Aromaticity of carbon nanotubes.

Carbon nanotubes are composed of cylindrical graphite sheets. Both nanotubes and graphite sheets are benzenoid derivatives composed of sp2 carbon atoms arranged in a hexagonal pattern. Therefore both systems are aromatic. The extent of the aromatic character of a molecule G (here benzenoids) can be explained in terms of the number of possible Kekulé structures in G. In this work the Kekulé structures in carbon nanotubes and the corresponding, rectangular, graphite-sheets the tubes might originate from, were enumerated. It was shown that (2,2), (3,3), and (4,4) carbon nanotubes are more aromatic than the corresponding, rectangular, planar structures. This explains why it might be more difficult to saturate nanotubes by addition reactions than the respective, "narrow", graphite sheets.