Classification of the tunneling-rotational energy levels of ammonia dimer in the molecular symmetry group

Abstract

The symmetry properties of NH3 dimer are analyzed in the molecular symmetry group and the physical assumptions underlying the choice of this group are detailed. Several low lying tunneling states are predicted. Two of these states transform as four-dimensional (G) representations of G36, the molecular symmetry group. These two states can have the same microwave selection rules as those found in the two experimentally observed states of NH3 dimer. That is, the microwave transitions can follow pure rotational selection rules (no tunneling splittings) even though interchange tunneling is included as a feasible motion in the molecular symmetry group. The observation of these selection rules is interpreted as internal rotation stabilizing the system against interchange tunneling. Hence, in these particular states the interchange tunneling is quenched. The group theory predicts this picture of ammonia dimer to be appropriate if certain internal rotation interactions are large compared to the interchange tunneling matrix element. These conclusions are derived through correlation diagrams which are used to provide physical insight into the nature of the two states of G symmetry and approximate electric dipole selection rules for microwave transitions arising from these states. The selection rules for the infrared transitions of this dimer which correlate to the ν2 fundamental of NH3 are also given.