Isotope Effects in Gas-Phase Chemistry - ACS Publications - American

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Chapter 11

Isotopic Study of the Mechanism of Ozone Formation

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N. Wessel Larsen, T. Pedersen, and J. Sehested Department of Chemistry, University of Copenhagen, H. C. Ørsted Institute, 5, Universitetsparken, DK-2100 Copenhagen, Denmark

Thompson and Jacox [1] reported, in a recent study using isotopic oxygen species, that apparently ozone formation in a matrix took place in a straightforward end­ -on manner. However rather surprisingly an isotopic study of the formation of ozone in the gas phase has not previously been undertaken. The most obvious reason for not undertaking an investigation would be to tacitly assume that the incoming oxygen atom had no choice but to attach itself at one of the ends of the oxygen molecule. Indeed our original motive for undertaking this investigation was to find out whether a symmetrically triangular (D ) form of ozone might be involved in the transition state. (We call it "cyclic ozone" following Wright [3] ). Cyclic ozone has a fairly long history in chemistry, see Wright [3], but it has never been observed. Quantum chemical calculations invariably seem to come up with either an excited state with D symmetry or with a dip in the higher energy ranges of the ground state surface with the same symmetry. See Murrell et. al. [4] and Burton [5]. To study the formation of ozone by means of isotopes would seem to be a good way of collecting evidence for the involvement of cyclic ozone in ozone formation. During the formation of ozone 0 and 0 must come down from the dissociation limit on the potential surface (or perhaps slightly above), and hence have a chance of falling into the D h region, provided it exists, but also assuming that it lies below the dissociation limit. If the mechanism of ozone formation is purely "end-on", then we expect only one isotopically substituted version of ozone in the reaction: 3h

3h

2

3

0 + QQ + M -> 0QQ + M 1 8

16

(1)

Q denotes 0 while Ο denotes 0 . If on the other hand the reaction passes via cyclic ozone, then we expect a mixture of products:

0097-6156/92/O502-0167S06.00/0 © 1992 American Chemical Society

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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168

Figure 1 :

ISOTOPE EFFECTS IN GAS-PHASE CHEMISTRY

The passage from the reactants to the ground state of ozone via the hypothetical,

cyclic intermediate. Supposing an equal propability for the breaking of each bond this channel will lead to a 2:1 ratio of the asymmetrical to the symmetrical isotopomer.

0 + QQ + M-> aOQQ + bQOQ + M

(2)

where the "isotopomer ratio" r = a/b (52 refers to the common mass of the ozone isotopomers) depends on the propability of the reacting species falling into the dip on the potential surface. In particular a ratio close to 2 is expected if all ozone formation takes place via cyclic ozone, assuming that the three bonds open up with equal propability to form ground state ozone, see Figure 1. 5 2

A third channel, which might be referred to by the self-explanatory term "insertion", is also conceivable: 0 + QQ + M - Q O Q + M

(3)

Bates [6] has proposed a socalled "flip-over" mechanism, which essentially amounts to the same thing as our cyclic mechanism. The only difference is whether the transition state is a minimum (our assumption) or just a shallow region on the potential surface, where the molecule is floppy. In Table 1 we show the very first results we obtained for the isotopomer yields. (The experimental details are reviewed in the Appendix.) As is evident, the isotopomer ratios are invariably close to 2 and rather insensitive to the nitrogen partial pressure, i. e. to the rate of ozone formation. 5 4

Somewhat to our surprise we found that 0 was present among the products and that there was more of O than was to be expected from the natural abundance of these isotopomers in the ozone. We realized that a wellknown exchange reaction [7] was taking place: 3

5 0

3

O + QQ -> OQ + Q

(4)

This reaction is in fact about 300 times faster than ozone formation (depending on pressure and temperature). Only when we started to do kinetic

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

11. LARSEN ET AL. Table 1:

Isotopic Study of Mechanism of Ozone Formation 169

Experimental abundances in pet. (percentages uncertain by .05 except for OOO

which is uncertain by .1) and isotopomer ratios (uncertain by .2) after UV-photolysis for 10 s of

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0

3

mixed with Q at room temperature and varying partial pressures of nitrogen. 2

Nitrogen partial pressure/Torr 476 134 256 64 0 96.4 93.3 97.6 99.4 96.7 1.03 1.39 1.37 1.60 1.53 0.49 0.64 0.60 0.71 0.65 2.10 2.17 2.28 2.25 2.35 0.75 1.18 1.50 1.97 2.07 0.38 0.55 0.70 0.84 0.91 1.97 2.15 2.14 2.35 2.27 0.62 0.84 1.72 1.93 2.41

