Constraints on small graphitic helices

A model for small helical carbon tubules is presented. A heptagon-pentagon defect in a perfect fullerene tubule, which is made up entirely of hexagons formed by nearest-neighbor carbon-carbon bonds, bends the tubule at a 30° angle, which is the angle between the axes of the two halves. The tubule circumferences should roughly match. Pentagons and heptagons should be isolated and separated as far as possible. These effects would favor long-range order along the helix, given a driving force for curling during the formation process.