Units Of Cm3 Molecule Research Articles

Rate Coefficients of Hydroxyl Radical Reaction with Dimethyl Ether and Methyl tert-Butyl Ether over an Extended Temperature Range

Rate coefficients of the reaction of hydroxyl (OH) radicals with CH3OCH3 (k1) and CH3OC(CH3)3 (k2) over an extended temperature range are reported. Measurements were performed using a laser photolysis−laser-induced fluorescence technique under slow flow conditions at a total pressure of 740 ± 10 Torr. Arrhenius plots of the data exhibited significant curvature and were fitted in the form of k(T) = ATB exp(−C/T). The resulting modified Arrhenius expressions (error limits ±2σ) that best described these extended temperature measurements and prior low-temperature measurements were (in units of cm3 molecule-1 s-1) k1(295−650 K) = (1.05 ± 0.10) × 10-17T2.0 exp[(328 ± 32)/T] and k2(293−750 K) = (1.15 ± 0.11) × 10-17T2.04 exp[(266 ± 41)/T]. Comparison of our measurements for k1 with previous, overlapping low-temperature measurements indicated generally good agreement. Our measurements for k2, although consistent with previous room temperature measurements, exhibited a larger temperature dependence than previously...

Rate constants for the gas-phase reaction of the hydroxyl radical with a series of dimethylbenzaldehydes and trimethylphenols at atmospheric pressure

Rate constants for three dimethylbenzaldehydes and two trimethylphenols have been determined for the OH reactions at 298±2 K and atmospheric pressure using a relative rate method. The OH reaction rate constants were placed on an absolute basis using the literature rate constant for 1,2,4-trimethylbenzene of (3.25±0.5)×10−11 cm3 molecule−1s−1). The measured rate constants were (in units of cm3 molecule−1 s−1) 2,4-dimethyl-benzaldehyde, (4.32±0.67)×10−11; 2,5-dimethylbenzaldehyde, (4.37±0.68)×10−11; 3,4-dimethylbenzaldehyde, (2.14±0.34)×10−11; and 2,3,5- trimethylphenol, (12.5±1.9)×10−11, 2,3,6-trimethylphenol, (11.8±1.8)×10−11. Using an average OH concentration of 8.7×105 molecule cm−3, the estimated atmospheric lifetimes are ca. 7.5 h for 2,4- and 2,5-dimethylbenzaldehydes, ca. 15 h for 3,4-dimethylbenzaldehyde, ca. 2.5 h for 2,3,5- and 2,3,6-trimethylphenols. The reactivities of the trimethylphenols exceed those of the dimethyl-benzaldehydes by more than a factor of 3. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 523–525, 1997.

Kinetic studies of OH reactions with Iso‐propyl, Iso‐butyl, Sec‐butyl, and Tert‐butyl acetate

The temperature dependence of the rate coefficients for the OH radical reactions with iso-propyl acetate (k1), iso-butyl acetate (k2), sec-butyl acetate (k3), and tert-butyl acetate (k4) have been determined over the temperature range 253–372 K. The Arrhenius expressions obtained are: k1=(0.30±0.03)×10−12 exp[(770±52)/T]; k2=(109±0.14)×10−12 exp[(534±79)/T]; k3=(0.73±0.08)×10−12 exp[(640±62)/T]; and k4=(22.2±0.34)×10−12 exp[−(395±92)/T] (in units of cm3 molecule−1 s−1). At room temperature, the rate constants obtained (in units of 10−12 cm3 molecule−1 s−1) were as follows: iso-propyl acetate (3.77±0.29); iso-butyl acetate (6.33±0.52); sec-butyl acetate (6.04±0.58); and tert-butyl acetate (0.56±0.05). Our results are compared with the previous determinations and discussed in terms of structure-activity relationships. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29: 683–688, 1997.

