Determination of the threefold internal rotation barrier in ArNH ₃

The two Σ and four Π states of the weakly bound, complex Ar-NH₃ correlating to j = 2, k = ± 1 ammonia have been observed by tunable far infrared difference frequency-microwave sideband spectroscopy. The results have been combined with published data to determine a new angular potential energy surface for the system. The barrier to threefold internal rotation of the NH₃ about its C₃ axis in the complex is estimated to be 25.606(24) cm^-1 near the minimum energy (T-shaped) configuration. The potential also exhibits maxima at both symmetric top configurations, with energies approximately 53 and 31 cm^-1, respectively above that of the global minimum. The location and splitting between the symmetric and antisymmetric Σ states are indicative of a strong interaction with another pair of unobserved states, most likely the first excited intermolecular stretch built on j = 1, k = ± 1 Ar-NH ₃.