Mass-independent isotope effects

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. 

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). 

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... 

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

Mass Dependent and Mass Independent Isotope Effects... 

Mass-Independent Isotope Effects in Atmospheric Compounds... 

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

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)... 

B.C. Hathorn, R.A. Marcus, An intramolecular theory of the mass-independent isotope effect for... 

The Physical Chemistry of Mass-independent Isotope Effects 161... 

There are several review papers on mass-independent chemical processes and their applications. Thiemens and Weston reviewed the progress in understanding the physical chemistry of gas-phase mass-independent processes and their observation on Earth and meteorites. Thiemens et al. (2001) reviewed the observations of mass-independent isotopic composition in various solid reservoirs of Earth and Mars, including both oxygen and sulfur isotopes. A more recent review (Thiemens, 2002) has summarized the theoretical and laboratory studies of the physical chemistry of mass-independent isotope effects and their observation on Earth and Mars, subsequent to the review of Thiemens et al. (2001). 

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. 

There now exist numerous observations of mass-independent isotopic compositions in nature. Most of these have recently been reviewed and will not be repeated here. When the first laboratory measurements of the mass-independent isotope effect were reported by Thiemens and Heidenreich (1983), their occurrence in nature was not expected, except possibly for the early solar system to produce the observed meteoritic CAI data. It is significant to note that, at present, all oxygen-bearing molecules in the atmosphere (except water) possess mass-independent isotopic compositions. These molecules include O2, O3, CO2, CO, N2O, H2O2, and aerosol nitrate and sulfate. Mass-independent sulfur isotopic compositions are also observed in aerosol (solid) sulfates and nitrates and sulfide and sulfate minerals from the Precambrian, Miocene volcanic sulfates, Antarctica dry valley sulfates, Namibian Gypretes, and Chilean nitrates. In addition, martian (SNC meteorites) carbonates and sulfates possess both mass-independent sulfur and oxygen isotopic compositions. These studies have been reviewed recently (Thiemens et al., 2001 Thiemens, 1999). 

Mojzsis, S.J., Coath, C.D., Greenwood, J.P., McKeegan, K.D., and Harrison, T.M., 2003. Mass-independent isotope effects in Archaean (2.5 to 3.8 Ga) sedimentary sulfides determined by ion microprobe anlysis. Geochim. Cosmochim. Acta, 67, 1635-58. 

There exist numerous recent review articles on the subject of mass-independent isotope effects and details are available in these articles.123 A key point of the culmination of observations is that the most important chemical issues in the environment today, and as expected in the future, may be studied utilizing mass-independent isotopic compositions. In the context of this report, this represents a future chemical frontier area of chemical-environmental research. 

Thiemerts M, Jackson T, Zipf EC, ErdmanPW, VanEgmond C (1995) Carbon dioxide and oxygen isotope anomalies in the mesosphere and stratosphere. Science 270 969-972 Thiemerts MH (1996) Mass-independent isotopic effects in chondrites the role of chemical processes. In RH Hewitts, RH Jones, ERD Scott (eds) Chondmles and the protoplanetary disk. Cambridge Urriversity Press, New York, p 107-118... 

Isotope effects are usually based on the differences between the masses of the atoms and therefore they are referred to sometimes as isotope-mass effects (the so-called mass-independent isotope effects, for example, the magnetic isotope effects, which are due to the differences between the nuclear spins of isotopes, will be discussed in O Sect. 15.8). The geometrical and electronic structures of isotopic molecules are much the same because isotopic nuclei differ only in the number of neutrons, while it is the number of protons that determines the nuclear charge and thus the electron distribution and molecular structure. This fact leads to a great simplification in theoretical calculations of isotope effects. 

See more about mass-independent isotope effects in Sect. 15.8 of Chap. 15, Vol. 2 (p. 2397). 

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