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

Abstract

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.