Ozone isotope effect

Kinetic Isotope Effect in Oxidation of Aliphatic Compounds by Ozone... 

The kinetic isotope effect proves the attack of ozone on the C—H bond and consequently the C—D bond of the oxidized compound. The values of the kinetic isotope effect (kH/kD) are collected in Table 3.6. 

Isotope Effects in Unimolecular Processes Mass Independent Isotope Fractionation and the Ozone Problem... 

An important aspect of the Gao-Marcus model is that it provides a theoretical structure for the understanding of quantum state density isotope effects in general, and is not specifically confined to the formation of ozone itself. This feature is important because as discussed above we are now aware that MIF s occur widely in nature. The theory aids in prediction of where MIF s will be likely found, and once found, in rationalizing how they were chemically produced. 

Thiemens, M. H. and Heidenreich, J. E. The mass independent fractionation of ozone. A novel isotope effect and its possible cosmochemical implications. Science 219, 1073 (1983). 

Thiemens MH, Heidenreich JE (1983) The mass independent fractionation of oxygen a novel isotope effect and its possible cosmochemical implications. Science 219 1073-1075 Thiemens MH, Jackson TL, Brenninkmeijer CAM (1995) Observation of a mass-independent oxygen isotopic composition in terrestrial stratospheric COj, the link to ozone chemisdy, and the possible occurrence in the Martian atmosphere. Geophys Res Lett 22 255-257 Timmes FX, Woosley SE, Weaver TA(1995) Galactic chemical evolution hydrogen through zinc. Astrophys J Suppl 98 617-658... 

Gale JD, Rohl. AL (2003) The General Utility Lattice Program (GULP). Mol Simul 29 291-341 Galy A, Bar-Matthews M, Halicz L, O Nions RK (2002) Mg isotopic composition of carbonate insight from speleothem formation. Earth and Planet Sci Lett 201 1-11 Gao YQ, Marcus RA (2002) On the theory of the strange and unconventional isotopic effects in ozone formation. J Chem Phys 116 137-154... 

A number of experimental and theoretical studies have focused on the causes of mass-independent fractionation effects, but as summarized by Thiemens (1999), the mechanism for mass-independent fractionations remains uncertain. The best studied reaction is the formation of ozone in the stratosphere. Mauersberger et al. (1999) demonstrated experimentally that it is not the symmetry of a molecule that determines the magnitude of enrichment, but it is the difference in the geometry of the molecule. Gao and Marcus (2001) presented an advanced model, which has led to a better understanding of nonmass-dependent isotope effects. 

Heidenreich JE, Thiemens MH (1983) A non-mass-dependent isotope effect in the production of ozone from molecular oxygen. J Chem Phys 78 892-895 Helman Y, Barkan E, Eisenstadt D, Luz B, Kaplan A (2005) Fractionation of the three stable oxygen isotopes by oxygen producing and consuming reactions in photosynthetic organisms. Plant Phys (2005) 2292-2298... 

Anderson, S. M K. Mauersberger, and J. Morton, The Ozone Molecule Isotope Effects and Electronic Structure, in Progress and Problems in Atmospheric Chemistry, Advanced Series in Physical Chemistry (J. R. Barker, Ed.), Vol. 3, pp. 473-499, World Scientific, Singapore, 1995. 

The experimental evidence obtained (116, 117) indicates that the central carbon of the acetal function becomes positively charged during the oxidation step. Furthermore, a rather high primary isotope effect (k /k = 6.5) has been measured (121). These results indicate that the reaction mechanism proceeds either via a direct hydride transfer yielding a dialkoxycarbonium ion and a hydrotrioxide ion which would collapse to the hydrotrioxide intermediate (162 - 163 165) (Fig. 20), or via an insertion of ozone in a 1,3-... 

Investigation of this reaction isolated in an N2 matrix at 10-20 K has shown that the apparent activation energy is smaller than 0.11 kcal/mol the unimolecular rate constant for N0-03 reactant complexes prepared in this way is 1.4 x 10-5 s-1 at 12 K. Experiments carried out using ozone enriched with 180 have revealed no observable isotope effect. 

