Interaction of the Benzenium Ion with Inert Ligands: IR Spectra of C 6 H 7 + L n Cluster Cations (L=Ar, N 2 , CH 4 , H 2 O)

IR photodissociation spectra of mass‐selected clusters composed of protonated benzene (C 6 H 7 + ) and several ligands L are analyzed in the range of the CH stretch fundamentals. The investigated systems include C 6 H 7 + Ar, C 6 H 7 + (N 2 ) n ( n =1–4), C 6 H 7 + (CH 4 ) n ( n =1–4), and C 6 H 7 + H 2 O. The complexes are produced in a supersonic plasma expansion using chemical ionization. The IR spectra display absorptions near 2800 and 3100 cm −1 , which are attributed to the aliphatic and aromatic CH stretch vibrations, respectively, of the benzenium ion, that is, the σ complex of C 6 H 7 + . The C 6 H 7 + (CH 4 ) n clusters show additional CH stretch bands of the CH 4 ligands. Both the frequencies and the relative intensities of the C 6 H 7 + absorptions are nearly independent of the choice and number of ligands, suggesting that the benzenium ion in the detected C 6 H 7 + L n clusters is only weakly perturbed by the microsolvation process. Analysis of photofragmentation branching ratios yield estimated ligand binding energies of the order of 800 and 950 cm −1 (≈9.5 and 11.5 kJ mol −1 ) for N 2 and CH 4 , respectively. The interpretation of the experimental data is supported by ab initio calculations for C 6 H 7 + Ar and C 6 H 7 + N 2 at the MP 2/6‐311 G(2df,2pd) level. Both the calculations and the spectra are consistent with weak intermolecular π bonds of Ar and N 2 to the C 6 H 7 + ring. The astrophysical implications of the deduced IR spectrum of C 6 H 7 + are briefly discussed.