STRUCTURES, DYNAMICS AND VIBRATIONS OF CYCLIC (H2O)(3) AND ITS PHENYLAND NAPHTHYL DERIVATIVES

The cyclic water trimer shows a fascinating complexity of its intermol ecular potential-energy surface as a function of the three intermolecu lar torsional coordinates: there are six isometric but permutationally distinct minimum-energy structures of C-1 symmetry, which can interco nvert by torsional motions via six isometric transition states, also o f C-1 symmetry. A second type of interconversion can occur through dif ferent torsional motions via two C-3 symmetric transition structures, and a third interconversion type via a planar C-3h symmetric transitio n 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 exam ined by resonant two-photon ionization spectroscopy in supersonic beam s. These clusters exhibit S-0 --> S-1 vibronic spectra with very diffe rent characteristics. These reflect the number of cluster structures f ormed, their low-frequency intermolecular vibrations and indirectly gi ve information about the cluster fluxionality.