The Experimental Determination of the Conformational Free Energy, Enthalpy, and Enthropy Differences for Alkyl Groups in Alkylcyclohexanes by Low Temperature Carbon-13 Magnetic Resonance Spectroscopy

Article abstract of DOI:10.1039/P29800000255

13C N.m.r. studies at low temperatures of solutions in CFCl3-CDCl3 of methylcyclohexane, <α-13C>-isopropylcyclohexane, cis-1-ethyl-4-methylcyclohexane, cis-1-isoprpyl-4-methylcyclohexane, and cis-1-ethyl-4-isopropylcyclohexane have yielded conformational enthalpy differences and conformational enthropy differences for methyl, ethyl, and isopropyl groups in alkylcyclohexanes.For -ΔH⁰, the averaged values obtained, in kcal mol^-1, were 1.75 (Me), 1.60 Et), and 1.52 (Prⁱ); for ΔS⁰, 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 -ΔH⁰ and ΔS⁰ values, the sequence of -ΔG⁰ values at 300 K, namely Prⁱ<*>Et<*>Me, is reversed at temperatures below ca. 40 K.

Full text of DOI:10.1039/P29800000255

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