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The Conformations of Cyclononane Dynamic Nuclear Magnetic Resonance and Force‐Field Calculations
Author(s) -
Anet Frank A. L.,
Krane Jostein
Publication year - 1980
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.198000055
Subject(s) - chemistry , force field (fiction) , conformational entropy , entropy (arrow of time) , nuclear magnetic resonance spectroscopy , crystallography , nuclear magnetic resonance , computational chemistry , thermodynamics , stereochemistry , molecule , physics , organic chemistry , artificial intelligence , computer science
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.