Pulsed Laser Photolysis-resonance Fluorescence Research Articles

Reaction of Cl Atom with c-C5F8 and c-C5HF7: Relative and Absolute Measurements of Rate Coefficients and Identification of Degradation Products.

Partially and fully fluorinated olefins are a class of compounds with relatively short atmospheric lifetimes and low 100-year global warming potentials, compared to their saturated predecessors, which are used or considered as refrigerants, propellants, solvents, and other end-uses. The cyclic unsaturated compounds c-C5F8 and c-C5HF7 are currently under consideration as etching agents for the semiconductor industry. In this study, we expand on our previous work on the reaction of the OH radical with c-C5F8 and c-C5HF7 and report the rate coefficients, k, for the gas-phase reaction of the Cl atom with c-C5F8 and c-C5HF7 over a range of temperature (245-367 K) and pressure (100-200 Torr of He or N2 and 0 to 4.8 Torr O2) using a pulsed laser photolysis-resonance fluorescence (PLP-RF) technique. In addition, a relative rate (RR) technique, employing multiple reference compounds, was used to study the Cl atom reactions at 296 K, 100 and 630 Torr (N2 or air) total pressure. Reaction rate coefficients, k1, of the Cl atom reaction with c-C5F8 were found to be independent of pressure, over the pressure range used in this work, with k1(296 K), derived as an average of results from the PLP-RF and RR techniques being (1.07 ± 0.02) × 10-12 cm3 molecule-1 s-1 and k1(T) = (7.76 ± 0.73) × 10-13 × (exp[(98 ± 26)/T]) cm3 molecule-1 s-1, where the quoted error limits represent the 2σ data precision. Rate coefficients, k2, for the Cl atom + c-C5HF7 reaction were measured to be k2(296 K) = (4.61 ± 0.10) × 10-12 cm3 molecule-1 s-1 and k2(T) = (7.42 ± 0.89) × 10-13 × (exp[(540 ± 32)/T]) cm3 molecule-1 s-1. The Cl atom temporal profiles, observed with the PLP-RF technique, indicate that the Cl atom with c-C5F8 and c-C5HF7 reactions lead to adduct formation. The equilibrium constants for adduct formation were derived in this work, and a second-law analysis was used to obtain ΔH and ΔS values of -18.5 ± 0.4 kcal mol-1, -30.9 ± 1.2 cal K-1 mol-1, and -13.9 ± 0.5 kcal mol-1, -27.6 ± 1.1 cal K-1 mol-1 for the c-C5F8 and c-C5HF7 reactions, respectively. The Cl-initiated degradation of c-C5F8 and c-C5HF7 in the presence of O2 was studied and stable products were identified via infrared spectroscopy using experimental or theoretically derived spectra from our previous OH reaction work. For c-C5F8, FC(O)CF2CF2CF2C(O)F and FC(O)C(O)F were observed with molar yields of 0.80 and 0.10, respectively. For c-C5HF7, we observed the formation of HC(O)CF2CF2CF2C(O)F and HC(O)C(O)F with a combined molar yield of 0.72. Carbonyl difluoride, F2CO, was also a major product in the decomposition of c-C5F8 and c-C5HF7. The oxidation mechanism of the Cl-initiated degradation of c-C5F8 and c-C5HF7 is discussed. Based on the combined findings from this and our previous work, the atmospheric implications from the use of c-C5F8 and c-C5HF7 are presented.

An experimental and theoretical study on the reaction of Cl with CF3CF2CH2OH

An absolute kinetic study of the reaction of Cl atoms with CF3CF2CH2OH is reported as a function of temperature (T = 268–377 K) and at a total pressure of 100 Torr by the Pulsed Laser Photolysis - Resonance Fluorescence (PLP-RF) technique. No pressure dependence of the rate coefficient for the title reaction, k, was observed between 50 and 200 Torr of He at 298 K. The derived Arrhenius expression in that T-range was k(T) = (2.18 ± 0.24) × 10−12 exp(–(333 ± 34)/T) cm3 molecule−1 s−1, where the uncertainties are ±2σ. From these results, the average tropospheric lifetime of CF3CF2CH2OH due to the reaction with Cl was estimated to be 50 years, considering a global Cl concentration of 103 atom cm−3 and an average temperature of 272 K. Additionally, a theoretical study of the Cl + CF3CF2CH2OH reaction has been carried out by ab initio Möller–Plesset second-order perturbation treatment with 6-311G** basis set to investigate the reaction mechanism. Molecular energies of the different critical points of the potential energy surface have been calculated at QCISD(T) level. The theoretical study shows that the H-atom abstraction from the –CH2– group is the most favourable reaction pathway.