A kinetic study of chlorine radical reactions with ketones by laser photolysis technique

Rate coefficients for reactions between Cl radicals and four ketones were determined at 294 ± 1 K with a relative rate method using a laser photolysis technique. The experiments were conducted in synthetic air in a flow system at atmospheric pressure. A mixture of Cl2/ClONO2 was photolyzed and the formation of NO3 through the reaction Cl + ClONO2 → Cl2 + NO3 was measured with and without ketones in the reaction mixture. The NO3 radical concentration was measured by optical absorption using a diode laser as the light source. The rate coefficients for the Cl-ketone reactions could then be evaluated. The following rate coefficients were obtained (in units of cm3 molecule−1 s−1): cyclohexanone (7.00 ± 1.15) × 10−11; cyclopentanone (4.76 ± 0.33) × 10−11; acetone (1.69 ± 0.32) × 10−12; and 2,3-butanedione (7.62 ± 1.66) × 10−13. The accuracy of the method employed was tested by using the well-studied reaction between Cl and methane and a rate coefficient of (9.37 ± 1.04) × 10−14 cm3 molecule−1 s−1 was obtained, which is in good agreement with previous work. The errors are at the 95% confidence level. The results in this work indicate that a carbonyl group in a ketone lowers the reactivity towards α-hydrogen abstraction by Cl radicals, compared to the corresponding Cl-alkane reactions. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 195–201, 1997.

Kinetic studies of OH reactions withIso-propyl,Iso-butyl,Sec-butyl, andTert-butyl acetate

The temperature dependence of the rate coefficients for the OH radical reactions with iso-propyl acetate (k1), iso-butyl acetate (k2), sec-butyl acetate (k3), and tert-butyl acetate (k4) have been determined over the temperature range 253–372 K. The Arrhenius expressions obtained are: k1=(0.30±0.03)×10−12 exp[(770±52)/T]; k2=(109±0.14)×10−12 exp[(534±79)/T]; k3=(0.73±0.08)×10−12 exp[(640±62)/T]; and k4=(22.2±0.34)×10−12 exp[−(395±92)/T] (in units of cm3 molecule−1 s−1). At room temperature, the rate constants obtained (in units of 10−12 cm3 molecule−1 s−1) were as follows: iso-propyl acetate (3.77±0.29); iso-butyl acetate (6.33±0.52); sec-butyl acetate (6.04±0.58); and tert-butyl acetate (0.56±0.05). Our results are compared with the previous determinations and discussed in terms of structure-activity relationships. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29: 683–688, 1997.

Ab Initio Study of the H + HONO Reaction: Direct Abstraction versus Indirect Exchange Processes

The mechanism of the H + HONO reaction (for which no experimental data are available) has been elucidated by ab initio molecular orbital calculations using modified G2 and BAC-MP4 methods. These results indicate that the reaction occurs predominantly by two indirect metathetical processes. One produces OH + HNO and H2O + NO from the decomposition of vibrationally excited hydroxyl nitroxide, HN(O)OH, formed by H atom addition to the N atom of HONO. The other produces H2O + NO from the decomposition of vibrationally excited dihydroxylamino radical, N(OH)2, formed by H atom addition to the terminal O atom. These indirect displacement processes are much more efficient than the commonly assumed, direct H-abstraction reaction producing H2 + NO2. A transition-state theory calculation for the direct abstraction reaction and RRKM calculations for the two indirect displacement processes give rise to the following rate constants, in units of cm3 molecule-1 s-1 for the 300−3500 K temperature range under atmospheric conditions: kH2 = 3.33 × 10-16T1.55 exp(−3328.5/T), kOH = 9.36 × 10-14T0.86 exp(−2500.8/T), kH2O = 1.35 × 10-17T1.89 exp(−1935.7/T), where the rate constant for H2O production represents the sum from both indirect displacement reactions.

