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

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 C_3h 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.