l, respectively. It is suggested that these hydrogen atom transfer reactions may involve a considerable degree of charge transfer. a-Bromoacetophenone also serves as a convenient bromine atom source in those cases where vicinal dibromides cannot be employed. The advantages and disadvantages of using probe techniques in the determination of absolute rate constants are discussed in some detail.
Ultraviolet absorption spectra and kinetics of CH3S and CH2SH radicals
Chemical Physics Letters, 1991
Acetonyl radicals and acetonylpcroxy radicals were produced by pulse radiolysis of gas mixtures with varying concentrations of CI-I&OCH, and Oz in 1 atm of SF6 to initiate tlte reactions ( 1) F+CI-IsCOCH3-+HFfCH@CH~, (2) ZCH&OCH,-products, (3 ) CHJCOCH2+02( + M)&I-ISCOCH202( + M) and (4) 2CHsCOCHLOL+products. The ultraviolet absorption spectrum of the acetonyl radical is composed of a fairly weak band centered at 3 15 nm and a stronger band in the range of 200-250 nm. The absorption cross section has been determined, a(310 nm)=(8.73~0.50)x10-1g cm* molecule-L. The values k2= (4.8f0.4) x lo-", k,= (1.5+0.3)x lo-'* and k4= (8.3f 1.6) x 10-'zcm3 molecule-' s-' at 298 K were derived by computer modelling of the observed kinetic features. 206 0009-2614/90/$03.50 0 1990 -Elsevier Science Publishers B.V. (North-Holland) volume 173, number 2,3 CHEMICAL PHYSICS LETTERS
The Journal of Chemical Physics, 2006
The A-X bands of the CH radical, produced in a 248 nm two-photon photolysis or in a supersonic jet discharge of CHBr 3 , have been observed via cavity ring-down absorption spectroscopy. Bromoform is a well-known photolytic source of CH radicals, though no quantitative measurement of the CH production efficiency has yet been reported. The aim of the present work is to quantify the CH production from both photolysis and discharge of CHBr 3 . In the case of photolysis, the range of pressure and laser fluences was carefully chosen to avoid postphotolysis reactions with the highly reactive CH radical. The CH production efficiency at 248 nm has been measured to be ⌽ = N͑CH͒ / N͑CHBr 3 ͒ = ͑5.0± 2.5͒10 −4 for a photolysis laser fluence of 44 mJ cm −2 per pulse corresponding to a two-photon process only. In addition, the internal energy distribution of CH͑X 2 ⌸͒ has been obtained, and thermalized population distributions have been simulated, leading to an average vibrational temperature T vib = 1800± 50 K and a rotational temperature T rot = 300± 20 K. An alternative technique for producing the CH radical has been tested using discharge-induced dissociation of CHBr 3 in a supersonic expansion. The CH product was analyzed using the same cavity ring-down spectroscopy setup. The production of CH by discharge appears to be as efficient as the photolysis technique and leads to rotationally relaxed radicals.
The Journal of Physical Chemistry, 1991
The gas-phase kinetics of the reactions of four partially halogenated methyl radicals (CH,Cl, CH2Br, CH21, and CHC12) with Cl, have been studied as a function of temperature using a tubular reactor coupled to a photoionization mass spectrometer. Radicals were homogeneously generated by pulsed 193-and/or 248-nm laser photolysis. Decays of the radical concentrations were monitored in time-resolved experiments as a function of [Cl,] to obtain bimolecular rate constants for the R + C1,-RCI + C1 reactions studied. The following Arrhenius expressions (k = A exp(-E/RT)) were obtained (the numbers in brackets are log (A/(" molecule-' s-')), E/(kJ mol-I); the temperature ranges are also indicated): R = CH2Cl [-11.82 f 0.12, 4.1 f 1.3, 295-719 K]; R = CH2Br [-11.91 * 0.14,2.4 f 1.4, 295-524 K]; R = CH21 [-11.94 * 0.19, 0.8 & 2.2, 295-524 K]; R = CHC12 [-12.07 * 0.15, 10.3 f 2.0, 357-719 K]. Errors are lu, including both random and an estimated 20% systematic error in the individual bimolecular rate constants. The Arrhenius parameters of these and two other R + C12 reactions are compared with theoretical determinations based on semiempirical AM1 calculations of transition-state energies, structures, and vibration frequencies. The calculations qualitatively reproduce the obsehed trends in both the Arrhenius A factors and in the activation energies. The use of molecular properties to account for reactivity differences among all the R + C12 reactions which have been studied to date are also explored using free-energy correlations with these properties.
