Water hexamer clusters: Structures, energies, and predicted mid-infrared spectra

We present an ab initio theoretical study of five low-energy isomers of the water hexamer {Chair, Cage(du)[1], Book, Prism, and Boat}, their intramolecular vibrations, binding energies D-e and dissociation energies D-0. Moller-Plesset second order perturbation calculations using the aug-cc-pVTZ basis set at aug-cc-pVDZ optimized geometries including vibrational zero point energy corrections predict Chair to be the most stable isomer, followed closely by Cage(du)[1] (+0.02 kcal/mol) and Book (+0.05 kcal/mol), while Prism is 0.15 kcal/mol higher. The Boat conformer is least stable at both the D-e and D-0 levels. The main focus is on the intramolecular normal modes of the five isomers. The calculated O-H stretching frequencies and intensities are compared to recent infrared spectra of water hexamer in supersonic jets, liquid-helium droplets and solid para-hydrogen matrices. The IR spectra indicate that Book and Chair are major species in the latter two environments and may also exist in supersonic jets. The (H2O)(6) gas phase interconversion equilibria are calculated and predict that the most abundant isomer is Chair below 8 K, Cage between 8-26 K, and Book above 26 K. Several of the low-frequency vibrational modes are identified as low-amplitude precursors of the Chair<---->Book<---->Cage isomerization pathways. (C) 2002 American Institute of Physics.