Knowledge Bank

The eigenstates of $NH3$ dimer are classified in the molecular symmetry group and the observable microwave spectrum for this system is predicted. Several symmetry groups are examined; each of which admits a different set of permutation-inversion operations. The physical assumptions underlying the choice of each group are detailed and discussed in terms of their acceptability. The microwave spectrum (including hyperfine structure and Stark effects) predicted by each symmetry group is compared with the experimentally observed $spectrum.1$ The observed spectrum for the primary isotope consists of two rigid rotor $J-J+1.K=0$ progressions arising from different vibrational states of the complex. Correlation diagrams are useful to provide physical insight into the nature of these two vibrational states. The microwave spectra of several isotopes of $NH3$ dimer have been obtained and the observed isotope effects are readily explained with the assumption of physioally reasobable barriers to internal rotation and inversion in the complex. Several recent theoretical $calculations2$ of the structure of $NH3$ dimer are discussed in the context of these results.