Microsolvation of HN2+ in argon:: Infrared spectra and ab initio calculations of Arn-HN2+ (n = 1-13)

Infrared (IR) photodissociation spectra of mass selected Ar-n-HN2+ complexes (n = 1-13) have been recorded in the 4 mu m spectral range in a tandem mass spectrometer. The dominant features are assigned to the nu(1) + m nu(s) (m = 1, 2) combination bands, where nu(1) corresponds to the intramolecular N-H stretch mode and nu(s) to the intermcrlecular stretching vibration of the first (proton-bound) Ar ligand. Systematic size-dependent complexation-induced frequency shifts and fragmentation branching ratios enabled the development of a consistent model for the cluster growth. The Ar-HN2+ dimer has a linear proton-bound structure and further Ar ligands fill two equatorial solvation rings around the linear dimer core, each of them containing up to five Ar atoms. The attachment of the 12th argon atom at the nitrogen end of HN2+ leads to the completion of the first solvation shell with an icosahedral structure. Weaker bands in the IR photodissociation spectra are attributed to less stable isomers. Comparison with previous studies of the related Ar-n-HOSi+ and Ar-n-HCO+ complexes reveals several similarities in the cluster growth. However, due to different charge distributions and anisotropies of the repulsive walls of the ionic cores, subtle differences occur in the order of shell filling as well as the occurrence and stability of isomeric structures. These differences are rationalized by two-dimensional intermolecular potential energy surfaces calculated at the MP2/aug-cc-pVTZ(#) level of theory.