Specific effects of heavy nuclei in chemical equilibrium

The article reports the results of numerical estimations of the nuclear size equilibrium isotope effects for some medium-weight and heavy elements obtained by using the following equation: ln alpha (fs)(i,j) = (kT)(-1) Delta rho (0)z(-1) f (z)(r(2))(ij), where alpha (fs)(i,j) are the equilibrium isotope separation factors corresponding to the nuclear size contribution of the nucleus field-shift for a given pair of isotopes I and j; Delta rho (0) is the difference of the nucleus electron density for a pair of compounds; z(-1)f (z) is the tabulated nuclear charge (z) function characterizing the nucleus of the element being the same for all isotopic nuclei; delta (r(2))(ij) is the isotopic shift of the nucleus effective charge radius in square for the isotopes I and j, and k is the Boltzmann constant. The values, Delta rho (0) were determined from either theoretical or experimentally determined isotope field shifts and from the data of Mossbauer spectroscopy. The (r(2))(ij) values were taken from experimental measurements followed by their standardization. The values of alpha (fs)(i,j) increase from Fe to U and achieve sufficiently high level for lanthanides. For heavier elements, these specific isotope effects begin to exceed the classical chemical equilibrium isotope separation factors.