Structures, dynamics and vibrations of cyclic (H2O)3and its phenyl and naphthyl derivatives

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Abstract
The cyclic water trimer shows a fascinating complexity of its intermolecular potential-energy surface as a function of the three intermolecular torsional coordinates: there are six isometric but permutationally distinct minimum-energy structures of C1 symmetry, which can interconvert by torsional motions via six isometric transition states, also of C1 symmetry. A second type of interconversion can occur through different torsional motions via two C3 symmetric transition structures, and a third interconversion type via a planar C3h symmetric transition structure. The equivalence of the six minima is broken if the ‘free’ H atom of one H2O molecule in the cluster is chemically substituted, yielding three distinct conformers, which occur in enantiomeric pairs. Not all three conformers are necessarily locally stable minima; this depends on the substituent. The phenol–(H2O)2, p-cyanophenol–(H2O)2, 1-naphthol–(H2O)2 and 2-naphthol–(H2O)2 clusters, which are the phenyl, p-cyanophenyl and naphthyl derivatives of (H2O)3, were examined by resonant two-photon ionization spectroscopy in supersonic beams. These clusters exhibit S0→ S1 vibronic spectra with very different characteristics. These reflect the number of cluster structures formed, their low-frequency intermolecular vibrations and indirectly give information about the cluster fluxionality.