Pulsed laser photolysis vacuum UV laser-induced fluorescence kinetic study of the reactions of Cl( 2P 3/2) atoms with ethyl formate, n-propyl formate, and n-butyl formate

Cl( 2P 3/2) atoms were produced by photolysis of Cl 2 at 351 nm in the presence of alkyl formates and monitored by LIF spectroscopy at 134.72 nm. Rate coefficients of k(Cl + C 2H 5OC(O)H) = (9.5 ± 0.3)×10 −12, k(Cl + n-C 3H 7OC(O)H) = (4.5 ± 0.3) × 10 −11 and k(Cl + n-C 4H 9OC(O)H) = (1.2 ± 0.1) × 10 −10 cm 3 molecule −1 s −1 were determined at 295 ± 2 K in 6–7 Torr of Ar diluent. These values are 15–30% lower than reported in a previous pulsed laser photolysis resonance fluorescence study. The discrepancy may reflect complications arising from the fact that the time scales for chlorine atom loss and regeneration were not sufficiently decoupled in the previous study.

Relative and absolute kinetic studies of 2-butanol and related alcohols with tropospheric Cl atoms

A newly constructed chamber/Fourier transform infrared system was used to determine the relative rate coefficient, k(i), for the gas-phase reaction of Cl atoms with 2-butanol (k(1)), 2-methyl-2-butanol (k(2)), 3-methyl-2-butanol (k(3)), 2,3-dimethyl-2-butanol (k(4)) and 2-pentanol (k(5)). Experiments were performed at (298 +/- 2) K, in 740 Torr total pressure of synthetic air, and the measured rate coefficients were, in cm(3) molecule(-1) s(-1) units (+/-2sigma): k(1)=(1.32 +/- 0.14) x 10(-10), k(2)=(7.0 +/- 2.2) x 10(-11), k(3)=(1.17 +/- 0.14) x 10(-10), k(4)=(1.03 +/- 0.17) x 10(-10) and k(5)=(2.18 +/- 0.36) x 10(-10), respectively. Also, all the above rate coefficients (except for 2-pentanol) were investigated as a function of temperature (267-384 K) by pulsed laser photolysis-resonance fluorescence (PLP-RF). The obtained kinetic data were used to derive the Arrhenius expressions: k(1)(T)=(6.16 +/- 0.58) x 10(-11)exp[(174 +/- 58)/T], k(2)(T)=(2.48 +/- 0.17) x 10(-11)exp[(328 +/- 42)/T], k(3)(T)=(6.29 +/- 0.57) x 10(-11)exp[(192 +/- 56)/T], and k(4)(T)=(4.80 +/- 0.43) x 10(-11)exp[(221 +/- 56)/T](in units of cm(3) molecule(-1) s(-1) and +/-sigma). Results and mechanism are discussed and compared with the reported reactivity with OH radicals. Some atmospheric implications derived from this study are also reported.

Influence of temperature in the kinetics of the gas-phase reactions of a series of acetates with Cl atoms

Abstract Absolute rate coefficients have been measured for the first time as a function of temperature for the gas phase reactions of chlorine atoms with a series of aliphatic acetates: methyl acetate ( k 1 ), ethyl acetate ( k 2 ), n -propyl acetate ( k 3 ) and n-butyl acetate ( k 4 ). The experiments were carried out using the pulsed laser photolysis-resonance fluorescence technique (PLP-RF), over the temperature range 265–383 K. The obtained kinetic data were used to derive the Arrhenius expressions: k 1 = ( 9.31 ± 1.02 ) × 1 0 - 12 exp [ - ( 359 ± 70 ) / T ] ; k 2 = ( 4.35 ± 0.65 ) × 1 0 - 12 exp [ ( 342 ± 92 ) / T ] ; k 3 = ( 2.22 ± 0.20 ) × 1 0 - 11 exp [ ( 217 ± 58 ) / T ] and k 4 = ( 5.41 ± 1.51 ) × 1 0 - 11 exp [ ( 245 ± 168 ) / T ] (in units of cm3 molecule−1 s−1). The rate constants obtained at room temperature were as follows: methyl acetate, 0.279±0.031; ethyl acetate, 1.37±0.20; n -propyl acetate, 4.60±0.41 and n -butyl acetate, 12.3±3.4 (in units 10−11 cm3 molecule−1 s−1). The results are discussed in terms of structure-reactivity relationships and the atmospherics implications are also analyzed.

