IR spectra of para-substituted phenol+–Ar cations: effect of halogenation on the intermolecular potential and O–H bond strength

IR spectra of isolated Ar complexes of the phenol, para-fluorophenol, and para-chlorophenol cations (Ph+–Ar, p-FPh+–Ar, p-ClPh+–Ar) are analyzed in the vicinity of the O–H stretch fundamental, ν1. Two isomers of each dimer are produced in the electron impact ion source and identified by their characteristic ν1 frequency, namely the H-bound global minimum and the less stable π-bound local minimum. The analysis of the ν1 frequencies indicates that the strength of the O–H bond in p-XPh+ increases by halogen substitution in the order X=H<F<Cl. Quantum chemical calculations support the interpretation of the experimental data.