Behavior of Isotope Mixtures in Electron-Transfer Reactions

2 Behavior of Isotope Mixtures in Electron-Transfer Reactions 

It is clear that deuterium as a substituent has the electron-donating effect. In other words, it can decrease electron affinity of the whole molecule. Potentials of reversible one-electron reduction for naphthalene, anthracene, pyrene, perylene, and their perdeuteriated counterparts indicate that the counterparts exhibit slightly more negative potentials (Goodnow and Kaifer 1990, Morris and Smith 1991). For example, the measurable differences in the reduction potentials are equal to -13 mV for the pair of naphthalene-naphthalene-dj or -12 mV for the pair of anthracene-anthracene-djo. The possible experimental error does not exceed 2 mV (Morris and Smith 1991). In another example, in DMF with 0.1 M n-Bu4NPFg, the deuterated pyrenes were invariably found to be more difficult to reduce than pyrene itself. The largest difference observed, 12.4 mV, was between perdeuteriated pyrene and pyrene bearing no deuterium at all with standard deviations between 0.2 and 0.4 mV (Hammerich et al. 1996). 

For example, a 4.5 1 mixture of CgHg + CgHg forms the 2 1 mixture of ( CgHg) -F ( CgHg) on reduction at 100°C by the half-stoichiometric amount of potassium in THF. Therefore, the heavier anion-radical amount is less than the amount of the heavier parental substrate in the initial mixture. The ESR data were reproduced by means of nuclear magnetic resonance (NMR) and mass-spectral methods after transformation of anion-radical mixtures into mixtures of corresponding neutral compounds and removal of the solvent. 

For the equation (CgHg) K+ + CgDg = CgHg + (CgDg) K+, the ratio of isotopomers before and after the electron transfer can be recalculated into the corresponding equilibrium constants. Table 2.1 gives the order of these constants and experimental conditions typical for their determination. 

Equilibrium Constants (ftgq) of Electron-Transfer Reactions 

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