Talk:Kinetic isotope effect

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This article was externally reviewed on December 14, 2005 by Nature. It was found to have 2 errors.

[edit] Errors in article

Two major errors are present.

1) There is currently no distinction between primary and secondary isotope effects. The kinetic isotope effect is simply the effect on the rate of the reaction of replacing one isotope with another.

A primary isotope effect occurs when bonds to the atoms in question are broken or formed during the rate-determining step of the reaction. A secondary effect is when the bond is broken/formed in a non-rate-determining step. The ratio of the rates of the lighter and heavier isotopes is indicative of whether the effect is primary or secondary. In H/D systems, primary effects have kH/kD (where k represents the rate constant) of 2 or more, while the ratio is 0.7-1.5 for secondary effects. For primary H/D effects, kH/kD values of greater than 20 indicate reliance of quantum mechanical tunnelling. Tunnelling is only a major factor for movement of H and electrons.

2) As implied above in the discussion of secondary H/D effects, the heavier isotope does not necessarily slow the reaction.

This information can be found in Carey & Sundberg's Advanced Organic Chemistry: Part A, 4th ed. Kluwer, 2000. (This book has been reprinted under the Springer imprint since 2004, when Springer purchased Kluwer).

Denise 66.71.92.124 05:33, 16 December 2005 (UTC)

Actually the isotope effect page is rife with error, and completely misses the fundamental experimental rationale for performing an isotopic substitution experiment....namely to elucidate reaction mechanism for the rate limiting event (i.e. free radical, deprotonation, and much, much more). Statements made surrounding (paraphased) 'reactions involving C-D bonds are typically XX slower than those with C-H bonds,' miss the above stated point, and are both fundamentally misleading and (for many reaction mechanisms) incorrect.

[edit] Errors ID'd by Nature, to correct

The results of what exactly Nature suggested should be corrected is out... italicize each bullet point once you make the correction. -- user:zanimum

Reviewer: Daniel Singleton, Associate Professor of Chemistry, Texas A&M University, College Station, USA.

1. When it says "In still other cases, the rate change may be due to subtle differences in the electronegativity of the two isotopes," the best scientists in the area would say that there is no convincing example of this. The statement is controversial at best, and I believe it to be simply wrong. 2. The final part of the discussion regarding relative mass versus absolute mass confuses issues. The discussion ultimately gives the wrong impression that the issue is the use of reduced masses rather than the fact that (1/1)^.5 differs from (1/2)^.5 much more than (1/12)^.5 differs from (1/13)^.5 Still, I find that the discussion is useful for the facts it presents in building its misleading argument.

[edit] What is being asked for

The reviewer notes that it is relative mass change that dominates. The reduced mass point of view is also helpful, but lss clear. The problem is: Can we make a statement about both without again confusing the two issues? -- Pinktulip 18:56, 2 January 2006 (UTC)