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An apparent new isotope effect in a molecular decomposition and implications for nature
Volume 172,number 5
CHEMICALPHYSICSLETTERS
I4 September 1990
An apparent new isotope effect in a molecular decomposition and implications for nature J. Wen and Mark H. Thiemens Department of Chemistry, B-01 7, Universify ofCalifornia, San Diego, La Jolla, CA 92093, USA
Received 29 January 1990;in final form 20 June 1990
The isotopic fractionation resulting from thermal ozone decomposition has been measured. The product Oz from gas phase thermal decomposition is equally enriched in I70 and “0 with respect to precursor 0,. The effect is not due to 0t02+M reaction, which is the source of another reported mass-independent isotope effect. Previous experiments have shown that visible light 0, decomposition produces isotopically light 02, thus, it appears that the source of the anomalous isotopic fractionation observed in thermal decomposition is the collisionaldecomposition process. This may then represent a new type of isotope effect not accounted for by theory. The possible consequencefor observation in nature is discussed.
1. Introduction isotope ratio measurements may be used to characterizea wide variety of processes;rangingfrom mechanistic resolution of chemical reactions to details of early solar system events. For high-precision measurements, isotope ratios are conventionally reported in the delta notation [ 11, where for example, the PO, in per mil (o/00),expresses the part per thousand deviation of a sample from a standard: Stable
where 34RJampleis the ratio of the masses34 ( ‘60’80) and 32 ( ‘60’60) in a sample,with respectto the same ratio in a defined standard ( 34RRstandard). The P70 refers in a similar fashion to the mass 33 ( ‘60’r0) to 32 ratio. Isotopic fractionations are generally dependent upon mass, which led Hulston and Thode [ 21 to propose that for meteoritic measurements of multi-isotopic ratios, distributions which are not attributable to a mass-dependent process must reflect nuclear intervention since chemical processesare not independent of mass. Later measurements of hightemperature minerals in carbonaceous chondritic meteorites revealed a 6’70=S’80 isotopic distribution [3] rather than 6”0=0.56’*0 observed for lunar and terrestrial materials and general mass-dependent fractionation processes [ 2 j. This was in416
terpreted as reflecting the admixture of essentially pure 160,presumably from supernova, into the presolar nebula [ 31. It was later experimentally demonstrated that ozone formation produces the same mass-independent isotopic distribution observed in the high-temperaturemeteoritic minerals and which may account for the meteoritic isotopic observations [ 4,5]. Later, Heidenreich and Thiemens demonstrated that the anomalous fractionation occurs during the actual 0) formation step [ 61 and a mechanism based upon an enhanced lifetime of the activated complex, [0: 1, for the isotopically asymmetric ozone molecules, 160’60’70 and ‘aO’60’B0, which leads to an increased probability of stabilization, was suggested [ 71. The dependence of the mass-independentfractionation process upon initial O2 isotopic abundance also suggeststhe role of symmetry [8,9], as do tunable diode laser absorption measurements of ozone isotopomers which demonstrate that asymmetric ‘60’60’B0possessestwice the enrichment as ‘a0’80’60 [lo]. Symmetrydependent isotopic fractionations have now been observed in the reactions of O+CO [ 11,121, during ozone formation followingO2photolysis [ 13,141and in the isotopes of sulfur followingthe reaction of SF, t SF5 [ 151. While an unambiguous resolution of the mechanism has remained elusive, it does seem clear that the fractionation is (1) related to symmetry
0009-2614/90/S 03.50 0 1990- Elsevier Science Publishers B.V. (North-Holland)
Volume 172, number 5
CHEMICAL PHYSICS LE-lTERS
properties and (2) occurs during a formation reaction. In situ mass spectrometric measurements of stratospheric ozone have shown that a large, and ag parently equal, enrichment of both '*Oand “0 (up to 41OOh)exists [ 161. These measurements further demonstrate the possible role of mass-independent isotopic fractionations in nature. A review of these processes and their possible role in nature has recently been given [ 171. Since mass-independentfractionations are of general interest, not only as probes of chemical kinetics and specific reaction features, but also due to their possible involvement in atmospheric and cosmochemical environments, it is important to carefully determine the relevant fractionation mechanisms.We report here the measurements of mass-independent isotopic fractionations in a well characterized, thermal ozone decomposition process.
