Gas-phase Formation of Cationic Fullerene/Amino Acid Clusters: Evidence for the "Magic Number" Chemical Reactivity of Fullerene Cations

An experimental and theoretical investigation on the chemical reactivity of fullerene cations (C- n (+), n = [36, 60]) with amino acid molecules (e.g., isoleucine, C6H13NO2) is performed. The results show that, in the gas phase, fullerene cations can react with amino acid molecules to form fullerene/amino acid cluster cations with high efficiency. The formation rate constants for the ion-molecule collision reactions between fullerene cations and isoleucine are estimated under the pseudo-first-order reaction condition. We find the formation rate constants increase gradually with a declining C-atom number of fullerene cations (C- n (+), n = [46, 60]), and depict a plateau in the reactivity for smaller fullerene cations (C- n (+), n = [36, 44]). More importantly, by comparing to its neighbor fullerenes, the magic number of C-atom counts (54, 58) that process with enhanced chemical reactivity are determined. We also obtained the molecular structures and binding energies for the fullerene-monoisoleucine adducts by quantum chemical calculations, which give a good explanation for the magic number chemical reactivity and the enhanced chemical reactivity of smaller fullerene cations. We infer that our results demonstrate the importance of ion-molecule reactions to the formation of large and complex fullerene-amino acid derivatives in the ISM. The high reactivity of fullerene species may indicate that amino acid molecules or other related prebiotic compounds can accrete on small interstellar carbon dust grains.