R. Simonaitis

Kinetics and Mechanism of the Reaction of O(3P) with Carbon Monoxide

The mercury‐photosensitized decomposition of N2O [to produce O(3P)] was studied in the presence of mixtures of CO and 2‐trifluoromethylpropene (TMP) at 298, 383, and 472°K to determine the relative rate constants for the reaction of O(3P) with the two gases. The absolute value of the rate constant with TMP was found to be 9.1 × 109 exp(−2,200/RT)M−1.sec−1 from a similar competitive study at the same temperatures between TMP and butene‐1, whose rate constant with O(3P) is relatively well known. The absolute value for the O(3P)+CO reaction was thus determined, and it was found to depend on pressure. The reaction was intermediate between second and third order. For any pressure, the reaction became more nearly second order as the temperature was raised. Expressions were obtained for the limiting low‐and high‐pressure rate constants: k0= 5.9 × 109 exp(−4100/RT)M−2.sec−1 with N2O as a chaperone and k∞= 1.6 × 107 exp(−2900/RT)M−1.sec−1. These results were interpreted in terms of the mechanism: O(3P)+CO+M⇄ lim −...

The photolysis of chlorofluoromethanes in the presence of O2 or O3 at 213.9 nm and their reactions with O(1D)

Abstract The photolysis of CFCl 3 at 213.9 nm and 25 °C in the presence of O 2 or O 3 gives CFClO and Cl 2 as products with Φ{CFClO} = 0.90 ± 0.15 and Φ{Cl 2 } = 0.50 – 0.63. In the O 3 system, −Φ {O 3 } increases from 2.75 at high total pressures to 4.6 at low total pressures. With CF 2 Cl 2 the photolysis was done only in the presence of O 2 ; Φ{CF 2 O} = 1.0 ± 0.2 and Φ{Cl 2 } = 0.52 – 0.66. These results indicate that the dominant photochemical process is chlorine atom ejection with a quantum yield near 1.0. For the reactions of the chlorofluoromethanes with O( 1 D), prepared from the photolysis of O 3 at 253.7 nm and 25 °C, the same products are obtained as in the photo-oxidation, and with the same yields. The quantum yields of O 3 removal are 5.7 ± 1 and 6.3 ± 1, respectively for the CFCl 3 and CF 2 Cl 2 systems. Thus the indicated dominant reaction path is chlorine atom abstraction by O( 1 D), with other paths (O 1 D deactivation or direct molecular formation of products) being negligible. Rate coefficients were obtained for the O( 1 D) reactions with O 3 , CO 2 , CFCl 3 , CF 2 Cl 2 , CF 3 Cl, and CCl 4 relative to N 2 O. The relative rate coefficients are: O 3 , 2.5; CO 2 , 0.65; CFCl 3 , 1.5; CF 2 Cl 2 , 1.2; CF 3 Cl, 0.52; CCl 4 , 2.1. The rate coefficients were also measured for the first five gases in the presence of He to remove the excess translational energy of the O( 1 D) atom. Except for O 3 , the same results were obtained in the presence and absence of He. However, for O 3 in the presence of He, the relative rate coefficient was 1.6. As a check on the reactivities of O( 1 D) with the chlorofluoromethanes, the competition with O 2 , rather than N 2 O, was studied in the O 3 O 2 —chlorofluoromethane system. By monitoring O 3 decay the relative rate coefficients for CFCl 3 , CF 2 Cl 2 , and CCl 4 relative to O 2 were found to be 4.04, 2.78, and 5.3 respectively. These results are consistent with those obtained from the N 2 O competition.

Perchloric acid: A possible sink for stratospheric chlorine

Abstract The possibility that chlorine may deplete stratospheric chlorine has received considerable attention recently. The only termination steps considered up to now involve HCl formation by chlorine atom attack on hydrogen-bearing molecules. We propose that an important removal mechanism for chlorine in the stratosphere may be the formation of HClO4 via the sequence of steps Cl + O2 + O3 → ClO3 + O2 ClO3 + OH → HClO4. In addition to being produced as often as HCl, HClO4 may be more stable to radical attack and thus a more efficient sink than HCl for stratospheric chlorine.

