Kinetics of the Reactions of O (3P) and Cl (2P) with HBr and Br2 (original) (raw)
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Kinetics of the reactions of Cl(2PJ) and Br(2P3/2) with O3
International Journal of Chemical Kinetics, 1990
A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the important stratospheric reactions Cl('P,) + O 3 C10 + O2 and Br('P,,,) + O3 -+ BrO + O2 as a function of temperature, The temperature dependence observed for the Cl('P,) + O3 reaction is nonArrhenius, but can be adequately described by the following two Arrhenius expressions (units are cm3 molecule-' s-', errors are 217 and represent precision only): k l ( T ) = (1.19 2 0.21) x lo-" exp [(-33 2 37)/T] for T = 189-269K and k,(T) = (2.49 2 0.38) x lo-'' exp[(-233 f 4 6 ) / n for T = 269-385 K. At temperatures below 230 K, the rate coefficients determined in this study are faster than any reported previously. Incorporation of our values for k,(T) into stratospheric models would increase calculated C10 levels and decrease calculated HCI levels; hence the calculated efficiency of C10, catalyzed ozone destruction would increase. The temperature dependence observed for the Br('P3 ') + O3 reaction is adequately described by the following Arrhenius expression (units are cm3 molecule-' s-', errors are 2 a and represent precision only): k,(n = (1.50 2 0.16) x 10.'' exp[(-775 * 30)/T] for T = 195-392 K. While not in quantitative agreement with Arrhenius parameters reported in most previous studies, our results almost exactly reproduce the average of all earlier studies and, therefore, will not affect the choice of k,(T) for use in modeling stratospheric BrO, chemistry.
Kinetics and Mechanism of the OH and OD Reactions with BrO
The Journal of Physical Chemistry A, 2001
The kinetics and mechanism of the reactions OH + BrO f products (1) and OD + BrO f products (2) have been studied in the temperature ranges of 230-355 K and 230-320 K, respectively, and at total pressure of 1 Torr of helium using the discharge-flow mass spectrometric method. The following Arrhenius expressions for the total rate constants have been obtained from the kinetics of BrO consumption in excess of OH(OD) radical: k 1 ) (1.65 ( 0.30) × 10 -11 exp{(250 ( 50)/T} cm 3 molecule -1 s -1 (with k 1 ) (3.8 ( 0.9) × 10 -11 cm 3 molecule -1 s -1 at T ) 298 K) and k 2 ) (1.7 ( 0.6) × 10 -11 exp{(230 ( 100)/T} cm 3 molecule -1 s -1 (with k 2 ) (3.7 ( 0.9) × 10 -11 cm 3 molecule -1 s -1 at T ) 298 K), where uncertainties are twice the standard deviation. From the kinetics of HBr formation, the upper limit of the rate constant of the reaction OH + BrO f HBr + O 2 (1b) has been determined at T ) 298 K: k 1b < 1.0 × 10 -12 cm 3 molecule -1 s -1 (k 1b /k 1 < 0.03 for the branching ratio of channel 1b). Similarly, for the reaction OD + BrO f DBr + O 2 (2b), the rate constant at T ) 298 K has been determined: k 2b ) (3.7 ( 1.8) × 10 -13 cm 3 molecule -1 s -1 (which corresponds to the branching ratio k 2b /k 2 ) (1.0 ( 0.5) × 10 -2 ). In addition, the rate constant of the reaction OD + DO 2 f D 2 O + O 2 (3) has been measured for the first time: k 3 ) (3.8 ( 0.9) × 10 -11 cm 3 molecule -1 s -1 at T ) 298 K. This work suggests that the additional HBr source from the OH + BrO reaction, although significant, does not appear to be sufficient to explain the difference between current modeled and observed stratospheric HBr concentrations.