Species or Ratios OOO OOQ OQO rso

OQQ QOQ QQQ

simulations did we realize that the isotopomer ratio r would end up being close to 2 also for the end-on or insertion mechanisms, when this process is taken into account. In the following section we analyse the situation in more detail and try to explain the strategy that we used in order find out which channel is actually followed. 5 2

A strategy a i m e d to influence the isotopomer ratios 5 0

5 2

The ratios r and r between the two isotopomers of O and of 0 may be stated in terms of the concentrations of the isotopomers as follows: 5 0

52

3

r

[

r 5

r r

0

0

Q

3

]

°-(ÔQÔi -

[

0

Q

Q

(51 ( 5 )

1

Î6)

( 6 )

" " [QOQ]

If the channel via cyclic ozone prevails, then the two ratios can only deviate from 2 to the extent that isotope effects make the breaking of one chemical bond more preferable than the breaking of a symmetrically non-equivalent bond, see Figure 1. If such is the case, then we would still anticipate a constant ratio, independent of the experimental conditions. (To simplify the following discussion we shall assume that the ratios will be exactly statistical, i.e. equal to 2 for this mechanism). If therefore, by therightchoice of experimental conditions, r and r can be caused to deviate significantly from the value 2, then this is evidence for either 5 0

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

5 2

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170

ISOTOPE EFFECTS IN GAS-PHASE CHEMISTRY

end-on addition (both ratios becoming larger than 2) or insertion (both ratios becoming smaller than 2). If the isotopomer ratios cannot be influenced, then it is more difficult to reach a definite conclusion. It can either mean that scrambling processes dominate, thus masking the end-on or insertion mechanisms, or that the mechanism is via cyclic ozone. In the following we shall assume that at any stage during the photolysis, the free atoms will become scrambled. This means that the ratio between the atomic concentrations will attain the same value as the ratio between amounts of the oxygen nuclides present: [0] [Q]

2[00] + [OQ1 2[QQ] + [OQ]

. V

;

This relation follows from the approximate steady-state conditions for the atoms: *[0][QQ] + i*[0][0Q] = *[Q][00] + \k[Q][OQ)

(8)

here k is the rate constant for the exchange reaction Eqn. (4) (left to right) while \ k is the rate constant for the opposite reaction (right to left, neglecting isotope effects). There are two approximations involved in the steady-state condition Eqn. (8): the neglect of ozone formation, and the neglect of ozone destruction, according to the process: Ο+0 - 20 3

(9)

2

Both approximations are very good indeed, since these processes occur on much larger time scales. Ozone formation is some three hundred times slower than exchange, and ozone destruction, is even slower. Obviously, if the dioxygen molecules also become scrambled, in the sense that the nuclides are statistically distributed among the molecular dioxygen species as well, then the ozone isotopomer ratios will both become equal to 2 irrespective of the mechanism. We believe that this was the situation in our early experiments, because the concentration of 0 (which was only present to the extent that it was produced by the photolysis) was insignificant at any time during the experiments, while OQ was formed at exactly the same rate as Q according to Eqn. (4). It then occurred to us, that we might delay the molecular part of the scram­ bling by having not only Q present, but by adding 0 in comparable quantity. This would delay the formation of the statistical amount of OQ, since the for­ mation of OQ from 0 would now be a process of substantial bulk. We have simulated the time development of the molecular concentrations in Figure 2. 2

2

2

2

The strategy in our final experiments has therefore consisted in letting the ozone photolysis take place in a mixture of approximately equal and relatively high concentrations of 0 and Q (numbers are found in the appendix). 2

2

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Isotopic Study of Mechanism of Ozone Formation 171

11. LARSEN ET AU

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dloxygena

V

0

25

SO

75

100

125

1»

175

200

Time / s Figure 2:

Simulation of the time development of the concentrations of the molecular dioxygen

species. The measurements were performed at 120 s. It is seen that at long times [OQ] approaches its "scrambled value" (there is continuous formation of O 2 due to ozone photolysis), while there is a range (0-125 s) in which it has not yet caught up. This is the preferable range for obtaining isotopomer ratios different from 2. Reprinted from ref. 2. Copyright 1991 Wiley.

