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A hydrated complex of 1,10-phenanthroline with Cu2+ cation was intercalated in the interlayer space of montmorillonite. This intercalation occurs initially by through a cation exchange mechanism in which the charge of the complex cation compensates the excess of the negative charge of the interlayer, then, once the cation exchange capacity (CEC) value has been reached, by direct adsorption of the sulfate salt of this complex (i.e. the cation together with its sulfate counterion). This material has showed interesting entrapping properties of gaseous phases and a peculiar chemical reactivity. However, the complete characterization and explanation of the formation of these materials is difficult with only experimental techniques. Hence, we used computational methods at atomic level to know how are the molecular structure of these complexes and their adsorption capacity of ammonia inside the interlayer confined space of montmorillonite for a better understanding of the experimental behaviour. First Principles calculations were performed based on Density Functional Theory (DFT). The intercalation of the phenanthroline-Cu(II) complex inside the nanoconfined interlayer of montmorillonite is energetically favourable in the relative proportion observed experimentally, being a cation exchange process. The further adsorption of the sulfate salt of the phenanthroline-Cu complex is also energetically possible. The adsorption of ammonia molecules in these montmorillonite-phenanthroline-Cu complexes was also favourable according with experimental behaviour.

This work aimed at studying the potentiality of interactions between kaolinite surfaces and a protein-fragment (350–370 amino acid units) extracted from the glycoprotein E1 in the transmembrane domain (TMD) of hepatitis C virus capsid. A computational work was performed for locating the potential electrostatic interaction sites between kaolinite aluminol and siloxane surfaces and the residues of this protein-fragment ligand, monitoring the possible conformational changes. This hydrated neutralized kaolinite/protein-fragment system was simulated by means of molecular modeling based on atomistic force fields based on empirical interatomic potentials and molecular dynamic (MD) simulations. The MD calculations indicated that the studied protein-fragment interacted with the kaolinite surfaces with an exothermic process and structural distortions were observed, particularly with the hydrophilic aluminol surface by favorable adsorption energy. The viral units isolation or trapping by the adsorption on the kaolinite nanoparticles producing structural distortion of the peptide ligands could lead to the blockage of the entry on the receptor and hence a lack of viral activity would be produced. Therefore, these findings with the proposed insights could be an useful information for the next experimental and development studies in the area of discovering inhibitors of the global challenged hepatitis and other pathogenic viruses based on the phyllosilicate surface activity. These MD studies can be extended to other viruses like the COVID-19 interacting with silicate minerals surfaces., Graphical abstract Unlabelled Image, Highlights • A protein of the Hepatitis C virus is adsorbed favorably on kaolinite surfaces. • Adsorption on kaolinite provokes changes of Hepatitis C virus. • Molecular dynamics are useful for exploring virus adsorption on mineral surfaces.

Methane has been detected on all planets of our Solar System, and most of the larger moons, as well as in dwarf-planets like Pluto and Eric. The presence of this molecule in rocky planets is very i ...

Ar ion sputtering on pyrite surfaces leads to the generation of sulfur vacancies and metallic iron. Our research shows that sputtering and annealing processes drive electrostatic changes on the pyrite surface, which play an important role in the molecular adsorption of glycine. While both chemical species (anion and zwitterion) adsorb on a sputtered pyrite surface, the anionic form of glycine is favoured. Nevertheless, in both treatments (sputtered or annealed surfaces), molecules evolve from zwitterionic to anionic species over time. Quantum mechanical calculations based in Density Functional Theory (DFT) suggest the energy required to generate vacancies increases with the number of vacancies produced, and the atomic charge of the Fe atoms that is next to a vacancy increases linearly with the number of vacancies. This leads to enhanced redox processes on the sputtered pyrite surface that favour the adsorption of glycine, which is confirmed experimentally by X-ray Photoemission Spectroscopy (XPS). We have investigated theoretically the efficiency of the adsorption process of the zwitterionic glycine onto vacancies sites: this reaction is exothermic, i.e. is energetically favoured and its energy increases with the number of defects, confirming the increased reactivity observed experimentally. The experiments show a treatment-dependent molecular selectivity of the pyrite surface., This work has been supported by the MINECO projects ESP2017- 89053 and PCIN-2017-098. The Instituto Nacional de Técnica Aeroespacial supported the work performed at CAB and the S.G.M PhD Thesis. This Project has been partially funded by the Spanish State Research Agency (AEI) Project No. MDM-2017-0737 Unidad de Excelencia “María de Maeztu”- Centro de Astrobiología (CSIC-INTA).

