Editing Kinetic isotope effect (section)

==== Steric isotope effects ====

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| {{center|<math>\frac{k_{(X=D)}}{k_{(X=H)}} = 1.15</math>}}
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The steric isotope effect (SIE) is a SKIE that does not involve bond breaking or formation. This effect is attributed to the different vibrational amplitudes of [[isotopologue]]s.<ref>{{GoldBookRef |file=S06001 |title=steric isotope effect}}</ref> An example of such an effect is the [[racemization]] of 9,10-dihydro-4,5-dimethylphenanthrene.<ref>{{cite journal
 | vauthors = Mislow K, Graeve R, Gordon AJ, Wahl GH
 |authorlink1=Kurt Mislow
 |year=1963
 |title=A Note on Steric Isotope Effects. Conformational Kinetic Isotope Effects in The Racemization of 9,10-Dihydro-4,5-Dimethylphenanthrene
 |journal=[[Journal of the American Chemical Society]]
 |volume=85 |issue=8 |pages=1199–1200
 |doi=10.1021/ja00891a038
|bibcode=1963JAChS..85.1199M
 }}</ref> The smaller amplitude of vibration for {{sup|2}}H than for {{sup|1}}H in C–{{sup|1}}H, C–{{sup|2}}H bonds, results in a smaller van der Waals radius or effective size in addition to a difference in the ZPE between the two. When there is a greater effective bulk of molecules containing one over the other this may be manifested by a steric effect on the rate constant. For the example above, {{sup|2}}H racemizes faster than {{sup|1}}H resulting in a SIE. A model for the SIE was developed by Bartell.<ref>{{Cite journal| vauthors = Bartell LS |date=1961-09-01|title=The Role of Non-bonded Repulsions in Secondary Isotope Effects. I. Alpha and Beta Substitution Effects.1 |journal=Journal of the American Chemical Society|volume=83|issue=17 |pages=3567–3571 |doi= 10.1021/ja01478a006 |bibcode=1961JAChS..83.3567B }}</ref> A SIE is usually small, unless the transformations passes through a transition state with severe steric encumbrance, as in the racemization process shown above.

Another example of the SIE is in the deslipping reaction of rotaxanes. {{sup|2}}H, due to its smaller effective size, allows easier passage of the stoppers through the macrocycle, resulting in faster deslipping for the deuterated [[rotaxane]]s.<ref>{{cite journal | vauthors = Felder T, Schalley CA | title = Secondary isotope effects on the deslipping reaction of rotaxanes: high-precision measurement of steric size | journal = Angewandte Chemie | volume = 42 | issue = 20 | pages = 2258–60 | date = May 2003 | pmid = 12772156 | doi = 10.1002/anie.200350903 }}</ref>

[[File:chemicalrotaxane.svg]]