Is the phenol•Ar complex van der Waals or hydrogen-bonded?: A REMPI and ZEKE spectroscopic study

The study of molecular complexes in their cationic states has been considerably advanced by the development of zero electron kinetic energy (ZEKE) photoelectron spectroscopy due to its high resolution and state selectivity. In this report, we apply ZEKE spectroscopy to the phenol Ar complex to investigate whether the system exists in multiple isomeric forms under supersonic jet conditions. ZEKE spectra are recorded via four intermediate levels of the S-1 excited state of the neutral. All four ZEKE spectra display vibrational progressions in the same three intermolecular modes, namely the b(x) bend (15 cm(-1)), the b(y) bend (25 cm(-1)) and the s(z) stretch (66 cm(-1)) which are consistent with a 'van der Waals' structure where the Ar atom binds above the aromatic ring. We discuss the absence of the hydrogen-bonding phenol.Ar structure, and suggest experimental routes that could be employed to observe this isomer. (C) 2000 Elsevier Science B.V. All rights reserved.