Perchlorates have been found widespread on the surface of Mars, their origin and degradation pathways are not understood to date yet. We investigate here, from a theoretical point of view, the potential redox processes that take place in the interaction of Martian minerals such as olivine, with anhydrous and hydrated perchlorates. For this theoretical study, we take as mineral substrate the (1 0 0) surface of forsterite and calcium perchlorate salt as adsorbate. Our DFT calculations suggests a reduction pathway to chlorate and chlorite. When the perchlorate has more than 4 water molecules, this mechanism, which does not require high-temperature or high energy sources, results in parallel with the oxidation of the mineral surface, forming magnesium peroxide, MgO, and in the formation of ClO, which through photolysis is known to form ClO-O. Because of the high UV irradiance that reaches the surface of Mars, this may be a source of O on Mars. Our results suggest that this process may be a natural removal pathway for perchlorates from the Martian regolith, which in the presence of atmospheric water for salt hydration, can furthermore lead to the production of oxygen. This mechanism may thus have implications on the present and future habitability of the Martian surface., Authors would like to acknowledge the contribution of the European COST Action CA17120 supported by the EU Framework Programme Horizon 2020, and the Spanish MINECO projects CGL2014-55230-R, FIS2016-77692-C2, PCIN-2017-098. MPZ acknowledges the partial support of the Spanish State Research Agency (AEI) Project No. MDM-2017-0737. E. E. acknowledges to Rafael Esteso for his help with the Graphical Abstract.

Muscovite (Ms) and phlogopite (Phl) belong to the 2:1 dioctahedral and trioctahedral layer silicates, respectively, and are the end members of Ms-Phl series minerals. This series was studied in the 2M1 polytype and modeled by the substitution of three Mg2+ cations in the Phl octahedral sites by two Al3+ and one vacancy, increasing the substitution up to reach the Ms. The series was computationally examined at DFT level as a function of pressure to 9 GPa. Cell parameters as a function of pressure and composition, and bulk moduli as a function of the composition agrees with the existing experimental results. The mixing Gibbs free energy was calculated as a function of composition. From these data, approximated solvi were calculated at increasing pressure. A gap of solubility is found, decreasing the gap of solubility at high pressure., This work was supported by Spanish MCINN and European FEDER grants CGL2008-02850/BTE, FIS2013-48444-C2-2P, FIS2016-77692-C2-2P, and PCIN-2017-098 and by the regional agency “Junta de Andalucía” for the RNM-264, -363, and -1897 PAI-grants.

Copper oxide nanostructures have great potential use in a plethora of nanotechnology applications including nanoelectronics, photovoltaics, sensors, electrochemistry, and pharmacology. In the present work we show how hierarchically nanostructured copper oxide and hydroxide may be prepared through self-assembly from CuSO4 salt and silicate solutions using the chemobrionic growth process of a chemical garden. Procedures were explored using the cupric salt in either solid (pellet and seed growth methods) or liquid phase (fluid injection techniques). Self-assembling nanostructures were characterized by means of environmental scanning electron microscopy (ESEM) with energy-dispersive X-ray spectroscopy (EDX) analysis, micro-Raman spectroscopy and X-ray diffraction. Our results show the formation of crystalline aggregates of copper oxide and hydroxide in complex hierarchical nanostructured forms including fans, flowers, petals, skeins, lentils, and sheaves. Analytical methods corroborate that these nanostructures may be selected in shape and chemical composition with the reaction conditions., We acknowledge the Spanish MINCINN project grants FIS201677692-C2-2P and PCIN-2017-098, along with European FEDER funds and the European COST Action CA17120.

