Infrared spectra of the phenol-Ar and phenol-N2 cations:: proton-bound versus π-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) frequency shifts upon complexation: the proton-bound global minimum (Delta nu(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 system. 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 bond due to the additional charge-quadrupole interaction. Ab initio calculations support the interpretation of the experimental data for both species. (C) 2000 Elsevier Science B.V. All rights reserved.