Abstract Submitted to the    NANOTUBE'04 Conference:

“Metallic” single-walled carbon nanotubes (SWNT’s) are considered prototypes of one-dimensional (1-D) conductors. Recent experimental reports revealed that SWNT’s aren’t true metals: small energy gaps have been measured and the density of electronic states near the Fermi energy at low temperatures was observed to be strongly suppressed. The physical origin of these observations is still under debate. Several mechanisms have been proposed to explain the opening of a band gap, including the nanotube curvature, by which the gap would depend on the tube diameter, intertube and other three-dimensional interactions, and Coulomb correlations. Peierls instability, which states that 1-D systems possess a band gap as a result of the lattice distortion, was discarded based upon symmetry considerations. Here we present conformational and electronic structure analyses, based upon quantum chemical calculations, in which nanotubes are treated as molecular materials. Comparisons are made wit h the structure of polyacetylene, an organic conjugated polymer that conducts electricity upon chemical doping. Results are consistent with a Peierls distortion of armchair (n,n) nanotubes and the opening of a small band gap, in analogy to trans-polyacetylene, while zigzag (n,0) nanotubes do not present a distorted conformation due to an intrinsic band gap, similar to cis-polyacetylene.