Muscovite (Ms) and phlogopite (Phl) belong to the 2:1 dioctahedral and trioctahedral layer silicates, respectively, and are the end members of Ms-Phl series minerals. This series was studied in the 2M

This work is a study of CO and H2O molecules as adsorbates that interact on the surface of olivine dust grains. Olivine (forsterite) is present on the Earth, planetary dust, in the interstellar medium (ISM) and in particular in comets. The composition of amorphous ice is very important for the interpretation of processes that occur in the solar system and the ISM. Dust particles in ISM are composed of a heterogeneous mixture of amorphous or crystalline silicates (e.g. olivine) organic material, carbon, and other minor constituents. These dust grains are embedded in a matrix of ices, such as H2O, CO, CO2, NH3, and CH4. We consider that any amorphous ice will interact and grow faster on dust grain surfaces. In this work we explore the adsorption of CO-H2O amorphous ice onto several (100) forsterite surfaces (dipolar and non-dipolar), by using first principle calculations based on density functional theory (DFT). These models are applied to two possible situations: i) adsorption of CO molecules mixed into an amorphous ice matrix (gas mixture) and adsorbed directly onto the forsterite surface. This interaction has lower adsorption energy than polar molecules (H2O and NH3) adsorbed on this surface; ii) adsorption of CO when the surface has previously been covered by amorphous water ice (onion model). In this case the calculations show that adsorption energy is low, indicating that this interaction is weak and therefore the CO can be desorbed with a small increase of temperature. Vibration spectroscopy for the most stable complex was also studied and the frequencies were in good agreement with experimental frequency values.

Some aspects of the crystal structure of illite are not understood properly yet, in spite of its abundance and significance as a component of soils, sediments, and low-grade metamorphic rocks. The present study aimed to explore the role of hydronium cations in the interlayer space of illite in a theoreticalexperimental approach in order to clarify previous controversial reports. The infrared spectroscopy of this mineral has been studied experimentally and by means of atomistic calculations at the quantum mechanical level. The tetrahedral charge is critical for the stability of the hydronium cations, the presence of which has probably been underestimated in previous studies. In the present study, computational studies have shown that the hydronium cations in aqueous solutions are likely to be intercalated in the interlayer space of illite, exchanging for K cations. During the drying process these cations are stabilized by hydrogen bonds in the interlayer space of illite.

The application of clay minerals in therapeutics is becoming important due to their structural and surface physicochemical properties. 5-aminosalicylic acid (5-ASA) is a very common pharmaceutical drug and is used worldwide. The interactions between the 5-ASA molecule and both the aluminol and siloxane surfaces of kaolinite are studied by means of atomistic calculations using force fields based on empirical interatomic potentials and quantum mechanics calculations based on density functional theory. A conformational analysis of 5-ASA has been performed and the anion of 5-ASA was also studied. The calculated adsorption energy values indicate that 5-ASA is likely to be adsorbed on the kaolinite surfaces with greater affinity to the aluminol surface. Hence, kaolinite may be considered as a promising pharmaceutical carrier of 5-ASA.

The presence of non-stoichiometric sites on the pyrite(100) surface makes it a suitable substrate for driving the chemical evolution of the amino acid glycine over time, even under inert conditions. Spectroscopic molecular fingerprints prove a transition process from a zwitterionic species to an anionic species over time on the monosulfide enriched surface. By combining experimental and theoretical approaches, we propose a surface mechanism where the interaction between the amino acid species and the surface will be driven by the quenching of the surface states at Fe sites and favoured by sulfur vacancies. This study demonstrates the potential capability of pyrite to act as a surface catalyst., This work has been supported by the MINECO project ESP2017-89053. The Instituto Nacional de Técnica Aeroespacial supported the work performed at CAB. EER is thankful to Javier Martin-Torres, Alfonso Hernández-Laguna and C. M. Pradier for their support and suggestions. This Project has been partially funded by the Spanish State Research Agency (AEI) Project No. MDM-2017-0737 Unidad de Excelencia “María de Maeztu”-Centro de Astrobiología (CSIC-INTA)., We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).

