Abstract
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.
Highlights
Carbon nanotubes (CNTs) are peri-condensed benzenoids, composed of sp2 carbon atoms, which in turn are ordered in a graphite-like, hexagonal pattern
The aim of this work was to compare the aromatic character of armchair and zigzag CNTs with the aromaticity of the corresponding, rectangular graphite sheets
Resonance energy per electron (REPE) was obtained by the Swinborne-Sheldrake equation
Summary
Carbon nanotubes (CNTs) are peri-condensed benzenoids, composed of sp2 carbon atoms, which in turn are ordered in a graphite-like, hexagonal pattern. REPE[2,2] 0.163 0.183 0.192 0.197 0.200 was devised by Gordon and Davison.12 Another algorithm for rectangular graphite sheets and incomplete, rectangular patterns was devised by Lukovits.13 Graph theoretical methods have been used to estimate the resonance energy in graphite.11,14 Enumeration of Kekulé structures in armchair CNTs was first accomplished by Lukovits et al.15 Besides accounting for the aromatic character of benzenoids, enumeration of K may be used to determine the weight of a given bond in a conjugated system by calculating the Pauling index.16
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