Polar effect in the reaction of CH 3 O with HBr (original) (raw)
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Kinetic study of the reaction of CH3O with Br and Br2
Reaction Kinetics and Catalysis Letters, 2002
The title reactions have been studied at room temperature by applying the discharge flow method coupled with laser induced fluorescence detection of methoxy radicals and resonance fluorescence detection of bromine atoms. The following rate constants were determined: CH 3 O + Br : products (1) k 1 (298 K) = (3.4 ± 0.4 (1)) × 10 13 cm 3 mol -1 s -1 , CH 3 O + Br 2 :products (2) k 2 (298 K) ≤ 5 × 10 8 cm 3 mol -1 s -1 .
Direct kinetic studies of the reactions Br+CH3OH and CH2OH+HBr: The heat of formation of CH2OH
Journal of Physical Chemistry, 1996
The chemical equilibrium Br + CH 3 OH h HBr + CH 2 OH (1, -1) has been studied by investigating the kinetics of the forward and reverse reactions. Excimer laser photolysis coupled with Br atom resonance fluorescence detection was used over the temperature range 439-713 K to obtain k 1 ) (3.41 ( 0.89) × 10 9 T 1.5 exp [-(29.93 ( 1.47) kJ mol -1 /RT] cm 3 mol -1 s -1 . The reverse reaction was studied with the fast flow technique, in the temperature range 220-473 K, using laser magnetic resonance for monitoring the CH 2 OH radicals. Thus, k -1 ) (1.20 ( 0.25) × 10 12 exp[(3.24 ( 0.44) kJ mol -1 /RT] was obtained. The kinetic results were compared with available literature data and possible causes of the deviations were discussed. Kinetic information on the foward and back reactions was combined to obtain the heat of formation for CH 2 OH. Both second-law and third-law procedures were used in the derivations, giving a recommended value of ∆ f H°2 98 (CH 2 OH) ) -16.6 ( 1.3 kJ mol -1 , which corresponds to the C-H bond dissociation energy of DH°2 98 (H-CH 2 OH) ) 402.3 ( 1.3 kJ mol -1 . These thermochemical data obtained from kinetic equilibrium studies agree within the error limits with current photoionization mass spectrometric and ab initio theoretical results.
Journal of Physical Chemistry A, 1999
Reactions of methyl radicals with hydrogen bromide CH 3 + HBr f CH 4 + Br (1) and bromine atoms CH 3 + Br f CH 3 Br (2) were studied using excimer laser photolysis-transient UV spectroscopy at 297 ( 3 K over the 1-100 bar buffer gas (He) pressure range. Methyl radicals were produced by 193 nm (ArF) laser photolysis of acetone, (CH 3 ) 2 CO, and methyl bromide, CH 3 Br. Temporal profiles of methyl radicals were monitored by UV absorption at 216.51 nm (copper hollow cathode lamp with current boosting). The yield of acetyl radicals in photolysis of acetone at 193 nm was found to be less than 5% at 100 bar He based on the transient absorptions at 222.57 and 224.42 nm. The measured rate constants for reaction 1 are k 1 ) (2.9 ( 0.7) × 10 -12 , (3.8 ( 1.5) × 10 -12 , and (3.4 ( 1.3) × 10 -12 cm 3 molecule -1 s -1 at the buffer gas (He) pressures of 1.05, 11.2, and 101 bar, respectively. The rate data obtained in this study confirmed high values of the previous (low pressure) measurements and ruled out the possibility of interference of excited species. The measured rate constant is independent of pressure within the experimental error. The rate constant of reaction of methyl radicals with bromine atoms (2) was determined relative to the rate constant of methyl radical self-reaction, CH 3 + CH 3 f C 2 H 6 (3) in experiments with photolysis of CH 3 Br: k 2 /k 3 ) 0.92 ( 0.32, 1.15 ( 0.30, and 1.65 ( 0.26 at 1.05, 11.2, and 101 bar He, respectively. On the basis of the literature data for reaction 3, this yields k 2
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.
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.
