Deshielding γ–gauche effects in 13C magnetic resonance: a comparison of the conformational behaviour of the acetyl group in cyclohexane and 5-substituted-1,3-dioxane systems

Low temperature (173 K) 13C spectra of acetylcyclohexane, 1-acetyl-1-methylcyclohexane, and 5-acetyl-5-methyl-1,3-dioxane are reported. Data permit the first direct measurement of −ΔG0 for the acetyl group bonded to cyclohexane. The value obtained, 1.02 ± 0.05 kcal/mol in CS2–CD2Cl2 solution, is considerably lower than that determined by equilibration methods. Factors contributing to observed axial–equatorial carbonyl carbon chemical shift differences for substituted cyclohexanes and 1,3-dioxanes are discussed.