The adsorption properties of clay minerals have been widely studied in several technological areas, due to their versatility, absorption capacity and catalytic properties. The interaction of amino acids with clay surfaces can be interesting due to their role in prebiotic scenarios. Different degrees of hydration and the adsorption of the glycine molecule and glycinium cation in the interlayer space of montmorillonite were investigated by means of calculations based on density functional theory (DFT). Our calculations show that the cation exchange of K+ by glycinium in the interlayer of hydrated K-montmorillonite is highly possible and exothermic. This supports previous experimental results by explaining the possible adsorption of glycine as a molecule and cation. Glycine is adsorbed in a zwitterionic form in the interlayer without being solvated with water. Besides, glycine and glycinium are highly exothermically adsorbed in the interlayer. The interlayer spacings under different conditions were in agreement with the experimental values. Hydrogen bonds and electrostatic interactions between molecules and surface atoms are responsible for this exothermicity. The IR spectra were calculated and compared with the experimental results showing interesting frequency shifts depending on the intermolecular interactions in the interlayer space of montmorillonite.

Titanium dioxide (TiO) is one of the most studied oxides in photocatalysis, due to its electronic structure and its wide variety of applications, such as gas sensors and biomaterials, and especially in methane-reforming catalysis. Titanium dioxide and olivine have been detected both on Mars and our Moon. It has been postulated that on Mars photocatalytic processes may be relevant for atmospheric methane fluctuation, radicals and perchlorate productions etc. However, to date no investigation has been devoted to modelling the properties of TiO adsorbed on olivine surface. The goal of this study is to investigate at atomic level with electronic structure calculations based on the Density Functional Theory (DFT), the atomic interactions that take place during the adsorption processes for formation of a TiO regolith. These models are formed with different titanium oxide films adsorbed on olivine (forsterite) surface, one of the most common minerals in Universe, Earth, Mars, cometary and interstellar dust. We propose three regolith models to simulate the principal phases of titanium oxide (TiO, TiO and TiO). The models show different adsorption processes i.e. physisorption and chemisorption. Our results suggest that the TiO is the most reactive phase and produces a strong exothermic effect. Besides, we have detailed, from a theoretical point of view, the effect that has the adsorption process in the electronic properties such as electronic density of states (DOS) and oxide reduction process (redox). This theoretical study can be important to understand the formation of new materials that can be used as support in the catalytic processes that occur in the Earth, Mars and Moon. Also, it may be important to interpret the present day photochemistry and interaction of regolith and airborne aerosols in the atmosphere on Mars or to define possible catalytic reactions of the volatiles captured on the Moon regolith., Authors acknowledges the Spanish MINECO projects CGL2014-55230-R, PCIN-2017-098, and FIS2016--77692-C2-2-P co-financed with European FEDER funds. We also would like to thank the generous support of Kempe and Wallenberg Foundations. MPZ, JMT, and EER recognize the ESA/Airbus/Air Liquide and Merck Space Exploration Masters challenge 2018 finalist award for Moon ISRU solutions for the Moon Alchemist proposal that uses Moon regolith with TiO2 for methane production.

The extraterrestrial origin of glycine has become a topic of great interest for astrobiology and it has been detected bonded to mineral surfaces in the dust of comet 81P/Wild 2 by the Stardust mission. The interactions of organic molecules with the dust grain surfaces shed light on possible routes for life from extraterrestrial space to Earth. The NH3 is one of the volatile components of cometary and interstellar ices. In this work we propose models that describe this scenario that consist of a quaternary system of one glycine molecule, a mixture of amorphous ammonia–water ice, and forsterite (100) surfaces (dipolar and nondipolar). Our quantum mechanical calculations show that the presence of amorphous ammonia/water ice increases the adsorption energy of glycine on the forsterite surface with respect to only amorphous water ice. In addition, we calculated the infrared (IR) frequencies to characterize the most reactive sites in the chemisorption processes with the mineral surface.

The interaction of molecules on the mineral surface is interesting in understanding the development of icy mantles on interstellar and interplanetary dust. The ice grains can freeze and cover the silicate cores, growing an amorphous ice mantle. In the interstellar medium, olivine is a silicate that has been found in many places in dust. Previously we have simulated the interaction between amorphous water ice and forsterite surface. In this work we describe a more realistic situation, by adding ammonia molecules in a model of amorphous dirty ice onto forsterite surface. The NH3 is a part of the volatile components of cometary and interstellar ices. We propose models that describe a mixture of amorphous ice (ammonia–water) and forsterite (100) surfaces (dipolar and nondipolar). Our quantum mechanical calculations show that the ammonia has a similar affinity (30 kcal/mol) to the forsterite surface as that of water (31 kcal/mol). We calculated also the infrared frequencies to characterize the most reactive si...