2. Experimental Ozone of known isotopic composition was generated, which develops a pressure of ~60 Torr at 23“C, and thermally decomposed in a z 75 cm’volume vessel as previously described [ 181. The initial ozone decomposition produces atomic and molecular oxygen. The fate of the atomic oxygen is to react with ozone to produce two O2molecules. The 0 +O, reaction is important only at long reaction times and
14 September 1990
0+0 is insignificantat all times. Followingreaction termination the product O2 and residual O3 were cryogenically separated as described previously [ 14,181 and analyzed mass spectrometrically. The residual Oa was measured after conversion to 02, All isotopic measurements are made on O2 gas using a triple collector Finnigan 251 mass spectrometer. Typical errors associated with 6l8O, 6”O measurements are within f 0.06Oh.
3. Results The experimental data are given in table 1. For convenience, we normalize to an initial O3 isotopic composition of 6170=6’80=0. The results are shown in a three isotope plot in fig. I. It may be.seen that thermal ozone decomposition produces isotopicallyheavy O2at temperaturesabove 7O”C,and light O2 at lower temperatures. Isotopic material balance is observed in all experiments. The same percent O3 to O2 conversion ( x20%) was utilized in order to compare the isotopic fractionation of the different temperature experiments.
4. Diseussion The present experiments clearly demonstrate that a mass-independent fractionation occurs during
Table 1 Experimental results of the ozone thermal decomposition at various temperatures
T (“C)
Time (min)
Produced 02 (pmol)
20 30 40 50 60 70 80 90 100 110 120
1080 820 395 252 180 90 60 30 14 8 4
66 74 82 62 67 65 70 72 56 68 65
Residual 0, 6’80
6”O
WJ)
(S)
-15.6 -14.8 -13.7 -11.2 -5.9 -4.4 5.1 10.1 17.9 21.9
-8.4 -7.5 -6.7 -4.5 -0.4 1.2 8.3 12.6 18.5 21.7
24.3
23.2
(wol)
277 275 265 288 282 276 281 275 295 278 286
6’80
61’0
(?w
(S)
3.1 4.0 4.3 2.4 1.4 1.1 -1.3 -2.6 -3.4 -5.4
2.0 2.0 2.1 1.0 0.1 -0.3 -2.1 -3.3 -3.5 -5.3 -5.2
-5.5
Slope
Yield
ti”/A6’8
(%)
0.54 0.51 0.49 0.40 0.07 -0.27 1.62 1.24 1.03 0.99 0.95
19 21 24 18 19 19 20 21 16 20 18
417
Volume 172,number 5
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CHEMICALPHYSICSLETTERS
0
IO
2-o
30
??o (so) Fig. I. The isotopic composition of the molecular oxygen from the thermal decomposition experiments.
thermal O3 decomposition. The observed fractionation must be due to the decomposition by either OS+ M or 0, t 0 since the 0 + O2recombination reaction is insignificant at conversions less than 30%. 4t low conversions, the low O2 concentration and the rapidity of the 0 + O3 reaction render the recombination unimportant. For experiment 1, for example, the ratio of Ot02 t M relative to O+O, is approximately 4% using rate constants from ref. [ 191. At 12O”C,where the maximum fractionation is observed, the O+Oz+M is only 22%. The observed effect then appears to be different from that previously observed which occurs during the formation step in the reactions O+O, [4-91, O+CO [ 11,121,and SF,+SF, [ 151. Given that the only significant experimental variables are temperature and reaction extent, data from table 1 imply that the isotopic composition is dependent upon decomposition temperature since all reactions are to the same conversion extent. From fig. 1 it is seen that the data define a straight line. For discussion,we might presume that this arises due to a two-component mixture, with a “high-temperature” component with 6’80=6’70=250~ and a lowtemperature one at 8”0= - 16, 6”0= -9O,60.Previous experiments where isotopically light O2 was produced [ 181 are consistent with the present results since those experiments were done in a 5 !Iflask where only a small portion ( x 200 cm3) was heated to z SO-60°C. The bulk temperature was no doubt considerably lower and most likely near room tem418
I4 September 1990
perature and therefore compare with the present experiments at TX 5O"C, where light O2 is produced. The observed 0, decomposition rate at a temperature of 90°C and higher agrees quite well with published measurements for homogeneous gas phase thermal decomposition [ 201. Therefore, the isotope effect observed in higher-temperatureexperiments is likely due to homogeneousgas phase reactions,which gave isotopically heavy O2 product with an equally enriched in 6”O and 6”O. At lower temperatures the decomposition rate is several orders of magnitude greater than publishedvalues.This is most likely due to wall decomposition, which is known to become important below about 70°C [20] and, even at 9O”C,a small wall component may be present. It is plausible then that the 6”0= - 16%~component at low temperature, and in the previous experiment [ 18] derives from a wall decomposition. From the experimentalobservationsalone, we may not determine the