Abstract: The geometries of Armchair single walled carbon nanotubes with different lengths and diameters, i.e . (3,3)/C₄₂ H₁₂ , (4,4)/C₅₆ H₁₆ , (5,5)/C₇₀ H₂₀ and (6,6)/C₈₄ H₂₄ were optimized by Gaussian 94 programme, and so did their cyclooxide isomers. The results showed that there were four isomers found for shorter and enlarged carbon nanotubes, which were named as Sloppy closed(S-closed), Sloppy-open(S-open), Horizontal closed(H-closed) and Horizontal open(H-open) isomers; and there were only three of them (without H-closed isomer) were found for longer and slimmer carbon nanotubes. After frequency analysis, there were no negative frequencies found for them, which indicated that these isomers were all stable ones. At the same time, we investigated the heat of formation( H_f), ionization potentials, distort energies, electron affinities energies and the energy gap of HOMO-LUMO of these carbon nanotubes and their corresponding cyclooxide isomers, and also discussed the relationship between the stability and these parameters. In addition, we explored sp² rehybridization of carbon atoms and strain force of these nanotubes using POAV programme.

Key words: Armchair single-walled carbon nanotubes, Heat of formation, Ionization potentials, Electron affinity energies, HOMO-LUMO energy gap, POAV analysis