T h e m e c h a n i s m o f ozone formation

The ratios observed in an experiment performed with visible light at room temperature are shown in Figure 3. As is evident from the figure, the ratios are both larger than 2, so that we may conclude that the mechanism is dominated by the end-on channel. The simulations were made under the assumption that there were no mass dependent isotope effects. The error ranges (9-13 % for r , 13-30 % for r ) do of course allow for contributions from other channels. In the figure we have made a simulation in which a contribution of 5 % insertion to the reaction has been assumed, 5 % is the limit, imposed by the error ranges, to which insertion might contribute to 50

52

the reaction. However alternatively we might have added 15 % of the channel via

the cyclic intermediate. (Since 1/3 becomes symmetrical). In the discussion we mention a series of new, as yet unpublished, experiments in which the number of data points has been increased, and where the temperature has been varied. In Figure 4 we show that the resulting simulations corroborate the findings of [2]. Discussion

We have presented experimental evidence for a mechanism of ozone formation in the gas phase, that is predominantly end-on, [2]. This evidence has been corroborated by a new series of experimental results (as yet unpublished, see

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

172

ISOTOPE EFFECTS IN GAS-PHASE CHEMISTRY

1

ι

ι

1

1

I

1

-

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Q

rso() r lb)

< 25

t

50

-

1 1 1

TcniC )

,

1 riltcTj ~0

• " - ^ — - — _

50

75

1

1 1

100

125

1

1

150

175

200

t/sec

Figure 3:

Simulations of the time development of the isotopomer ratios r

5 0

and r 2 for 5

VIS-photolysis experiment assuming: (a) the end-on mechanism, (b) 95 % end-on plus 5 % insertion and (c) 100 % insertion. The dashed line indicates the time (120 s) where the measure­ ments are performed. Also shown are the the experimental ratios with error bars. Reprinted from ref. 2. Copyright 1991 Wiley.

below). It is important to emphasize that our conclusion is based on the fact that we have been able to cause the isotopomer ratios to increase significantly above the value 2, by the strategy discussed above. It does not rely, therefore, on absolute determinations of the isotopic abundances in the resulting mixtures. It is worth noting that in our first experiments see Table 1, where we at­ tempted to influence the reactions by manipulating the nitrogen pressure, we were unable to obtain results differing significantly from 2. At that stage we actually thought that we had evidence for the channel via the cyclic transition state. As implied above (see also [2] for details) we rely on a calibration mixture for our measurements on the microwave spectrometer. This is a drawback, which we have not sofar been able to avoid, although an attempt has been made, which is explained in the second part of the appendix. At the moment we attempt to come around this difficulty along two lines: Firstly we are trying to measure a larger set of microwave lines. Secondly we are working on obtaining the rotational spectrum of a calibration mixture, of iso­ topically substituted ozone species, in the far-IR range using our high-resolution Fourier Transform spectrometer (Bruker FS120HR). In both cases we aim at applying the absolute line-strength for concentration assessments. The calcula­ tion of absolute line strength, in the rotational part of the spectrum, requires knowledge about the permanent dipole moment and the partition functions. The permanent dipole moment for the parent molecule has been measured, while its components in the principal axis systems for the isotopically substituted species

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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11. LARSEN ET AL.

0

20

40

60

Isotopic Study of Mechanism of Ozone Formation 173

80

100

120

KO

0

20

40

60

80

100

120

KO

Time/IOs

Time/IOs

0

50

100

150

200

250

300

350

400

Time/IOs

Figure 4:

Simulations of the time development of the isotopomer ratios r^o and r 2 for three, 5

more recent VIS-photolysis experiments, run at different temperatures. The experimental set-up is different from that of [2], that is the reason why the time scale is different. The low temperature experiments b) and c) were simulated using the newly found temperature dependence for the scrambling process - see discussion and appendix, a) 10 ° C ; b) -70 ° C ; c) -130 ° C

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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174