absolute magnitude of fractionation associated with the decomposition since for the size of O3 reservoir, there is a variation of the initial isotopic composition with reaction extent. A thorough kinetic analysisshowsthat it is z 5OWin 6”0, 6180, favoring heavy isotopes in the 0, [ 211. Regardinga possible mechanismfor this effect we may, at present, make only qualitative suggestionsas to which of the two decomposition channels produce the anomalous fractionation. It might be considered that the source of the mass-independent fractionation is the 0+03 step, since the short-lived O...Os transition state may be subject to the symmetry constraints suggested previously [ 7,81. However, previous experiments [22] demonstrated that ozone photolysis by visible light produces O2 which is isotopically light, rather than heavy (with respect to precursor 0,). The significantdifferencebetweenthe photolysis experiments and the present is the initial mode of ozone decomposition. We may not exclude fractionation differences which arise due to energy differences in the reaction O+O, from photolysis versus thermal decomposition. Comparison of the photolysis and thermal decomposition results, particularly at higher temperatures where the homogeneous reaction dominates, qualitatively suggeststhat the source of the anomalous fractionation is in the initial O3 decomposition; though 0+ O3 is not quantitatively excluded. It thus appears that it is the col-
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CHEMICAL PHYSICS LETTERS
lisional decomposition process which is the source of the anomalous fractionation. Since the present results are for a strictly thermal, ground-state, collisional-driven process, and thus are not the simple reverse of the recombination process of 0 t OZt M previously studied. Visible light O3 photolysis, which is energetically more similar to the reverse, produces isotopically light OZ. A theoretical analysis of 0, thermal decomposition by Kaye [23] based upon well accepted unimolecular decomposition theory predicts that isotopically light, mass dependently fractionated O2 should be produced and as we in fact observe in O3 photolysis and wall decomposition. There is, at present, no theory which quantitatively accounts for a mass-independent isotopic fractionation during a thermal, unimolecular decomposition process, though application of vibrational symmetry factors to RRKM theory has been suggested [24]. In the rovibrational excitation of CO2 by collisions with hot deuterium atoms a strong propensity for population of odd-J states is observed and which is attributed to symmetry of the COz wavefunctions, resulting in oscillations in cross sections as a function of rotational quantum number [25]. If, for the present experiments, different cross sections for the collisionally induced decomposition (0, t M) exist for the symmetric ( ‘60’60’60) and asymmetric species ( ‘60’60’70, ‘60’60’80) a mass-independent fractionation may result. The results would require that the cross section for the asymmetric isotopomers be larger than for symmetric and equal to one another. At present, however, there is no theoretical basis for such requirement. Based on a series of elegant O3 photodissociation experiments, Valentini [26] has suggested that a mass-independent fractionation may arise due to selection rules during nonadiabatic processes. While these results are of general interest, they most likely do not explain the present results, which are adiabatic. Another consideration is a fractionation arising due to the anomalous isotopic structure of 0,. It is known that O3 during its formation is preferentially enriched in the asymmetric 160160’70, ‘60’60’s0 species, with respect to ‘60180’60, ‘60’70’60 [lo]. It is not known, however, whether this unusual structure is responsible for the observed decomposition
14 September1990
effect. A kinetic analysis [21] suggests that the isotopomeric distribution would not give rise to the present observation; however, further experiments with isotopically enriched ozone would be useful in resolving this question. A similar approach has been successful in documenting the role of symmetry in the 0 t O2 + M recombination reaction [ 91. Given that we observe light 0, formation during photolysis and at low temperatures (presumably wall decomposition) it still must be concluded that anomalous fractionation arises from the gas phase decomposition. At present, it may only be stated with certainty that a mass-independent fractionation is clearly evident in the thermal ozone decomposition and is apparently a different mechanism than previously reported for formation reactions. Further experiments investigating the effect of pressure and third-body composition will be of importance in resolving the mechanism. The effect of initial isotopic abundance