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

Abstract

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.