US EPA

1439376 

Journal Article 

Nuclear size and shape effects in chemical reactions. Isotope chemistry of the heavy elements 

Bigeleisen, J 

1996 

Yes 

Journal of the American Chemical Society
ISSN: 0002-7863
EISSN: 1520-5126 

118 

15 

3676-3680 

WOS:A1996UF65700017 

The theory of the isotopic enrichment factor is extended to
include hyperfine splitting and the nuclear field shift. It is shown that hyperfine splitting is
an order of magnitude too small to explain the anomaly in the U-238/U-235 separation in the U
(III)-U(VI) exchange reaction. The ''anomalous mass effect'' in this reaction and the related
U(IV)-U(VI) exchange reactions are shown to be related to the nuclear field shift of the electron
energy levels. Calculations of the effects of these shifts exactly reproduce the odd-even
staggering in the U(IV)-U(VI) exchange reaction and the separation factors for the even-even
nuclei. In the U(IV)-U(VI) exchange reactions the nuclear field effect is three times as large as
the absolute value and of opposite sign to the vibrational energy term. It is the nuclear field
shift which leads to a preference of the U(IV) for the heavy isotope in each of these exchange
reactions. A revision of the reduced partition function ratios of uranium ions in solution, which
takes into account the nuclear field shift, is presented.