The number of spin symmetry species of tunnelling molecular groups

It has been conventional for many years to discuss the quantum rotational motion of small symmetric molecular groups like CH3and NH4in a way which does not properly antisymmetrise state functions for single pair exchanges of nuclei, on the grounds that single pair exchanges occur too slowly to affect the energy levels. Though this is true, it also leads to a spurious factorisation of two-dimensional E nuclear spin symmetry species into two one-dimensional species Eaand Eb. This has led to widespread misconceptions and some erroneous interpretation of observations in terms of population differences of these fictitious species. It has also presented an obstacle to the development of a satisfactory description of the transition between quantum tunnelling motion at low temperatures and thermally activated hopping motions at high temperatures. Simple model calculations on methyl groups are presented to illustrate the true nature of the E species.