Molecular structures determined by intramolecular attractive steric interactions. Dynamic NMR and molecular mechanics investigation of 1,6-dimethylcyclo-octatetraene

Abstract

1,6-Dimethylcyclo-octatetraene equilibrates with 1,4-dimethylcyclo-octatetraene by a bond-shift process which is slow on the NMR timescale at ambient temperature. The greater stability of the former valence isomer (ΔG₀= 0.081 kcal mol^–1 at 20 °C in perdeuteriobenzene solution) is attributed to attractive steric interaction between methyl groups. NMR spectra of the equilibrium mixture are considered in detail with a view to characterising other 1,6 ⇌ 1,4-disubstituted cyclooctatetraene equilibria. Diagnostically useful differences in spectra are demonstrated, while some apparently clear differences are due to second-order effects. Molecular mechanics calculations agree with experimental results as to the preferred valence isomer and confirm that it has greater methyl–methyl attractive steric interactions. There are three slightly different conformations present in similar relative amounts for each valence isomer due to methyl rotation, and NMR results do not disagree with this suggestion.