Interaction of the benzenium ion with inert ligands:: IR spectra of C6H7+-Ln cluster cations (L = Ar, N2, CH4, H2O)

IR 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+-(N-2)(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 sigma 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+-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) (approximate to9.5 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+-N-2 at the MP2/ 6-311 G(2df,2pd) level. Both the calculations and the spectra are consistent with weak intermolecular pi bonds of Ar and N-2 to the C6H7+ ring. The astrophysical implications of the deduced IR spectrum Of C6H7+ are briefly discussed.