An Electron Spin Resonance Study of the 9,10-Dihydrophenanthrene Radical Anion

Abstract

The e.s.r. spectrum of 9,10-dihydrophenanthrene radical ion has been obtained in ether solvents at a number of temperatures between –106 and +51 °C. In dimethoxyethane–tetrahydrofuran mixtures below –80 °C, six triplet splittings are found with mean coupling constants of 5.36, 3.36, 2.66, 0.70, 0.26 and 0.15 oe. Above this temperature broadening occurs with some lines of the hyperfine spectrum due to motion resulting from the interconversion between the two equivalent conformers of the non-planar molecule ion. While the ring proton coupling constants are not strongly dependent on temperature, those due to the four methylene protons are, and interconversion between the conformers broadens the lines so that a 1:4:1 triplet is observed at low temperatures with a coupling of 2.97 oe. This is the sum of the axial and equatorial methylene proton splittings of a particular conformer. Near room temperature the motion is sufficiently rapid to give exchange narrowed lines and the process has an activation energy of 4.5 kcal/mol–1. From these observations it was possible to assign the coupling constants to specific protons in the molecule and to determine the dihedral angle between the rings as 21°.