Reaction Mechanism. The proposed mechanism for the ozone-hydrosilane reaction (7) shown in Equation 2, as deduced by analyzing and correlating data on relative rates, substituent effects, deuterium isotope effects, low temperature NMR, and ultraviolet spectroscopy for a range of hydrosilanes, is a multistep one as follows ... 

In view of the possibility that attack by ozone may proceed via oxygen insertion into the Si-H o-bond (state l), the known reaction of dichlorocarbene with the Si-H bond (32) was taken as a model for such insertion. This reaction is postulated to proceed via the three-center transition state 2, which is structurally analogous to 1. The isotope effect, ku/ki>, for insertion of dichlorocarbene into (n-C4H9)3SiH and (n-C4H9)SiD, determined in the same manner as for their ozonations, was found to be 1.23 (10). The large difference in the ku/kjy values... 

These data suggest that the initial step in the reaction of ozone with the Si-H bond is the reversible formation of a silicon-ozone complex. This cannot be the rate step since p would have to be positive (nucleophilic attack) and no primary isotope effect would be predicted. To eliminate the statistical factor for the di- and trihydrosilanes, attack by ozone on the hydridic proton from within the complex must be much more favorable than direct encounter and reaction with uncomplexed ozone and Si-H. 

Step(s) Bi and B2—Hydrogen Abstraction. How the ozonation proceeds from the complex is now considered. If direct o-bond insertion (1) is eliminated on the basis of isotope effects as discussed above, then 5 and 6 are the viable alternatives. The transition state 5 could collapse to form the silicon hydroperoxide 7, while transition state 6 could collapse to form the silicon hydrotrioxide 8 (path Bi) alternatively, 6 could collapse directly to ion or radical pairs (path B2). The transformation 5 -> 7 is not meant to suggest that atomic oxygen is the other product. Since the reaction order in ozone has not been determined, the fate of the other oxygen atom(s) is moot. 

R.A. Marcus, An intramolecular theory of the mass-independent isotope effect for ozone, n. Numerical implementation at low pressures using a loose transition state, /. Chem. Phys. 113 (2000)... 

Y.Q. Gao, R.A. Marcus, Strange and unconventional isotope effects in ozone formation. Science 293... 

There are also well known isotope effects associated with odd nuclear spin, spin-electron spin coupling, e.g., [10]. Such an effect that would favor formation over even spin nuclei absent in ozone formation, presumably because the ozone formation occurs on a singlet electronic state surface While there are some 27 electronic states of an O/O2 pair that in principle could be involved in ozone formation, only the lowest singlet state of O3 is deep enough to be sufficiently long-lived to be collisionally stabilized to lead to O3 formation at atmospheric pressure. Only it was assumed [12-15] to play a significant route to O3 formation under those conditions. The "no reaction" posed by the other entrance chaimels reduced the calculated rate of O3 formation accordingly [15]. 

The reaction CO -I- OH CO2 -I- H is important for controlling the CO and OH concentration in the atmosphere and has an anomalous A O [11,51] and an O isotope effect has also been measured in [52]. In theoretical studies on this reaction results have been obtained for C [53], H [53] and O [54] isotope effects. Inasmuch as the intermediate in this reaction, HOCO, has no symmetry with respect to isotopes, the symmetry effect that appears in the ozone and the CAI problems is now absent. Any anomalies that appear in A O are then due to other reasons. Available three-isotope plots for this reaction are too sparse to define its slope reliably. The theory [53] provides an understanding of the H/D and data, and its... 

This chapter focuses upon some recent observations of mass-independent isotopic processes in nature. As discussed by Thiemens et al. (2001) and Thiemens (2002), there exist other mass-independent isotope effects in nature that derive from non-ozone reactions. For example, CO2 photolysis produces a large mass-independent isotope effect that, in part, may account for observations in the SNC (martian) meteorites and the synthesis of their secondary minerals. UV photolysis of SO2 produces new isotopic fractional effect. An accompanying mass-independent isotopic composition determines the evolution of oxygen in the Earth s earliest atmosphere. 

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