AbstractThesynandantirotamers ofo‐tolyl‐di‐tert‐butylcarbinol, 2a and 2b, respectively, have been studied by1H NMR at 200 MHz and by natural abundance13C NMR at 50 MHz.1H‐{1H} NOE enhancement factors are consistent with the known structures and the calculated geometries of these compounds. The relaxation time,T1, of the 2‐Me protons in 2b is unexpectedly higher than that for 2a. The13C relaxation times of the 2‐Me and the quaternary carbon of thetert‐butyl group are also both higher in 2b than in 2a, suggesting that the rotation of these groups is faster in the less stable isomer. The activation energies fort‐Bu rotation, measured by1H DNMR, agree with this conclusion. Further confirmation is provided by theoretical calculation of the 2‐Me andt‐Bu rotation barriers based on Allinger's MM2 force field. Comparison of measured ΔG‡ values from this work and from the literature with MM2‐calculated ΔH‡ values indicates that this force field systematically underestimates rotation barriers in open‐chain systems by