Dolomite (CaMg(CO3)2) is a sedimentary mineral that is found in the majority of natural carbonate rocks. Dolomite has been used in the treatment of polluting agents, and as a potential adsorbent of...

In recent years glycine, the simplest amino acid, has become a topic of great importance in astrobiology owing to its unambiguous detection in comet 81P/Wild 2 dust by Stardust mission. A possible end state for glycine in comets is by forming a molecular bond with some species of cometary dust. Many questions remain about of how glycine survives the extreme conditions prevailing in the interstellar medium (ISM). The detailed chemical interactions occurring on the dust grain surfaces with organic molecules are of great interest, as they shed light on possible routes of life to Earth from extraterrestrial space. In a previous work we simulated the interaction between glycine molecules and a forsterite surface and found that the interaction occurs through a chemisorption process. In this paper, we try to describe a more realistic situation, by adding an ice component to the mixture. We propose two models for that interaction. In the first model, neutral glycine is adsorbed on a mineral surface covered by a thin layer of amorphous ice, while in the second model neutral glycine and ice are adsorbed simultaneously onto the mineral surface. We find that this second model is the most likely scenario, although the particular adsorption process depends on the kind of surface. For a dipolar surface the adsorption proceeds through an associative process. In the case of a non-dipolar surface the adsorption process is both associative and dissociative and the chemisorption process is stronger. For the stable systems, we calculated the infrared frequencies to characterize the most reactive sites in the chemisorption processes. We observed important shifts frequencies that have relationship with the position of the main vibrational modes of the CO and COO− moieties, which react chemically with the mineral surface.

Glycine, the simplest amino acid, has been detected in the carbonaceous chondrites of several meteorites, and as well as in the comet Wild 2 embedded in the aerogel panel returned by the Stardust mission. A possible end state for glycine in comets is for it to form a molecule bond with another species of cometary dust. This paper aims to study the chemical interaction of a glycine molecule in its neutral and protonated form with the (100) surface of forsterite, a well-known cometary dust component, through electronic structure calculations based on the Density Functional Theory (DFT). Two models are investigated for the surface: dipolar and non-dipolar. The interaction of forsterite–glycine occurs via the two adsorption processes: physisorption and chemisorption, which depend on the configuration of the amino acid with the surface. When the amino acid reacts chemically with the surface (chemisorption process) the energies range from 58 to 96 kcal/mol. The calculated frequencies and normal modes of vibration of the most chemisorbed structures are in agreement with previous work in other surfaces.

Potential energy curves and different adsorption sites of Ni on Anatase (0 0 1) surface, at increasing coverage, were computed at singlet, triplet, and quintuplet multiplicity states. Coverage was increased until a complete adlayer is reached. A density functional theory (DFT) based method (periodic D Mol 3 included in MS package) was used. At the lowest coverage of our system (one Ni per 2 × 2 × 1 unit cells) different adsorption sites were found at the holes on the surface. At the triplet multiplicity state, two deep minima were found at 0.55 and 0.11 A from the bridging oxygen plane (BOP) on the surface with adsorption energies of 3.5 and 3.74 eV, respectively. Both of these minima showed spin transfer from the Ni to the Ti atoms at the relaxed slab surfaces. With singlet multiplicity a BOP minimum was found at −0.33 A. For high coverages, spin multiplicity of the system turned out to be the singlet. This shows similar trends alike in the clustering of Ni on Anatase (0 0 1) surface. Calculated surface energy decreased until full coverage, and the surface energy per Ni also decreased up to saturation value at full coverage.

