Termolecular Channels Research Articles

Measurements of the kinetics of the OH‐initiated oxidation of methyl vinyl ketone and methacrolein

AbstractThe mechanisms of the OH‐initiated oxidation of methyl vinyl ketone and methacrolein have been studied at 300 K and 100 Torr total pressure, using a turbulent flow technique coupled with laser‐induced fluorescence detection of the OH radical. The rate constants for the OH + methyl vinyl ketone and OH + methacrolein reactions were measured to be (1.78 ± 0.08) × 10−11 and (3.22 ± 0.10) × 10−11 cm3 molecule−1 s−1, respectively, and were found to be in excellent agreement with previous studies. In the presence of O2 and NO, the OH radical propagation and the loss of OH through radical termination resulting from the production of methyl vinyl ketone‐ and methacrolein‐based alkyl nitrates were measured at 100 Torr total pressure and compared to the simulations of the kinetics of these reaction systems. The results of these experiments are consistent with an overall rate constant of (2.0 ± 1.3) × 10−11 cm3 molecule−1 s−1 for both the methyl vinyl ketone‐based peroxy radical + NO and methacrolein‐based peroxy radical + NO reactions, each with branching ratios of 0.90 ± 0.10 for the bimolecular channel (oxidation of NO to NO2) and 0.10 ± 0.10 for the termolecular channel (production of methyl vinyl ketone‐ and methacrolein‐based alkyl nitrates). © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 12–25, 2003

Measurements of the kinetics of the OH‐initiated oxidation of isoprene

The mechanism of the OH‐initiated oxidation of isoprene has been studied at 300 K and 100 and 150 Torr total pressure using a turbulent flow technique coupled with laser‐induced fluorescence detection of the OH radical. The rate constant for the reaction of OH with isoprene was measured to be (10.8 ± 0.5) × 10−11 cm3 molecule−1 s−1 at 150 Torr total pressure and independent of pressure between 100 and 150 Torr, in excellent agreement with most previous absolute measurements. In the presence of O2 and NO, propagation of OH radicals and loss of OH through radical termination resulting from the formation of organic nitrates were measured at 150 Torr total pressure and compared to simulations of the kinetics of this reaction system. The results of these experiments are consistent with an overall rate constant of (1.1 ± 0.8) × 10−11 cm3 molecule−1 s−1 for the reaction of NO with isoprene‐based hydroxyalkyl peroxy radicals, with branching ratios of 0.85 ± 0.10 for the bimolecular channel (oxidation of NO to NO2) and 0.15 ± 0.10 for the termolecular channel (formation of organic nitrates). Although the organic nitrate yield reported here is the result of indirect measurements, it suggests that isoprene may be a more significant sink of NOx than previously estimated.

Kinetics and Mechanism of the BrO Self-Reaction: Temperature- and Pressure-Dependent Studies

The flash photolysis/UV absorption technique has been used to study the self-reaction of BrO radicals over the temperature range 222−298 K and the pressure range 100−760 Torr of N2 or O2. Two chemical sources of BrO radicals were used: photolysis of Br2 in the presence of excess ozone and photolysis of O2 in the presence of excess Br2. The overall rate constant, k1, for the BrO self-reaction (defined by −d[BrO]/dt = 2k1[BrO]2) was found to be temperature and pressure independent at T ≥ 250 K, with k1 = (2.88 ± 0.20) × 10-12 cm3 molecule-1 s-1. At temperatures below 250 K, k1 was found to be pressure dependent, due to the emergence of a new termolecular channel of the BrO self-reaction 1c, −1c forming the BrO dimer, Br2O2 (BrO + BrO + M ⇌ Br2O2 + M). Channel-specific rate constants were determined for the two bimolecular channels of the BrO self-reaction above 250 K, giving for (1a) (BrO + BrO → 2Br + O2) k1a = (5.31 ± 1.17) × 10-12 exp{(−211 ± 59)/T} cm3 molecule-1 s-1 and for (1b) (BrO + BrO → Br2 + O2) k1b = (1.13 ± 0.47) × 10-14 exp{(983 ± 111)/T} cm3 molecule-1 s-1. Below 250 K, the overall rate coefficient of the two bimolecular channels is reduced as the dimer forming channel emerges. At 235 and 222 K, rate constants for the formation (k1c) and decomposition (k-1c) of Br2O2 were recorded. Using the values for K1c, ΔHr for reaction 1c was estimated as −58.6 ± 0.1 kJ mol-1. A UV absorption spectrum attributed to Br2O2 was also recorded over the wavelength range 300−390 nm. The cross section of the smooth Br2O2 spectrum was found to be 1.2 × 10-17cm2 molecule-1 at 320 nm. These results are rationalized in terms of a mechanism of the BrO self-reaction that shows competition, at low temperatures, between collisional quenching and unimolecular dissociation of an excited BrOOBr* intermediate. The rate constant for the reaction of oxygen atoms with molecular bromine was also determined in the course of these experiments [O + Br2 → BrO + Br (5)], giving k5 = (5.12 ± 1.86) × 10-13 exp{(989 ± 91)/T} cm3 molecule-1 s-1. All errors are 1σ. The atmospheric implications of these results are discussed.

