Steering, orbital

Orbitals, atomic, see Atomic orbitals Orbitals, molecular, see Molecular orbitals Orbital steering mechanism, 220-221 Oxyanion intermediates, 172,181,185,210 Oxyanion hole, 181... 

The Thorpe-Ingold effect 208 Effects on the formation of larger rings 216 The relief of ground-state strain 217 Orbital steering 222... 

Attempts have been made to account for the rate enhancements in intramolecular catalysis on the basis of an effective concentration of 55 M combined with the requirement of very precise alignment of the electronic orbitals of the reacting atoms orbital steering. Although this treatment does have the merit of emphasizing the importance of correct orientation in the enzyme-substrate complex, it overestimates this importance, because, as we now know, the value of 55 M is an extreme underestimate of the contribution of translational entropy to effective concentration. The consensus is that although there are requirements for the satisfactory overlap of orbitals in the transition state, these amount to an accuracy of only 10° or so.23-24 The distortion of even a fully formed carbon-carbon bond... 

The Thorpe-Ingold or gem-Dialky l Effect—Stereopopulation Control or Orbital Steering... 

Storm, D.R. and Koshland, D. E. (1970) A source for the special catalytic power of enzymes orbital steering, Proc. Natl. Acad. Sci. USA 66, 445-452. 

Stereospecific addition of CN at a chelated imine - orbital steering... 

The enzyme binds the substrate(s) (and cofactor) in such a way that the susceptible bonds are (a) in close proximity to the catalytic group on the active side (called the entropy effect) and (b) so oriented in relation to the catalytic group that the transition state is readily formed (orbital steering). 

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