Theoretical study of reactions of N2O with NO and OH radicals

The reactions of N2O with NO and OH radicals have been studied using ab initio molecular orbital theory. The energetics and molecular parameters, calculated by the modified Gaussian-2 method (G2M), have been used to compute the reaction rate constants on the basis of the TST and RRKM theories. The reaction N2O + NO N2 + NO2 (1) was found to proceed by direct oxygen abstraction and to have a barrier of 47 kcal/mol. The theoretical rate constant, k1 = 8.74 × 10−19 × T2.23 exp (−23,292/T) cm3 molecule−1 s−1, is in close agreement with earlier estimates. The reaction of N2O with OH at low temperatures and atmospheric pressure is slow and dominated by association, resulting in the HONNO intermediate. The calculated rate constant for 300 K ≤ T ≤ 500 K is lower by a few orders than the upper limits previously reported in the literature. At temperatures higher than 1000 K, the N2O + OH reaction is dominated by the N2 + O2H channel, while the HNO + NO channel is slower by 2–3 orders of magnitude. The calculated rate constants at the temperature range of 1000–5000 K for N2O + OH N2 + O2H (2A) and N2O + OH HNO + NO (2B) are fitted by the following expressions: in units of cm3 molecule −1s−1. Both N2O + NO and N2O + OH reactions are confirmed to enhance, albeit inefficiently, the N2O decomposition by reducing its activation energy. © 1996 John Wiley & Sons, Inc.

Kinetics of the self-reaction of CF2radical and its reaction with H2, O2, CH4and C2H4over the temperature range 295–873 K

Laser flash photolysis, coupled with time-resolved UV absorption detection, has been used to measure rate coefficients for the self-reaction of CF2: CF2+ CF2→ C2F4(1) and its reaction with the molecular species H2, O2, CH4 and C2H4 over the temperature range 295–873 K. The CF2 radicals were produced by the 193 nm laser photolysis of C2F4. At room temperature, values of k1=(4.01 ± 0.51)× 10–14 cm3 molecule–1 s–1 were obtained for the rate coefficient of reaction (1) and σ249=(1.61 ± 0.21)× 10–17 cm2 molecule–1 for the absorption cross-section of CF2 at 249 nm, where the spectral resolution was 0.5 nm (FWHM). The value of k1 is in good agreement with previous room-temperature measurements, but the value of σ249 is almost a factor of two lower, reflecting the lower spectral resolution used in this study. The absorption cross-section, σ249, was found to decrease with increasing temperature, whereas the rate coefficient, k1, exhibits a positive temperature dependence. The temperature dependence of k1 at 760 Torr N2, when combined with earlier shock-tube determinations at 1200 and 1400 K, could be fitted to a modified Arrhenius expression: k1=(1.39 ± 0.20)× 10–19T2 exp[(347 ± 56)/T] cm3 molecule–1 s–1for the temperature range 295–1400 K. The uncertainties refer to random error in the parameters at the 95% confidence level. Systematic errors are estimated to be ± 10%. However, the variation of k1 with pressure at 873 K indicated that the reaction had not quite reached its high-pressure limit at 760 Torr, so this expression can only be regarded as an empirical expression for the temperature dependence at atmospheric pressure. At lower temperatures, k1 had clearly reached its high-pressure limiting value at 760 Torr. No evidence could be found at any temperature for products other than C2F4, implying that recombination is the only channel occurring.No reaction was observed between CF2 and any of the molecular reagents studied at temperatures up to 873 K. Upper limits were derived for the rate coefficient of CF2 with each reagent at several temperatures. AT 873 K, the values are (in units of cm3 molecule–1 s–1) < 5 × 10–19 for H2, < 3 × 10–17 for O2, < 4 × 10–19 for CH4 and < 2 × 10–16 for C2H4.Experiments performed using the 193 nm photolysis of C3F6 as a source of CF2 suggested another reactive and absorbing species is generated in rather higher yield than CF2, at variance with earlier studies. The results could be rationalised by assuming the species to be C2F3 which reacts with CF2 with a rate coefficient estimated to be ca. 2 × 10–13 cm3 molecule–1 s–1 at 295 K.The implications of the very low reactivity of CF2 with molecular species, compared with its reactivity with radical species, O, H and OH, are discussed in the context of the role played by the CF2 radical in the chemistry of flame inhibition by fluorinated hydrocarbons considered as potential halon replacements.

