Borden predicted measurable heavy-atom isotope effects in the ring opening of cyclopropylcarbinyl radical. In my blog post on this paper, I concluded with the line:
Borden hopes that experimentalists will reinvestigate this
problem (and hopefully confirm his predictions).
Well, in a recent paper where Borden collaborates with Singleton, these predictions are confirmed!1
There is a sizable kinetic isotope effect for breaking the ring bond to a 12C over a bond to a 13C atom, up to 16% at -100 °C. The KIE predicted without including tunneling are dramatically below the experimental values, but incorporation of tunneling in the computated KIEs match up with experiment with an error no greater that 0.7%. The Arrhenius plot of ln KIE vs. 1/T shows enhanced isotope effects when tunneling is included, very nice agreement between the experimental and tunneling-corrected KIEs and curvature – all indicative of heavy atom tunneling. Lastly, the ring open product (1-butene) is the observed major product (62%) at -100 °C; the minor product is methylcyclopropane. In the absence of heavy-atom tunneling, 1-butene would be the minor product (28%).
References
(1) Gonzalez-James, O. M.; Zhang, X.; Datta, A.; Hrovat, D. A.; Borden, W. T.; Singleton, D. A. J. Am. Chem. Soc., 2010, 132, 12548-12549, DOI: 10.1021/ja1055593.
InChIs
Cyclopropylcarbonyl radical: InChI=1/C4H7/c1-4-2-3-4/h4H,1-3H2
InChIKey=RMCDUNHIVVEEDD-UHFFFAOYAR
1-butene: InChI=1/C4H8/c1-3-4-2/h3H,1,4H2,2H3
InChIKey=VXNZUUAINFGPBY-UHFFFAOYAZ
Henry Rzepa responded on 25 Oct 2010 at 1:01 am #
i feel this comment is rather tangential, but I hope Steve allows it anyway. In the astrophysics community, this article is raising much interest, particularly in the form of commentaries on blogs. The physicists often seem ahead of the chemical community (in areas such as open publishing), so it comes as no surprise to note that the journal (more accurately an archive) where the article is published supports automated pingbacks from all the blogs commenting on it. It is certainly an interesting extra layer of peer review, and one I urge chemistry journals to certainly investigate, if not adopt.
I am reminded of a meeting a year or so ago between myself and the chair (in the UK, the head) of my department. He asked what visibility I had achieved in the previous year. Well, publish papers in reputable journals I replied. No, he replied, I don’t want to know that, I want to know what the press said about your papers! What is important nowadays is not what you publish, but what other people say (in public) about what you publish. To re-work the Oscar Wilde aphorism, if there is one thing worse than people talking about you behind your back, it’s people not talking about you behind your back. Perhaps pingbacks might make my boss happy?