- Laser (248 nm) flash photolysis and pulse radiolysis of CF2Br2 in aqueous solution: Atmospheric implications
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Henry's law constant (Kh) of CF2Br2 was measured to be 0.058 M atm-1 at room temperature. The steady state as well as laser (248 nm) flash photolysis of CF2Br2 was carried out in aqueous solution. The formation of Br2.- radicals involving two primary photodissociation channels was observed in the flash photolysis. One channel produced CF2Br. and Br. radicals, and the other led to the formation of Br- and H+. The spectrum and reactions of CF2Br. were studied in aqueous solution by pulse radiolysis. The CF2Br. radical was found to absorb in the 230-270 nm region with a molar absorbance (ε) of 710 M-1 cm-1 at λmax = 245 nm. The rate constants of some of the reactions were measured at 299 K as follows: k (eaq- + CF2Br2) = 1.20 ± 0.13 × 1010, 2κ (CF2Br. + CF2Br.) = 1.4 ± 0.4 × 109, κ(CF2Br. + O2) = 1.9 ± 0.5 × 109 M-1 s-1. The CF2BrO2. radical absorbed in the 245-360 nm region with ε = 593 M-1 cm-1 at λmax = 265 nm. An apparatus used to measure Kh, and a closed-loop flow system used to carry out the flash photolysis/pulse radiolysis of sparingly soluble gases, like CF2Br2, are reported.
- Saini, Rameshwar D.,Dhanya, Suresh,Nath Das, Tomi
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- Measuring the rate constant of the reaction between chlorine atoms and CHF2Br by Cl atom resonance fluorescence
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The rate constant of the reaction between Cl atoms and CHF2Br has been measured by chlorine atom resonance fluorescence in a flow reactor at temperatures of 295–368 K and a pressure of ~1.5 Torr. Lining the inner surface of the reactor with F-32L fluoroplastic makes the rate of the heterogeneous loss of chlorine atoms very low (khet ≤ 5 s–1). The rate constant of the reaction is given by the formula k = (4.23 ± 0.13) × 10–12e(–15.56 ± 1.58)/RT cm3 molecule–1 s–1 (with the activation energy in kJ/mol units). The possible role of this reaction in the extinguishing of fires producing high concentrations of chlorine atoms is discussed.
- Larin,Spasskii,Trofimova,Proncheva
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p. 308 - 312
(2016/07/06)
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- Photodissociation dynamics of the reaction CF2Br2 + hv → CF2 + 2Br. Energetics, threshold and nascent CF2 energy distributions for λ = 223-260 nm
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The dissociation dynamics of the reaction CF2Br2 + hv → CF2 + 2 Br have been studied for a variety of dissociation energies, E(diss) = 460-535 kJ mol-1 (corresponding to λ = 260-223 nm). The laser induced fluorescence spectrum of nascent CF2 products was measured for various dissociation energies within this range. Analysis of the spectra yielded the CF2 vibrational distribution and average rotational energy. The translational energy of CF2 was measured via the Doppler broadening of various fully resolved rovibronic transitions. The most detailed analysis of energy disposal in the CF2 fragments was carried out at E(diss) = 486 kJ mol-1 (or λ = 246 nm). At this energy each degree of freedom of CF2 had an average energy of E(vib) = 0.4 ± 0.2 kJ mol-1, E(rot) = 2.5 ± 0.5 kJ mol-1, and E(trans) = 24 ± 3 kJ mol-1. These CF2 energies, coupled with the available thermochemical data, allow us to determine unambiguously that CF2 production must be accompanied by the production of two atomic Br fragments. A photofragment excitation spectrum of CF2Br2, probing for the production of CF2 fragments, provided a reaction threshold of 460 ± 3 kJ mol-1 (corresponding to 260 ± 1.5 nm). The range of previously published reaction enthalpies varies from 392 to 438 kJ mol-1, all of which are substantially below the observed threshold. Additionally, at E(diss) = 486 kJ mol-1, the energy of the CF2 fragment was 27 kJ mol-1 on average, already in excess of the available 26 kJ mol-1, and without considering the kinetic energy of the recoiling Br atoms. We rationalise these data by proposing that the reaction might have a small barrier in the exit channel. The observed threshold corresponds to the top of the barrier (460 kJ mol- 1), while the final energy in the fragments is determined by the asymptotic reaction energy (~ 424 kJ mol-1). Simple dynamical models are presented to show that the proposed mechanism is reasonable. Key future experiments and calculations are identified that would enable a clearer picture of the dynamics of this reaction.
- Cameron, Melanie R.,Johns, Stephen A.,Metha, Gregory F.,Kable, Scott H.
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p. 2539 - 2547
(2007/10/03)
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- Equilibrium and Rate Constants of the Reactions RBr ? R + Br and Br + RBr ? Br2 + R (R = CH2Cl, CHCl2, CCl3, and CBrF2) at 300-700°C
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The equilibrium (Ke,l = k1/k-1) and rate constants of the reactions RBr k5?k-5 R + Br and Br + RBr k2?k-2 R + Br2 (R = R = CH2Cl, CHCl2, CCl3, and CBrF2) at 500-700°C are determined by the method of isothermal pyrolysis of the bromides CH2BrCl, CHBrCl2, CBrCl3, and CBr2F2. In all cases the use of the theoretical point Ke,5 (T = ∞) = A5 allowed more precise determination of Δ00 = ED(R-Br) than in all previous studies.
