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Understanding the Mechanisms of Organic, Organometallic, and Biorganic Reactions |
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Understanding the Mechanisms of Organic, Organometallic, and Biorganic Reactions |
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Chemists at Texas A&M University, College Station, have devised a way to employ NMR to simultaneously measure small kinetic isotope effects of all atoms involved in a reaction, using natural abundances of the elements [J. Am. Chem. Soc. 117, 9357 (1995)]. According to the researchers, the technique may make determination of all kinetic isotope effects routine features of investigating reactions. The key step in the method of organic chemistry professor Daniel A. Singleton and graduate student Allen A. Thomas is to isolate the starting materials after the reaction has gone almost to completion. In this way, positive isotope effects show up as great enrichment at each atom position, whereas negative effects show up as great depletion - differences easily seen in NMR. They demonstrated their method with a study of the Diels-Alder reaction of maleic anhydride with isoprene. At 98.9% completion, they distilled off the unreacted isoprene and determined the amount of deuterium and carbon-13 at each position, using the presumably uninvolved methyl group as an internal standard.