and symmetry should also be investigated along with spectroscopic measurements of the relevant isotopomeric symmetries during the decomposition process. Further resolution of the mechanism will be of importance, not only for the inherent physical-chemical interest but for the possible application to understanding natural phenomena. It is known that an up to 410% “0 enrichment in stratospheric OS exists [ 161, yet laboratory experiments produce a ~85% single-stage IsO enrichment [ 141. It is not known how the 4 10% enrichment is produced, and resolution of the details of fractionation mechanisms associated with both the formation and destruction of 0, are needed. In meteoritic components, objects which have been exposed to heating and, at least some dissociation, could then produce a 6 “O= 6 “0 fractionation provided the decomposition was similar to the present experiments. Chondrules could possibly be a candidate for such processes since they are clearly objects which are extensively heated and vaporized, undergo some material loss and are subsequently quenched. If concomitant dissociation of an oxygen-bearing species possessing terminal oxygen atoms occurred (e.g. OSiO) a 6’70=6’80 fractionation might occur. A 6’70=6’80 fractionation in fact is observed in size-separated chondrules from the Dhajala chondrite (H3 ) and in individual chondrules from unequilibrated ordinary chondrites [ 27 1. 419
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CHEMICALPHYSICSLETTERS
These compositions are presently thought to reflect exchange between differing nebula dust-gas reservoirs [ 17,271. Since the precise mechanism for chondrule formation is not known, it may not be stated at present if the decomposition fractionation mechanism is responsible for the meteoritic chondrule (or inclusion) observations, but may be considered as a possibility.Further measurements of the oxygen isotopic composition of size separated chondrules from different meteoritic classes will be of importance. In summary, we have shown that a mass-independent fractionation occurs during simple, gas-phase, thermal O3 decomposition with equal “0, I80 en-
richment in the product O1 at temperatures above 90°C. The effect is not attributable to O+O, tM recombination and thus, appears to be a different fractionation mechanism than previously reported. At present, no theory accounts for the present observations. The simplicity of the effect may render it a significant and observable process in nature.
Acknowledgement We wish to thank R.N. Zare, K. Mauersberger, J. Kaye, and R.N. Clayton for helpful and thoughtful comments. Support from the NSF is gratefully acknowledged (Grant No. ATM87-21051).
References [ I] H. Craig, Geochim. Cosmochim. Acta 12 (1957) 133. [2] J.R. Hulston and H.G. Thode, I. Geophys. Res. 70 (1965) 3475. [ 3 ] R.N. Clayton, L. Grossman and TX. Mayeda, Science I82 ( 1973) 485.
420
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141M.H. Thiemens and J.E. Heidenreich III, Science 219 (1983) 1073. [5] J.E. Heidenreich III and M.H. Thiemens, J. Chem. Phys. 78 (1983) 892. 161J.E. Heidenreich III and M.H. Thiemens, Geochim. Cosmochim. Acta 49 ( 1985) 1303. [7] J.E. Heidenreich III and M.H. Thiemens, J. Chem. Phys. 84 (1986) 2129. [S] J. Yang and S. Epstein, Geochim. Cosmochim. Acta 51 (1987)2011. [9] J. Morton, B. Schueler and K. Mauersberger,Chem. Phys. Letters 154 (1989) 143. [lo] S.M. Anderson, J. Morton and K. Mauersberger,Chem. Phys. Letters 156 ( 1989) 175. [ 111SK. Bhattacharyaand M.H. Thiemens,Z. Naturforsch.44a (1989) 435. [ 121S.K. Bhattacharyaand M.H. Thiemens,Z. Naturforsch.44a (1989) 811. [ 131M.H. Thiemens and T. Jackson, Geophys. Res. Letters 14 (1987) 624. [ 141M.H. Thiemens and T. Jackson, Geophys. Res. Letters 15 (1988) 639. [ 151S.K. Bains-Sahotaand M.H. Thiemens, J. Chem. Phys. 90 (1989) 6099. [ 161K. Mauersberger,Geophys. Res. Letters 14 ( 1987) 80. [ 171M.H. Thiemens, in: Meteorites and the early solar system, eds. J. Kerridge and M.S. Matthews (University of Arizona Press, Tucson, 1988) pp. 899-923. [ 181SK. Bhattacharya and M.H. Thiemens, Geophys. Res. Letters 15 ( 1988) 9. [ 191W.B.DeMore et al., JPL Publication 87-41 ( 1987). [20] S.W. Benson and A.E. Axworthy Jr., J. Chem. Phys. 26 (1957) 1718. [21] J. Wen and M.H. Thiemens, J. Geophys. Res. (1990), sumitted for publication. [22] J. Wen andM.H. Thiemens, EOS 70 (1989) 1034. [23]J.A.Kaye,J.Geophys.Res.91 (1986) 7865. [241 M. Park, SM. Hongand J. Yang,in: Abstractsfrom the 52nd Annual Meteoritical Society Conference, Vienna, Austria (1989). 1251J.F. Hershbqer, S.A.Hewitt, G.W. FlynnandR.E. Weston, J. Chem. Phys. 88 ( 1988) 7243. [26] J.J. Valentini,J. Chem. Phys. 86 (1987) 6757. [ 27] R.N. Clayton et al., in: Protostars and planets, Vol. 2, eds. D.C. Blackand MS. Mattews (University of Arizona Press, Tucson, 1985).
