Isotopic enrichments via altered first and second solution electron affinities

Stevenson, Gerald R.; Espe, Matthew P.; Reiter, Richard C.

doi:10.1021/ja00279a016

Cited by 23 publications

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“…Stevenson and co-workers have found very large, secondary isotope effects on the equilibrium constant for the reduction of aromatic hydrocarbons to their radical anions 1e. This is the equilibrium constant for the reaction shown in eq 1; and K (1) = 3.85 at 173 K corresponds to a 0.47 kcal/mol smaller electron affinity for benzene- d 6 than for benzene.…”

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Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-d₆ and Cyclooctatetraene-d₈

et al. 1997

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B3LYP/6-31+G* density functional calculations have been performed in order to understand why large, equilibrium isotope effects have been observed in the reduction of benzene to the corresponding radical anion but not in the reduction of cyclooctatetraene (COT) to COT•-. The calculations reproduce these experimental trends. However, the calculations also find that reduction of planar COT, like that of benzene, has a substantial, equilibrium isotope effect. Therefore, the small isotope effect, both calculated and observed, for reduction of COT is due to an inverse isotope effect on the planarization of the tub-shaped ring in COT. The isotope effect computed for forming planar (D 8 h ) triplet COT/COT-d 8 at 173 K is 0.38. This is the predicted isotope effect on the adiabatic singlet−triplet splitting in COT. The isotope effect computed for forming planar (D 4 h ) singlet COT/COT-d 8 at the same temperature is 0.41. This is the predicted isotope effect on ring inversion. The reason why planarization of COT has an inverse isotope effect is discussed, and an experimental test of the prediction of an inverse isotope effect on ring inversion of COT is proposed.

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“…Stevenson and co-workers have found very large, secondary isotope effects on the equilibrium constant for the reduction of aromatic hydrocarbons to their radical anions 1e.…”

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confidence: 99%

Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-d₆ and Cyclooctatetraene-d₈

et al. 1997

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“…The possibility that one manifestation of an isotope effect is in the existence of an energy difference for the identical redox transformation between two different isotopically labeled isomers has been suggested by Stevenson and co-workers (1)(2)(3)(4)(5)(6)(7)(8). This is based on their observations that the equilibrium constant (Koq) for the electron selfexchange reaction [1] differs, in some cases dramatically, from unity A-+ *A = A + *A [1] where *A and A are the heavy and light isotopic isomers, respectively, and A-and *A-their corresponding one-electron reduction products.…”

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Direct Voltammetric Determination of the Equilibrium Isotope Effect in Naphthalene‐h8/d8, Anthracene‐h10/d10, and Benzophenone‐12CO/13CO

