Interaction of the Benzenium Ion with Inert Ligands: IR Spectra of C6H7+Ln Cluster Cations (L=Ar, N2, CH4, H2O)

Abstract

AbstractIR photodissociation spectra of mass‐selected clusters composed of protonated benzene (C6H7+) and several ligands L are analyzed in the range of the CH stretch fundamentals. The investigated systems include C6H7+Ar, C6H7+(N2)n (n=1–4), C6H7+(CH4)n (n=1–4), and C6H7+H2O. 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 CH stretch vibrations, respectively, of the benzenium ion, that is, the σ complex of C6H7+. The C6H7+(CH4)n clusters show additional CH stretch bands of the CH4 ligands. Both the frequencies and the relative intensities of the C6H7+ absorptions are nearly independent of the choice and number of ligands, suggesting that the benzenium ion in the detected C6H7+Ln 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 N2 and CH4, respectively. The interpretation of the experimental data is supported by ab initio calculations for C6H7+Ar and C6H7+N2 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 N2 to the C6H7+ ring. The astrophysical implications of the deduced IR spectrum of C6H7+ are briefly discussed.