Kinetic isotope effect usual range

Taken as a whole, the literature on isotope effects in mass spectrometry exhibits two salient features. The values of isotope effects for different molecules and different experimental conditions vary greatly and the isotope effects for some decompositions are very large (>100). These isotope effects are based on ion abundances, as has already been emphasised, but the kinetic isotope effects if measured would show not dissimilar variety and magnitudes. Both features arise because the range of internal energies encountered in reactive ions is very wide and the isotope effects are dependent upon internal energy, usually increasing... 

There exist (4, 5, 8, 9, 27) simple direct relations, between isotope effect, structure, and force field, which do not necessarily require a complete knowledge of all molecular parameters and avoid the solution of the secular equation. These relations are, however, approximations restricted to limited ranges of temperature. [Newer approximation methods, based on expansions in Jacobi polynomials, are applicable over wide ranges of temperatures (6, i6).] In the past, before the ready availability of fast digital computers, tests of the validity of these approximations were usually fairly limited in nature, but recent extensive tests on model calculations of kinetic isotope effects have been carried out 23, 28). In addition, extensive tests of power-series approximations (not considered in the present paper) have now been performed (6,16). 

We might expect that the maximum hydrogen primary kinetic isotope effect for a reaction in which a C-H bond is broken would be h/ d = 7, with smaller values for reactions that are either endothermic or exothermic. Indeed, the usual range for primary kinetic isotope effects is about 5—8. However, a ratio of A h/A d of 25 was seen in one case, and a value of 13,000 was reported in one unusual situation. Obviously, our analysis of the possible magnitude of h/ d has been oversimplified. Two factors were specifically ignored above ... 

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