Molecular mechanics conformational analysis of cyclononane using the RIPS method and comparison with quantum‐mechanical calculations

The recently reported Random Incremental Pulse Search (RIPS) technique has been used to probe the conformational energy surface of cyclononane. The stochastic method permits searching of the potential energy surface for all minimum‐energy conformations. The search located all previously reported structures together with three additional conformations that were not found by earlier, primitive searching techniques. Two of these structures are high‐nergy skew forms, and the third is a low‐energy conformer that should contribute significantly to the overall equilibrium set of cyclononane conformations. The global minimum has been found to be the D₃ symmetrical twist chair‐boat (TBC) form in accordance with previous studies. The newly discovered low‐energy structure, which lies only 2.2 kcal/mol above the global minimum, has been designated twist chair‐twist chair (TCTC). The two higher energy conformers are skewed chair‐chair (SCC) and skewed boat‐boat (SBB) forms that are 5.7 kcal/mol and 10.4 kcal/mol above the global minimum, respectively. The seven reported conformations were reanalyzed quantum mechanically (AM1), and a comparison between MM2 and AM1 results is presented.