Infrared spectra of the phenol–Ar and phenol–N2 cations: proton-bound versus π-bound structures

Infrared spectra of the phenol–Ar/N2 cations (Ph+–Ar/N2), produced in an electron impact ion source, are analyzed in the vicinity of the O–H stretch fundamental, ν1. For Ph+–Ar two isomers are identified by their ν1 frequency shifts upon complexation: the proton-bound global minimum (Δν1=−65 cm−1) and the π-bound local minimum (Δν1=+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 π–electron system. The larger frequency shift of proton-bound Ph+–N2 (Δν1=−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.