Proton magnetic resonance study of rotational isomerism in vinylcycloalkanes

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Abstract
The vicinal coupling constant, J(12), between the vinyl CH and the ring CH protons in vinylcyclohexane was calculated from a ‘partial molecule’ six‐spin system. The 100 and 270 MHz results are in good agreement; those at 60 MHz were, however, still inaccurate in this approximation. J(12) increases with increasing solvent polarity and decreasing temperature. The energy difference between the s‐trans and gauche conformers in both C2Cl4 and perdeuterioacetone solvents is 879 ± 167 J mol−1 (210±40 cal mole−1). The s‐trans conformer is the most stable, in contrast to the isoelectronic cyclohexylcarboxyaldehyde where the gauche rotamers are lower in energy.