ISOTOPE EFFECTS IN GAS-PHASE CHEMISTRY

may be calculated by simple vector algebra. The partition functions for all the species may be obtained from the rotational constants and a general force field, data which are also available. The reason why we put so much emphasis on the calibration procedure is, that we wish to contribute to the solution of a number of unsolved problems in the atmospheric behaviour of heavy ozone. These problems have arisen through the findings of Maursberger and collaborators [10], [11] and [12], Abbas et. al. [13], Goldberg et. al. [14] and Thiemens et. al. [15] of unexpected enhancements of 50-ozone in stratosphere as well as in the laboratory. Such enhancements were originally predicted by Cicerone and McCrumb [16] but Kaye and Strobel [17] and Kaye [18] questioned the predictions, pointing out that the exchange process Eqn. (4) would render any major enrichment impossible, leaving room for only very minor, mass dependent effects. We aim at doing laboratory studies of absolute isotopomer ratios. (As opposed to the relative values we are presently using). Such ratios are crucial in order to understand how the observed enhancements arise. Even though we have not as yet accomplished reliable, absolute measurements of individual isotopomer abundances for 50- and 52-ozone, we have been able to improve our calibration (see appendix), so that we can at least compare enhancements obtained for total abundances with those of Morton, Barnes, Schueler and Maursberger [12]. Results of scrambling experiments at three different temperatures are shown in Table 2. (The scrambling experiments are described in the first part of the appendix.) While Morton et al [12] use mass spectrometry to ascertain the reference composition of the original oxygen mixture, relative to which the enhancements are calculated, we have had to use the isotope composition of the collected ozone as the reference composition (using the same formula as Morton et al in [11] for the calculation of the enhancements). We believe that the uncertainty stemming from this source is small compared to that originating from the experiments. It should be kept in mind, that the enhancements are relatively more uncertain than the abundances, because they are ratios between two abundances divided by ratios between two standard abundances. Table 2: Abundances and enhancements of isotopomers of ozone (both in %), obtained by scrambling using VIS-photolysis and measured by microwave spectroscopy applying a new calibration method, described in the appendix. Enhancement of OOO is zero by definition. Abundance / Enhancement 20°C

Isotopomer

-70°C

OOO OOQ+OQO OQQ+QOQ QQQ

14.1(.5)/0(0) 41.0(1.7)/14(10) 34.4(1.6)/12(4) 8.5(1.6)/-3(7)

13.4(.5)/0(0) 42.0(1.8)/21(11) 34.2(1.6)/15(4) 8.7(1.6)/1(8)

70°C

14.1(.5)/0(0) 42.6(1.8)/15(10) 37.3(1.6)/15(4) 9.0(1.6)/-5(7)

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Isotopic Study of Mechanism of Ozone Formation175

11. LARSEN ET AL.

We can compare with Morton et al [12] for 50-ozone only. For this isotopomer we find from their Fig. 3 -70°C : 8%, 20°C : 11%, 70°C : 14%. Within our large error bars - and their smaller - we seem to be in fair agreement with the en­ hancements previously found for 50-ozone produced by photolysis. We would now like to discuss the low temperature experiments performed at 143 Κ in [2]. These presented us with a puzzling discrepancy between simulated isotopomer ratios and the corresponding experimental values. In one experiment we determined the values r = 2.8(0.3) and r = 7.3(2.1), while the simu­ lated values were r = 4.9 and r = 13.3. We speculated that the reason for such a large discrepancy had to do with lack of knowledge of the temperature dependence of either the exchange process or the process of ozone formation. The relation for the temperature dependence of the process of ozone formation was extended to low temperatures by Hippler, Rahn and Troe [19]. It was found to predict a value at 143 K, not deviating much from the value we had been using. So the focus had to be put on the exchange process, which was until then believed (but not measured) to be independent of temperature [9] at least for temperatures at or above room temperature. We have examined this process at 143 Κ (see appendix for details) and found, very much to our surprise, that its temperature dependence is pronounced at lower temperatures, and that it mimicks that of ozone remarkably. This means,

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5 0

5 2

5 0

5 2

among other things, that the rate of exchange increases with decreasing tempéra-

ture. With this relation for the rate of exchange, we are now able to simulate also the low temperature experiments reasonably well, as is evident from Figure 4. (The new measurements underlying the figure aimed at putting more data points - spanning a suitable time domain - onto the graphs. The data are as yet unpublished, proper publication awaits one of our - hopefully successful - new calibration procedures. The original calibration method has been used to obtain the results.) This new finding is very interesting, since it points towards a relationship between ozone formation and exchange. If we consider the results in relation to the theoretical considerations also presented in the paper by Hippler et. al. [19], then it appears that our experiments take place in a pressure/temperatureregime, where the function of the third body is to form a Van der Waals-type complex before the final reaction takes place, rather than to act as an energy transport agent after the reaction has taken place. In the light of this theory one might suggest that the reaction sequences leading to exchange or ozone formation are the following: Q+O ^Q.Oa;

(10)

a

Q 0

2

- Q O + 0;

Q ·0 +M 2

Q 0 + M; 2

k

(11)

2

k

3

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

(12)

176

ISOTOPE EFFECTS IN GAS-PHASE CHEMISTRY

Assuming that the the concentration of the Van der Waals complex reaches a steady-state we obtain the concentration of the complex: [ Q l

Q

0

°

*i-[Q][0»)