Morris¹,

Smith²

1991

J. Electrochem. Soc.

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Measurable differences in the reduction potentials (AE) of naphthalene-h8 vs. naphthalene-d8 (-13 + 2 mV) and anthracene-hi0 vs. anthracene-d10 (-12 + 2 mV) have been determined by cyclic voltammetry in 0.1M tetra n-butyl ammonium fiuorborate (TBABF4)-tetrahydrofuran (THF). These results corroborate earlier reports of the existence of a large equilibrium isotope effect for these deuterated polyacene molecules. These hE values correspond to isotope separation factors (a) at room temperature of 1.7 for the naphthalene isomers and 1.6 for the anthracene isomers. A much smaller potential difference is found for benzophenone-~2CO vs. benzophenone-~CO (-2 -+ 2 mV) in 0.1M TBABF4 dimethoxyethane (DME). This result is inconsistent with previous reports of a large isotope effect for this and related isotopically substituted carbonyl-containing moieties. Consideration of ion-pairing effects in these low dielectric solvents visa vis existing theories of equilibrium isotope effects suggests that the conditions employed in the present study should have been the most favorable for enhancement of an isotope effect in solution.The possibility that one manifestation of an isotope effect is in the existence of an energy difference for the identical redox transformation between two different isotopically labeled isomers has been suggested by Stevenson and co-workers (1-8). This is based on their observations that the equilibrium constant (Koq) for the electron selfexchange reaction [1] differs, in some cases dramatically, from unitywhere *A and A are the heavy and light isotopic isomers, respectively, and A-and *A-their corresponding one-electron reduction products. In the work of Stevenson et al., either chemical separation of the reaction mixture followed by mass-spectral analysis or direct electron pragmatic resonance (EPR) or nuclear magnetic resonance (NMR) curve-fitting analysis of the reaction mixture has been used to determine Keq for [1] and thereby the magnitude of the isotope effect. If, in fact, differences in redox potentials are a manifestation of an isotope effect, then a careful electrochemical determination of the reduction potential for a pure sample of each of the isotopic isomers provides the most unambiguous and unequivocal verification of the existence of the isotope effect. In addition, the difference in the reduction potential for the isomers provides the most reliable means of determining the equilibrium constant for reaction [1] and thereby the maximum possible isotope separation factor, a. This is because the results from electrochemical experiments are not dependent on or clouded by potential inefficiencies in subsequent chemical separation processes or difficulties inherent in theoretical simulations of complex EPR/NMR spectral patterns.We have undertaken such electrochemical experiments for several of the isotopically labeled organic molecules studied previously by Stevenson and report these results here. Additional electrochemical work is currently underway in our laboratories to explore the existence...

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“…reaction 1) between organic isotopic isomers1-3 allows the separation P D, of the isotopic isomers of the organic substrates.4-6 Reaction 1 and the analogous reactions for other organic isotopic iso-mers1-6 are shifted from unity due to the fact that the changes in the oxidation states of the organic substrates are coupled to changes in their zero point vibrational energies. 7 We now report that a nonunity equilibrium constant is also associated with an analogous equilibrium involving the change of the oxidation state of the cation instead of the anion, which can be utilized to enrich the isotopes of the metal.…”

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Separation of Isotopes of Sodium during Anion Radical Production

Stevenson¹,

Halvorsen²,

Reidy³

et al. 1992

Anal. Chem.

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The physical separation of 22Na and 23Na Is realized when a mixture of these two Isotopes are used to reduce naphthalene (C10H() to Its anion radical. When (C10H8) dissolved In tetrahydrofuran Is exposed to a sodium metal mirror containing "Na, a competition for electrons exists between the Isotopes of the solvated sodium cation [*M(t) + C10H8*~,M+(tokl) ^M(<) + C10Ht*VM+(Mkl)]. After separation of the anion radical solution from the unoxldlzed metal, It was shown that the radioactive sodium Is concentrated In the anion radical phase (analysis via scintillation counting and atomic absorption).The separation factor (a) describing the room-temperature separation of 22Na from MNa Is 1.30 ± 0.02 (error reported as a standard deviation from five separate experiments). This value falls to 1.19 ± 0.05 when the reduction Is carried outIn the presence of 18-crown-6 and Is within experimental error of unity when the solvent Is dhnethoxyethane.

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Isotopic enrichments via altered first and second solution electron affinities

Cited by 23 publications

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Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-d₆ and Cyclooctatetraene-d₈

Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-d₆ and Cyclooctatetraene-d₈

Direct Voltammetric Determination of the Equilibrium Isotope Effect in Naphthalene‐h8/d8, Anthracene‐h10/d10, and Benzophenone‐12CO/13CO

Separation of Isotopes of Sodium during Anion Radical Production

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Isotopic enrichments via altered first and second solution electron affinities

Cited by 23 publications

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Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-d6 and Cyclooctatetraene-d8

Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-d6 and Cyclooctatetraene-d8

Direct Voltammetric Determination of the Equilibrium Isotope Effect in Naphthalene‐h8/d8, Anthracene‐h10/d10, and Benzophenone‐12CO/13CO

Separation of Isotopes of Sodium during Anion Radical Production

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Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-d₆ and Cyclooctatetraene-d₈

Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-d₆ and Cyclooctatetraene-d₈