ESR and ENDOR studies on the 2,2′-binaphthyl radical anion in solution

Abstract

ESR and ENDOR studies of the 2,2′-binaphthyl radical anion in tetrahydrofuran in the temperature range −50 to −90°C showed the presence of only one paramagnetic species with no evidence of ion pairing or of hindered rotation such as occurs in the phenyl naphthalenes. Extended Hückel molecular orbital calculations of energy as a function of the dihedral angle between the two naphthyl parts suggest that the species has four stable conformations with angles of 45, 140, 220, and 315° and jumps rapidly among them at a rate in the range of 10 7 to 10 9 sec −1. The proton coupling constants calculated by the McLachlan method, together with the experimental values, provided an independent estimate of the stable angles which agreed closely with the values derived from the energies. As in other cases involving the nonplanar bonding of aromatic rings these calculations are only partially successful in predicting the coupling constants. Neither inversion or near degeneracy of the orbitals nor breakdown of σ−π separability seems able to account for the discrepancies. The g value of the radical was measured. It is slightly larger than the value predicted by Stone's theory in contrast to a number of non-planar radicals which have smaller values.