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 aprot ic (c(H circle plus) < 0,1 ppm) solution can be measured in an especia lly designed and sealed glass apparatus and provide information on unk nown facets of the microscopic pathway through the network of interdep endent equilibria. For tetraphenyl-p-benzoquinone in tetrahydrofuran, single-electron reduction by a sodium metal mirror produces a red solu tion and, unexpectedly, after addition of 2.2.2. cryptand, contact wit h a potassium metal mirror generates a green (!) one. For both, ESR/EN DOR spectra prove the presence of tetraphenyl-p-benzoquinone radical a nion. UV/VIS measurements provide the clue: In the equilibrium reveale d by repetetive spectra recording, M(solv)(.circle minus) + Me(solv)(c ircle plus) = [M(.circle minus)... Me(circle minus)](solv), the radica l 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.