Reaction of O(3P) with the alkyl iodides: CF3I, CH3I, CH2I2, C2H5I, 1-C3H7I and 2-C3H7I

The kinetics of the reactions between O(3P) and various alkyl iodides (RI) was investigated using the pulsed laser photolysis–resonance fluorescence (PLP–RF) technique at temperatures between 223 and 363 K. The reactions studied were O(3P) + CF3I (1), O(3P) + CH3I (2), O(3P) + CH2I2 (3), O(3P) + C2H5I (4), O(3P) + 1-C3H7I (5), and O(3P) + 2-C3H7I (6). The Arrhenius expressions (units of cm3 molecule−1 s−1) obtained were: k1(223–363 K) = (8.73 ± 1.29) × 10−12exp{−(216 ± 38)/T}, k2(223–363 K) = (9.88 ± 1.67) × 10−12exp{(183 ± 50)/T}, k3 (256–363 K) = (7.36 ± 0.31) × 10−11 cm3 molecule−1 s−1, k4(223–363 K) = (1.58 ± 0.22) × 10−11exp{(239 ± 39)/T} k5(223–363 K) = (1.10 ± 0.17) × 10−11exp{(367 ± 42)/T}, and k6(223–363 K) = (1.84 ± 0.31) × 10−11exp{(296 ± 46)/T}. With the exception of k1 and k3, all the other rate coefficients display a negative temperature dependence, which highlights the importance of association complex formation in reactions of O(3P) + RI.

A kinetic study of the reaction of Cl with a series of linear and ramified ketones as a function of temperature

The reactions of Cl atoms with 2,3-butadione (k1), 2-butanone (k2), 2-pentanone (k3), 2-hexanone (k4), 3-methyl-2-butanone (k5), 4-methyl-2-pentanone (k6) and 5-methyl-2-hexanone (k7), were investigated for the first time as a function of temperature (266–380 K), over the pressure range 20–200 Torr, using pulsed laser photolysis-resonance fluorescence (PLP-RF). The Arrhenius expressions for the seven reactions are proposed. The temperature dependence of the studied reactions is discussed; the reaction of Cl atoms with 2,3-butadione exhibits a positive temperature dependence whereas the corresponding reactions with 2-pentanone, 2-hexanone, 4-methyl-2-pentanone and 5-methyl-2-hexanone exhibit a negative temperature dependence and those with 2-butanone and 3-methyl-2-butanone shows only a slightly positive and a negative temperature dependence, respectively. Finally, the results and atmospheric implications are discussed and compared with the reactivity towards the OH radical.

Temperature-dependent quantum yields for O(3P) and O(1D) production from photolysis of O3 at 248 nm

A pulsed laser photolysis–resonance fluorescence technique was employed independently by two laboratories to measure ΦO3P, the quantum yield for production of O(3P) from O3 photolysis at 248 nm, between 196 and 427 K. The agreement between the two studies is very good, and the combined results are adequately represented by the function ΦO3P= (0.115 ± 0.030) × exp((35 ± 60)/T) where the uncertainties are 2σ. Within experimental uncertainties, the new results are in agreement with previously reported room temperature results as well as with the single previous temperature dependence study, and greatly reduce the uncertainties in ΦO3P(T) and ΦO1D(T) (=1− ΦO3P(T)) especially at temperatures other than room temperature. The yield of O(3P) in the reaction of O(1D) with O3 is shown to be greater than unity at room temperature and below, and to increase slightly with decreasing temperature.

Gas-phase chemistry of atmospheric Cl atoms: a PLP-RF kinetic study with a series of ketones

The pulsed laser photolysis-resonance fluorescence technique (PLP-RF) has been used to determine the absolute rate coefficients for the gas-phase reactions of Cl atoms with a series of ketones at room temperature (298 K). The rate coefficients obtained (in units of cm 3 molecule −1 s −1) are: 2-butanone (3.30±0.20)×10 −11, 2,3-butadione (4.92±0.44)×10 −13, 2-pentanone (4.57±0.28)×10 −11, 3-pentanone (4.50±0.32)×10 −11, 2-hexanone (6.54±0.58)×10 −11, 3-hexanone (6.69±0.62)×10 −11 and cyclohexanone (6.75±0.52)×10 −11. The measured values were independent of pressure over the range 20–200 Torr. These rate coefficients were compared with previous studies carried out by different techniques, when data were available. Results and atmospheric implications are discussed and compared with the reactivity with OH radicals.

A Pulsed Laser Photolysis-Resonance Fluorescence Kinetic Study of the Atmospheric Cl Atom-Initiated Oxidation of Propene and a Series of 3-Halopropenes at Room Temperature

Absolute rate coefficient measurements have been carried out for the reactions of Cl atoms with propene and a series of 3-halopropenes, at room temperature (298 ± 2) K using a newly constructed laser photolysis-resonance fluorescence (PLP-RF) system. The rate coefficients obtained (in units of cm3 molecule−1 s−1) are: propene (1.40± 0.24) ×10−10, 3-fluoropropene (4.92 ± 0.42) ×10−11, 3-chloropropene (7.47 ± 1.50) × 10−11, 3-bromopropene (1.23± 0.14) ×10−10 and 3-iodopropene (1.29± 0.15) ×10−10. In order to test this new system, the reactions of Cl atoms with acetone and isoprene have also been studied and compared with data previously reported. The rate coefficients determined at room temperature for these last two reactions are (2.93 ± 0.20) ×10−12 cm3 molecule−1 s− 1 and (3.64± 0.20)×10−10 cm3 molecule−1 s−1, respectively. The measured values were independent of pressure over the range 20–200 Torr. The influence of the different halogen atoms substituents on the reactivity of these alkenes with Cl atoms as well as the atmospheric implications of these measurements are studied and discussed for the first time in this work and compared with the reactivity with NO3 and OH radicals.

