Conformational analysis. Part 36. A variable temperature 13C NMR study of conformational equilibria in methyl substituted cycloalkanes

The conformational equilibria in six methylcycloalkanes have been investigated using the temperature dependence of the 13C chemical shifts at temperatures above the coalescence point. Using conformationally homogeneous compounds as standards the temperature dependence was analysed to give the conformer energy differences ΔH (ax–eq). These were for methylcyclohexane 1.92 kcal mol−1, 2- and 3-methylcyclohexanone 1.58 and 1.35 kcal mol−1, 3-methyltetrahydropyran 1.50 kcal mol−1 and 2- and 4-methylpiperidine >2.6 and 2.30 kcal mol−1. These values are compared with previous literature values and theoretical values from molecular mechanics and ab initio calculations. They are in very good agreement with previous literature values and the ab initio values, but the values from MM calculations show larger differences. The only exceptional case is 2-methylcyclohexanone in which the ab initio value of 2.38 kcal mol−1 is much larger than the energy difference in chloroform solution determined here (1.58 kcal mol−1) which is in good agreement with previous literature data in solution. Sterically hindered solvation in the equatorial conformer is the most likely cause of this difference. It is concluded that this technique represents a novel and useful approach to conformational analysis.