Site-specific interaction between hydrocarboncations and inert ligands: IR spectra of isomeric C3H3+–L dimers (L = Ne,Ar,O2,N2,CO2)

Infrared (IR) photodissociation spectra of isomeric C3H3+–L dimers with the inert ligands L = Ne, Ar, O2, N2, and CO2 are recorded in the spectral range of the C–H stretch fundamentals. At least two C3H3+ isomers are generated by electron impact ionization of allene (H2CCCH2) in the cluster ion source, namely the cyclopropenyl (c-C3H3+) and propargyl (H2CCCH+) cations. Both C3H3+ ions form weakly-bound adducts with all ligands considered and the attraction in these ion–ligand complexes is dominated by induction and electrostatic forces. In general, the intermolecular interaction in both c-C3H3+–L and H2CCCH+–L increases in the order Ne < Ar < O2 < N2 < CO2, in line with the polarizabilities, quadrupole moments, and proton affinities of L. Rovibrational analysis of the ν4 vibrations of c-C3H3+–L involving molecular ligands reveals several competing structural binding motifs, including the formation of H-bonds (L = N2, CO2, O2), C-bonds (L = CO2), and π-bonds (L = O2). The preferred binding site and details of the equilibrium structure strongly depend on L. In contrast, the observed H2CCCH+–L dimers feature all H-bonds between L and the acetylenic proton. Ab initio calculations for c-C3H3+–L and H2CCCH+–L at the MP2(full)/6-311G(2df,2pd) level provide additional information about the intermolecular potential energy surface of these ion–ligand complexes and confirm the interpretation of the experimental IR spectra. Photofragmentation branching ratios observed for c-C3H3+–(CO2)n with n = 1–3 allow the dissociation energy of the intermolecular H-bonds and C-bonds to be estimated as D0(H/C) = 1300 ± 300 cm−1.