1 2

)

.,*i~[Q)[0»]

-^+ifc +ifc3[Mr

f

l (

t^+jb,

2

3 1

) 3

)

Hippler et. al. formulated their socalled "Radical Complex"(RC)-mechanism in terms of an unspecified M. What we suggest is that M be 0 . This suggestion is advanced partly because it explains the relationship we have established between exchange and ozone formation, partly because the biradical 0 ( Σ ~ ) is more likely to form a relatively long-lived Van der Waals complex with the atom Q( P) than is a closed shell molecule like N . The primary argument used by Hippler et. al. for the RC-mechanism was based on the temperature dependence for ozone formation. The theoretically pre­ dicted T " / dependence was sufficiently close to the experimentally determined T" -dependence to corroborate the mechanism, whereas a T" -dependence was required theoretically for the "Energy Transfer"-(ET)-mechanism. If we assume, in accordance with Hippler et. al., that the temperature de­ pendence of is ~ T ~ , those of and k ~ T° and that of k ~ T " , then the resulting temperature dependence of the exchange process will become ~ T ~ / , while that of ozone formation becomes ~ y - e ^ b t in accordance with the experimental findings within their uncertainties. (We must also assume fc [M] ). So the fit cannot be characterised by a total rms, but the relative rms is close to 1 with our uncertainty estimates, as it should be. The obvious implication is that this calibration method cannot be used to determine the individual abundances of the symmetric/asymmetric forms of 50and 52-ozone. However, it may still be used to determine the total abundances of the isotopomers 48, 50, 52 and 54. This is in fact done when we calculate enhancements in the discussion. In conclusion the new method does not allow us to improve on our previous results, but it does make a comparison with other methods feasible, when only the total abundances are concerned, this is done in the discussion. (

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2

Literature Cited 1. Thompson, W.E. and Jacox, M.E., J. Chem. Phys. 91, 3826-3837, (1989). 2. Wessel Larsen, N . , Pedersen, T., and Sehested, J., Int. J . Chem. Kin 23, 331-343 (1991). 3. Wright, J . S . , Canad. J . Chem. 51, 139-146 (1973). 4. Murrell, J . N . , Carter, S., Farantos, S.C., Huxley, P. and Varandas, A.J.C., "Molecular Potential Functions", John Wiley & Sons Ltd., 1984. 5. Burton, P.G., J . Chem. Phys. 71, 961-972, (1979). 6. Bates, D.R., J . Chem. Phys. 93, 8739-8744 (1990). 7. Ogg, R.A., Jr. and Sutphen, W.T., Discuss. Faraday Soc. 17, 47-54 (1954). 8. "Chemical Kinetics and Photochemical Data for Use in Stratospheric Modelling", Evaluation no. 9, JPL-publication 90-1. NASA Jet Propulsion Laboratory, 1990. 9. Anderson, S.M., Klein, F.S. and Kaufman, F . , J. Chem. Phys. 83, 1648-55 (1985). 10. Mauersberger, Κ., Geophys. Res. Lett 8, 935-937 (1981). 11. Morton, J., Schueler, B. and Mauersberger, Κ., Chem. Phys. Lett. 154, 143-145 (1989).

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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ISOTOPE EFFECTS IN GAS-PHASE CHEMISTRY

12. Morton, J., Barnes, J., Schueler, B. and Mauersberger, K., J. Geophys. Res. 95, 901-907 (1990). 13. Abbas, M.M., Guo, J., Carli, B., Mencaraglia, F., Carlotti, M. and Nolt, I.G., J. Geophys. Res. 92, 231239 (1987). 14. Goldman, Α., Murcray, F . J . , Murcray, D.G., Kosters, J.J., Rinsland, C.P., Flaud, C.P., Camy-Peyret, C. and Barbe, Α., J. Geophys. Res. 94, 8467-8473 (1989). 15. Heidenreich, J.E. III, Thiemens, M.H., J. Chem. Phys. 84, 2129-2136 (1985). 16. Cicerone, R.J. and McCrumb, J . L . , Geophys. Res. Lett. 7, 251-254 (1980). 17. Kaye, J. and Strobel, D.F., J. Geophys. Res. 88, 84478452 (1983). 18. Kaye, J., J. Geophys. Res. 91, 7864-7874 (1986). 19. Hippler, H., Rahn, R., Troe, J., J. Chem. Phys. 93, 6560-6569 (1990). RECEIVED May 11, 1992

In Isotope Effects in Gas-Phase Chemistry; Kaye, Jack A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.