An electron spin resonance study of ring inversion in the radical anions of peri-bridged naphthalenes and singly bridged biphenyls

The radical anions of 2,3-dihydrophenalene, 1H,3H-naphtho[1,8-cd]pyran and 6,7-dihydro-5H-dibenzo[a,c]-cyoloheptene were generated by alkali metal reduction and their e.s.r. spectra recorded at several temperatures. Symmetric line width broadening due to the modulation of the ?-axial and equatorial proton splittings were observed. By analysing the line shape variations with temperature, the activation parameters for the inversion of the aliphatic ring could be determined. A comparison of the experimental activation energies for the radical anions with those estimated by force field calculations on the parent compounds shows that the addition of an unpaired electron to the ?-system gives rise to a large change in the inversion barrier in the bridged biphenyl, while no variation seems to occur in the bridged naphthalene. This behaviour is rationalized in terms of the ?-resonance energies of the open and closed shell systems.