Splitting of degenerate orbitals of dibenzyl and tetrabenzyl adducts of C-60: ESR of the radical anions and the rotation barriers of benzyl groups

The degenerate E-2(1u) ground state and the thermally accessible low-lying (2)A(2u), excited state of C-60(.-) give a broad ESR signal (Delta(H)(msl) = 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 temp erature (Delta H-msl = 6.5 G at 133 K). The I-h symmetry of the parent C-60 is lowered by the introduction of two benzyl groups to C-60, to give 1,4-( C6H5CH2)(2)C-60, which leads to the larger g value (2.0004) of 1,4-(C6H5CH2 )(2)C-60(.-) than that of C-60(.-) (g = 1.9984) and a much smaller linewidt h (Delta H-msl = 2.5 G at 213 K) which is independent of temperature. An ev en smaller Delta H-msl value (0.17 G) and a larger g value (2.0011) are obs erved in the tetrabenzyl C-60 adduce radical anion, 1,4,10,24-(C6H5CH2)(4)C -60(.-). This is ascribed to a large splitting of the degenerate t(1u) orbi tals caused by introduction of four benzyl groups to C60 In this case, a hy perfine structure due to two nonequivalent protons of only one benzyl group (aH(1) = 0.31 G, aH(2) = 0.11 G) is observed and this is consistent with t he predicted localized spin density at the C2 position next to the C1 carbo n 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 CI position is slow on the ESR ti me scale. The slow rotation rates of the benzyl groups in 1,4-dibenzyl addu cts of C-60 and the activation parameters were determined from a line broad ening of the methylene protons in the H-1 NMR spectra with increasing tempe rature.