- Skorobogatov,Dymov,Nedozrelova
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p. 1777 - 1785
(2007/10/03)
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- Competition between unimolecular C-Br-bond fission and Br2 elimination in vibrationally highly excited CF2Br2
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The competition between C-Br-bond fission and three-center elimination of molecular bromine (Br2) in highly excited CF2Br2 molecules has been studied under collision-free conditions.Transient resonantly enhanced multiphoton ionization (REMPI) was used to monitor Br(2P1/2) and Br(2P3/2) formation during and after infrared (IR) mutiphoton excitation of CF2Br2; time-resolved laser-induced fluorescence (LIF) spectroscopy was employed for the detection of transient CF2 after Br2 elimination.Direct time-resolved measurements of the sum of afterpulse reaction rates, absolute product yields for the CF2 and Br(2P3/2) channels as well as absorbed energies per excitation pulse were used to characterize parts of the vibrational energy distribution P(E) established after IR multiphoton excitation and to determine rate coefficients and branching ratios for the elimination and dissociation reaction as a function of the average internal energy .The existence of both channels, the dissociation and the elimination channel, has been confirmed.A comparison of the experimental data with statistical adiabatic channel model calculations (SACM) enabled us to determine the threshold energies E0(J=0) for the unimolecular Br2 elimination 0(J=0) = 19 070+/-500 cm-1> and the C-Br bond fission 0(J=0) = 20 700+/-500 cm-1>, the two possible pathways of the reaction.
- Abel, B.,Hippler, H.,Lange, N.,Schuppe, J.,Troe, J.
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p. 9681 - 9690
(2007/10/02)
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- Chemistry of CFn(1+) (n = 1-3) Ions with Halocarbons
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The gas-phase reactions of CF(1+), CF2(1+), and CF3(1+) with the halocarbons CF3Cl, CF3Br, CF3I, CF4, and C2F6 have been studied using a variable-temperature-selected ion flow tube (VT-SIFT) instrument at 300 and 496 K.The ion CF(1+) reacts rapidly with CF3X (X = Cl, Br, I) producing the ions CF2X(1+).In the reaction of CF(1+) with CF3Cl, CF3(1+) is also produced as a minor product.Curvature was observed in the pseudo-first-order kinetics plots for the reactions of CF(1+) with CF4 and C2F6.In both cases the curvature is attributed to the presence of two or more CF(1+) states (probably vibrational) of differing reactivities toward the perfluorocarbon of interest.This conclusion is supported by our observation of charge transfer from CF(1+) to NO, a reaction which is endothermic by 15 kJ/mol for the ground state of CF(1+).CF(1+) is unreactive with O2, N2, and Xe.The reactions of CF2(1+) with CF3X yield CF3(1+) and CF2X(1+) for X = Cl and Br; for X = I, CF2I(1+) and CF3I(1+) are produced.The overall reactions proceed at approximately the collision rate at 300 and 496 K, and the branching ratios are not strongly dependent on temperature.The reactions of CF2(1+) with CF4 and C2F6 produce CF3(1+) and C2F5(1+), respectively.The rate constants decrease significantly with increasing temperature.CF2(1+) reacts rapidly by charge transfer with NO.The reaction of CF2(1+) with O2, producing CF2O(1+), is inefficient.CF2(1+) is unreactive with N2.CF3(1+) reacts with CF3X (X = Cl, Br, I) at rates below the collision values, producing a single ionic product, CF2X(1+).While the rate constants for the reactions of CF3(1+) with CF3X increase in the series with increasing CF3X mass, the rate constants for reaction with each CF3X decrease sharply with increasing temperature.A mechanism is proposed in which the reaction proceeds on a double-well potential energy surface.No reaction was observed for the CF3(1+)/CF4 system.CF3(1+) appeared to react very slowly with C2F6 and NO, producing C2F5(1+) and NO(1+), respectively, but reactions with impurities in the neutral reagents cannot be ruled out as the source of these ions.CF3(1+) is unreactive with O2 and N2.
- Morris, Robert A.,Viggiano, A. A.,Doren, Jane M. Van,Paulson, John F.
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p. 2597 - 2603
(2007/10/02)
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- Ultrafast absorption spectroscopy of photodissociated CF2Br2: Details of the reaction mechanism and evidence for anomalously slow intramolecular vibrational redistribution within the CF2Br intermediate
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Ultrafast time-resolved absorption spectroscopy in the hard ultraviolet has been used to investigate the photodissociation of gas-phase CF2Br2 photolyzed at 248 nm.The broadband spectra obtained in the 250-265 nm region have shown that absorption of a single photon activates a two-step sequential elimination of the molecule's two bromine atoms, leaving the product CF2 radical in the ground or first-excited vibrational state of its ν2 bending mode.The spectra also demonstrate the direct detection of the vibrationally hot CF2Br intermediate species itself.We interpret the 6 ps time scale over which the diffuse CF2Br spectrum evoles as evidence for slow intramolecular vibrational redistribution within this molecule.
- Gosnell, T. R.,Taylor, A. J.,Lyman, J. L.
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p. 5949 - 5953
(2007/10/02)
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- FREE RADICAL ADDITION TO OLEFINS-5
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Part 5 deals with addition of difluorobromomethyl radicals to trifluoroethylene and ethylene. The photochemical reaction of CF//2Br//2 with CHFCF//2 and CH//2CH//2 has been examined in a series of gas-phase experiments. A mechanism for the radical chain addition which takes place is proposed. The variation in the rate of formation of termination products, CF//2BrCF//2Br and CF//2BrCHFCF//2CF//2Br, with reactant concentration is explained if five important termination reactions are taken into account and a long-lived excited state of the CF//2Br//2 is accepted. Arrhenius parameters for the addition of CF//2Br radicals to the olefins have been derived.
- TEDDER JM,WALTON JC
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p. 1135 - 1144
(2008/10/08)
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