Isotopes, independent

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

Abstract The theoretical framework needed for interpretation of kinetic isotope effects on unimolecular reactions is reviewed. Application to the satisfactory rationalization of the theoretically puzzling mass independent isotope effect observed for oxygen isotope fractionation in extraterrestrial samples is described. 

The last parts of this chapter will deal with the so-called mass independent isotope effects which have been rationalized in recent years to be a consequence of unimolecular processes and which have become one of the most fascinating of present day isotope effect studies. 

Mass Independent Isotope Fractionation in the Laboratory, the Stratosphere, and the Troposphere... 

Theory of Mass Independent Isotope Fractionation of Ozone... 

The work of Thiemens, Mauersberger, and their collaborators has now definitively established a pervasive mass independent isotope fractionation for oxygen containing molecules found in the atmosphere, upper atmosphere, and in space. These results, initially surprising because they are inconsistent with ordinary equilibrium... 

Marcus, R. A. Mass independent isotope effect in the earliest processed solids in the solar system. A possible chemical mechanism../. Chem. Phys. 121, 8201 (2004). 

Thiemens, M. H. History and applications of mass-independent isotope effects. Ann. Rev. Earth Planet. Sci. 34, 217 (2006). 

Elements beyond zinc. Despite extensive investigations carried out in parallel with the FUN inclusions, clear-cut mass-independent isotopic heterogeneities (e.g., more than 5 cr of a single measurement) have not been identified yet in normal inclusions for the rest of the elements besides extinct radioactivities. Nevertheless, there are consistent hints for excesses of about 2 e in Zr (Harper et al. 1991 Schonbachler et al. 2003) which is an r-process isotope for which some coupling may exist with neutron-rich statistical equilibrium (Meyer 1994). 

Thiemens MH (1988) Heterogeneity in the nebula evidence from stable isotopes. In Meteorites and die Early Solar System. Kerridge JF and Matthews MS (eds) University of Arizona Press, Tucson, p 899-923 Thiemens MH (1999) Mass independent isotope effects in planetary atmospheres and die early solar system. Science 283 341-345... 

Thiemens M, Savarino J, Farquhar J, Bao HM (2001) Mass-independent isotopic compositions in terrestrial and extraterrestrial solid and their applications. Accounts Chem Res 34 645-652... 

Weston RE (1999) Anomalous or mass-independent isotope effects. Chemical Reviews 99 2115-2136... 

Efforts to measure Mo isotope variations, using thermal ionization mass spectrometry (TIMS), can be traced back to the early 1960s. Early studies were provocative Murthy (1962 1963) reported that the Mo isotope composition in some iron meteorites deviated from that of others, and from terrestrial Mo, by 10 %o/amu, while Crouch and Tuplin (1964) reported mass dependent variations and mass independent isotopic anomalies among terrestrial molybdenites. As discussed below, Wetherill (1964) subsequently demonstrated isotopic uniformity among iron meteorites to better than 1 %o/amu. It is likely that earlier reports were affected by analytical artifacts due to inadequate correction of Mo isotope fractionation during analysis. 

Temperature-independent isotope effects per se do not indicate tunneling... 

There are ways in which temperature-independent isotope effects can arise in the absence of tunneling. 

Kinetic complexity can produce apparently temperature-independent isotope effects. For example, a rise in temperature produces a smaller intrinsic isotope effect, in agreement with the conventional expectations of Chart 3, for an isotope-sensitive step that is partially rate limiting. If at the same time the rise in temperature makes other steps relatively more rapid so that the isotope-sensitive step then becomes more nearly rate-limiting, then the intrinsic isotope effect will be more fully expressed (Chart 4). If these effects roughly balance, then the isotope effect may appear to be independent of temperature while in fact fully in accord with semiclassical expectations. Seymour and Klinman have discussed in detail the problem of kinetic complexity in isotope-effect temperature dependences. 

Mass Dependent and Mass Independent Isotope Effects... 

Mass-independent isotopic fractionations are widespread in the earth s atmosphere and have been observed in O3, CO2, N2O, and CO, which are all linked to reactions involving stratospheric ozone (Thiemens 1999). For oxygen, this is a characteristic marker in the atmosphere (see Sect. 3.9). These processes probably also play a role in the atmosphere of Mars and in the pre-solar nebula (Thiemens 1999). Oxygen isotope measurements in meteorites demonstrate that the effect is of significant importance in the formation of the solar system (Clayton et al. 1973a) (Sect. 3.1). 

Mass-Independent Isotope Effects in Atmospheric Compounds... 

Mass independent isotope compositions have been observed in a number of atmospheric molecules such as ozone, CO2, N2O, and CO (Thiemens 1999, 2006) as shown in Fig. 3.35. 

N2O possesses a large variable mass-independent isotope composition, which also requires a mass-independent process - a source, sink, or exchange reaction (Thiemens 1999). 

Thiemens MH (1999) Mass-independent isotope effects in planetary atmospheres and the early solar system. Science 283 341-345... 

Explain how mass-dependent and mass-independent isotopic fractionation of oxygen isotopes interact to produce the range of compositions that we observe in cosmochemical materials. 

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