Kinetic Studies of OH Reactions with a Series of Acetates

Absolute rate constants have been measured for the gas phase reactions of hydroxyl radicals with a series of aliphatic acetates: methyl acetate (k1), ethyl acetate (k2), n-propyl acetate (k3), n-butyl acetate (k4), and n-pentyl acetate (k5). Experiments were carried out using the pulsed laser photolysislaser induced fluorescence technique over the temperature range 243−372 K. The obtained kinetic data were used to derive the Arrhenius expressions, and since the Arrhenius plots were slightly curved they were fitted, using a three-parameter expression. The Arrhenius expressions obtained are: k1 = (0.53 ± 0.09) × 10-12 exp[−(128 ± 102)/T] ; k2 = (0.48 ± 0.09) × 10-12 exp[(397 ± 103)/T]; k3 = (1.03 ± 0.12) × 10-12 exp[(370 ± 69)/T] ; k4 = (2.10 ± 0.28) × 10-12 exp[(299 ± 81)/T], and k5 = (2.75 ± 0.46) × 10-12 exp[(302 ± 102)/T] (in units of cm3 molecule-1 s-1). At room temperature, the rate constants obtained (in units of 10-12 cm3 molecule-1 s-1) were as follows: methyl acetate, 0.32 ± 0.03; ethyl acetate...

Kinetics of the Sn(3P0) Reactions with CO2 and O2 over Wide Temperature Ranges

Rate coefficients for the title reactions have been measured by the high-temperature fast-flow reactor (HTFFR) technique. The relative concentrations of Sn ground-state atoms were monitored by laser-induced fluorescence (LIF) for Sn + CO2 → SnO + CO and by either LIF or atomic resonance absorption for the Sn + O2 → SnO + O reaction. Results from these two diagnostic methods agree very well. We obtained, in units of cm3 molecule-1 s-1, k1(690−1600 K) = 2.3 × 10-10 exp(−9100 K/T) and k2(380−1840 K) = (5.1 × 10-13)(T/K)0.79 exp(−437 K/T), with 2σ accuracy limits of about ±25%. The results are compared to other metal atom reactions with these oxidants. All have similarly large preexponentials. The magnitudes of the exponential terms are shown to be typical for a spin-forbidden and a spin-allowed reaction, respectively.

Atmospheric Lifetimes of HFC-143a and HFC-245fa: Flash Photolysis Resonance Fluorescence Measurements of the OH Reaction Rate Constants

Rate constants for the reactions of hydroxyl radicals with CH3CF3 (HFC-143a) and CHF2CH2CF3 (HFC-245fa) have been measured using the flash photolysis resonance fluorescence technique over the temperature range 273−370 K. A data analysis procedure is presented which should minimize rate constant errors introduced by the possible effects of radical diffusion. The following Arrhenius expressions have been derived for the reactions of OH with CH3CF3 and CHF2CH2CF3, respectively (in units of cm3 molecule-1 s-1): × 10-12 exp{−(1979 ± 65)/T} and × 10-12 exp{−(1331 ± 43)/T}. With these values, the atmospheric lifetimes for the two HFCs have been estimated to be 51 and 7.4 years, respectively. An error analysis is presented from which the rate constant uncertainty at any temperature can be calculated.

