<i>Ab</i> <i>initio</i> studies of cyclic water clusters (H2O)<i>n</i>, <i>n</i>=1–6. I. Optimal structures and vibrational spectra

The optimal structures and harmonic vibrational frequencies of cyclic water clusters, (H2O)n, have been determined at the Hartree–Fock (for n=2–6) and second order perturbation theory (for n=2–4) levels of theory with an augmented correlation consistent double zeta basis set. At the MP2 level this basis set yields very accurate results for the structure, dipole moment, and polarizability of the water monomer as well as results of comparable accuracy for the structure, binding energy, and harmonic vibrational frequencies of the water dimer. The optimal structure of (H2O)4 and the harmonic frequencies of (H2O)3,4 are the first ones reported at a correlated level for these species. Analysis of the structural trends reveals that the separation between neighboring oxygen atoms decreases exponentially with increasing cluster size. The predicted R0(O–O) for the ring hexamer is less than 0.02 Å shorter than the interoxygen separation in ice Ih. Furthermore, the trends in the harmonic vibrational frequencies suggest that, for large clusters, the intramolecular bends are blue shifted by ∼70 cm−1 with respect to the monomer frequency. The frequencies corresponding to the ‘‘free’’ OH stretches show little (≤50 cm−1) shift to the blue for n=2–6, whereas the ones corresponding to the ‘‘bridge’’ hydrogens are shifted to the red by ∼500 cm−1 with respect to the average of the harmonic stretching frequencies in water.