Hydroxyl radical rate constants and photolysis rates of .alpha.-dicarbonyls
Environmental Science & Technology, 1983
Photolysis rates of glyoxal, methylglyoxal, and biacetyl and OH radical reaction rate constants for glyoxal and methylglyoxal have been determined at 298 f 2 K in an environmental chamber, by using the photolysis of CH30NO-air mixtures to generate OH radicals. The OH radical rate constants obtained were (1.15 f 0.04) X lo-'' and (1.73 f 0.13) X lo-'' cm3 molecule-l s-' for glyoxal and methylglyoxal, respectively. The photolysis rates of glyoxal, methylglyoxal, and biacetyl increased throughout this series, and average quantum yields for the wavelength region 1290 nm of 0.029 f 0.018,0.107 f 0.030, and 0.158 f 0.024 were derived for glyoxal, methylglyoxal, and biacetyl, respectively. In addition, upper limits to the rate constants for the reaction of O3 with glyoxal and methylglyoxal of <3 X and <6 X cm3 molecule-' s-l, respectively, were obtained at 298 f 2 K. These data will serve as needed input to chemical kinetic computer modeling studies of the aromatic hydrocarbons. Additionally, the a-dicarbonyls also photolyze: of the dicarbonyls and cyclohexane with-0(8P) atoms and o3 were negligible, and since dilution due to sampling was
Kinetics and mechanism of the reaction of acetonyl radical, CH3C(O)CH2, with Br2
Chemical Physics Letters, 2013
The low pressure fast discharge flow method with laser induced fluorescence detection of CH 3 C(O)CH 2 was employed to study the kinetics of the reaction CH 3 C(O)CH 2 + Br 2 → CH 3 C(O)CH 2 Br + Br (1) at 298, 323 and 365 K. The rate coefficient at room temperature is k 1 = (2.33 ± 0.04 (2σ)) × 10-12 cm 3 molecule-1 s-1 , which increases slightly with increasing temperature. Quantum chemistry (G2) and theoretical rate theory (conventional TST) computations have supplied results in qualitative agreement with experiment. The relatively slow rate of reaction (1) can be due to the resonance stabilization of the acetonyl radical.
The Journal of Physical Chemistry A, 2010
We report experimental and computational studies of the photolysis of atmospherically important 1,2dibromoethanes (1,2-C 2 X 4 Br 2 ; X) H, F) in Ar matrixes at 5 K. Using the pulsed deposition method, we find that significant conformational relaxation occurs for 1,2-C 2 H 4 Br 2 (EDB; observed anti/gauche ratio)30:1) but not for 1,2-C 2 F 4 Br 2 (TFEDB; anti/gauche) 3:1), which is traced to a larger barrier to rotation about the CC bond in the latter. Laser photolysis of matrix-isolated EDB at 220 nm reveals the growth of infrared bands assigned to the gauche conformer and C 2 H 4-Br 2 charge transfer complex (both as major products), and the C 2 H 4 Br radical and C 2 H 3 Br-HBr complex as minor (trace) products. The presence of the C 2 H 4-Br 2 complex is confirmed in the UV/visible spectrum, which shows an intense charge transfer band at 237 nm that grows in intensity upon annealing. In contrast to previous reports, our experimental and computational results do not support a bridged structure for the C 2 H 4 Br radical in either the gas phase or matrix environments. We also report on the laser photolysis of matrix-isolated TFEDB at 220 nm. Here, the dominant photoproducts are the anti and gauche conformers of the C 2 F 4 Br radical, the vibrational and electronic spectra of which are characterized here for the first time. The increase in yield of radical for TFEDB vs EDB is consistent with the stronger C-Br bond in the fluoro-substituted radical species. The photochemistry of the C 2 F 4 Br radical following excitation at 266 nm was investigated and found to lead C-Br bond cleavage and formation of C 2 F 4. The implications of this work for the atmospheric and condensed phase photochemistry of the alkyl halides is emphasized.
The Journal of Chemical Physics, 2007
The photodissociation of 2-bromopropene at 193 nm produces C 3 H 5 radicals with a distribution of internal energies that spans the threshold for the secondary decomposition of the 2-propenyl radicals into C 3 H 4 + H. Just above this threshold, the decomposition rate is on the nanosecond time scale, and in the present study, time-resolved velocity-map ion imaging is used to gain insight into this process. The results provide information on the energy dependence of the secondary dissociation process. In addition, comparison of the results with theoretical predictions of the energy dependence of the dissociation rate provides information on the branching between fragment rotational and vibrational energies in the primary photodissociation process.
The Journal of Physical Chemistry A, 1999
Cavity ring-down spectroscopy (CRDS), end-product analysis, and ab initio calculations have determined absorption cross sections, rate coefficients, reaction mechanisms, and thermochemistry relevant to the addition of halogen atoms to propargyl chloride and propargyl bromide. Halogen atoms were produced by laser photolysis, and the addition reaction products were probed at a variable delay by CRDS using a second laser pulse. We report the continuum spectra of C 3 H 3 Cl 2 (1,2-dichloroallyl), C 3 H 3 ClBr (1-chloro-2-bromoallyl), and C 3 H 3 Br 2 (1,2-dibromoallyl) radicals between 238 and 252 nm and the absorption cross sections, σ 240 (C 3 H 3 -Cl 2 ) ) (4.20 ( 1.05) × 10 -17 cm 2 molecule -1 and σ 242 (C 3 H 3 Br 2 ) ) (1.04 ( 0.31) × 10 -17 cm 2 molecule -1 . When the observed data are fit to complex reaction schemes, the 298 K rate coefficients for formation of 1,2-dihaloallyl radicals at 665 Pa were found to be k(Cl + C 3 H 3 Cl) ) (1.2 ( 0.2) × 10 -10 cm 3 molecule -1 s -1 and k(Br + C 3 H 3 Br) ) (2 ( 1) × 10 -12 cm 3 molecule -1 s -1 . At 298 K and 665 Pa the self-reaction rate coefficients of these radicals were found to be k(C 3 H 3 Cl 2 + C 3 H 3 Cl 2 ) ) (3.4 ( 0.9) × 10 -11 cm 3 molecule -1 s -1 and k(C 3 H 3 Br 2 + C 3 H 3 Br 2 ) ) (1.7 ( 1.1) × 10 -11 cm 3 molecule -1 s -1 . The listed uncertainties are twice the standard deviation of individual determinations, and those for rate coefficients include the uncertainty of the appropriate absorption cross section. † NIST/NRC Postdoctoral Associate 1995-1997.