Water molecules in different crystal surroundings: Vibrational O–H frequencies from a b i n i t i o calculations

A b initio quantum-mechanical calculations of anharmonic frequencies for the water O–H vibrations have been performed for a series of crystalline hydrates. In each case, the potential-energy curve for the uncoupled water O–H stretch was derived at the Mo/ller–Plesset MP2 level. Nearest neighbors of the water molecule were explicitly included in the supermolecule and the rest of the surroundings were mimicked by point charges to reproduce the crystal field out to infinity. The time-independent Schrödinger equation for the motion of the proton in this potential well was solved variationally and the frequency was obtained from the energy difference between the 0 and 1 eigenstates. Computed frequencies can be directly compared with existing infrared data for isotope-isolated water molecules in these hydrates. The compounds selected (LiClO4⋅3H2O, LiHCOO⋅H2O, LiOH⋅H2O ) exhibit experimental O–H frequency shifts in a wide range, from −150 down to −930 cm−1. Good agreement is found between experimental and theoretical frequencies (experimental values in parentheses): 3596 (3556) for LiClO4⋅3H2O, 3129 (3112) and 3488 (3390) for LiHCOO⋅H2O, and 2817 (2775) cm−1 for LiOH⋅H2O. Correlation curves of typical H-bond parameters such as ν(O–H) vs R(H⋅⋅⋅O), re (O–H) and νOH/νOD have been computed and compared with experiment. The vibrational intensities are also discussed.