Microwave rotational spectroscopic investigation of the Ar2–NH3 van der Waals trimer

Abstract

Rotational spectra of five isotopomers of the Ar₂–NH₃ van der Waals trimer were recorded between 3.9 and 21 GHz using a pulsed jet, Balle–Flygare type Fourier transform microwave spectrometer. The observed microwave transitions are b-type transitions within the ground internal rotor state of the complex. ¹⁴N nuclear quadrupole hyperfine structures of the rotational transitions were resolved and analyzed. The resulting nuclear quadrupole coupling constants are sensitive to the orientation and large amplitude internal rotation of the NH₃ moiety within the trimer. For each of the three deuterated isotopomers, a tunnelling splitting due to the inversion of ammonia was observed. The spectroscopic constants obtained from the spectral fits were used to estimate the structure of the complex and to discuss the internal rotation and inversion dynamics of NH₃ within the Ar₂–NH₃ trimer. The results are compared with previous microwave studies of the Ar–NH₃ dimer and Ar₃–NH₃ tetramer and with a recent combined microwave and ab initio investigation of the Ne₂–NH₃ trimer.