Very low HOH bending frequencies, II. Quantum chemical study of the water bending potential in compounds of the MKPO4 center dot H2O type

In an attempt to get a deeper quantitative insight into the reasons behind the occurrence of very low delta (HOH) frequencies (appreciably lower than that of free water) in compounds of the type MKPO4.H2O (where M is an eleme nt of {Mg, Ni, Co, Mn}), several quantum chemical models for the delta (HOH ) potential were proposed, within the finite cluster and finite-field appro aches. The vibrational potential energy curves for the uncoupled HOH bendin g vibrations were obtained from pointwise energy calculations at the semiem pirical PM3 and ab initio Hartree-Fock SCF levels of theory with various ba sis sets. The one-dimensional vibrational Schrodinger equations were solved using second-order perturbation theory. Both fundamental anharmonic and ha rmonic S(HOH) vibrational frequencies, as well as the anharmonicity constan ts, were computed. According to the quantum chemical results, the overall r ed shift of the fundamental delta (HOH) frequency in the studied systems ma y be basically attributed to variations in the harmonic force constant ("fl attening of the potential well"), while the vibrational anharmonicity is of much lesser importance. (C) 2001 Elsevier Science B.V. All rights reserved .