Nuclear field shift effects

Eujii T, Suzuki D, Gunjii K, Watanabe K, Moriyama H, Nishizawa K (2002) Nuclear field shift effect in the isotope exchange reaction of chromium(III) using a crown ether. J Phys Chem A 106 6911-6914 Gale JD (2001) Simulating the crystal structures and properties of ionic materials from interatomic potentials. Rev Mineral Geochem 42 37-62... 

Nuclear volume effects, sometimes referred to as nuclear field shift effects, are believed to be one cause of mass-independent isotope fractionation [46]. Nuclei of isotopes differ from one another only in their number of neutrons. Self-evidently, this provides the isotopes with a different mass, but this may also give rise to differences in the size and shape of the nuclei among the isotopes. The nuclei of nuclides with an odd number of neutrons are often smaller than they should be based on the mass difference relative to those of the neighboring nuclides with an even number of neutrons [47]. These differences in nuclear shape and size, and thus charge density, affect the interaction between the nucleus and the surrounding electron cloud. The resulting difference between the isotopes in terms of density and shape of the electron cloud results in slight differences in the efficiency with which they participate in chemical reactions [48]. 

Fujii, T., Moynier, F., and AlbarMe, F. (2009) The nuclear field shift effect in chemical exchange reactions. Chem. Geol, 267, 139-158. 

A small amount of theoretical work has been published on the fractionation of uranium isotopes. As mentioned above, Schauble [64] demonstrated that equilibrium isotope fractionation between species of the heaviest elements is not dominated by differences in bond vibrational frequencies, as they are for lighter elements, but by the nuclear field shift effect. This effect is due to interactions between electron shehs, espedahy s shells, that have high electron density near the very large nuclei of heavy atoms. The heavier isotopes partition into those species with fewer s electrons or in which s electrons are shielded by more p, d, or f electrons. Schauble [64] presented calculations for various ojddation states and species of T1 and Hg and the same general conclusions apply to U. Calculations for uranium species were presented at a conference by Schauble ]73]. The largest fractionations are predicted to occur when U(IV) and U(VI) species equilibrate, with values of au(iv) u(vi) as large as 0.0012 at 273 K [Au(i iu(vi) l-2%o at 0°C]. U(IV) has two 5f electrons that apparently shield s electrons from the isotopically... 

More recently, Epov et al. [65] reviewed what is known about mechanisms of fractionation not due to mass-dependent differences in bond vibrational frequencies. These mechanisms include the nuclear field shift effect, but also the nuclear spin effect, which results from interaction between the magnetic field associated with a nucleus with nonzero spin (such as 2 U) and the magnetic fields associated with electron spin angular momentum. They pointed out that so far, no data unambiguously reflect this effect, but it is possible that fractionation of U in the contexts described here results from the nuclear spin effect, rather than the nuclear field shift effect. 

For molecules containing light atoms, we accordingly neglect this effect of finite nuclear volume or field shift, but other effects prevent exact application of isotopic ratios that one might expect on the basis of a proportionality with in formula 13 instead of total F. For this reason we supplement term coefficients in formula 8 for a particular isotopic species i with auxiliary coefficients [54],... 

Chemical shift - A small change in the energy levels (and hence in the spectra associated with these levels) resulting from the effects of chemical binding in a molecule. The term is used in fields such as NMR, Mossbauer, and photoelectron spectroscopy, where the energy levels are determined primarily by nuclear or atomic effects. 

In addition to the mass variation by isotopic substitution, the nuclear size will vary slightly giving rise to small changes in the Coulomb interaction between the electrons and the nucleus. This isotope effect which is called field shift in the theory of atomic spectra [78Hei] can be traced back to a similar form of... 

Nuclear Magnetic Resonance. A definitive review of interproton allylic spin-spin coupling has been made which contains much of relevance to alicyclic systems. Study of the n.m.r. spectra of 36 axially and equatorially substituted cyclo-hexanols has identified some unexpected chemical shift effects. Substituent-dependent additive shielding increments were used successfully to calculate the chemical shift of methine H atoms present, so confirming the observation of some axial H atom shifts at lower fields than the equatorial H atom resonances, i.e. an inversion of the accepted behaviour. 

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