CHEMICALPHYSICSLETTERS
I4 September 1990
An apparent new isotope effect in a molecular decomposition and implications for nature J. Wen and Mark H. Thiemens Department of Chemistry, B-01 7, Universify ofCalifornia, San Diego, La Jolla, CA 92093, USA
Received 29 January 1990;in final form 20 June 1990
The isotopic fractionation resulting from thermal ozone decomposition has been measured. The product Oz from gas phase thermal decomposition is equally enriched in I70 and “0 with respect to precursor 0,. The effect is not due to 0t02+M reaction, which is the source of another reported mass-independent isotope effect. Previous experiments have shown that visible light 0, decomposition produces isotopically light 02, thus, it appears that the source of the anomalous isotopic fractionation observed in thermal decomposition is the collisionaldecomposition process. This may then represent a new type of isotope effect not accounted for by theory. The possible consequencefor observation in nature is discussed.
1. Introduction isotope ratio measurements may be used to characterizea wide variety of processes;rangingfrom mechanistic resolution of chemical reactions to details of early solar system events. For high-precision measurements, isotope ratios are conventionally reported in the delta notation [ 11, where for example, the PO, in per mil (o/00),expresses the part per thousand deviation of a sample from a standard: Stable
where 34RJampleis the ratio of the masses34 ( ‘60’80) and 32 ( ‘60’60) in a sample,with respectto the same ratio in a defined standard ( 34RRstandard). The P70 refers in a similar fashion to the mass 33 ( ‘60’r0) to 32 ratio. Isotopic fractionations are generally dependent upon mass, which led Hulston and Thode [ 21 to propose that for meteoritic measurements of multi-isotopic ratios, distributions which are not attributable to a mass-dependent process must reflect nuclear intervention since chemical processesare not independent of mass. Later measurements of hightemperature minerals in carbonaceous chondritic meteorites revealed a 6’70=S’80 isotopic distribution [3] rather than 6”0=0.56’*0 observed for lunar and terrestrial materials and general mass-dependent fractionation processes [ 2 j. This was in416
terpreted as reflecting the admixture of essentially pure 160,presumably from supernova, into the presolar nebula [ 31. It was later experimentally demonstrated that ozone formation produces the same mass-independent isotopic distribution observed in the high-temperaturemeteoritic minerals and which may account for the meteoritic isotopic observations [ 4,5]. Later, Heidenreich and Thiemens demonstrated that the anomalous fractionation occurs during the actual 0) formation step [ 61 and a mechanism based upon an enhanced lifetime of the activated complex, [0: 1, for the isotopically asymmetric ozone molecules, 160’60’70 and ‘aO’60’B0, which leads to an increased probability of stabilization, was suggested [ 71. The dependence of the mass-independentfractionation process upon initial O2 isotopic abundance also suggeststhe role of symmetry [8,9], as do tunable diode laser absorption measurements of ozone isotopomers which demonstrate that asymmetric ‘60’60’B0possessestwice the enrichment as ‘a0’80’60 [lo]. Symmetrydependent isotopic fractionations have now been observed in the reactions of O+CO [ 11,121, during ozone formation followingO2photolysis [ 13,141and in the isotopes of sulfur followingthe reaction of SF, t SF5 [ 151. While an unambiguous resolution of the mechanism has remained elusive, it does seem clear that the fractionation is (1) related to symmetry
0009-2614/90/S 03.50 0 1990- Elsevier Science Publishers B.V. (North-Holland)
Volume 172, number 5
CHEMICAL PHYSICS LE-lTERS
properties and (2) occurs during a formation reaction. In situ mass spectrometric measurements of stratospheric ozone have shown that a large, and ag parently equal, enrichment of both '*Oand “0 (up to 41OOh)exists [ 161. These measurements further demonstrate the possible role of mass-independent isotopic fractionations in nature. A review of these processes and their possible role in nature has recently been given [ 171. Since mass-independentfractionations are of general interest, not only as probes of chemical kinetics and specific reaction features, but also due to their possible involvement in atmospheric and cosmochemical environments, it is important to carefully determine the relevant fractionation mechanisms.We report here the measurements of mass-independent isotopic fractionations in a well characterized, thermal ozone decomposition process.
