Nuclear magnetic resonance spectroscopy. Variable-temperature 13C and 19F study of the chair-chair interconversion of 1,1,3,3-tetramethylcyclohexane and gem-difluoro-1,1,3,3-tetramethylcyclohexanes

Abstract

A variable-temperature, proton noise-decoupled, 13C nmr spectral study of the chair-chair interconversion in 1,1,3,3-tetramethylcyclohexane has demonstrated the utility of 13C 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 13C spectra of 1,1,3,3-tetramethylcyclohexane with those determined from a variable-temperature 19F 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 13C chemical shifts for 1,1,3,3-tetramethylcyclohexane reflect the importance of the methyl-methyl 1,3-diaxial interactions.