Resolving a three-decade misassignment in hydroxylated C60 chemistry

Water-soluble functionalized fullerenes commonly referred to as fullerenols, fullerols or polyhydroxy fullerenes are widely used in photonics, catalysis, and biomedicine, yet their molecular structures have been assumed to consist solely of hydroxyl groups for nearly three decades. This assumption remains despite persistent mismatches between calculated and experimental vibrational and optical spectra as well as expected and observed chemical reactivity. Here we combine Fourier transform infrared (FTIR) and absorption (UV-Vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), targeted chemical derivatization, and computational quantum chemistry to resolve this discrepancy. We show that only a polyoxy-fullerene architecture incorporating counterion-coordinated carbonyl and hemiketal groups alongside hydroxyls reproduces both the characteristic FTIR features and the experimental UV-Vis absorption profile. A purely hydroxylated fullerene model fails to capture the dominant FTIR band and asymmetric ultraviolet absorption. Oxime-formation experiments chemically validate the presence of carbonyl and hemiketal groups. This structural reassignment resolves long-standing inconsistencies in fullerene chemistry, corrects a pervasive misinterpretation in the literature, and establishes a framework for rationally tuning the optical and chemical properties of functionalized nanocarbons.