Splitting of degenerate orbitals of dibenzyl and tetrabenzyl adducts of C₆₀: ESR of the radical anions and the rotation barriers of benzyl groups

The degenerate ²E_1u ground state and the thermally accessible low-lying ²A_2u excited state of C₆₀^̇- give a broad ESR signal (δH_ms1 = 30.9 G at 213 K) which has a smaller g value (1.9984) as compared to the free spin value (2.0023) and linewidth which decreases with decreasing temperature (δH_ms1 = 6.5 G at 133 K). The I_h, symmetry of the parent C₆₀ is lowered by the introduction of two benzyl groups to C₆₀ to give 1,4(C₆H₅CH₂)₂C₆₀, which leads to the larger g value (2.0004) of 1,4-(C₆H₅CH₂)₂C₆₀ ^̇- than that of C₆₀̇ (g = 1.9984) and a much smaller linewidth (δH_ms1 = 2.5 G at 213 K) which is independent of temperature. An even smaller δH_ms1 value (0.17 G) and a larger g value (2.0011) are observed in the tetrabenzyl C₆₀ adduct radical anion, l,4,10,24-(C₆H₅CH₂)₄C ₆₀^̇- This is ascribed to a large splitting of the degenerate t_1u orbitals caused by introduction of four benzyl groups to C₆₀-̇ In this case, a hyperfine structure due to two nonequivalent protons of only one benzyl group (aH₁ = 0.31 G, aH₂ = 0.11 G) is observed and this is consistent with the predicted localized spin density at the C2 position next to the C1 carbon to which a benzyl group is attached. The observation of two nonequivalent hyperfine coupling constants due to the benzyl methylene protons indicates that rotation of the benzyl group at the C1 position is slow on the ESR time scale. The slow rotation rates of the benzyl groups in 1,4-dibenzyl adducts of Ceo and the activation parameters were determined from a line broadening of the methylene protons in the ¹H NMR spectra with increasing temperature.