Infrared spectra of the H-bound and π-bound isomers of the phenol–argon cation

Abstract

Infrared photodissociation spectra of the phenol–argon cation (Ph +–Ar) are recorded in the vicinity of the O–H stretch vibration ( ν 1) of bare Ph +. Two isomers are identified by their characteristic complexation-induced frequency shifts, Δ ν 1. In H-bound Ph +–Ar, the Ar atom forms an ionic hydrogen bond with the O–H group of Ph + leading to a significant red shift, Δ ν 1∼−70 cm −1. In the π-bound Ph +–Ar isomer, the Ar atom is attached to the π-electron system of Ph +, resulting in a much smaller shift, Δ ν 1∼+2 cm −1. The Ph +–Ar complexes are produced in an electron impact ion source. The relative ν 1 band intensities of both isomers depend strongly on the source conditions and reveal that H-bound Ph +–Ar is the global minimum, while the π-bound structure is a local minimum. Ab initio calculations at the UMP2/6-311G(2df,2pd) level predict binding energies of D e∼660 and ∼400 cm −1 for the H-bound and π-bound minima, with an isomerization barrier of V b∼150 cm −1 from π-bound toward H-bound Ph +–Ar.