The inhibition of photochemical smog—III. Inhibition by Diethylhydroxylamine, N-methylaniline, triethylamine, diethylamine, ethylamine and ammonia

Additional inhibitors for the conversion of NO to NO2 in C3H6-NO-O2 irradiated mixtures have been tested at 25°C These mixtures initially contained about 16 mTorr C3H6, 8 mTorr NO, 0.012 mTorr NO2, additive and enough O2 to bring the total pressure to 100 Torr. The NO2 pressure was monitored photometrically. In the absence of additive, the NO2 pressure first increases with irradiation time reaching a maximum conversion at about 12 min. As the irradiation time increases beyond 12 min, the NO2 pressure drops. Ten-min irradiations were done with various amounts of diethylhydroxylamine ((C2H5)2NOH), N-methylamline (C6H5NHCH3), triethylamine ((C2H5)3N), diethylamine ((C2H5)2NH), ethylamine (C2H5NH2), and ammonia (NH3) added. The NO2 pressure was reduced to one-half its value in the absence of inhibitor with 1.5 mTorr N-methylaniline, 2.5 mTorr (C2H5)2NOH, 6 mTorr (C2H5)3N or 6.0 mTorr (C2H5)2NH added. Ammonia showed no inhibition, and C2H5NH2 gave slight inhibition. Studies of the per cent conversion of NO to NO2 vs irradiation time were done with either 8.1 mTorr (C2H5)3N, 6.0 mTorr (C2H5)2NH, 2.2 mTorr (C2H5)2NOH, and 2.4 mTorr N-methylaniline added. The times for maximum per cent conversion of NO to NO2 were shifted from the additive absent value of 12 min to 15, 18, 22 and 22 min, respectively. In addition the per cent conversion at the maximum was also slightly suppressed with additive present. There is a dark reaction between (C2H5)2NOH and NO2 which follows the rate law −d[NO2]dt = k[NO2][(C2H5)2NOH]. The rate coefficient k = 4.5 × 10−18cm3 s−1 at 25°C is too small for the dark reaction to significantly influence the photochemical inhibition studies.

The inhibition of photochemical smog. I. Inhibition by phenol, benzaldehyde, and aniline.

In urban atmospheres hydrocarbons promote the conversion of NO to NO2 under the influence of sunlight, ultimately giving rise to photochemical smog. The conversion results from a long chain process with HO radicals as the chain carrier. If this chain could be interrupted by suitable radical traps, the formation of photochemical smog would be inhibited. In this paper we report the results of studies using phenol, benzaldehyde, and aniline as inhibitors. Mixtures containing 16 mTorr C3H6, 8 mTorr NO, ∼85 Torr 02, and the addi tives were irradiated at 25°C. The NO2 pressure was monitored photometrically. In the absence of additive, the NO2 pressure first increases with irradiation time reaching a maximum conversion corresponding to 70% of the NO at 1 2 minutes. As the radiation time is lengthened, the NO2 pressure drops. With the additive present, the formation of NO2 is delayed. The time to reach the maximum percent conversion of NO to N02 becomes 20, 22, 31, and 40 minutes respectively, for 13 mTorr C6H5OH...

The Reactions of CCl3O2 with NO and NO2 and the thermal decomposition of CCl3O2NO2

Abstract The reactions of CCl 3 O 2 with NO and NO 2 have been studied by steady-state photolysis of Cl 2 HCCl 3 O 2 N 2 NONO 2 mixtures at ≈ 1 atm: CCl 3 O 2 + NO → CCl 3 O + NO 2 (3), CCl 3 O 2 NO 2 (+ M) ⇌ CCl 3 O 2 NO 2 (+ M) (4, −4); k 4 / k 3 = 0.68, independent of temperature. The thermal decomposition of CCl 3 O 2 NO 2 via the back reaction (−4) was studied from 268–298 K at 1 atm total pressure (mainly N 2 ). The Arrhenius expression is k −4 = 3.7 × 10 15 exp(−11 000/ T )s −1 , though both pre-exponential factor and activation energy may be somewhat larger in the high-pressure regime. CCl 3 O 2 NO 2 should be produced in the stratosphere from reaction (4). In the stratosphere at 220 K, the thermal lifetime of CCl 3 O 2 NO 2 will be > 16 days.

