Photochemical studies of atomic species (F, Br, O) in solution (original) (raw)
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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.
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.
Chemosphere, 1996
Laboratory experiments with H202/UV oxidation processes and photolysis at 253.7 nm wavelength have been carried out on dilute aqueous solutions (Co = 0.1 to 3.0 raM) of trifluorobenzene derivatives (1,3,5trifluorobenzene, 1,2,3 and 1,2,4-trifluorobenzene) and of ct,ct,tx-trifluorotoluene in the presence and in the absence of dissolved oxygen. The analyses of fluoride ions content during the oxidation experiments showed that the first steps lead to the production of about 2 tool of F-/mol of trifluorobenzene decomposed and of 1 mol of F-/mol of trifluorotoluene decomposed. Kinetic studies lead to the determination of the quantum yield for the photolysis of 1,3,5-trifluorobenzene, 1,2,3 and 1,2,4-trifluorobenzene (~ = 0.011, 0.010 and 0.015 respectively), and of trifluorotoluene (~ = 0.015). The rate constants for the reaction of hydroxyl radicals with these molecules, determined under specific experimental conditions, were found to range from 3.7 109 to 4.9 109 M-l.s-1). GC/MS analyses carried out on extracts at different irradiation time (UV, H202/UV) lead to the identification of numerous by-products from trifluorobenzene and trifluorotoluene. They consist mostly in hydroxylated and dehalogenated compounds. Dimers have also been observed during photolysis. Moreover, experiments carried out under oxygen limiting conditions revealed the formation of other compounds. For each case studied, a detailed mechanism involving radical intermediates and the different reaction sequences is proposed.
Photochemistry of Bromofluorobenzenes
The Journal of Physical Chemistry A, 2006
The photochemistry of low lying excited states of six different fluorinated bromobenzenes has been investigated by means of femtosecond laser spectroscopy and high level ab initio CASSCF/CASPT2 quantum chemical calculations. The objective of the work was to investigate how and to what extent light substituents, position on the benzene ring and number, would influence the dissociation mechanism of bromobenzene. In general, the actual position of a fluorine atom affects the dissociation rate to a less extent than the number of fluorine atoms. A clear connection between a lowering of a repulsive πσ* relative to a bound ππ* state and the number of fluorine substituents exists, and the previously suggested model of coupling between dissociation rate and relative location of bound and repulsive state still holds for these molecules. A more elaborate examination of the electronic structure of the excited states in bromobenzenes than previously reported is presented.
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 Physical Chemistry A, 2010
Nascent emission and laser-induced dispersed fluorescence spectra of products or intermediates from the multiphoton photolysis reaction of bromomethanes (CHBr 3 , CHBr 2 Cl, CHBrCl 2 , and CH 2 Br 2) at 266 nm were recorded in a slow flow cell. Electronically excited species including CH (A 2 ∆, B 2 Σ-, and C 2 Σ +), C 2 (d 3 Π g), and atomic Br (4 D J and 4 P J) were observed in the nascent emission spectra. Free radicals such CHBr or CHCl were also successfully found using laser-induced dispersed fluorescence spectroscopy. The reactive intermediate, CHBr, was seen only in the photolysis of CHBr 3 , whereas CHCl was only discovered when the precursor was CHBr 2 Cl or CHBrCl 2. More experiments including the power dependence and temporal waveform measurements were conducted. The present study reports the first direct measurements of the intermediate products in the multiphoton photodissociation reaction of these bromomethanes at 266 nm. Nascent emission spectra following the photolysis at longer near-ultraviolet wavelengths (280 and 355 nm) were also acquired. On the bassis of these results, the multiphoton photodissociation mechanism of these bromomethanes at 266 nm can be confirmed.
Photochemistry of fluorinated compounds: reaction between FCO and CF3CF2 radicals
Journal of Photochemistry and Photobiology A: Chemistry, 2002
CF 3 CF 2 C(O)F is a suitable source to generate the title radicals which can recombine to give back the precursor or lead to the formation of C 4 F 10 , CF 2 O and CO. We performed the photolysis of CF 3 CF 2 C(O)F pure and in the presence of c-C 6 H 12 or (FCO) 2 (oxalylfluoride), following the concentration of the different species by FTIR spectroscopy. The rate constant for the reaction CF 3 CF 2 + FCO → CF 3 CF 2 C(O)F was obtained through a simulation using both our experimental data and bibliography data available. The value found is (6.8 ± 0.8) × 10 −12 cm 3 molecule −1 s −1 and it is not substantially different from the rate constant (4.2 ± 0.5) × 10 −12 cm 3 molecule −1 s −1 obtained for the reaction of CF 3 and FCO radicals. Thus, it was concluded that the length of the carbon chain does not affect the mechanism nor the rate constant value when it is compared with the CF 3 + FCO system.
p -Nitrobenzenesulfenate Esters as Precursors for Laser Flash Photolysis Studies of Alkyl Radicals
The Journal of Organic Chemistry, 2002
A series of p-nitrobenzenesulfenate esters was used in laser flash photolysis (LFP) studies to generate alkoxyl radicals that fragmented to give the (2,2-diphenylcyclopropyl)methyl radical. Rate constants for the -scission reactions increased as a function of the carbonyl compound produced in the fragmentation reaction in the order CH 2 O < MeCHO < Me 2 CO < PhCHO < Ph 2 CO and increased with increasing solvent polarity. For alkoxyl radicals that fragment to produce benzaldehyde and benzophenone, the -scission reactions are faster than 1,5-hydrogen atom abstractions when the incipient carbon radical is as stable as a secondary alkyl radical, and this entry to carbon radicals can be used in LFP kinetic studies.
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.