Vibrational spectroscopy and theory of the protonated benzene dimer and trimer.

Protonated benzene cluster ions, H(C(6)H(6))(2)(+) and H(C(6)H(6))(3)(+), are produced in a pulsed electrical discharge source coupled to a supersonic expansion. Mass-selected complexes are investigated with infrared photodissociation spectroscopy in the 1000-3200 cm(-1) region using the method of argon tagging. The IR spectra of H(C(6)H(6))(2)(+)-Ar and H(C(6)H(6))(3)(+)-Ar contain broad bands in the high frequency region resulting from CH-π hydrogen bonds. Sharp peaks are observed in the fingerprint region arising from the ring modes of both the C(6)H(7)(+) and C(6)H(6) moieties. M06-2X calculations have been performed to investigate the structures and vibrational spectra of energetically low-lying configurations of these complexes. H(C(6)H(6))(2)(+) is predicted to have three nearly isoenergetic conformers: the parallel displaced (PD), T-shaped (TS), and canted (C) structures [Jaeger, H. M.; Schaefer, H. F.; Hohenstein, E. G.; Sherrill, C. D. Comput. Theor. Chem. 2011, 973, 47-52]. A comparison of the experimental dimer spectrum with those predicted for the three isomers suggests an average structure between the TS and PD conformers, which is consistent with the low energy barrier predicted to separate these two structures. No evidence is found for the C dimer even though it lies only 1.2 kcal/mol above the PD dimer. Although the trimer is also computed to have many low lying isomers, the IR spectrum limits the possible species present.