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

Infrared photodissociation spectra of the phenol-argon cation (Ph+-Ar) are recorded in the vicinity of the O-H stretch vibration (nu (1)) of bare Ph+. Two isomers are identified by their characteristic complexation-induced fr equency shifts, DeltaV(1). In H bound Ph+-Ar, the Ar atom forms an ionic hy drogen bond with the O-H group of Phi leading to a significant red shift, D elta nu (1), similar to - 70 cm(-1). In the a-bound Ph+ -Ar isomer, the Ar atom is attached to the pi -electron system of Ph+, resulting in a much sma ller shift, Delta nu (1) similar to + 2 cm(-1). The Ph+-Ar complexes are pr oduced in an electron impact ion source. The relative nu (1) band intensiti es of both isomers depend strongly on the source conditions and reveal that II-bound Ph+-Ar is the global minimum, while the a-bound structure is a lo cal minimum. Ab initio calculations at the UMP2/6-311 G(2df,2pd) level pred ict binding energies of D-e similar to 660 and similar to 400 cm(-1) for th e H-bound and pi -bound minima, with an isomerization barrier of V-b simila r to 150 cm(-1) from a-bound toward pi -bound Ph+-Ar. (C) 2001 Elsevier Sci ence B.V. All rights reserved.