Ab initio calculation of carbon clusters. II. Relative stabilities of fullerene and nonfullerene C24

Abstract

Chemical stabilities of six low-energy isomers of C 24 derived from global-minimum search are investigated. The six isomers include one classical fullerene ͑isomer 1͒ whose cage is composed of only five-and six-membered rings ͑5 / 6-MRs͒, three nonclassical fullerene structures whose cages contain at least one four-membered ring ͑4-MR͒, one plate, and one monocyclic ring. Chemical and electronic properties of the six C 24 isomers are calculated based on a density-functional theory method ͑hybrid PBE1PBE functional and cc-pVTZ basis set͒. The properties include the nucleus-independent chemical shifts ͑NICS͒, singlet-triplet splitting, electron affinity, ionization potential, and gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital ͑HOMO-LUMO͒ gap. The calculation suggests that the neutral isomer 2, a nonclassical fullerene with two 4-MRs, may be more chemically stable than the classical fullerene ͑isomer 1͒. Analyses of molecular orbital NICS show that the incorporations of 4-MRs into the cage considerably reduce paratropic contributions from HOMO, HOMO-1, and HOMO-2, which are mainly responsible for the sign change in NICS from positive for isomer 1 ͑42͒ to negative ͑−19͒ for isomer 2, although C 24 clusters satisfy neither 4N + 2 nor 2͑N +1͒ 2 aromaticity rule. Anion photoelectron spectra of four cage isomers, one plate, one monocyclic ring, and one tadpole isomer, as well as three bicyclic ring isomers are calculated. The simulated photoelectron spectra of monoand bicyclic rings ͑with C 1 symmetry͒ appear to match the measured HOMO-LUMO gap ͑between the first and second band in the experimental spectra͒ ͓S. Yang et al., Chem. Phys. Lett. 144, 431 ͑1988͔͒. Nevertheless, the nonclassical fullerene isomers 3 and 4 apparently also match the measured vertical detachment energy ͑2.90 eV͒ reasonably well. These results suggest possible coexistence of nonclassical fullerene isomers with the mono-and bicyclic ring isomers of C 24 − under the experimental conditions.

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H2formation

“…For coronene, the theoretically calculated energies vary from 7.02 eV for C 24 H 12 to 8.25 eV for C 24 . This last value also agrees with the calculations of An et al (2008) for C 24 . The decrease of the IP with hydrogenation has also been reported for anthracene (C 14 H 10 ) and naphthalene (C 10 H 8 ) first dehydrogenated and first superhydrogenated states, where ∆IP becomes more constant as the PAH gets larger (Kapinus 2004).…”

“…An et al (2008) reports an EA of 1.89 eV for C 24 , which agrees with the ∼0.1 eV variation per dehydrogenation derived from the IPs. Therefore, we also consider the relation given by Eq.…”

From C58to C62and back: Stability, structural similarity, and ring current

“…In fact, as noted earlier, some isomers with one or two squares are favoured over classical cages for C 24 and C 26 [18,19]; by extension it might be expected that a favoured square-containing isomer would be more likely for C 58 than the larger C 62 .…”

“…Previous studies have suggested that energies of carbon cages rise rapidly with the numbers of both sub-hexagonal [18,19] and larger [20] faces.…”

“…Inclusion of square faces can be favourable for small fullerenes [18,19], where it can reduce the number of pentagon-pentagon contacts. However, for C 60 , the cost of inclusion of the square face is evidently not offset by the reduction in N 55 .…”