Infrared Spectra of C3H3 +-N2 Dimers:  Identification of Proton-Bound c-C3H3 +-N2 and H2CCCH+-N2 Isomers

Mid-infrared photodissociation spectra of mass selected C3H3 +-N2 ionic complexes are obtained in the vicinity of the C−H stretch fundamentals (2970−3370 cm-1). The C3H3 +-N2 dimers are produced in an electron impact cluster ion source by supersonically expanding a gas mixture of allene, N2, and Ar. Rovibrational analysis of the spectra demonstrates that (at least) two C3H3 + isomers are produced in the employed ion source, namely the cyclopropenyl (c-C3H3 +) and the propargyl (H2CCCH+) cations. This observation is the first spectroscopic detection of the important c-C3H3 + ion in the gas phase. Both C3H3 + cations form intermolecular proton bonds to the N2 ligand with a linear −C−H···N−N configuration, leading to planar C3H3 +-N2 structures with C 2 v symmetry. The strongest absorption of the H2CCCH+-N2 dimer in the spectral range investigated corresponds to the acetylenic C−H stretch fundamental (ν1 = 3139 cm-1), which experiences a large red shift upon N2 complexation (Δν1 ≈ −180 cm-1). For c-C3H3 +-N2, the strongly IR active degenerate antisymmetric stretch vibration (ν4) of c-C3H3 + is split into two components upon complexation with N2: ν4(a1) = 3094 cm-1 and ν4(b2) = 3129 cm-1. These values bracket the yet unknown ν4 frequency of free c-C3H3 + in the gas phase, which is estimated as 3125 ± 4 cm-1 by comparison with theoretical data. Analysis of the nuclear spin statistical weights and A rotational constants of H2CCCH+-N2 and c-C3H3 +-N2 provide for the first time high-resolution spectroscopic evidence that H2CCCH+ and c-C3H3 + are planar ions with C 2 v and D 3 h symmetry, respectively. Ab initio calculations at the MP2(full)/6-311G(2df,2pd) level confirm the given assignments and predict intermolecular separations of R e = 2.1772 and 2.0916 Å and binding energies of D e = 1227 and 1373 cm-1 for the H-bound c-C3H3 +-N2 and H2CCCH+-N2 dimers, respectively.