Electron transfer and ion pair formation .48. Titrations of tetraphenyl-p-benzoquinone with sodium and potassium metals in aprotic solutions

UV/VIS and ESR spectra of electron transfer reaction products in aprotic (c(H circle plus) < 0,1 ppm) solution can be measured in an especially designed and sealed glass apparatus and provide information on unknown facets of the microscopic pathway through the network of interdependent equilibria. For tetraphenyl-p-benzoquinone in tetrahydrofuran, single-electron reduction by a sodium metal mirror produces a red solution and, unexpectedly, after addition of 2.2.2. cryptand, contact with a potassium metal mirror generates a green (!) one. For both, ESR/ENDOR spectra prove the presence of tetraphenyl-p-benzoquinone radical anion. UV/VIS measurements provide the clue: In the equilibrium revealed by repetetive spectra recording, M(solv)(.circle minus) + Me(solv)(circle plus) = [M(.circle minus)... Me(circle minus)](solv), the radical anion is green (v(m) = 16900 cm(-1)]) and the contact ion pair red (v(m) = 18900 cm(-1)). On ion pair formation, therefore, the excitation energy of the radical anion increases by 0.25 eV.