2. Experimental Ozone of known isotopic composition was generated, which develops a pressure of ~60 Torr at 23“C, and thermally decomposed in a z 75 cm’volume vessel as previously described [ 181. The initial ozone decomposition produces atomic and molecular oxygen. The fate of the atomic oxygen is to react with ozone to produce two O2molecules. The 0 +O, reaction is important only at long reaction times and
14 September 1990
0+0 is insignificantat all times. Followingreaction termination the product O2 and residual O3 were cryogenically separated as described previously [ 14,181 and analyzed mass spectrometrically. The residual Oa was measured after conversion to 02, All isotopic measurements are made on O2 gas using a triple collector Finnigan 251 mass spectrometer. Typical errors associated with 6l8O, 6”O measurements are within f 0.06Oh.
3. Results The experimental data are given in table 1. For convenience, we normalize to an initial O3 isotopic composition of 6170=6’80=0. The results are shown in a three isotope plot in fig. I. It may be.seen that thermal ozone decomposition produces isotopicallyheavy O2at temperaturesabove 7O”C,and light O2 at lower temperatures. Isotopic material balance is observed in all experiments. The same percent O3 to O2 conversion ( x20%) was utilized in order to compare the isotopic fractionation of the different temperature experiments.
4. Diseussion The present experiments clearly demonstrate that a mass-independent fractionation occurs during
Table 1 Experimental results of the ozone thermal decomposition at various temperatures
T (“C)
Time (min)
Produced 02 (pmol)
20 30 40 50 60 70 80 90 100 110 120
1080 820 395 252 180 90 60 30 14 8 4
66 74 82 62 67 65 70 72 56 68 65
Residual 0, 6’80
6”O
WJ)
(S)
-15.6 -14.8 -13.7 -11.2 -5.9 -4.4 5.1 10.1 17.9 21.9
-8.4 -7.5 -6.7 -4.5 -0.4 1.2 8.3 12.6 18.5 21.7
24.3
23.2
(wol)
277 275 265 288 282 276 281 275 295 278 286
6’80
61’0
(?w
(S)
3.1 4.0 4.3 2.4 1.4 1.1 -1.3 -2.6 -3.4 -5.4
2.0 2.0 2.1 1.0 0.1 -0.3 -2.1 -3.3 -3.5 -5.3 -5.2
-5.5
Slope
Yield
ti”/A6’8
(%)
0.54 0.51 0.49 0.40 0.07 -0.27 1.62 1.24 1.03 0.99 0.95
19 21 24 18 19 19 20 21 16 20 18
417
Volume 172,number 5
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0
IO
2-o
30
??o (so) Fig. I. The isotopic composition of the molecular oxygen from the thermal decomposition experiments.
thermal O3 decomposition. The observed fractionation must be due to the decomposition by either OS+ M or 0, t 0 since the 0 + O2recombination reaction is insignificant at conversions less than 30%. 4t low conversions, the low O2 concentration and the rapidity of the 0 + O3 reaction render the recombination unimportant. For experiment 1, for example, the ratio of Ot02 t M relative to O+O, is approximately 4% using rate constants from ref. [ 191. At 12O”C,where the maximum fractionation is observed, the O+Oz+M is only 22%. The observed effect then appears to be different from that previously observed which occurs during the formation step in the reactions O+O, [4-91, O+CO [ 11,121,and SF,+SF, [ 151. Given that the only significant experimental variables are temperature and reaction extent, data from table 1 imply that the isotopic composition is dependent upon decomposition temperature since all reactions are to the same conversion extent. From fig. 1 it is seen that the data define a straight line. For discussion,we might presume that this arises due to a two-component mixture, with a “high-temperature” component with 6’80=6’70=250~ and a lowtemperature one at 8”0= - 16, 6”0= -9O,60.Previous experiments where isotopically light O2 was produced [ 181 are consistent with the present results since those experiments were done in a 5 !Iflask where only a small portion ( x 200 cm3) was heated to z SO-60°C. The bulk temperature was no doubt considerably lower and most likely near room tem418
I4 September 1990
perature and therefore compare with the present experiments at TX 5O"C, where light O2 is produced. The observed 0, decomposition rate at a temperature of 90°C and higher agrees quite well with published measurements for homogeneous gas phase thermal decomposition [ 201. Therefore, the isotope effect observed in higher-temperatureexperiments is likely due to homogeneousgas phase reactions,which gave isotopically heavy O2 product with an equally enriched in 6”O and 6”O. At lower temperatures the decomposition rate is several orders of magnitude greater than publishedvalues.This is most likely due to wall decomposition, which is known to become important below about 70°C [20] and, even at 9O”C,a small wall component may be present. It is plausible then that the 6”0= - 16%~component at low temperature, and in the previous experiment [ 18] derives from a wall