Third-body dependence of rate coefficients for ozone formation in 16O–18O mixtures

Guenther, Juergen; Krankowsky, D.; Mauersberger, K.

doi:10.1016/s0009-2614(00)00550-9

Cited by 47 publications

(42 citation statements)

References 30 publications

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“…The results are summarized elsewhere. 1 The experiments involve a ''mass-independent'' isotope effect in ozone formation in scrambled systems and contrastingly different experimental results [25][26][27][28][29] in unscrambled systems. The latter show, instead, dramatic unconventional mass-dependent effects.…”

Section: Introductionmentioning

confidence: 99%

On the theory of the strange and unconventional isotopic effects in ozone formation

Gao

Marcus

2002

The Journal of Chemical Physics

149

186

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The strange mass-independent isotope effect for the enrichment of ozone and the contrastingly unconventional strong mass-dependent effect of individual reaction rate constants are studied using statistical ͑RRKM͒-based theory with a hindered-rotor transition state. Individual rate constant ratios of recombination reactions and enrichments are calculated. The theory assumes ͑1͒ an ''-effect,'' which can be interpreted as a small deviation from the statistical density of states for symmetric isotopomers, compared with the asymmetric isotopomers, and ͑2͒ weak collisions for deactivation of the vibrationally excited ozone molecules. A partitioning effect controls the recombination rate constant ratios. It arises from small differences in zero-point energies of the two exit channels of dissociation of an asymmetric ozone isotopomer, which are magnified into large differences in numbers of states in the two competing exit channel transition states. In enrichment experiments, in contrast, this partitioning factor disappears exactly ͓Hathorn and Marcus, J. Chem.Phys. 112, 9497 ͑2000͔͒, and what remains is the -effect. Both aspects can be regarded as ''symmetry driven'' isotopic effects. The two experiments, enrichments and rate constant ratios, thus reveal markedly different theoretical aspects of the phenomena. The calculated low-pressure ozone enrichments, the low-pressure recombination rate constant ratios, the effects of pressure on the enrichment, on the individual recombination rate constant ratios, and on the recombination rate constant are consistent with the experimental data. The temperature dependence of the enrichment and of the recombination rate constant ratios is discussed and a variety of experimental tests are proposed. O at 130 K and 300 K is used for testing or providing information on the nature of a variationally determined hindered-rotor transition state. The theory is not limited to ozone formation but is intended to apply to other reactions where a symmetrical stable or unstable gas phase molecule may be formed.

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Section: Introductionmentioning

confidence: 99%

On the theory of the strange and unconventional isotopic effects in ozone formation

Gao

Marcus

2002

The Journal of Chemical Physics

149

186

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“…15%͒ non-Rice-Ramsperger-KasselMarcus ͑RRKM͒ effect gave rise to a dramatic effect, the so-called mass-independent isotope effect, in ozone formation. Recently, experiments on specific elementary reaction steps, [25][26][27][28] in contrast with scrambled systems, showed dramatic mass-dependent effects. This dichotomy between the behavior of individual recombination steps and of scrambled systems is treated in the present paper, using the formalism given in paper I.…”

Section: Introductionmentioning

confidence: 99%

An intramolecular theory of the mass-independent isotope effect for ozone. II. Numerical implementation at low pressures using a loose transition state

Hathorn

Marcus

2000

The Journal of Chemical Physics

104

112

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A theory is described for the variation in the rate constants for formation of different ozone isotopomers from oxygen atoms and molecules at low pressures. The theory is implemented using a simplified description which treats the transition state as loose. The two principal features of the theory are a phase space partitioning of the transition states of the two exit channels after formation of the energetic molecule and a small ͑ca. 15%͒ decrease in the effective density of states, ͓a ''non-Rice-Ramsperger-Kassel-Marcus ͑RRKM͒ effect''͔, for the symmetric ozone isotopomers ͓B. C. Hathorn and R. A. Marcus, J. Chem. Phys. 111, 4087 ͑1999͔͒. This decrease is in addition to the usual statistical factor of 2 for symmetric molecules. Experimentally, the scrambled systems show a ''mass-independent'' effect for the enrichments ␦ ͑for trace͒ and E ͑for heavily͒ enriched systems, but the ratios of the individual isotopomeric rate constants for unscrambled systems show a strongly mass-dependent behavior. The contrasting behavior of scrambled and unscrambled systems is described theoretically using a ''phase space'' partitioning factor. In scrambled systems an energetic asymmetric ozone isotopomer is accessed from both entrance channels and, as shown in paper I, the partitioning factor becomes unity throughout. In unscrambled systems, access to an asymmetric ozone is only from one entrance channel, and differences in zero-point energies and other properties, such as the centrifugal potential, determine the relative contributions ͑the partitioning factors͒ of the two exit channels to the lifetime of the resulting energetic ozone molecule. They are responsible for the large differences in individual recombination rate constants at low pressures. While the decrease in for symmetric systems is attributed to a small non-RRKM effect , these calculated results are independent of the exact origin of the decrease. The calculated ''mass-independent'' enrichments, ␦ and E, in scrambled systems are relatively insensitive to the transition state ͑TS͒, because of the absence of the partitioning factor in their case ͑for a fixed non-RRKM ͒. They are compared with the data at room temperature. Calculated results for the ratios of individual isotopomeric rate constants for the strongly mass-independent behavior for unscrambled systems are quite sensitive to the nature of the TS because of the partitioning effect. The current data are available only at room temperature but the loose TS is valid only at low temperatures. Accordingly, the results calculated for the latter at 140 K represent a prediction, for any given . At present, a comparison of the 140 K results can be made only with room temperature data. They show the same trends as, and are in fortuitous agreement, with the data. Work is in progress on a description appropriate for room temperature.

