Dynamical solvent effects on conformational isomerization of cyclohexane and 1,1-difluorocyclohexane

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Abstract
The effect of pressure on the conformational isomerization of cyclohexane in n-pentane-d12 solvent has been investigated by using the high-resolution, high-pressure nuclear magnetic resonance (NMR) technique. The isomerization rate of cyclohexane increases with increasing pressure at 218 K. The experimental data are interpreted in terms of stochastic models for isomerization reactions, and the results show that cyclohexane isomerization in n-pentane-d12 exhibits behavior typical for low-friction, energy controlled regime. The effect of temperature and pressure on the conformational isomerization of 1,1-difluorocyclohexane in solution has been investigated with the use of 19F Fourier transform (FT) NMR spectroscopy. The solvents used in this study are n-pentane, acetone-d6, and methylcyclohexane. Similarly, as in the case of cyclohexane, the ring inversion in 1,1-difluorocyclohexane is accelerated when pressure is increased, and this pressure dependence is nonlinear. The interpretation of the experimental data for 1,1-difluorocyclohexane depends critically on the estimate of ΔV‡TST which value is not known at the present time. Therefore, it is impossible to determine whether isomerization of 1,1-difluorocyclohexane exhibits the Kramers turnover.