Conformations of large cycloalkanes: cyclooctadecane, cyclononadecane and cycloicosane.

The conformations of cyclooctadecane, cyclononadecane, and cycloicosane were generated by a stochastic program that works in conjunction with MM2. The shapes of the rings are analyzed in terms of previous work by Dale and others, and in terms of distributions of energies, torsion angle distributions, and torsion angle sequences. A new shape element called the 'nick' has been discovered, and it seems to be increasingly important with 18-membered and larger rings. Previously suggested relationships between ring size and energy distribution were observed, and a geometrical explanation is provided for the relative distributions of stable conformations in 16-, 18-, and 20-membered rings.