Knowledge Bank

Intermolecular forces and microsolvation processes in $NH3+-Arn$ complexes (n=1-6) are studied by infrared photodissociation spectroscopy in a tandem mass spectrometer. Analysis of the rovibrational structure yields a proton bound dimer equilibrium structure with $C2v$ symmetry. Observed tunnelling splittings are attributed to hindered internal rotation of the planar $NH3+$ cation within the dimer. Systematic frequency shifts observed in the spectra of larger clusters provide a detailed picture of the cluster growth. The first three Ar ligands form equivalent linear proton bonds, leading to highly symmetric cluster structures with $C2v$ or $D3h$ symmetry. The next two Ar atoms bind to the $2pz$ orbital of the central N atom on opposit e sites of the $C3$ axis, leading to structures with $C3v$ and $D3h$ symmetry. Ab initio calculations support the interpretation of the experimental data.