Science
Photolysis produces isotope enrichment Nuclei with magnetic moments catalyze radical electron spin reversals and influence choice of reaction paths on a selective basis Chemists at Columbia University have found a technique to concentrate isotopes photochemically that may lead to general methods for separating isotopes of many elements. Specifically, Dr. Nicholas J. Turro and Dr. Bernhard Kraeutler, with partial photolysis of dibenzyl ketone in surfactant solution, have produced residual ketone whose content of carbon-13 at the carbonyl carbon is up to double the original amount [J. Am. Chem. Soc, 100, 7432 (1978)]. The investigators explain their observations on the basis of the magnetic mo-
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ment of carbon-13 and confinement of free radical pairs within surfactant micelles. Photolysis of dibenzyl ketone produces phenylacetyl and benzyl radicals. The newly generated free radical pair is in the triplet state—electron spins are the same. Ordinarily, the radical pair cannot recombine because of incompatibility of spin-paired electrons. In that case, the phenylacetyl radical loses carbon monoxide to form another benzyl radical, the two benzyl radicals combine, and the product is 1,2-diphenylethane. If the carbonyl carbon of the phenylacetyl radical is carbon-13, however, the free electron is strongly influenced by the magnetic moment of this isotope. The natural abundance of carbon-13 in all carbon is 1.11%. In such a case, interaction with the carbon-13 nucleus may reverse the spin of the electron before the phenylacetyl and benzyl radicals have drifted too far apart or before the phenylacetyl radical has lost carbon monoxide. With one electron spin reversed, the
radical pair is in the singlet state—the electron spins are opposite. The phenylacetyl and benzyl radicals thus are rendered compatible for recombination to dibenzyl ketone. Carbon-12, which has no magnetic moment, cannot interact in the same way with the spin state of an electron. Hence, there is a tendency for residual dibenzyl ketone in the reaction mixture to become enriched in carbon-13. Indeed, in benzene solution, the Columbia team finds enrichments of up to 5%. The more dramatic enrichments are produced when the ketone is photolyzed while trapped within micelles in trimethylcetylammonium chloride surfactant solutions. Micelles are spherical structures formed as surfactant molecules orient themselves in water with the nonpolar cetyl chains directed inward away from the watery environment and the cationic trimethylammonium groups facing outward toward the solvent. Turro and Kraeutler say that when dibenzyl ketone is photolyzed within the
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Nov. 13, 1978 C&EN 23
Columbia's Kraeutler (left) and Turro with apparatus used to determine the effect of a superimposed magnetic field on the magnetic effects of carbon-13 nuclei
micelles, the radicals may rebound several times from inner micelle walls, providing increased time for interaction of the phenylacetyl electron with the carbonyl carbon-13, if present. Photolysis of 0.05M dibenzyl ketone in 0.005M trimethylcetylammonium chloride for five hours produced 92% conversion to diphenylethane. Nuclear magnetic resonance and mass spectrometric analysis showed that carbon-13 content at the carbonyl carbon of residual dibenzyl ketone had increased from 1.11% to 2.50%, a 125% enrichment. Similarly, photolysis of dibenzyl ketone, with the carbonyl carbon already enriched to 47.6% carbon-13, produced enrichment of residual ketone to 63.7%, a 34% increase. The Columbia chemists say this method of isotope enrichment may be extended to any isotope with a magnetic moment. Strategies would involve photochemical or thermal reactions that proceed along one pathway in the triplet state and by another route in the singlet state. Atoms with magnetic moments would tend to shift the reaction along one path, whereas those without magnetic moments would be guided preferentially to alternative products. One such useful separation would be oxygen-17, which has a magnetic moment, from oxygen-16 and -18, which have none. Natural abundance of oxygen-17 is only
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0.0373%. Similarly, uranium-235, which has a magnetic moment, is useful in nuclear reactors but is present in only 0.715% natural abundance. Uranium-235 has no magnetic moment and comprises 99.28% of natural uranium. In addition to commercial isotope separations, the technique may be applied to exploring mechanisms of reactions or the nature of micelle structure, Turro and Kraeutler say. Use of compounds enriched at one molecular site with a magnetic isotope may provide information about whether reactions proceed by free radicals closely associated with one another in space and/or capable of interconversion between singlet and triplet states. The chemists' results also indicate that application of external magnetic fields may be capable of influencing outcomes of reactions that proceed along two alternative pathways. For example, when photolysis was carried out in a 15,000gauss magnetic field, carbonyl carbon-13 content of dibenzyl ketone already enriched to 47.6% was increased only to 54.3%, compared with an increase to 63.7% with no field applied. This experiment indicates that the magnetic field influenced photolysis to proceed to a greater extent by formation of diphenylethane than by recombination of radicals to dibenzyl ketone. •
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