decomposition. From the experimentalobservationsalone, we may not determine the absolute magnitude of fractionation associated with the decomposition since for the size of O3 reservoir, there is a variation of the initial isotopic composition with reaction extent. A thorough kinetic analysisshowsthat it is z 5OWin 6”0, 6180, favoring heavy isotopes in the 0, [ 211. Regardinga possible mechanismfor this effect we may, at present, make only qualitative suggestionsas to which of the two decomposition channels produce the anomalous fractionation. It might be considered that the source of the mass-independent fractionation is the 0+03 step, since the short-lived O...Os transition state may be subject to the symmetry constraints suggested previously [ 7,81. However, previous experiments [22] demonstrated that ozone photolysis by visible light produces O2 which is isotopically light, rather than heavy (with respect to precursor 0,). The significantdifferencebetweenthe photolysis experiments and the present is the initial mode of ozone decomposition. We may not exclude fractionation differences which arise due to energy differences in the reaction O+O, from photolysis versus thermal decomposition. Comparison of the photolysis and thermal decomposition results, particularly at higher temperatures where the homogeneous reaction dominates, qualitatively suggeststhat the source of the anomalous fractionation is in the initial O3 decomposition; though 0+ O3 is not quantitatively excluded. It thus appears that it is the col-
Volume 172, number5
CHEMICAL PHYSICS LETTERS
lisional decomposition process which is the source of the anomalous fractionation. Since the present results are for a strictly thermal, ground-state, collisional-driven process, and thus are not the simple reverse of the recombination process of 0 t OZt M previously studied. Visible light O3 photolysis, which is energetically more similar to the reverse, produces isotopically light OZ. A theoretical analysis of 0, thermal decomposition by Kaye [23] based upon well accepted unimolecular decomposition theory predicts that isotopically light, mass dependently fractionated O2 should be produced and as we in fact observe in O3 photolysis and wall decomposition. There is, at present, no theory which quantitatively accounts for a mass-independent isotopic fractionation during a thermal, unimolecular decomposition process, though application of vibrational symmetry factors to RRKM theory has been suggested [24]. In the rovibrational excitation of CO2 by collisions with hot deuterium atoms a strong propensity for population of odd-J states is observed and which is attributed to symmetry of the COz wavefunctions, resulting in oscillations in cross sections as a function of rotational quantum number [25]. If, for the present experiments, different cross sections for the collisionally induced decomposition (0, t M) exist for the symmetric ( ‘60’60’60) and asymmetric species ( ‘60’60’70, ‘60’60’80) a mass-independent fractionation may result. The results would require that the cross section for the asymmetric isotopomers be larger than for symmetric and equal to one another. At present, however, there is no theoretical basis for such requirement. Based on a series of elegant O3 photodissociation experiments, Valentini [26] has suggested that a mass-independent fractionation may arise due to selection rules during nonadiabatic processes. While these results are of general interest, they most likely do not explain the present results, which are adiabatic. Another consideration is a fractionation arising due to the anomalous isotopic structure of 0,. It is known that O3 during its formation is preferentially enriched in the asymmetric 160160’70, ‘60’60’s0 species, with respect to ‘60180’60, ‘60’70’60 [lo]. It is not known, however, whether this unusual structure is responsible for the observed decomposition
14 September1990
effect. A kinetic analysis [21] suggests that the isotopomeric distribution would not give rise to the present observation; however, further experiments with isotopically enriched ozone would be useful in resolving this question. A similar approach has been successful in documenting the role of symmetry in the 0 t O2 + M recombination reaction [ 91. Given that we observe light 0, formation during photolysis and at low temperatures (presumably wall decomposition) it still must be concluded that anomalous fractionation arises from the gas phase decomposition. At present, it may only be stated with certainty that a mass-independent fractionation is clearly evident in the thermal ozone decomposition and is apparently a different mechanism than previously reported for formation reactions. Further experiments investigating the