Talk:Kinetic isotope effect

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.

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.

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)

Equillibrium Isotope Effects

Wikipedia has no entry for equillibrium isotope effects and they are not mentioned on this page. Should a new article be drafted or should an addendum be made to this one?12.217.244.228 (talk) 04:03, 11 March 2008 (UTC)Daniel

• I would opt for inclusion in current page V8rik (talk) 21:12, 11 March 2008 (UTC)

• • I have today added a section at Equilibrium constant#Effect of isotopic substitution, as well as a link from the See also list of this article. Better 9 years late than never! Dirac66 (talk) 02:19, 31 May 2017 (UTC)

KIEs are not necessarily observed only in the RDS

Hi User:Dirac66, I removed the lines stating the necessity of KIE's appearing in the RDS, because depending on how the experiment is done (see Expts B and C, inter and intramolecular competition experiments) the KIE can be observed even if it happens after the RDS. The specialist literature is replete with examples of that. Please see Simmons and Hartwig's recent Angew. Chem. review for the distinction between the three kinds of KIE experiments. https://www.ncbi.nlm.nih.gov/pubmed/22392731. In short, only the first type of experiment where overall rate constants are measured necessarily pertain to the RDS. Alsosaid1987 (talk) 06:54, 8 December 2018 (UTC)

Dead link in sources

While trying to find sources for a report I'm writing I found that source 40 " Kwan E. "CHEM 106 Course Notes - Lecture 14 - Computational Chemistry" (PDF). Retrieved 2 November 2013." no longer links to a usable page.

The link http://isites.harvard.edu/fs/docs/icb.topic1312284.files/Lecture%2014%20-%20Computational%20Chemistry%20II.pdf Leaves me to a page telling me: The iSites platform has been retired.

Is it possible for someone to retrieve a working version of this link or is it lost to time and should be removed?

--Speederzzz (talk) 12:37, 29 June 2021 (UTC)

Double standard

I noticed a double standard in this article. It calls the nitrogen KIE (¹⁴N versus ¹⁵N), "k₁₄/k₁₅", denoting each isotope by its mass number. But it calls the hydrogen KIE (normal hydrogen, ¹H; versus deuterium, ²H), "k_H/k_D", denoting each isotope with a separate symbol. If the notation wer consistent, the hydrogen KIE would be called "k₁/k₂". (Using the separate letter method for nitrogen [or carbon etc. for that matter] is not practical, because there isn't really a separate symbol for different isotopes.) Solomonfromfinland (talk) 17:22, 7 June 2024 (UTC)

Hydrogen is the only element whose isotopes are sometimes given different names: deuterium and tritium. This is because the mass ratios are more different from 1 than for any other element, and therefore the isotope effects are more important than for any other element. See Deuterium#Chemical symbol. Dirac66 (talk) 01:39, 8 June 2024 (UTC)

“Hydrogen is the only element whose isotopes are sometimes given different names”. Not quite true. Helium-2, which is probably an intermediate step in nuclear fusion, is called a diproton, tho this name technically refers to the nucleus alone. Helium-4 may be called an alpha particle; tho this, again, technically refers to the nucleus alone, and may be taken to imply that said nucleus is from a recent alpha decay. See “Chemical symbol#Symbols for named isotopes”. In the early days of radiochemistry, nuclides in decay chains, often got their own names, apparently because people didn’t realize they wer, in many cases, isotopes of the same element. At least one of those isotope-names is still significantly used: thoron, ²²²Rn (see “Isotopes of radon”). In fact, because of this, they decided that tennessine had to hav the chemical symbol Ts rather than the more appropriate Tn.

However, it is obvious that the 2/1 mass ratio of ²H and ¹H is the steepest of any two stable isotopes of an element. Therefor, their KIE should be the biggest of any stable isotopes, so it is understandable that ²H would be called by a dedicated name, deuterium. However, using numbers to denote isotopes of e.g. nitrogen in KIE’s, but separate symbols for isotopes of hydrogen, is still technically a double standard, because you ar using different notations for the same thing: isotopes of the same element. See “Deuterium#Chemical symbol”. It says: IUPAC prefers the symbol “²H” over “D”, to prevent problems with alphabetic sorting of chemical formulas. I strongly agree with IUPAC’s recommendation, for many reasons. And, k₁/k₂, for hydrogen KIE, would obviously be more in line with IUPAC’s recommendation, than k_H/k_D. (IUPAC’s recommendation doesn’t mean you shouldn’t call an isotope by a separate name, it just means that you shouldn’t call it by a separate chemical symbol; and i see no conflict between calling an isotope by a separate name, but not by a separate chemical symbol.)

However, even tho i strongly oppose calling deuterium by the so-called chemical symbol “D”, i am fine with using “D” for deuterium in initialisms; with the understanding that “D” here is an initial, not a chemical symbol. (Equivalent for T = tritium; okay as an initial, but not as a so-called chemical symbol.) DTF = deuterium–tritium fusion. DHR = deuterium/hydrogen ratio (should said initialism be used in this article, or the related article “Hydrogen isotope biogeochemistry”?). After all, we willingly hav initialisms where a letter denotes one element, even tho said letter is properly the chemical symbol of a different element. YBCO = yttrium barium copper oxide; B is properly boron, C is properly carbon. MSG = monosodium glutamate; S is properly sulfur. Solomonfromfinland (talk) 07:40, 31 July 2024 (UTC)

Expression

Could the following expressions be used in this article?:

• StKIE = steric KIE (St, rather than plain S, so you know it isn't secondary KIE)
• To avoid the double standard (see my post above) of e.g. calling the nitrogen KIE "k₁₄/k₁₅" (writing in the mass number; see "Isotopes of nitrogen"), but calling the hydrogen KIE "k_H/k_D" (calling each isotope by a separate symbol); i suggest an alternate notation for hydrogen KIE: "k_1H/k_2H", with the 1 and 2 being mass numbers, of course; mass numbers should be superscript, but a super-/subscript within a super-/subscript, is a hassle to typeset, and in this case is not necessary for legibility. Writing simply "k₁/k₂", might be confusing, because it might be taken as a generic "n₁, n₂"–type expression, where the subscripts simply distinguish two different objects, or denote two different instances of the same quantity.

Solomonfromfinland (talk) 03:03, 4 August 2024 (UTC)