Atmospheric Chemistry of 1,1,1-Trichloroethane: UV Spectra and Self-Reaction Kinetics of CCl3CH2 and CCl3CH2O2 Radicals, Kinetics of the Reactions of the CCl3CH2O2 Radical with NO and NO2, and the Fate of the Alkoxy Radical CCl3CH2O

cm3 molecule-' s-'. The reaction of CCl3CH202 radicals with NO gives CC12CH20 radicals. In the atmosphere, >96% of the CCl3CH20 radicals react with 02 to give CC13CH0. The rate constants for the reactions of CC13CH202 radicals with NO and NO2 were determined to be k6 L 6.1 x cm3 molecule-' s-' and k7 = (6.5 f 0.4) x lo-'' cm3 molecule-' s-', respectively. As a part of this work, the rate constant for the reaction between F atoms and CCl3CH3 was determined to be k3 = (6.8 f 1.5) x lo-', cm3 molecule-' s-'. A relative rate technique was used to measure rate constants for the reactions of C1 atoms with: CCl3CH3, (9.9 f 2.0) x lo-? CCl3CH0, (6.0 f 1.3) x lo-',; and CC13CH20H (3.0 f 0.6) x (units of cm3 molecule-' s-') at 296 K. Results are discussed in the context of the atmospheric chemistry of 1, 1 ,l -trichloroethane. and k50bs = (4.7 f 1.6) x

The temperature dependence of the OH radical reactions with some aromatic compounds under simulated tropospheric conditions

AbstractThe temperature dependence of the rate coefficients for the OH radical reactions with toluene, benzene, o‐cresol, m‐cresol, p‐cresol, phenol, and benzaldehyde were measured by the competitive technique under simulated atmospheric conditions over the temperature range 258–373 K. The relative rate coefficients obtained were placed on an absolute basis using evaluated rate coefficients for the corresponding reference compounds. Based on the rate coefficient k(OH + 2,3‐dimethylbutane) = 6.2 × 10−12 cm3 molecule−1s−1, independent of temperature, the rate coefficient for toluene kOH = 0.79 × 10−12 exp[(614 ± 114)/T] cm3 molecule−1 s−1 over the temperature range 284–363 K was determined. The following rate coefficients in units of cm3 molecule−1 s−1 were determined relative to the rate coefficient k(OH + 1,3‐butadiene) = 1.48 × 10−11 exp(448/T) cm3 molecule−1 s−1: o‐cresol; kOH = 9.8 × 10−13 exp[(1166 ± 248)/T]; 301–373 K; p‐cresol; kOH = 2.21 × 10−12 exp[(943 ± 449)/T]; 301–373 K; and phenol, kOH = 3.7 × 10−13 exp[(1267 ± 233)/T]; 301–373 K. The rate coefficient for benzaldehyde kOH = 5.32 × 10−12 exp[(243 ± 85)/T], 294–343 K was determined relative to the rate coefficient k(OH + diethyl ether) = 7.3 × 10−12 exp(158/T) cm3 molecule−1 s−1. The data have been compared to the available literature data and where possible evaluated rate coefficients have been deduced or updated. Using the evaluated rate coefficient k(OH + toluene) = 1.59 × 10−12 exp[(396 ± 105)/T] cm3 molecule−1 s−1, 213–363 K, the following rate coefficient for benzene has been determined kOH = 2.58 × 10−12 exp[(−231 ± 84)/T] cm3 molecule−1 s−1 over the temperature range 274–363 K and the rate coefficent for m‐cresol, kOH = 5.17 × 10−12 exp[(686 ± 231)/T] cm3 molecule−1 s−1, 299–373 K was determined relative to the evaluated rate coefficient k(OH + o‐cresol) = 2.1 × 10−12 exp[(881 ± 356)/T] cm3 molecule−1 s−1. The tropospheric lifetimes of the aromatic compounds studied were calculated relative to that for 1,1,1‐triclorethane = 6.3 years at 277 K. The lifetimes range from 6 h for m‐cresol to 15.5 days for benzene. © 1995 John Wiley &amp; Sons, Inc.