effect of pressure and third-body composition will be of importance in resolving the mechanism. The effect of initial isotopic abundance and symmetry should also be investigated along with spectroscopic measurements of the relevant isotopomeric symmetries during the decomposition process. Further resolution of the mechanism will be of importance, not only for the inherent physical-chemical interest but for the possible application to understanding natural phenomena. It is known that an up to 410% “0 enrichment in stratospheric OS exists [ 161, yet laboratory experiments produce a ~85% single-stage IsO enrichment [ 141. It is not known how the 4 10% enrichment is produced, and resolution of the details of fractionation mechanisms associated with both the formation and destruction of 0, are needed. In meteoritic components, objects which have been exposed to heating and, at least some dissociation, could then produce a 6 “O= 6 “0 fractionation provided the decomposition was similar to the present experiments. Chondrules could possibly be a candidate for such processes since they are clearly objects which are extensively heated and vaporized, undergo some material loss and are subsequently quenched. If concomitant dissociation of an oxygen-bearing species possessing terminal oxygen atoms occurred (e.g. OSiO) a 6’70=6’80 fractionation might occur. A 6’70=6’80 fractionation in fact is observed in size-separated chondrules from the Dhajala chondrite (H3 ) and in individual chondrules from unequilibrated ordinary chondrites [ 27 1. 419
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These compositions are presently thought to reflect exchange between differing nebula dust-gas reservoirs [ 17,271. Since the precise mechanism for chondrule formation is not known, it may not be stated at present if the decomposition fractionation mechanism is responsible for the meteoritic chondrule (or inclusion) observations, but may be considered as a possibility.Further measurements of the oxygen isotopic composition of size separated chondrules from different meteoritic classes will be of importance. In summary, we have shown that a mass-independent fractionation occurs during simple, gas-phase, thermal O3 decomposition with equal “0, I80 en-
richment in the product O1 at temperatures above 90°C. The effect is not attributable to O+O, tM recombination and thus, appears to be a different fractionation mechanism than previously reported. At present, no theory accounts for the present observations. The simplicity of the effect may render it a significant and observable process in nature.
Acknowledgement We wish to thank R.N. Zare, K. Mauersberger, J. Kaye, and R.N. Clayton for helpful and thoughtful comments. Support from the NSF is gratefully acknowledged (Grant No. ATM87-21051).
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141M.H. Thiemens and J.E. Heidenreich III, Science 219 (1983) 1073. [5] J.E. Heidenreich III and M.H. Thiemens, J. Chem. Phys. 78 (1983) 892. 161J.E. Heidenreich III and M.H. Thiemens, Geochim. Cosmochim. Acta 49 ( 1985) 1303. [7] J.E. Heidenreich III and M.H. Thiemens, J. Chem. Phys. 84 (1986) 2129. [S] J. Yang and S. Epstein, Geochim. Cosmochim. Acta 51 (1987)2011. [9] J. Morton, B. Schueler and K. Mauersberger,Chem. Phys. Letters 154 (1989) 143. [lo] S.M. Anderson, J. Morton and K. Mauersberger,Chem. Phys. Letters 156 ( 1989) 175. [ 111SK. Bhattacharyaand M.H. Thiemens,Z. Naturforsch.44a (1989) 435. [ 121S.K. Bhattacharyaand M.H. Thiemens,Z. Naturforsch.44a (1989) 811. [ 131M.H. Thiemens and T. Jackson, Geophys. Res. Letters 14 (1987) 624. [ 141M.H. Thiemens and T. Jackson, Geophys. Res. Letters 15 (1988) 639. [ 151S.K. Bains-Sahotaand M.H. Thiemens, J. Chem. Phys. 90 (1989) 6099. [ 161K. Mauersberger,Geophys. Res. Letters 14 ( 1987) 80. [ 171M.H. Thiemens, in: Meteorites and the early solar system, eds. J. Kerridge and M.S. Matthews (University of Arizona Press, Tucson, 1988) pp. 899-923. [ 181SK. Bhattacharya and M.H. Thiemens, Geophys. Res. Letters 15 ( 1988) 9. [ 191W.B.DeMore et al., JPL Publication 87-41 ( 1987). [20] S.W. Benson and A.E. Axworthy Jr., J. Chem. Phys. 26 (1957) 1718. [21] J. Wen and M.H. Thiemens, J. Geophys. Res. (1990), sumitted for publication. [22] J. Wen andM.H. Thiemens, EOS 70 (1989) 1034. [23]J.A.Kaye,J.Geophys.Res.91 (1986) 7865. [241 M. Park, SM. Hongand J. Yang,in: Abstractsfrom the 52nd Annual Meteoritical Society Conference, Vienna, Austria (1989). 1251J.F. Hershbqer, S.A.Hewitt, G.W. FlynnandR.E. Weston, J. Chem. Phys. 88 ( 1988) 7243. [26] J.J. Valentini,J. Chem. Phys. 86 (1987) 6757. [ 27] R.N. Clayton et al., in: Protostars and planets, Vol. 2, eds. D.C. Blackand MS. Mattews (University of Arizona Press, Tucson, 1985).