| Abstract: | In simple cases of methyl group internal rotation, the barrier to internal rotation is equal to the $V_{3}$ coefficient in the standard equation of the potential function. In the past few years, methyl internal rotation potentials have been reported with significant $V_{6}$ contributions, of which p-toluic acid is just one example.\footnote{E.G. Schnitzler, et al., J. Phys. Chem. A 121 (2017) 8625} For $|V_6| << |V_{3}|$, the barrier is still $|V_3|$. However, if $0 < |V_{3}/V_{6}| < 4$, there are now two different barriers because there are two different potential minima (or maxima), and none of the barriers is equal to $|V_{3}|$ or $|V_{6}|$. Of course, corresponding effects are also present in systems with two or more internal rotors. However, in these systems additional minima and/or maxima may occur when potential interaction terms become significant, e.g. when $V_{33}$ and/or $V'_{33}$ have magnitudes similar to $V_{3}$ in a molecule like acetone (molecular symmetry group $G_{36} = [33]C_{2v}$). A number of examples for 1-D and 2-D systems are given and some consequences for the spectroscopy are discussed. |
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