Fourier transform infrared and Raman spectroscopic study of chromatographically isolated LiΓ₆₀ and LiΓ₇₀

IR and Raman investigations have been carried out on purified LiΓ₆₀ and LiΓ₇₀ produced by low-energy ion implantation. The structural interpretations, based on theoretical considerations and comparison of the obtained vibrational spectra with those of different known fullerene oligomers, have shown that neither of the chromatographically isolated endohedral species (LiΓ₆₀₍₇₀₎) correspond to monomeric nonderivatized molecules. We ascribe the two different fractions of LiΓ₆₀ to a double-bonded dimer and a trimer, presumably with the shape of closed triangle. The infrared and Raman spectra of the LiΓ₇₀ compound are similar to those of (C₇₀)₂, and we therefore propose for this species the structure of a double-bonded dimer (LiΓ₇₀)₂. The IR spectra of the LiΓ₆₀ fractions prepared with ⁶Li and ⁷Li isotopes are identical except for the position of broad bands at about 450-600 cm^-1, originating from vibrational-rotational movement of the Li⁺ cation inside the carbon cage.