Isotope effects phenomenological

Values of kH olki3. o tend to fall in the range 0.5 to 6. The direction of the effect, whether normal or inverse, can often be accounted for by combining a model of the transition state with vibrational frequencies, although quantitative calculation is not reliable. Because of the difficulty in applying rigorous theory to the solvent isotope effect, a phenomenological approach has been developed. We define <[), to be the ratio of D to H in site 1 of a reactant relative to the ratio of D to H in a solvent site. That is. 

Secondary isotope effects measure transition-state structure 37 Quantum tunneling in enzyme-catalyzed reactions breakthroughs 42 Experimental phenomenology of quantum tunneling in enzyme-catalyzed... 

A different experimental approach to the relative importance of one-center and two-center epimerizations in cyclopropane itself was based on the isomeric l-13C-l,2,3-d3-cyclopropanes165"169. Here each carbon has the same substituents, one hydrogen and one deuterium, and should be equally involved in stereomutation events secondary carbon-13 kinetic isotope effects or diastereotopically distinct secondary deuterium kinetic isotope effects may be safely presumed to be inconsequential. Unlike the isomeric 1,2,3-d3-cyclo-propanes (two isomers, only one phenomenological rate constant, for approach to syn, anti equilibrium), the l-13C-l,2,3-d3-cyclopropanes provide four isomers and two distinct observables since there are two chiral forms as well as two meso structures (Scheme 4). Both chiral isomers were synthesized, and the phenomenological rate constants at 407 °C were found to be k, = (4 l2 + 8, ) = (4.63 0.19)x 10 5s l and ka = (4kl2 + 4, ) = (3.10 0.07) x 10 5 s 1. The ratio of rate constants k, kl2 is thus 1.0 0.2 both one-center and two-center... 

The mechanistic and phenomenological rate constants in equation 2 have the same meanings as those in equation 1 and Z12 are the mechanistic secondary isotope effects for formation of the biradical and correlated double rotation respectively. 

Stern, M. J., Weston, R. E. J. (1974) Phenomenological manifestations of quantum-mechanical tunneling. III. Effect on relative tritium-deuterium kinetic isotope effects, J. Chem. Phys. 60, 2815-2821. 

For isobutyl alcohol, no increase in the rate coefficient is observed at the initial stage of purging and afier return to the initial alcohol-helium flow, the rate coefficient reaches only a half of the previous level (Fig. 6b). The kinetic effects (8a-d,j, k) observed during purging for various butyl alcohols and catalysts, as well as the phenomenology of isotopic exchange mentioned later, are quantitatively describable in terms of the reaction mechanism discussed in the next section. 

Phenomenologically the mass effect of electromigration can be written, in a linear approximation in relative isotopic differences, as (25)... 

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