Nuclear Volume Effects on Chemical Reactions

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I consider the nuclear volume effect, a new cause of isotope fractionation, from relativistic molecular orbital theory. Four-component Dirac-Coulomb Hartree-Fock method with Gaussian type nucleus model well reproduces the experimental isotope fractionation factor of uranium isotope exchange reaction. Hence, we could theoretically confirm that nuclear volume difference of isotope really cause the isotope fractionation as the scale of the experiment. I also discuss the relationship of ligand coordination and nuclear volume effect. Since the stronger ligand locates closer to the uranium, it provides larger nuclear volume effect. In atomic isotope spectra, it is known that the field shift(nuclear volume effect in atomic spectra) is proportional to the electron charge density on nucleus. However, this is not clear for molecular system and isotope fractionation factor. Hence I analyze the electron density at the nucleus for all molecules calculated and found that the density and isotopic energy difference show highly linearity. It means the analysis of electron density at the nucleus is also possible for the discussion of isotope fractionation caused by nuclear volume effect.