Experimental 300 K Measurement of the Rate Constant of the Reaction OH + BrO → Products
The Journal of Physical Chemistry, 1996
The results reported herein are believed to be the first experimental measurements of the rate constant for the reaction OH + BrO f products (eq 1), which was found to be (7.5 (4.2) × 10-11 cm 3 molecule-1 s-1 (2σ) at 300 K and 1 Torr. The mean value is 7 times larger than the estimate in the NASA stratospheric database, which currently finds widespread use to model the chemistry that controls stratospheric ozone concentrations. The reactant radicals were prepared in separate flow reactors and mixed in the main flow reactor. OH was prepared by F + H 2 O f OH + HF, and BrO was prepared by passing dilute mixtures of He/Br 2 /O 2 through a microwave discharge. The composition of the gas mixture was adjusted empirically to minimize the effluent concentration of Br 2. Beam-sampling mass spectrometry supplemented by chemical titration techniques was used to measure atom and radical concentrations. The rate constant for reaction 1 was obtained from a least-squares fit of the observed BrO concentrations as a function of time to a numerical model of relevant reactions. Known values were used for all other rate constants while k 1 was fitted. Just three reactions significantly affect the fitted value of k 1 : OH + BrO f Br + HO 2 (eq 1a), OH + Br 2 f HOBr + Br (eq 2), and BrO + BrO f products (eq 6). The mechanism of reaction 1 is believed to be OH + BrO f [HOOBr] # f Br + HO 2 , ∆H R)-10 kcal mol-1 (eq 1a) and OH + BrO f [HOOBr] # f HBr + O 2 , ∆H R)-48 kcal mol-1 (1b), where [HOOBr] # denotes a short-lived vibrationally excited addition complex. It is argued that eq 1a is the predominant and perhaps exclusive product channel, with eq 1b hindered by a large activation energy for access to the HBr + O 2 products. The magnitude of k 1 , approximately one-half of the gas kinetic limit, is attributed to the promotion of efficient spin-orbit mixing of singlet and triplet surfaces in the [HOOBr] # complex by the heavy Br atom.
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.
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 .
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
Absolute Rate Constants for the Reactions of Cl Atoms with CH 3 Br, CH 2 Br 2 , and CHBr 3
The Journal of Physical Chemistry A, 1997
The rate constants for the reactions of chlorine atoms with the complete series of the three bromomethanes CH 3 Br (1), CH 2 Br 2 (2), and CHBr 3 (3) were measured in a very low pressure reactor, employing a microwave discharge for the generation of Cl atoms with mass spectrometric detection of reactants and products. The experiments were performed in the temperature range 273-363 K and at total pressures ∼1 mTorr. The reactions proceed via hydrogen atom transfer leading to HCl product and the corresponding bromomethyl radicals. Their rate constant expressions are (in cm 3 molecule-1 s-1): k 1) (1.66 (0.14) × 10-11 exp(-1072 (46/T), k 2) (0.84 (0.15) × 10-11 exp(-911 (101/T), and k 3) (0.43 (0.11) × 10-11 exp(-809 (142/T). The activation energy of the reaction decreases with additional bromine substitution, which is attributed to the gradual weakening of the corresponding C-H bond strength. Ab initio theoretical calculations performed at the MP2/6-31++G(2d,2p) level of theory suggest C-H bond strengths for CH 3 Br, CH 2 Br 2 , and CHBr 3 of 416.58, 407.03, and 396.60 kJ mol-1 , respectively.
Temperature dependence of the total reaction rates for Cl+HI and Cl+HBr
The Journal of Chemical Physics, 1999
Thermal reaction rate constants have been determined for the reactions Cl+HI and Cl+HBr in the temperature range 220–400 °K. The rates vary slowly with temperature. For Cl+HI the effective reaction cross section reaches a maximum of 31 Å2 near 300 °K. A tentative reaction model is proposed in which the attacking halogen atom is attracted to the halogen end of the hydrogen halide and then rotation of the hydrogen, with little or no activation energy, completes the reaction.
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).
Kinetic study of the reactions of Br2 with OH and OD
International Journal of Chemical Kinetics, 1999
The kinetics of the reactions OH ϩ Br 2 : HOBr ϩ Br (1) and OD ϩ Br 2 : DOBr ϩ Br (3) have been studied in the temperature range 230-360 K and at total pressure of 1 Torr of helium using the discharge-flow mass spectrometric method. The following Arrhenius expressions were obtained either from the kinetics of product formation (HOBr, DOBr) in excess of Br 2 over OH and OD or from the kinetics of Br 2 consumption in excess of OH and OD: