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

Abstract

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.