Alfonso Hernández-Laguna

Muscovite (Ms) and phlogopite (Phl) series mineral is studied in the 2M₁polytype and modeled by the substitution of three Mg²⁺cations in the three octahedral sites of Phl [KMg₃(Si₃Al)O₁₀(OH)₂] by two Al³⁺and one vacancy, increasing the substitution up to reach the Ms [KAl₂□(Si₃Al)O₁₀(OH)₂]. The series was computationally examined at DFT using Quantum ESPRESSO, as a function of pressure from − 3 to 9 GPa. Crystal structure is calculated, and cell parameters, and geometry of atomic groups agree with experimental values. OH in the Mg²⁺octahedrons are approximately perpendicular to the (001) plane, meanwhile when they are in Al³⁺, octahedral groups are approximately parallel to this plane. From Quantum Theory of Atoms in Molecules, the atomic basins are calculated as a function of the pressure, K⁺and basal O show the largest volumes. The bulk excess volume (Vxs) and the excess atomic volumes are analyzed as a function of the composition and the pressure. K⁺, basal and apical O Vxs show a behavior similar to the bulk Vxs as a function of the composition, keeping qualitatively this behavior as a function of pressure; substituent atoms do not show a Vxs behavior similar to the bulk and their effect consequently is mostly translated to atoms in the interlayer space. Atomic compressibilities are also calculated. Atomic compressibilities are separated in the different sheets of the crystal cell. Atomic moduli of K and basal O are the lowest and the ones behaving as the bulk modulus of the series. The atomic bulk modulus of the H’s is different depending of their position with respect to the (001) plane.

The behavior of aragonite (CaCO3), strontianite (SrCO3 ), cerussite (PbCO3) andwitherite (BaCO3 ) at increasing pressure have been studied up to 6 GPa using density func-tional theory with plane waves. A parallelism of the orthorhombic carbonates with theclosed-packed AsNi structure is considered in our analysis, being the CO2− groups not3centered in the interstice of the octahedron. The decomposition of the unit cell volume intoatomic contributions by using the Quantum Theory of Atoms in Molecules has allowed theanalysis of the bulk modulus in atomic contributions. The bulk, axes, interatomic distancesand atomic compressibilities are calculated. The largest compression is on the c crystallo-graphic axis and the c linear modulus has a linear function with the mineral bulk modulus(K0 ). Many of the interatomic distances moduli of the alkaline earth (AE) carbonates show linear functions with the bulk modulus; however, the whole series (including cerussite) onlygives linear functions when K 0 is related either with the CC distances modulus or the mod-ulus of the distances of the C to the faces of the octahedron perpendicular to c. These lastdistances are the projections of the Metal-Oxygen (MO) distances to the center of the octa-hedron. K 0 AE carbonates also show linear functions with the atomic moduli of their cations,however, the whole series show a linear relation with the atomic modulus of C atoms. There-fore, the whole series highlight the importance of the C atoms and their interactions in themechanism of compression of the orthorhombic carbonate series.

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.

Organic peroxides are interesting compounds with a broad range of properties from antimalarial and antimicrobial activities to explosive character. In this work the gas-phase thermolysis reaction mechanism of the 3,6-dimethyl-1,2,4,5-tetroxane (DMT) is studied by DFT calculations, considering axial–axial, axial–equatorial, and equatorial–equatorial position isomers. The critical points of the singlet (S) and triplet (T) potential energy surfaces (PES) are calculated. Three mechanisms are considered: i) S-concerted, ii) S-stepwise, and iii) T-stepwise. The first intermediate of the reaction through S-stepwise-PES is a diradical open structure, o, yielding, as products, two molecules of acetaldehyde and one of O₂ in the S state. The S-stepwise-mechanism gives exothermic reaction energies (Er) in the three position isomers. The S-concerted mechanism yields very high activation energies (Ea) in comparison with those of the S-stepwise mechanism. In the T-stepwise mechanism, a triplet open structure (T-o) is first considered, yielding an Er 12 kcal mol^–1 more exothermic than that of the S-mechanisms. The S-o and T-o are similar in structure and energies; therefore, a crossing from the S- to T-PES is produced at the o intermediate as a consequence of a spin–orbit coupling. The highest Ea is the first step after o intermediate, and thus it is considered the rate limiting step. Therefore, the Er at the T-PES is more in agreement with the Er of the exothermic experimental diperoxide products. Ea, Er, and O···O distances are studied as a function of the number of methyl groups and the position isomerization.

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.

The effect on the vibrational spectrum of the hydroxy groups in dioctahedral 2:1 phyllosilicates of the isomorphous cation substitution of Mg²⁺ by Al³⁺ in the octahedral sheet was investigated at the DFT level. Ortho, meta and para Mg²⁺ configurational polymorphs were defined. The theoretical vibration frequencies of OH groups depend significantly on the nature of the cations they are joined with. Theoretical values are spread out over narrow ranges: 3,612–3,626 cm⁻¹ for ν(AlOHMg), 3,604–3,606 cm⁻¹ for ν(AlOHAl), and 3,657–3,660 cm⁻¹ for ν(MgOHMg); 803–830 cm⁻¹ for δ(AlOHMg), 877 cm⁻¹ for δ(AlOHAl), and 693–711 cm⁻¹ for δ(MgOHMg), in agreement with known experimental values. From the intensities of the XOHY bands, we observe that the vibrational adsorptivities of the ν(OH) vibrations are not the same for all XOHY groups, and that ν(MgOHMg) absorptivity is much lower than that of ν(AlOHAl). These theoretical results should be taken into account in quantitative analysis of experimental vibrational studies in clay minerals, introducing different molar extinction coefficients in the Lambert-Beer law to determine the relative concentrations of both cationic arrangements.

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.

Cyclic organic peroxides are a broad and highly sought-after class of peroxide compounds that present high reactivity and even explosive character. The unusually high reactivity of these peroxides can generally be attributed to the rupture of O–O bonds. Cyclic diperoxides are a very interesting series of substituted compounds in which tetroxane is the most prominent member. Gas-phase thermolysis of the simplest substituted member of the series [3-methyl-1,2,4,5-tetroxane or methylformaldehyde diperoxide (MFDP)] has been observed to yield one acetaldehyde, one formaldehyde, and one oxygen molecule as reaction products. DFT at the 6-311 + G** level of theory using the BHANDHLYP correlation–exchange functional was applied via the Gaussian09 program to calculate the critical points of the potential energy surface (PES) of this reaction. Equatorial and axial isomers were studied. The singlet state PES of MFDP was calculated, and an open diradical structure was found to be the first intermediate in a stepwise reaction. Two PESs were subsequently obtained: singlet state (S) and triplet state (T) PESs. After that, two alternative stepwise reactions were found to be possible: 1) one in which either an acetaldehyde, or 2) formaldehyde molecule is initially formed. For second one, exothermic reactions were observed for both the S and T PESs. The reaction products include a oxygen molecule in either S or T state, with the T reaction being the most exothermic. When calculations were performed at the CASSCF(10,10)/6-311 + G** level, spin–orbit coupling permitted S to T crossing at the open diradical intermediate stage, a non-adiabatic reaction was observed, and lower activation energies and higher exothermicity were generally seen for the T PES than for the S PES. These results were compared with the corresponding results for tetroxane. The spin–orbit coupling of MFDP and tetroxane yielded identical values, so it appears that the methyl substituent does not have any effect on this coupling.