The experimental determination of the conformational free energy, enthalpy, and entropy differences for alkyl groups in alkylcyclohexanes by low temperature carbon-13 magnetic resonance spectroscopy

13 C N.m.r. studies at low temperatures of solutions in CFCl3–CDCl3 of [Me-13C]methylcyclohexane, [α-13C]-isopropylcyclohexane, cis-1-ethyl-4-methylcyclohexane, cis-1-isopropyl-4-methylcyclohexane, and cis-1-ethyl-4-isopropylcyclohexane have yielded conformational enthalpy differences and conformational entropy differences for methyl, ethyl, and isopropyl groups in alkylcyclohexanes. For –ΔH°, the averaged values obtained, in kcal mol–1, were 1.75 (Me), 1.60 (Et), and 1.52 (Pri); for ΔS°, the averaged values, in cal mol–1 K–1, were –0.03 (Me), 0.64 (Et), and 2.31 (Pri). The determined values are in good agreement with those calculated by the Allinger group for the gas phase, using a method based on classical mechanics. As a result of the opposing trends for –ΔH° and ΔS° values, the sequence of –ΔG° values at 300 K, namely Pri > Et > Me, is reversed at temperatures below ca. 40 K.