Investigation of the internal dynamics of NH3 van der Waals complexes with rare gas atoms: Rotational spectra and ab initio calculations.
Jennifer van Wijngaarden
Universität Basel, Institut für Physikalische Chemie,
Klingelbergstrasse 80, CH-4056 Basel, Switzerland
Wolfgang Jäger
Department of Chemistry, University of Alberta, Edmonton AB T6G 2G2,
Canada
Microwave rotational spectra of seven different van der Waals complexes consisting of one, two, and three rare gas (Rg) atoms bound to one NH3 molecule were measured between 3.8 GHz and 25 GHz using a pulsed molecular beam Fourier transform microwave spectrometer. The spectra are complicated by the internal rotation and inversion of the NH3 subunit within the clusters. The rotational constants obtained from fitting the ground state spectra of the RGn–NH3 (n = 1, 2, 3) complexes were used to estimate the Rg–Rg and Rg–NH3 bond lengths in each species. The 14N nuclear quadrupole hyperfine structures were analyzed and the resulting coupling constants were used to derive information about the orientation and internal dynamics of the NH3 moiety in the various Rg cluster environments. In the spectra of the deuterated isotopomers, inversion tunnelling splittings were observed and qualitatively related to the energy differences between the two inversion states in each complex. The experimental results were complemented by the construction of ab initio intermolecular potential energy surfaces for the Nen–NH3 (n = 1, 2, 3) clusters using coupled cluster [CCSD(T)] theory. In addition to predicting the minimum energy structures of the three Ne containing clusters, a comparison of the topologies of the potential energy surfaces provides an estimate of the relative barriers to the internal rotation and inversion of the NH3 moiety as a function of the Rg cluster size.