General Predictions on Kinetic Isotope Effects

General Predictions on Kinetic Isotope Effects Equation 11.20 and simplified formulations like Eq. 11.21 permit us to make some general predictions about isotope effects. Since / has been defined so that the subscripts 1 and 2 refer to the light and heavy isotopes, respectively, v2L. Moreover 

If the isotopic reactants are atoms, then / — /B = 1 and the condition becomes 

The latter requirement is easily satisfied by reactions involving hydrogen atoms. 

Estimates of maximum regular and inverse isotope effects may be made quite simply. For simplicity of notation the classical symmetry number factors will be omitted. In order to arrive at maximal values of regular isotope effects, we assume no isotope effect in the activated complex, i.e., /+ = 1. This assumption is equivalent to no isotopic binding in the activated complex. The maximum value of jv2L is (m2/m1) /l, where the m s are the masses of the isotopic atoms. Values of / for various isotopic species have been tabulated in the literature.49 Using the values of / for strongly bonded species, one may then estimate maximum values of isotope effects. These are tabulated in Table I which is 

TABLE I. Estimated Maximum Ratios in Specific Rate Constants 

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