Temperature dependence of the isotope chemistry of the heavy elements.

Abstract

The temperature coefficient of equilibrium isotope fractionation in the heavy elements is shown to be larger at high temperatures than that expected from the well-studied vibrational isotope effects. The difference in the isotopic behavior of the heavy elements as compared with the light elements is due to the large nuclear isotope field shifts in the heavy elements. The field shifts introduce new mechanisms for maxima, minima, crossovers, and large massindependent isotope effects in the isotope chemistry of the heavy elements. The generalizations are illustrated by the temperature dependence of the isotopic fractionation in the redox reaction between U(VI) and U(IV) ions.Stern and colleagues (1, 2) have studied the temperature dependence of equilibrium isotope fractionation for the elements hydrogen, carbon, nitrogen, and oxygen over the temperature range 20-2000 K; they used a computational approach to the problem. The vibrational frequencies of isotopomers of a large number of compounds of the above elements were calculated using the Wilson FG matrix method. Input parameters were valence force constants, F matrices, and the masses, bond distances, and bond angles of the molecules, G matrices. The calculated frequencies were then substituted into the Bigeleisen-Mayer equation (3) for the isotope fractionation factor. In accord with theoretical expectations, Stern and colleagues (1, 2) found, in all cases, that the logarithm of the fractionation factor obeys a 1/T law at low temperature and a 11T2 law at high temperature. The chemical basis for these two limiting temperature dependences can be readily understood through the Bigeleisen-Mayer free energy function.

Research potential and limitations of trace analyses of cremated remains

“…Isotopic fractionation effects which occur under natural, physiological conditions (thermodynamic or enzyme related) cease to occur at higher temperatures, since heat provides the entire system with so much energy that breakage of intramolecular bonds, regardless of whether a light or a heavy isotope is involved, proceeds easily. A plot of ln a (fractionation factor) versus 1/T (temperature) will have a slope of zero at high temperatures [65]. As long as the heat exposure takes place at oxidizing conditions as conducted in our experiment, cremation leads to the formation of oxidized bone components (cf.…”

Uranium isotopes fingerprint biotic reduction

“…Indeed, for very heavy elements, such as U, isotope fractionation is dominated by a mass-independent fractionation mechanism. The nuclei of U isotopes differ in size and shape, and due to their large size, electron orbital energies are altered, leading to a phenomenon known as the nuclear field shift (19,25). During reduction reactions, the added valence electrons are impacted by the nuclear field shift, resulting in varying energies of reaction for distinct isotopes.…”

Natural fractionation of 238U/235U

“…Although relative mass differences are rather small, surprisingly large (per-mil level) isotope variations, are observed even for very heavy elements, such as Hg and Tl (e.g., Rehkämper et al, 2002;Bergquist and Blum, 2007). Significant equilibrium isotope fractionation for these elements is possible because of the ''nuclear field shift'' effect (Bigeleisen, 1996;Schauble, 2007), a fractionation mechanism related to nuclear volume rather than to mass that operates independently of traditional, vibrationally-derived, mass-dependent fractionation. According to the modeling of Schauble (2006), this volume effect should also produce significant U isotope fractionation on the ‰ level at typical environmental temperatures (%25°C).…”