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Infrared photodissociation spectra of the ionic complexes CH3+–Arn (n=1–8) have been recorded in the vicinity of the ν3 asymmetric stretching vibration of the CH3+ monomer. The CH3+–Ar dimer has also been investigated in the spectral range of the first CH stretching overtones, resulting in the characterization of its 2ν1, ν1+ν3, and 2ν3 vibrational states at the level of rotational resolution. The spectrum of CH3+–Ar is consistent with a pyramidal C3v minimum structure of the complex predicted by ab initio calculations at the MP2 level, whereby the Ar atom is attached to the empty 2pz orbital of the CH3+ moiety. The rotationally resolved ν3 spectrum of the CH3+–Ar2 trimer indicates that the two Ar atoms are located on opposite sides of the CH3+ moiety on the C3 axis, with significantly differing intermolecular C–Ar bond lengths. The splittings observed in the trimer spectrum are attributed to a tunneling motion between two equivalent C3v minimum configurations via a symmetric D3h transition state. The spectra of larger clusters (n⩾3) lack rotational resolution, however the positions and profiles of the ν3 band suggest that the additional Ar atoms are weakly attached to CH3+–Ar2 trimer, which acts as the effective nucleation center for the cluster growth. The stretching fundamentals of the CH3+ ion core in the CH3+–Arn clusters are intermediate between those of the methyl radical and the methyl cation, implying a substantial charge transfer from the rare gas atoms to the unoccupied 2pz orbital of CH3+.

Rotationally resolved infrared spectra of the ν1 and ν3 N−H stretching vibrations of the Ar−NH2 + radical ionic complex have been observed by means of photodissociation spectroscopy. The analysis of the rotational structure shows that the complex has a 3Σ- ground electronic state with a linear or quasi-linear proton-bound structure Ar−H−N−H+ characterized by an intermolecular center of mass separation of 3.085 Å. The origins of the ν1 and ν3 bands were determined as 2803.65(2) and 3287.36(2) cm-1, and the frequency of the intermolecular stretch vibration, νs, as 170.4(6) cm-1. Ab initio calculations performed at the UMP2 level of theory confirm that the quasi-linearity and the diradical character of NH2 + in its electronic ground state are not changed upon Ar complexation. The calculated properties of the intermolecular bond of the complex (D e = 1773 cm-1, RAr - H ∼ 2.01 Å, νs ∼ 185 cm-1) and the predicted complexation induced frequency shifts for ν1 and ν3 are in good agreement with the experimental results.

Infrared (IR) photodissociation spectra of mass selected Ar n −HN2 + complexes (n = 1−13) have been recorded in the 4 μm spectral range in a tandem mass spectrometer. The dominant features are assigned to the ν1 + mνs (m = 1, 2) combination bands, where ν1 corresponds to the intramolecular N−H stretch mode and νs to the intermolecular 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.