STRUCTURES AND MOLECULAR-PROPERTIES OF CH ARGE-PERTURBED MOLECULES .54. PYRIDINIUM-TETRAPHENYLCYCLOPENTADIENIDE BETAINE AND THE SINGLE-ELECTRON OXIDATION TO ITS RADICAL-CATION

The single-crystal structure of the dark-blue pyridinium-tetraphenylcy clopentadienide betaine shows the pyridine ring and the phenyl substit uents twisted between 37 and 55 degrees, indicating reduced pi interac tions between cos(2) omega = 0.6 and 0.4. According to AM1 calculation s based on the structure coordinates, the total charges of the pyridin ium cation acceptor and the cyclopentadienyl anion donor amount to +0. 62 and -0.53. This pronounced betaine zwitterionic character causes a 0.5 eV (!) bathochromic shift from 22000 cm(-1) in aqueous to 18000 cm (-1) in n-heptane solution, which can be correlated with the respectiv e E(T) solvent parameters. Cyclo-voltammetry in H2CCl2/0.1 M R(4)N(cir cle plus)ClO(4)(circle minus) yields an irreversible reduction potenti al at -1.5 V and two oxidation potentials at +0.36 and +1.28 V, of whi ch the lower one is reversible. With F3CCOOcircle minus Ag-circle plus , therefore, a radical cation can be generated and is characterized by ESR/ENDOR spectra. The coupling constants are satisfactorily reproduc ed by McLachlan calculations, which also suggest that most of the spin population should be localized within the five-membered ring.