Rate coefficients for quenching of Cl(2P1/2) by various atmospheric gases

Rate coefficients for quenching of the upper spin–orbit state of chlorine Cl(2P1/2) by a variety of collision partners have been measured at room temperature. The method used was laser flash photolysis of NOCl or ClONO2 with resonance fluorescence detection of chlorine atoms using a strongly reversed resonance lamp for preferential detection of Cl(2P1/2). The rate coefficients were (units of cm3 molecule–1 s–1): CF4, (2.3 ± 0.3)× 10–11; CF2Cl2, (3.3 ± 0.5)× 10–10; CH4, (2.2 ± 0.3)× 10–11; ClONO2, (1.7 ± 0.3)× 10–10; O2, (3.5 ± 0.5)× 10–13; N2, (5.0 ± 1.5)× 10–15; He, (6.0 ± 1.0)× 10–14; Ar, (3.0 ± 1.0)× 10–16. These values are in good overall agreement with recent determinations which used IR laser detection of Cl(2P1/2). Quenching by helium was found to be unusually fast, explaining in part discrepancies with earlier studies which used resonance absorption detection. The reaction with CH4 was shown to proceed exclusively by quenching, and not to involve enhanced reactivity of Cl(2P1/2) relative to the ground-state species. Additionally, the yield of Cl(2P1/2) from the photolysis of chlorine nitrate was found to be 30% of the overall chlorine atom yield at 308 nm. The rate coefficient for the reaction of Cl(2P3/2) with NOCl was measured to be (9.6 ± 1.5)× 10–11 cm3 molecule–1 s–1.

Kinetics of reactions of ground state nitrogen atoms (4S3/2) with NO and NO2

The discharge flow technique has been used with resonance fluorescence detection of N atoms to study the fast radical‐radical reaction of ground state nitrogen atoms (4S3/2) with NO and NO2. The rate constants obtained are (in units of cm3 molecule−1 s−1) k1 = (2.2±0.2) × 10−11exp[(160±50)/T] in the temperature range 213 K ≤ T ≤ 369 K for N + NO → N2 + O and k2 = (5.8±0.5) × 10−12 exp [(220±50)/T] in the temperature range 223 K ≤ T ≤ 366 K for N + NO2 → N2O + O. The reported error limits are at the 95% confidence level. The reaction kinetics are consistent with other radical‐radical reactions, essentially no enthalpic barrier is observed. Substitution of the measured rate of R1 for the value recommended hi the latest Jet Propulsion Laboratory compendium [DeMore et al., 1992] results in a small change in the concentration of ozone predicted in a two‐dimensional photochemical model. Modeled ozone concentrations are higher (approximately 1%) in the high‐latitude upper stratosphere as a result of a 3–10% reduction in the calculated concentrations of NOy.

Kinetics of the DS-radical reactions with CINO and NO2

The rate constants for the reactions of the DS radical with NO2 (reaction 1) and ClNO (reaction 2) have been measured using the discharge-flow technique at 2 torr total pressure of helium. The DS radical was monitored by laser-induced fluorescence. The reactions were found to have the following bimolecular rate constants (95% confidence level, in units of cm3 molecule−1 s−1): This expression for k1 is found to be in excellent agreement with one of several previous studies. The magnitude of k2 is examined within the framework of a well-established reactivity trend. © 1994 John Wiley & Sons, Inc.