Kinetics of the Reactions of O (3P) and Cl (2P) with HBr and Br2
A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of reactions (1)-(4) as a function of temperature. (1) O(3P) + Br2-BrO + Br('P,,,) (255-350 K) (2) Cl('P) + Br, -BrCl + Br('P,,) (298-401 K) (3) O(3P) + HBr -OH + Br('PJ) (250-402 K) (4) Cl('P) + HBr -HC1 + Br('PJ) (257-404 K) In all cases, the concentration of the excess reagent, i.e., HBr or Br,, was measured in situ in the slow flow system by UV-visible photometry. Heterogeneous dark reactions between XBr ( X = H or Br) and the photolytic precursors for Cl('P) and O(3P) (CI2 and 03, respectively) were avoided by injecting minimal amounts of precursor into the reaction mixture immediately upstream from the reaction zone. The following Arrhenius expressions summarize our results (errors are 2u and represent precision only, units are cm3 molecule-' s-'): k , = (1.76 t_ 0.80) x lo-" exp[(40 2 100)/T]; k 2 = (2.40 2 1.25) X lo-'' exp[-(144 2 1 7 6 ) / T ] ; k 3 = (5.11 2 2.82) x lo-'' exp[-(1450 2 160)/T1; k, = (2.25 2 0.56) X 10.'' exp[-(400 & 80)/n. The consistency (or lack thereof) of our results with those reported in previous kinetics and dynamics studies of reactions (1)-(4) is discussed.
Kinetics of the reaction of H(2S) with HBr
International Journal of Chemical Kinetics, 1992
The rate constants for the reaction H + HBr -H 2 + Br were measured between 217 and 383 K using pulsed laser photolysis of HBr and cw resonance fluorescence detection of H( 2 S). The temporal profiles of the product Br atoms were also monitored to obtain the rate constant at 298 K. The yield of Br from the reaction was determined to be unity. The rate coefficient as a function of temperature is given by the Arrhenius expression, k 1 ~ (2.96 ± 0.44) X 10-11 exp(-(460 ± 40)/T) cm 3 molecule-1 s-1 . The quoted errors are at the 95% confidence level and include estimated systematic errors. Our results are compared with those from previous direct measurements.
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.
Reactivity of the CHBr2+ Dication toward Molecular Hydrogen
The Journal of Physical Chemistry A, 2006
Structural aspects as well as the stability and reactivity of the CHBr 2+ dication are studied both experimentally and theoretically. Translational energy distributions of the CHBr + products from charge transfer between CHBr 2+ and Kr indicate that the dication exists in two isomeric forms, H-C-Br 2+ and C-Br-H 2+. In the reaction of CHBr 2+ with H 2 , the dominant channel corresponds to proton transfer leading to CBr + + H 3 +. Other reaction channels involve the formation of the intermediates CH 3 Br 2+ and CH 2 BrH 2+ , respectively. Both of the latter dications can either lose a proton to form CH 2 Br + or undergo a spin-isomerization followed by cleavage of the C-Br bond. The proposed mechanisms are supported by DFT calculations and deuterium labeling experiments.
The Journal of Physical Chemistry A, 2006
Kinetics of the reaction Br + CH 2 ClBr T CHClBr + HBr (1,-1) were studied experimentally in the forward direction. The absolute reaction kinetics method of laser flash photolysis coupled with Br atom resonance fluorescence detection and three different relative-rate methods with gas-chromatographic analysis were applied to carry out the experiments. The rate constants determined were found to obey the Arrhenius law in the wide temperature range of T) 293-785 K providing the kinetic expression k 1) (2.8 (0.1) × 10 13 exp[-(47.6 (0.3) kJ mol-1 /RT] cm 3 mol-1 s-1 (the errors given refer to 1σ precision). An ab initio direct dynamics method was used to study reaction (1,-1) theoretically. The electronic structure information including geometries, gradients, and force constants was obtained at the MP2 level of theory; and energies were improved at higher theoretical levels. Rate constants were calculated using the canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200-1000 K. Theory substantially underestimates k 1 compared to experiment. The agreement was found good with k-1 reported previously predicting positive temperature dependence. The experimental kinetic parameters were utilized in thermochemical calculations yielding the recommended standard enthalpy of formation of ∆ f H°2 98 (CHClBr)) (140 (4) kJ mol-1 (with 2σ accuracy given).