Quantum mechanical calculations based on the density functional theory (DFT) are used to study the crystal structures of dioctahedral 2:1 phyllosilicates. The isomorphous cation substitution is investigated by exploring different substitutions of octahedral Al3+ by Mg2+ or Fe3+, tetrahedral substitution of Si4+ by Al3+, and different interlayer cations (IC) (Na+, K+, Ca2+, and Mg2+). Samples with different kinds of layer charges are studied: only tetrahedrally charged, only octahedrally charged, or mixed octahedral/tetrahedral charged. The effect of the relative arrangements of these substitutions on the lattice parameters and total energy is studied. The experimental observation of segregation tendency of Fe3+ and dispersion tendency of Mg2+ in the octahedral sheet is reproduced and explained with reference to the relative energies of the octahedral cation arrangements. These energies are higher than those due to the IC/tetrahedral and IC/octahedral relative arrangements. The tetrahedral and octahedral substitutions that generate charged layers also tend to be dispersed. The octahedral cation exchange potentials change with the IC-charge/ionic radius value.

Trans-vacant and cis-vacant polymorphs of smectites and illites were distinguished by studying different cation substitutions in octahedral and tetrahedral sheets and in the interlayer. The standard Kohn-Sham self-consistent density functional method was used in the generalized gradient approximation (GGA) with numerical atomic orbitals as the basis set. The calculations reproduce the differences in the lattice parameters between the cis-vacant and trans-vacant configurations as observed from experimental studies of phyllosilicates. This theoretical approach is a useful tool for predicting crystallographic properties that must be calculated for smectites and illites because they cannot be determined experimentally in these clay minerals, especially the cis-vacant and trans-vacant configurations. The effect of cation substitutions in the octahedral and tetrahedral sheets on various structural features is also presented. The calculated effects are consistent with experimental results. The energy differences between the cis-vacant and trans-vacant polymorphs for a given composition are lower than the energy changes produced by the relative cation distributions in the octahedral sheet. Nevertheless, in the most illitic samples the trans-vacant arrangement is more stable than the cis-vacant form, in accord with experimental studies.

The crystal structure of triclinic pyrophyllite and its dehydroxylate derivative was studied with quantum mechanical calculations. The standard Kohn-Sham self-consistent density functional theory (DFT) was used through a linear-scaling DFT method with periodical boundary conditions in the generalized gradient approximation (GGA) with numerical atomic orbitals as the basis set. The calculations reproduce the lattice parameters found experimentally in pyrophyllite and its dehydroxylate derivative. 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 frequencies of different vibration modes were calculated and compared with experimental data; the results show a good agreement. The dehydroxylation process, including different intermediates of this reaction, was investigated theoretically. The energetic differences are according to the thermodynamics of the experimental process. The semidehydroxylate derivative is identified, for the first time, as an important intermediate in this process, clarifying previous questions concerning the mechanism reported from the experimental data.

The spatial geometry and local environment of hydroxyl groups of the cis-vacant (cv) crystal polymorph of dioctahedral 2:1 phyllosilicates are studied by computational methods, doing especial emphasis on the hydrogen bonds and electrostatic interactions of the hydroxyl groups with the neighbor atoms. Different types of phyllosilicates are explored: with only tetrahedral charge, with only octahedral charge, with simultaneous octahedral and tetrahedral substitution, and with different interlayer cation (IC). The effect of these interactions on the spectroscopic properties of these hydroxyl groups is also studied. All results are compared with the trans-vacant (tv) crystal forms of these minerals. Frequency differences between cv and tv polymorphs are smaller than those due to the local environments of these OH groups. This means that the changes in the interactions of the different local environments of each OH group are greater than the cv/tv differences.

This work is dedicated in memoriam to Dr. Vicente Botella., The hydroxy groups of the crystal lattice of dioctahedral 2:1 phyllosilicates were investigated by means of quantum-mechanical calculation. The standard Kohn-Sham self-consistent density functional theory (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 substitutions and the interlayer charge on the hydroxyl group properties was also studied. These structures represent different cation pairs among Al3+, Fe3+ and Mg2+ 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 frequencies of different vibrational modes of the OH group [(OH), (OH) and (OH)] were calculated and compared 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., The authors are also grateful to E. Artacho for his fruitful discussion, and to the MCYT by the financial support (grants BTE2000-1146-CO2-01 and PPQ2001-2932