Orientation of OH groups in kaolinite and dickite: Ab initio molecular dynamics study

Ab-initio density-functional molecular dynamics simulations and full relaxa tion of all atomic positions are used to reconcile the crystal structures w ith IR spectra of dickite and kaolinite. The relaxation of atomic positions preserves the accepted space group symmetries. A pair of two hydroxyl grou ps oriented parallel to the layer is formed in structures of both dickite a nd kaolinite producing the high-frequency components of the OH-stretching f requencies. Other hydroxyls make relatively strong interlayer hydrogen bond s and produce down-shifted stretching bands. All hydroxyl groups are involv ed in effective hydrogen bonds, The OH-stretching frequencies depend linear ly on the strength of the hydrogen bond.