Knowledge Bank

The theory of vibration-large amplitude internal motion interaction in $molecules1$ is used to calculate these contributions to the effective Eamiltonian for internal rotation when the internal rotor has a three fold axis of symmetry. Calculations are made for four isotopic species of methyl alcohol, two isotopic species of acetaldehyde and trifluoroacetaldehyde, and perfluoroacetyl fluoride. The dynamic contribution to the potential energy coefficient $V6$ ranges from -3 to $-6cm-1$. The potential energy coefficient $V3$ shows a dependence of a few tens of $cm-1$ upon the state of the perpendicular vibrations. A cos6$\tau$ dependence of the reduced torsional coefficient is of the order of 0.05% of $b\tau \tau$. The mixing of internal rotation with other vibrations cannot account for the decrease in barrier for methyl alcohol with deuterium substitution. Actually the calculation gives an increase in barrier of comparable magnitude to the decrease observed for $CD3OH$ compared to $CH3OH$.