The first trial to explore the orig

The first trial to explore the origin of the isotope effects in chemical exchange reactions was carried out by Clewett and Schaap [16], who suggested that the isotope effects in a chemical exchange reaction are due to a slight difference in the affinity of the isotopes for a given molecule or complex due to minor variances in the internal energies, mainly vibrational energy, of the molecule. Based on the quantum molecular vibration energy, Bigeleisen formulated the method to calculate the isotope exchange equilibrium constant from spectroscopic data [17]. This method was used to calculate the equilibrium constant of the isotopic exchange of many elements ranging from hydrogen to uranium. Unfortunately, this method could not explain the anomalous isotope effects of the odd isotopes 233U [15] and 235U [14] among the other uranium even isotopes. This anomaly was found to be similar to the odd–even staggering of the isotope shift in the atomic spectra. According to the new theory derived by Bigeleisen, this anomaly is believed to be due to the field shift [18]. Later on, similar odd–even isotope effects were found in Gd [2], Zn [3], [4], [19], [20] and [21], Nd [9], [10] and [11] and Cd [22].