Infrared spectra of the phenol-Ar and phenol-N-2 cations: proton-bound versus pi-bound structures

infrared spectra of the phenol-Ar/N-2 cations (Ph+-Ar/N-2), produced in an electron impact ion source, are analyzed in the vicinity of the O-H stretch fundamental, nu(1). For Ph+-Ar two isomers are identified by their nu(1) f requency shifts upon complexation: the proton-bound global minimum (Delta n u(1) = - 65 cm(-1)) and the pi-bound local minimum (Delta nu(1) = + 2 cm(-1 )). The former isomer represents the first aromatic ion-rare gas (Rg) dimer where the Rg atom does not prefer binding to the aromatic pi-electron syst em. The larger frequency shift of proton-bound Ph+-N-2 (Delta nu(1) = - 169 cm(-1)) compared to Ph+-Ar is consistent with a stronger intermolecular bo nd due to the additional charge-quadrupole interaction. Ab initio calculati ons support the interpretation of the experimental data for both species. ( C) 2000 Elsevier Science B.V. All rights reserved.