Examination of the product channels in the reactions of NH(a 1Δ) with H2 and D2

A flash photolysis study (193 nm) of HNCO has been conducted and the mechanisms of the reactions NH(a 1Δ)+H2→NH2+H(1) and NH(a 1Δ)+D2→products (2) have been examined in detail at 295±3 K by monitoring NH(a 1Δ), H, D, NH2, and their D substituents via the laser induced fluorescence technique. From the pseudo-first-order analysis of the decay rate for NH(a 1Δ), rate constants have been determined as k1=(3.96±0.17)×10−12 and k2=(2.62±0.08)×10−12. (All the rate constants are expressed in units of cm3 molecule−1 s−1.) These rate constants are consistent with those determined from the time dependence of H and D atoms: they are k1=(3.76±0.70)×10−12 and k2=(2.78±0.17)×10−12. No pressure dependence has been observed for 10–100 Torr He. The branching fraction for H and D atoms as products for reaction (2) has been found to be [H]/[D] =0.24/0.76, where D production is more abundant than statistically predicted. This indicates that reaction (2) is dominated by insertion of NH(a 1Δ) into the D2 bond, but vibrational energy of the reaction intermediate NHD2 is still localized in newly formed N–D bonds before it passes through the exit barrier into NHD+D or ND2+H channels. NH2(X̃ 2B1) was observed in (0,0,0) and (0,1,0) vibrational states as a product of reaction (1), and the observed time dependence of both vibrational states could be satisfactorily simulated by solving the master equation for vibrational relaxation of NH2. This analysis has indicated that the vibrational energy partitioning in the product NH2 is nearly statistical.

Absolute rate coefficients for the reaction between nitrate radicals and some cyclic alkenes

Absolute rate coefficients for the reaction between NO3 and cyclopentene, cyclohexene and 1-methylcyclohexene have been determined to be (5.9 ± 1.1)× 10–13(295 K), (6.3 ± 1.3)× 10–13(294 K) and (1.5 ± 0.5)× 10–11(293 K)(units of cm3 molecule–1 s–1), respectively, using the fast-flow–discharge technique. The activation energies for the NO3+ cyclohexene and 1-methylcyclohexene reactions were 1 ± 4 and 0 ± 5 kJ mol–1. The simple cyclic alkenes react slightly faster than (Z)-but-2-ene which contains a similar active structural element. The rate coefficient for reaction with 1-methylcyclohexene is close to that for Δ3-carene or 2-carene but significantly greater than for α-pinene.

Absolute rate constants for elementary reactions between chlorine atoms and CHFzC1, CH3CFCI:, CHaCFaC1 and CH2FCF3 at 297± 2 K

The reactions of chlorine atoms with CHF2CI, CH3CFCI2, CH3CF2CI and CH2FCF3 have been investigated at 297 ± 2 K and 3–6 Torr under pseudo-first-order conditions with an excess of chlorine atoms by means of the discharge flow technique coupled to a mass spectrometer (DF/MS). The following absolute rate constants were obtained in units of cm3 molecule−1 s−1: HCFC-22, (1.7±0.2)×10−5; HCFC-141b, (2.1+0.2)×10−15; HCFC-142b, (5.6±2.0)×10−16 and HFC-134a, (1.6 ± 0.3) × 10−15. Quoted errors are ± 2σ. These values are in good agreement with those determined previously by relative or absolute methods. The role of these reactions under tropospheric conditions is discussed.

Determination of the Rate Constants for the Gas Phase Reactions of NO3 with H, OH and HO2 Radicals at 298 K

AbstractThe reactions of NO3 with H (1), OH (2) and HO2 (3) radicals have been investigated at 298 K using a discharge flow system equipped with mass spectrometric and fluorescence detectors. The rate constants k(1) and k(2) were determined by numerical modeling of experimental [OH] profiles from reactions (1) and (2). In the reaction of NO3 with HO2 (3) the growth and decay of [OH] as well as the decay of [HO2] were measured. Comparison of these two profiles provided evidence for two different reaction paths (3a) and (3b), for which the rate constants were determined by numerical modeling. Experiments carried out in a wide range of reactant concentrations yielded in units of cm3 molecule−1 s−1 k(1) = (0.94 ± 0.2) · 10−10, k(2) = (1.22 ±0.35) · 10−11, k(3a) = (2.5 ± 0.7) · 10−12, and k(3b) = (1.9 ± 0.8) · 10−12. The results are compared with literature data.