Kinetics and mechanism of the chlorine oxide ClO + ClO reaction: pressure and temperature dependences of the bimolecular and termolecular channels and thermal decomposition of chlorine peroxide

The kinetics and mechanism of the CIO + CIO reaction and the thermal decomposition of CIOOCI were studied using the flash photolysis/long path ultraviolet absorption technique. Pressure and temperature dependences were determined for the rate coefficients for the bimolecular and termolecular reaction channels, and for the thermal decompositon of CIOOCI.

A further investigation of the reaction of C 2H +2 with H 2

The reactions of C 2H + 2 with H 2 and C 2D + 2 + D 2 have been studied in a selected ion flow drift tube (SIFDT) over a range of centre-of-mass energies E c from thermal (300K) to a few tenths of an electron volt. The reactions proceed along parallel bimolecular and termolecular channels producing C 2H + 3 (C 2D + 3) and C 2H + 4 (C 2D + 4). The relative importance of the bimolecular channel increases and that of the termolecular channel decreases with increasing E c in accordance with previous data. The rates for the bimolecular reactions increase as the C=C stretching vibrations in the reactant ion are excited in collisions with the helium carrier gas atoms in the SIFDT at higher E c values, consistent with other work in which these vibration modes are selectively excited using laser ionisation. The question is addressed as to whether the atom abstraction reactions are exothermic or endothermic. Based on the new experimental data, some recent ion trap data and previous data on the reaction of C 2H + 2 with H atoms, it is concluded that the reactions are slightly endothermic, and it is tentatively suggested that a potential barrier also exists to the reactions. The interstellar implications of these results are briefly referred to.

Association reactions at low pressure. IV. The HC3N+/HC3N system

The reactions between HC3N+ and HC3N, and between HC5N+ and HC3N have been examined at pressures from 1×10−7 to 1×10−3 Torr by ion cyclotron resonance mass spectrometry. The reaction between HC3N+ and HC3N has both a bimolecular reaction path and a termolecular reaction path. The overall bimolecular reaction rate coefficient was found to be 1.3×10−9 cm3 s−1. The primary product, HC5N+, represents 90% of the product ions, while the minor products HC6N+2 and H2C6N+2 each represent 5%. The termolecular association rate coefficient was 3.7×10−24 cm6 s−1 with He as the third body. From double resonance experiments the mean lifetime of the collision complex was determined to be 180 μs. HC5N+ was found to react with HC3N and form the adduct ion H2C8N+2 through both bimolecular and termolecular channels. The bimolecular rate coefficient was 5.0×10−10 cm3 s−1 and the termolecular rate coefficient was observed to be 1.2×10−22 cm6 s−1 with HC3N as the third body. With He as the stabilizing molecule, the termolecular rate coefficient was 6.0×10−24 cm6 s−1. The mean unimolecular lifetime of the collision complex was estimated to be ≥15 μs and the mean radiative lifetime for the radiative association channel was found to be ≥89 μs. HC5N+ was found to be formed with excess internal energy and did not react by bimolecular association until relaxed by several nonreactive collisions with HC3N.

Rate coefficient for the termolecular channel of the self-reaction of chlorine monoxide

The rate coefficient for the reaction ClO+ClO{sup M}{yields}Cl{sub 2}O{sub 2} has been measured by using a flash photolysis/UV absorption technique at temperatures from 200 to 263 K and pressures from 25 to 600 Torr in N{sub 2}, O{sub 2}, He, and SF{sub 6}. Conventional single-wavelength detection and diode array spectrometry have both been employed in monitoring the reactant and product species. The UV absorption cross section of ClO has been measured in the same temperature range, from 220 to 350 nm, by using the diode array, and at 282.65 nm (the maximum of the ClO (9,0) band) by using the single-wavelength detection. The reactions of chlorine atoms with O{sub 3} and Cl{sub 2}O have been used to produce ClO. The title reaction is believed to play a role in the loss of ozone from the Antarctic stratosphere during the austral spring. At the temperatures and pressures relevant to the Antarctic stratosphere, our measured values agree with the recent work of Sander et al. and are considerably smaller than the values obtained by Hayman et al.

The importance of three-body processes to reaction kinetics at atmospheric pressures. III. Reactions of He/sub 2//sup +/ with selected atomic and molecular reactants

For pt.II see ibid., vol.22, p.47-50 (1986). Results are presented from studies of the reactions He/sub 2//sup +/ with five species displaying a wide variation of molecular structures and polarizabilities: Ar, Xe, Ne, O/sub 2/, and CO/sub 2/. Measurements were made in the afterglows of preionized discharges into the reactant gas mixtures diluted in 1-6-atm pressures of helium. Effective rates of reaction were obtained that could be separated into contributions from bimolecular and termolecular channels. The latter generally dominated, showing no evidence of saturation up to 6 atm pressure of diluent. These results continue to confirm earlier reports that the initial capture step in the reaction is not limited by the Langevin rate as usually applied. Such super-Langevin rates appear to be a general phenomenon at high pressures, at least for the reactions of He/sub 2//sup +/. In the most extreme case examined, He/sub 2//sup +/+CO/sub 2/, the effective rate of reaction contributed by all channels was found to exceed Langevin by a factor of five at 6-atm pressure of diluent. >