We propose a model to calculate the electronic structure of hydrotalcite-like compounds by using periodic boundary conditions and ab initio density functional theory (DFT). The proposed method to build up layered double hydroxides (LDHs) was tested for Zn 2/3 Al 1/3 (OH) 2 Cl 1/3 • 2 / 3 H 2 O, Zn 2/3 Al 1/3 (OH) 2 (CO 3 ) 1/6 • 4 / 6 H 2 O, and Mg 2/3 Al 1/3 (OH) 2 (CO 3 ) 1/6 • 4 / 6 H 2 O with 3R 1 polytype. In the model, the occupation of cationic sites in hydroxide layers is ordered and the interlayer anions and water molecules form a film between the layers. Direct comparison with experimental structural parameters shows good agreement. The a parameter is close to that in brucite for all three LDHs. The c parameter is smaller (about 1 Å) for the LDHs with CO 3 2as a consequence of its strong interaction with hydroxide layers. Those interactions were evidenced by the difference of density and vibrational analysis. The intercalated water molecules have small mobility and interact strongly with one another and with interlayer anions and hydroxide layers. These interactions cause the downshift in the calculated vibrational wavenumbers of water O-H stretching modes below 3420 cm -1 and are consistent with the reported infrared spectra of hydrotalcite-like compounds. The calculated formation enthalpies for LDHs with carbonate are in agreement with the previously reported trend. The biggest difference between theoretical and experimental values is 2 kcal/mol. The calculated formation Gibbs energies are negative. The zero point energy is important to evaluate ∆H, but the formation entropy does not affect the Gibbs free energy significantly.

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This ab initio study was performed to better understand the correlation between intercalated water molecules and layered double hydroxides (LDH), as well as the changes that occur by the dehydration process of Zn-Al hydrotalcite-like compounds containing Cl⁻ and CO₃²⁻ counterions. We have verified that the strong interaction among intercalated water molecules, cointercalated anions, and OH groups from hydroxyl layers is reflected in the thermal stability of these compounds. The Zn(2/3)Al(1/3)(OH)₂Cl(1/3)·2/3H₂O hydrotalcite loses all the intercalated water molecules around 125 °C, while the Zn(2/3)Al(1/3)(OH)₂(CO₃)(1/6)·4/6H₂O compound dehydrates at about 175 °C. These values are in good agreement with experimental data. The interlayer interactions were discussed on the basis of electron density difference analyses. Our calculation shows that the electron density in the interlayer region decreases during the dehydration process, inducing the migration of the Cl⁻ anion and the displacement of the hydroxyl layer from adjacent layers. Changes in these compound structures occur to recover part of the hydrogen bonds broken due to the removal of water molecules. It was observed that the chloride ion had initially a lower Löwdin charge (Cl(-0.43)), which has increased its absolute value (Cl(-0.58)) after the water molecules removal, while the charges on carbonate ions remain invariant, leading to the conclusion that the Cl⁻ anion can be more influenced by the amount of water molecules in the interlayer space than the CO₃²⁻ anion in hydrotalcite-like compounds.

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