Rate constants for the gas‐phase reactions of chlorine atoms with a series of ketones

The pulsed laser photolysis-resonance fluorescence technique has been used to determine the absolute rate coefficient for the Cl atom reaction with a series of ketones, at room temperature (298 ± 2) K and in the pressure range 15–60 Torr. The rate coefficients obtained (in units of cm3 molecule−1 s−1) are: acetone (3.06 ± 0.38) × 10−12, 2-butanone (3.24 ± 0.38) × 10−11, 3-methyl-2-butanone (7.02 ± 0.89) × 10−11, 4-methyl-2-pentanone (9.72 ± 1.2) × 10−11, 5-methyl-2-hexanone (1.06 ± 0.14) × 10−10, chloroacetone (3.50 ± 0.45) × 10−12, 1,1-dichloroacetone (4.16 ± 0.57) × 10−13, and 1,1,3-trichloroacetone (<2.4 × 10−12). © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 62–66, 2000

Rate constants for the gas-phase reactions of chlorine atoms with a series of ketones

The pulsed laser photolysis-resonance fluorescence technique has been used to determine the absolute rate coefficient for the Cl atom reaction with a series of ketones, at room temperature (298 ± 2) K and in the pressure range 15–60 Torr. The rate coefficients obtained (in units of cm3 molecule−1 s−1) are: acetone (3.06 ± 0.38) × 10−12, 2-butanone (3.24 ± 0.38) × 10−11, 3-methyl-2-butanone (7.02 ± 0.89) × 10−11, 4-methyl-2-pentanone (9.72 ± 1.2) × 10−11, 5-methyl-2-hexanone (1.06 ± 0.14) × 10−10, chloroacetone (3.50 ± 0.45) × 10−12, 1,1-dichloroacetone (4.16 ± 0.57) × 10−13, and 1,1,3-trichloroacetone (<2.4 × 10−12). © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 62–66, 2000

Kinetic studies of Cl atom reactions with series of alkanes using the pulsed laser photolysis-resonance fluorescence method

The rate coefficients for the reactions of Cl atoms with CH4 (k1), C2H6 (k2), C3H8 (k3), and n-C4H10 (k4), have been determined using a newly built pulsed laser photolysis-resonance fluorescence (PLP-FR) apparatus. The reactions were studied at room temperature (297 ±2) K and P = 60 Torr of helium. The rate constants k2, k3 and k4 were measured in the presence of oxygen. The obtained values are: k1 = (1.07±0.03)×10-13; k2 = (6.5±0.3)×10-11; k3 = (1.31±0.03)×10-10; k4 = (2.25±0.06)×10-10 (in cm3 molecule-1 s-1). These results are compared to the literature data and provide a validation of the newly built PLP-FR apparatus in our laboratory.

Kinetic Studies on the Reactions of Hydroxyl Radicals with Diethers and Hydroxyethers

Rate constants for the reactions of OH radicals with a series of diethers and hydroxyethers have been determined at 298 ± 2 K. Rate measurements were made using a pulsed laser photolysis resonance fluorescence technique at total pressures of ∼100 Torr and a conventional photolytic relative rate method at atmospheric pressure. The temperature dependencies of the rate constants for four diethers were also studied over the temperature range 230−372 K using pulsed laser photolysis resonance fluorescence. The rate data for reaction of OH with the diethers show significant deviations from simple structure−activity relationships. Evidence is presented that suggests that these deviations may be a consequence of stabilization of the reaction transition states by hydrogen bonding.

Rate constants for the reactions of hydroxyl radicals and chlorine atoms with halogenated aldehydes

Rate constants for the reactions of OH radicals and chlorine atoms with halogenated aldehydes of the type CX 3 CHO (X=H, Cl, or F) have been determined at 298±2 K. The OH radical rate data were determined using a pulsed laser photolysis resonance fluorescence technique at total pressures in the region 15-100 Torr and a conventional photolytic relative rate technique at atmospheric pressure. The chlorine atom rate data were determined using the relative rate method only. The rate data for reaction with both OH radicals and chlorine atoms indicate that increasing halogen atom substitution decreases the rate constant for reaction. Evidence is presented which suggests that the deactivation of these compounds is due mainly to polar effects in the transition states of the reactions induced by the halomethyl substituents, rather than decreassed overall reaction enthalpy