Isomorphous substitution effect on the vibration frequencies of hydroxyl groups in molecular cluster models of the clay octahedral sheet

The geometrical features and electronic structure of molecular clusters mod els of two octahedrally coordinated cations in edge-sharing octahedra were studied by means of Hartree-Fock ab initio molecular orbital calculations a t LANL2DZ and 6-31+G* levels. These models represent the different cation p airs among Al3+, Fe3+, and Mg2+ of the octahedral sheet of clays. These mod els reproduce the experimental values of the main geometrical features in t he corresponding minerals. The vibrational frequencies of the bridging hydr oxyl groups (M-OH-M') were calculated and compared with experimental data. A good agreement between theoretical and experimental results was found. Th e relative differences of nu(OH) and delta(OH) frequencies calculated among these (M-OH-M') cation pairs are similar to the experimental behavior in c lays. Theoretical gamma(OH) frequencies were also calculated and presented as an estimation of the experimental. Correlations between the atomic weigh ts and the atomic Mulliken charges of the cations with the experimental and theoretical OH vibration frequencies have been also determined and a simil ar behavior was found.