A magnetic resonance study of the monoanions of cyclooctatetraene and 1,3,5,7-tetramethyl cyclooctatetraene, produced in single crystals of K₂C₈H₈ · diglyme, Rb₂C₈H₈ · diglyme and K₂(CH₃)₄C₈H₄ · (diglyme)₂

Abstract

Single-crystal E.S.R. studies of the monoanions of cyclooctatetraene (COT) and tetramethyl cyclooctatetraene (TMCOT) produced by X-irradiation of M₂COT·diglyme (Dg) (M=K, Rb) and K₂TMCOT·2Dg, respectively, are reported. The experimental g tensors of these anions are compared with the principal values calculated for the free COT anion. From the hyperfine interactions at room temperature it could be concluded that the COT anion rapidly rotates around its eightfold axis. The spectra at 20 K (where the rotation is slow on the E.S.R. time scale) reveal an alternating spin density of 20 per cent at the eight carbon atoms. An interpretation for this is given in terms of crystal-field interactions and vibronic coupling.

From the temperature behaviour of the E.S.R. spectrum of the TMCOT anions it is shown that the frequency of the methyl group rotation falls below the hyperfine splitting frequencies between 50 K and 20 K. From the analysis of the hyperfine splitting at different temperatures it can be concluded that the splitting between the ground state and the lowest excited state is at least 1200 cm^-1.