Contribution of nuclear size and shape to chemical enrichment of iron isotopes

The isotopic enrichment process of iron isotopes by a chemical exchange reaction of liquid-liquid extraction using a crown ether, dicyclohexano-18-crown-6 was investigated. An enrichment factor of Fe-54 relative to Fe-56 was epsilon(54,56) =0.000 +/- 0.003, while that of Fe-58 to Fe-56 was epsilon(58,56) = -0.032 +/- 0.004, instead of their similar mass differences. This showed the breakdown of the Bigeleisen-Mayer approximation which states the isotope effect is proportional to the mass difference in the chemical exchange reaction. The difference of nuclear size and shape, namely the field shift in atomic spectrum was found to dominate the enrichment factors of the iron isotopes. Contribution of the nuclear size and shape was -0.199 to the enrichment factor observed for Fe-57 to Fe-56, epsilon(57,56) = -0.0055 +/- 0.0004, and that of the nuclear spin was 0.049.