EPR and AM1 study of the structure of the radical anion of beta-ionone

The structure of the radical anion of beta-ionone has been investigated by EPR and AM1 molecular modeling methods as a part of a study to determine th e structure and magnetic properties of intermediates of one-electron transf er reactions of carotenoids. Analysis of the temperature dependence of the EPR spectrum of the radical anion shows that the largest hyperfine coupling of 16 G previously assigned to the 7-H proton should be assigned to the 4- H proton in the axial orientation. The previous assignment failed to accoun t for rate of the cyclohexene ring inversion. Comparing the resolved EPR sp ectra at various temperatures indicates that the coupling of 7-H proton equ als approximate to 9.5 G. The structure of the radical anion predicted by A M1 molecular modeling is in agreement with this analysis, From the temperat ure dependence the rate of the cyclohexene ring inversion in the radical an ion at room temperatures is approximately 6 x 10(7) s(-1) with an activatio n energy of about 7 kcal/mol.