Nuclear Magnetic Resonance Spectroscopy. Variable-Temperature 13 C and 19 F Study of the Chair-Chair Interconversion of 1,1,3,3-Tetramethylcyclohexane and gem -Difluoro-1,1,3,3-Tetramethylcyclohexanes

A variable-temperature, proton noise-decoupled, 13 C nmr spectral study of the chair-chair interconversion in 1,1,3,3-tetramethylcyclohexane has demonstrated the utility of 13 C spectra for probing conformational equilibria. There is a much larger chemical-shift difference between the carbons of axial and equatorial methyl groups than between the protons of these methyl groups, 133 Hz compared to 7.5 Hz. Comparison of the activation parameters determined from the 13 C spectra of 1,1,3,3-tetramethylcyclohexane with those determined from a variable-temperature 19 F study on 2,2,6,6-tetramethyl-1,1-difluorocyclohexane shows that the introduction of a gem -difluoro group into the ring causes significant steric interactions. The 13 C chemical shifts for 1,1,3,3-tetramethylcyclohexane reflect the importance of the methyl-methyl 1,3-diaxial interactions.