Adsorption of hydrogen on neutral and charged fullerene: Experiment and theory

Helium droplets are doped with fullerenes (either C-60 or C-70) and hydrogen (H-2 or D-2) and investigated by high-resolution mass spectrometry. In addition to pure helium and hydrogen cluster ions, hydrogen-fullerene complexes are observed upon electron ionization. The composition of the main ion series is (H-2)(n)HCm+ where m = 60 or 70. Another series of even-numbered ions, (H-2)(n)C-m(+), is slightly weaker in stark contrast to pure hydrogen cluster ions for which the even-numbered series (H-2)(n)(+) is barely detectable. The ion series (H-2)(n)HCm+ and (H-2)(n)C-m(+) exhibit abrupt drops in ion abundance at n = 32 for C-60 and 37 for C-70, indicating formation of an energetically favorable commensurate phase, with each face of the fullerene ion being covered by one adsorbate molecule. However, the first solvation layer is not complete until a total of 49 H-2 are adsorbed on C-60(+); the corresponding value for C-70(+) is 51. Surprisingly, these values do not exhibit a hydrogen-deuterium isotope effect even though the isotope effect for H-2/D-2 adsorbates on graphite exceeds 6%. We also observe doubly charged fullerene-deuterium clusters; they, too, exhibit abrupt drops in ion abundance at n = 32 and 37 for C-60 and C-70, respectively. The findings imply that the charge is localized on the fullerene, stabilizing the system against charge separation. Density functional calculations for C-60-hydrogen complexes with up to five hydrogen atoms provide insight into the experimental findings and the structure of the ions. The binding energy of physisorbed H-2 is 57 meV for H2C60+ and (H-2)(2)C-60(+), and slightly above 70 meV for H2HC60+ and (H-2)(2)HC60+. The lone hydrogen in the odd-numbered complexes is covalently bound atop a carbon atom but a large barrier of 1.69 eV impedes chemisorption of the H-2 molecules. Calculations for neutral and doubly charged complexes are presented as well. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790403]