Hydrogen bonding in (substituted benzene)center dot(water)(n) clusters with n <= 4

Infrared ion-depletion spectroscopy, a double resonance method combining vi brational predissociation with resonant two-photon ionization (R2PI) spectr oscopy, has been applied to study mixed clusters of the type (substituted b enzene) (H2O)(n) with n less than or equal to 4. The UV chromophores were p -difluorobenzene, fluorobenzene, benzene, toluene, p-xylene and anisole. Fr om the IR depletion spectra in the region of the OH stretching vibrations i t could be shown that the water molecules are attached as subclusters to th e chromophores. Size and configuration of the subclusters could be deduced from the IR depletion spectra. In the anisole . (H2O)(1) (and) (2) complexe s the water clusters form an ordinary hydrogen bond to the oxygen atom of t he methoxy group. In all other mixed complexes a pi-hydrogen bond is formed between one of the free OH groups of a water subcluster and the pi-system of the chromophore. According to the strength of this interaction the frequ ency of the respective absorption band exhibits a characteristic red-shift which could be related to the total atomic charges in the aromatic ring. (C ) 1998 Elsevier Science B.V. All rights reserved.