Reaction Coordinate Diagrams and Isotope Effects

Various images of three- and two-dimensional reaction coordinate diagrams. 

Diagram showing that a residual ZPE difference between C-H and C-D bonds at the transition state diminishes the isotope effect. 

The Use of Primary Kinetic Isotope Effects to Probe the Mechanism of Aliphatic Hydroxylation by Iron(III) Porphyrins One reaction that is uncommon in organic chemistry but is common in biological systems in the hydroxylation of alkanes to form alcohols. Cytochrome P-450, a heme containing enzyme, catalyzes this reaction. Large primary isotope effects are found, as well as an absence of carbocation-like skeletal rearrangements and loss of stereochemistry. These observations led researchers to conclude that a radical cage mechanism is operative. 

A radical cage mechanism is one in which radicals are created and react together before diffusing apart. 

Several models of this heme enzyme are based upon substituted porphyrins. For example, compound i (oxidized Fe in porphyrin ligand shown) catalyzes the oxidation of norbornane to norboran-2-ol with iodosobenzene 

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