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“…11). No direct experimental data on ozone isotope enrichments in a CO 2 bath are available and data on isotopomer specific rate coefficients in ozone formation (Guenther et al, 2000) do neither support nor disprove this hypothesis. While such an effect would qualitatively agree with the observations, it is not sufficient to quantitatively explain the strong dependency on ρ.…”

Section: Discussionmentioning

confidence: 99%

“…This may imply that the presence of CO 2 itself has an effect on the final equilibrium. On the one hand, it is possible that the higher stabilization efficiency of CO 2 compared to O 2 or N 2 in the formation of O 3 (∼2.7 Guenther et al, 2000) leads to a 2.7 times larger effective pressure (610 at ρ=0 instead of 225 hPa). Analysing available (Morton et al, 1990;Thiemens and Jackson, 1990) and new data on the effect of pressure on the isotope effects in ozone formation, this would correspond to a reduction of the equilibrium values by 17‰ and 22‰ for 17 O and 18 O, respectively (Tuzson and Janssen, 2007, see also Eq.…”

Section: Discussionmentioning

confidence: 99%

Investigations of the photochemical isotope equilibrium between O2, CO2 and O3

2007

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Abstract. Contrary to tropospheric CO 2 whose oxygen isotopic composition follows a standard mass dependent relationship, i.e. δ 17 O∼0.5 δ 18 O, stratospheric CO 2 is preferentially enriched in 17 O, leading to a strikingly different relation: δ 17 O∼1.7 δ 18 O. It has been shown repeatedly that the isotope anomaly is inherited from O 3 via photolytically produced O( 1 D) that undergoes isotope exchange with CO 2 and the anomaly may well serve as a tracer of stratospheric chemistry if details of the exchange mechanism are understood. We have studied the photochemical isotope equilibrium in UV-irradiated O 2 -CO 2 and O 3 -CO 2 mixtures to quantify the transfer of the anomaly from O 3 to CO 2 at room temperature. By following the time evolution of the oxygen isotopic compositions of CO 2 and O 2 under varying initial isotopic compositions of both, O 2 /O 3 and CO 2 , the isotope equilibria between the two reservoirs were determined. A very strong dependence of the isotope equilibrium on the O 2 /CO 2 -ratio was established. Equilibrium enrichments of 17 O and 18 O in CO 2 relative to O 2 diminish with increasing CO 2 content, and this reduction in the equilibrium enrichments does not follow a standard mass dependent relation. When molecular oxygen exceeds the amount of CO 2 by a factor of about 20, 17 O and 18 O in equilibrated CO 2 are enriched by (142±4)‰ and (146±4)‰, respectively, at room temperature and at a pressure of 225 hPa, independent of the initial isotopic compositions of CO 2 and O 2 or O 3 . From these findings we derive a simple and general relation between the starting isotopic compositions and amounts of O 2 and CO 2 and the observed slope in a three oxygen isotope diagram. Predictions from this relation are compared with published laboratory and atmospheric data.

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Third-body dependence of rate coefficients for ozone formation in 16O–18O mixtures

Cited by 47 publications

References 30 publications

On the theory of the strange and unconventional isotopic effects in ozone formation

On the theory of the strange and unconventional isotopic effects in ozone formation

An intramolecular theory of the mass-independent isotope effect for ozone. II. Numerical implementation at low pressures using a loose transition state

Investigations of the photochemical isotope equilibrium between O<sub>2</sub>, CO<sub>2</sub> and O<sub>3</sub>

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Third-body dependence of rate coefficients for ozone formation in 16O–18O mixtures

Cited by 47 publications

References 30 publications

On the theory of the strange and unconventional isotopic effects in ozone formation

On the theory of the strange and unconventional isotopic effects in ozone formation

An intramolecular theory of the mass-independent isotope effect for ozone. II. Numerical implementation at low pressures using a loose transition state

Investigations of the photochemical isotope equilibrium between O&lt;sub&gt;2&lt;/sub&gt;, CO&lt;sub&gt;2&lt;/sub&gt; and O&lt;sub&gt;3&lt;/sub&gt;

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Investigations of the photochemical isotope equilibrium between O<sub>2</sub>, CO<sub>2</sub> and O<sub>3</sub>