The Conformations of Cyclononane Dynamic Nuclear Magnetic Resonance and Force‐Field Calculations

Iterative force‐field calculations for cyclononane are reported for four conformations and five transition states for conformational interconversions. The strain energies (in kcal/mol) relative to the [333] conformation are as follows: energy minima, [333], 0; [144], 1.2; [225], 1.7; [234], 3.2; conformational transition states, [ 1 323], 4.4; [ 1 224], 6.0; [ 1 233], 7.0; [ 1 8], 8.3; [ 1 134], 9.9. The 13 C NMR spectrum of cyclononane shows two dynamic NMR effects at low temperatures and reveals the presence of three different conformations. At – 173.°C, the major conformation (≃ 95%) is [333] and the minor conformation (≃ 5%) is [225]. The third conformation, [144], is present in a very small amount (≃ 1% at – 95°C) and is only barely detectable by its broadening effect near – 95°C. The [225] has an entropy which is 3.5 eu greater than that of the [333], mainly because of the high symmetry of the latter conformation. As a result of the entropy difference, Δ G 0 for the [333] to [225] equilibrium is quite temperature dependent and the equilibrium constant is very strongly temperature dependent. It is estimated that at room temperature there are roughly 40% of the [333], 50% of the [225], and 10% of the [144] in cyclononane.