Mass-independent fractionation

A kinetic system involving at least three isotopes of a given element is needed to identify a so-called mass independent isotope effect. Take for example 16O, 17O and 18O. Normally one would expect the rate of reaction of a system involving 17O to be about halfway inbetween the rates of reactions of 16O and 18O. If that is not the case, it is termed a mass-independent effect. Isotopic substitution can effect the symmetry of the system, and it is possible that the nuclear spin of the isotopes will be different. Other than these two effects, the only effect that isotopic substitution could have on reaction rate would be one arising from mass, and since the mass independent effects are observed in some systems where these two causes can be ruled out, some have questioned the wisdom of calling these effects 'mass independent'. Nonetheless the effects are real as evidenced by 'three isotope plots.' The anomalous distribution of 17O and 18O in ozone was discovered in the laboratory by Thiemens, and Mauersberger has documented these distributions in stratospheric ozone. Recently the Nobel Prize-winning chemical theorist Rudy Marcus has described how these distributions arise based on symmetry and changes in zero point energy.