Computations of model narrow nanotubes closed by fragments of smaller fullerenes and quasi-fullerenes

New type of carbon nanotubes - narrow nanotubes - has recently been observed with diameters of 4-5Å. It has been postulated that the narrow nanotubes are closed by fullerene fragments of C₂₀ and C₃₆. This paper presents computational results on related model nanotubes with stoichiometries such as C₈₀, C₈₄, C₉₆, C₁₀₈, or C₁₂₀. The computations were carried out at the PM3, AM1, SAM1, HF/3-21G, HF/4-31G, and B3LYP/6-31G* levels. Two C₃₆ fullerenes were considered, D_6h and D_2d. At the PM3 level and with the C₈₄ nanotube stoichiometry, the D_2d cage closure gave a lower energy (by 185kcal/mol and a diameter of 5.42Å). There is another possible candidate, a C₃₂ cage with D_4d symmetry (two four-membered rings). At the PM3 level and with the C₉₆ nanotube stoichiometry, the D_4d closure (with a diameter of 5.43Å) had energy lower by 210kcal/mol than that of the D_6h nanotube closure. On the other hand, four-membered rings should not play a significant role for narrow nanotubes with a diameter of 4Å, where the dodecahedron-related closure should be exclusive. Still narrower nanotubes are briefly discussed.