Vibronic spectroscopy of jet-cooled hydrogen-bonded clusters - Gerhards, M.; Kimpfel, B.; Pohl, M.; Schmitt, M.; Kleinermanns, K. (Elsevier)
Mass-selected, two-photon resonant ionisation spectra of supersonically cooled p-cresol . (H"2O)"n and phenol . (H"2O)"n are reported. Cluster spectra with one, two and three water molecules attached can be unambiguously assigned.A monotonic shift of the electronic spectra with increasing cluster size is not observed here. The spectrum of p-cresol . (H"2O)"1, is red-shifted relative to the free p-cresol spectrum, while the p-cresol . (H"2O)"2","3 electronic origins are blue-shifted relative to p-cresol . (H"2O)"1 but still lie on the red side of the monomer. Simple, highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) considerations based on an initio calculations show that this can be explained by the inductive effect exerted on the O-atom of p-cresol, which acts as proton donor and acceptor in the H-bonding. The blue shift of n-π* transitions of H-bonded chromophores with carbonyl groups like CH"2O . (H"2O)"1 can be explained similarly.Vibrational spectra of supersonically cooled complexes of p-cresol with H"2O and CH"3OH were further analysed by dispersed fluorescence and stimulated emission, detected by two-colour ionisation dip. In p-cresol . (H"2O)"1, progressions of the intermolecular cluster stretch vibration and its combination bands with intramolecular cluster vibrations were observed with similar frequencies in the S"0 and S"1 states. In p-cresol . (H"2O)"3, further intense intermolecular bands arise, namely the hydrogen-bridge bending and torsion vibrations. This can be attributed to the lower symmetry of these clusters. Ab initio quantum chemical calculations show p-cresol . (H"2O)"3 to have a higher H-bond stretch frequency than p-cresol . (H"2O)"1, because its (unsymmetric) cyclic structure is more rigid.A characteristic pattern of CH"3 torsional bands in p-cresol is observed with complex structures due the CH"3 torsion-overall rotation interaction. Although their appearance is similar, the spacing of the monomer and cluster lines is very different, giving direct information on cluster structures from the rotational constants and moment of inertia.
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Keywords: V; U