The photolysis of ccl4 in the presence of o2 or o3 at 213.9 nm, And the reaction of o(1d) with ccl4

Abstract When CCl4 is photolyzed at 25°C with 213.9 nm radiation in either the presence of O2 or O3 the products are CCl2O, Cl2, and an unidentified compound. At low total pressure, φ{CCl2O} = 2.0, but this value drops to 1.0 for [CCl4] ∼ 50 Torr and [O2] or [N2] = 700 Tort. φ{Cl2} is reasonably invariant to pressure at ∼ 1.3 – 1.4. The results are interpreted in terms of an excited molecule mechanism which proceeds entirely by: at low pressures, with singlet CCl2 being produced. At higher pressures CCl4★ is quenched and CCl2 production is inhibited, though it may (and probably is) replaced by production of CCl3 + Cl. The O(1D) reaction with CCl4 at 25°C gives CCl2O and Cl2 as the exclusive products. The O(1D) was produced from O3 photolysis at 253.7 nm. The quantum yields are invariant to reaction conditions and are φ{CCl2O} = 0.87 ± 0.2 and φ{Cl2} = 1.1 ± 0.2. The O3 consumption is the same, or slightly higher than in the absence of CCl4. The three possible reaction paths are: Reaction (13a) was shown to be an important, and possibly the exclusive path, whereas reaction (13c) is unimportant and proceeds

The Cl2 photosensitized decomposition of O3: the reactions of ClO and OClO with O3

Abstract Cl2 was photolyzed in the temperature range 254 – 296 K with 366 nm radiation in the presence of O3· O3 was removed with quantum yields of 5.8 ± 0.5, 4.0 ± 0.3, 2.9 ± 0.3 and 1.9 ± 0.2 at 24, 10,0 and—21 °C respectively, independent of the initial O3 or Cl2 removal quantum yields were 0.11 ± 0.2 at 24 °C for Cl2 conversions of about 30%, much higher than expected from mass balance considerations based on the initial quantum yield of 0.089 ± 0.013 for OClO formation at 24 °C. The final chlorine-containing product was Cl2O7 which was observed. It was produced at least in part through the formation of OClO as an intermediate which was also observed with an initial quantum yield of Φi[;OClO]; = 2.5 × 103 exp [;—(3025 ± 625)/T]; independent of [O3] or Ia. The results are consistent with the reaction sequence The relative importance of the channels for reaction (2) at 296 K are the following: k2a/k2 = 0.63; k2b/k2 = 0.34; k2c/k2 = 0.0032. Also, k2c/k2b = 2.5 × 103 exp [;—(3025 ± 625)/T];. The upper limit for the rate coefficient was found to be less than 1 × 10−18 cm3 s−1 for channels (24a) and (4b): The addition of nitrogen had no effect, but oxygen reduced —Φ[;O3]; for unknown reasons and several possibilities are discussed. At temperatures below 296 K the equilibrium becomes apparent. The reaction of OClO with O3 was also studied by direct mixing of OClO and O3 in a quartz vessel in the temperature range 253 – 296 K, and in the Cl2O3 system by monitoring OClO decay in the dark in the temperature range 264 – 296 K: The Arrhenius rate coefficient recommended for reaction (5) is The low values of k4 and k5 obtained in this study indicate that reactions (4) and (5) are probably not important in atmospheric chemistry.

Ultraviolet absorption spectra of HO2NO2, CCl3O2NO2, CCl2FO2NO2, and CH3O2NO2

Abstract The ultraviolet absorption spectra of HO 2 NO 2 , CCl 3 O 2 NO 2 , CCl 2 FO 2 NO 2 , and CH 3 O 2 NO 2 were measured from 210 to 280 nm. The spectra are similar, and the absorption intensity increases as the wavelength decreases with a shoulder at ≈ 255 nm. Assuming that every absorption leads to photodissociation, the photodissociation lifetimes will be ≲1 day in the lower stratosphere and

The Inhibition of Photochemical Smog

Additional inhibitors for the conversion of NO to NO2 in C3H6—NO—02 irradiated mixtures have been tested at 25°C. These mixtures initially contained 16 mTorr C3H6, 8 mTorr NO, 0.012 mTorr NO2, additive, and enough O2 to bring the total pressure to 100 Torr. The NO2 pressure was monitored photometrically. In the absence of additive, the NO2 pressure first increases with irradiation time reaching a maximum conversion at about 15 minutes. As the irradiation time increases beyond 15 min, the NO2 pressure drops. Before adding the inhibitors, runs were done with 10 Torr of CO added, and in these runs the conversion was speeded so that the maximum in NO2 pressure occurred at 10 min. This enhancement in conversion rate is considered to be diagnostic for the presence of HO radicals. Next 10-min irradiations were done with various amounts of hexafluorobenzene (C6F6), nitrobenzene (C6H5NO2), or naphtha lene (C10H8) added. The NO2 pressure was reduced to one-half its value in the absence of inhibitor with 270 mTorr C...