ORIGINAL PAPER V. Botella    V. Timon  E. Escamilla-Roa A. Herna´ ndez-Laguna  C. I. Sainz-Dı´az Hydrogen bonding and vibrational properties of hydroxy groups in the crystal lattice of dioctahedral clay minerals by means of first principles calculations Received: 14 July 2003 / Accepted: 8 April 2004 Abstract The hydroxy groups of the crystal lattice of dioctahedral 2:1 phyllosilicates were investigated by means of quantum-mechanical calculation. The stan- dard Kohn-Sham self-consistent density functional the- ory (DFT) method was applied using the generalized gradient approximation (GGA) with numerical atomic orbitals and double-zeta polarized functions as basis set. Isomorphous cation substitution of different cations in the octahedral and tetrahedral sheet was included along with several interlayer cations reproducing experimental crystal lattice parameters. The effect of these substitu- tions and the interlayer charge on the hydroxyl group properties was also studied. These structures represent different cation pairs among Al 3+ , Fe 3+ and Mg 2+ in the octahedral sheet of clays joined to OH groups. The geometrical disposition of the OH bond in the crystal lattice and the hydrogen bonds and other electrostatic interactions of this group were analyzed. The frequen- cies of different vibrational modes of the OH group [m(OH), d(OH) and c(OH)] were calculated and com- pared with experimental data, finding a good agreement. These frequencies depend significantly on the nature of cations which are joined with, and the electrostatic interactions with, the interlayer cations. Besides, hydrogen-bonding interactions with tetrahedral oxygens are important for the vibrational properties of the OH groups; however, also the electrostatic interactions of these OH groups with the rest of tetrahedral oxygens within the tetrahedral cavity should be taken into account. The cation substitution effect on the vibration modes of the OH groups was analyzed reproducing the experimental behaviour. Keywords Smectites  Quantum mechanics  Vibration frequencies  Hydrogen bonds Introduction Clay minerals are important in cosmetics, oil drilling, toxic and radioactive waste disposal, catalysis, drug and agrochemical delivery, as additives for polymeric mate- rials etc. These minerals play a key role in the fate and mobility of contaminants in natural systems, and they are especially interesting as barrier in nuclear waste deposits. The 2:1 phyllosilicates have a layer structure where two tetrahedral layers sandwich a sheet of octahedrally coordinated cations. These minerals present a great diversity of compounds because of their capacity for isomorphous substitution, Al 3+ by Mg 2+ , Fe 3+ , and Fe 2+ in the octahedral sheet, and Si 4+ by Al 3+ in the tetrahedral sheet. These substitutions can produce a negative charge that is compensated by the presence of cations in the interlayer space (Brindley and Brown 1984). Many XRD studies of 2:1 clay structures are reported, but they give no indication of the proton positions within the hydroxyl groups. Although using techniques of neutron diffraction they have been able to obtain some meaningful results in mica minerals (Catti et al. 1994). The study of the structure and properties of these hydroxyl groups is interesting, because they can play a major role in the catalytic activity of these min- erals, and in their interactions with water, other mole- cules and cations. Additionally, the octahedral cations can play a significant role in sorption and dissolution phenomena in many minerals, where OH groups can have varying effects on the reactivity of mineral surface (Schindler and Stumm 1987). Phys Chem Minerals (2004) 31: 475–486 Ó Springer-Verlag 2004 DOI 10.1007/s00269-004-0398-7 V. Botella  V. Timon Instituto de Estructura de la Materia (CSIC), C/Serrano, 113(bis), 28006 Madrid, Spain Present address: V. Timon Science Labs., University of Durham, South Road, Durham DH1 3LE, UK E. Escamilla-Roa  A. Herna´ndez-Languna  C.I. Sainz-Dı´az (&) Estacio´n Experimental del Zaidı´n (CSIC), C/ Profesor Albareda 1, 18008-Granada, Spain E-mail: sainz@eez.csic.es   Dr. V. Botella passed away last February