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Vibronic spectroscopy of jet-cooled hydrogen-bonded clusters

1992; Elsevier BV; Volume: 270; Linguagem: Inglês

10.1016/0022-2860(92)85036-g

1872-8014

Markus Gerhards, B. Kimpfel, Marvin N. Pohl, Michaël Schmitt, Karl Kleinermanns,

Spectroscopy and Quantum Chemical Studies

Mass-selected, two-photon resonant ionisation spectra of supersonically cooled p-cresol · (H2O)n and phenol · (H2O)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 · (H2O)1, is red-shifted relative to the free p-cresol spectrum, while the p-cresol · (H2O)2,3 electronic origins are blue-shifted relative to p-cresol · (H2O)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 CH2O · (H2O)1 can be explained similarly. Vibrational spectra of supersonically cooled complexes of p-cresol with H2O and CH3OH were further analysed by dispersed fluorescence and stimulated emission, detected by two-colour ionisation dip. In p-cresol · (H2O)1, progressions of the intermolecular cluster stretch vibration and its combination bands with intramolecular cluster vibrations were observed with similar frequencies in the S0 and S1 states. In p-cresol · (H2O)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 · (H2O)3 to have a higher H-bond stretch frequency than p-cresol · (H2O)1, because its (unsymmetric) cyclic structure is more rigid. A characteristic pattern of CH3 torsional bands in p-cresol is observed with complex structures due the CH3 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.