Interaction of C3H3+ isomers with molecular nitrogen:: IR spectra of C3H3+-(N2)n clusters (n=1-6)

The intermolecular interaction and microsolvation process. of isomeric C3H3+ ions in molecular nitrogen are characterized by infrared (IR) photodissociation spectroscopy of C3H3+-(N-2)(n) complexes (n = 1-6) and quantum chemical calculations (n = 0-4). The rovibrational analysis of the C3H3+-N-2 spectrum unambiguously reveals the presence of (at least) two C3H3+ isomers in the ion source, namely the propargyl (H2CCCH+) and the cyclopropenyl (C-C3H3+) cations. Analysis of the cluster size-dependent vibrational frequency shifts and splittings, the photofragmentation branching ratios, and the results of density functional calculations provides a consistent picture of the microsolvation of c-C3H3+ and H2CCCH+ in inert nitrogen. In the most stable c-C3H3+-(N-2)(n) complexes, the first three N-2 ligands form (nearly) linear and equivalent proton bonds to the three protons of c-C3H3+, leading to highly symmetric planar structures with C-2v (n = 1, 2) and D-3h symmetry (n = 3). After completion of this first solvation subshell at n = 3, further N-2 ligands form weaker intermolecular bonds to the C atoms of the nearly planar c-C3H3+-(N-2)(3) ion core. The dissociation energies of the H-bonds and C-bonds in C-C3H3+-(N-2)(n) are estimated as D-0(H) = 900 +/- 130 cm(-1) and D-0(C) = 860 +/- 170 cm(-1), respectively. In the most stable H2CCCH+-N-2 complex, the N-2 ligand forms a linear ionic H-bond to the acetylenic C-H group of H2CCCH+, leading to a planar structure with C-2v symmetry. The calculations suggest that the next two ligands bind to the protons of the CH2 group giving rise to planar structures with C-s (n = 2) and C-2v symmetry (n = 3), and these structures are compatible with the observed IR spectra. (Int I Mass Spectrom 218 (2002) 281-297) (C) 2002 Elsevier Science B.V. All rights reserved.