CONSTRAINTS ON SMALL GRAPHITIC HELICES

A model for small helical carbon tubules is presented. A heptagon-pent agon defect in a perfect fullerene tubule, which is made up entirely o f hexagons formed by nearest-neighbor carbon-carbon bonds, bends the t ubule at a 30 degrees angle, which is the angle between the axes of th e two halves. The tubule circumferences should roughly match. Pentagon s and heptagons should be isolated and separated as far as possible. T hese effects would favor long-range order along the helix, given a dri ving force for curling during the formation process.