F.J. Torres

Ab initio study of the vibrational spectrum and related properties of crystalline compounds; the case of CaCO3 calcite

Abstract The static and high frequency dielectric tensors, Born effective charges, vibrational spectrum at the Γ point, TO-LO splitting and IR intensities of calcite CaCO3 have been calculated with the periodic ab initio CRYSTAL program, with five different basis sets of increasing size and four different Hamiltonians (HF, LDA, PBE, B3LYP). B3LYP is shown to perform better than the other options, in particular of LDA and PBE that are often used for the calculation of the vibrational spectrum of crystalline solids. When comparing B3LYP and experimental frequencies, the mean absolute difference is as small as 8.5 cm-1; this number reduces to 4.8 cm-1 if the two lowest experimental frequencies, that we suspect to be affected by a relatively large error, are excluded from statistics. Static and high frequency dielectric tensors, as well as IR intensities computed with the same hybrid scheme (B3LYP) compare quite favourably with experiment. The full set of modes is characterized by various tools including isotopic substitution, “freezing” one of the two subunits (Ca2+ or CO32-) and graphical representations. A general tool has been implemented, that permits the automatic generation of the animation of the full set of modes starting from the CRYSTAL output (available at www.crystal.unito.it/vibs/calcite).

Ab initio simulation of the IR spectra of pyrope, grossular, and andradite

IR spectra of pyrope Mg3Al2Si3O12, grossular Ca3Al2Si3O12 and andradite Ca3Fe2Si3O12 garnets were simulated with the periodic ab initio CRYSTAL code by adopting an all‐electron Gaussian‐type basis set and the B3LYP Hamiltonian. Two sets of 17 F1u Transverse Optical (TO) and Longitudinal Optical (LO) frequencies were generated, together with their intensities. Because the generation of LO modes requires knowledge of the high frequency dielectric constant ε∞ and Born effective charges, they were preliminary evaluated by using a finite field saw‐tooth model and well localized Wannier functions, respectively. As a by‐product, the static dielectric constant ε0 was also obtained. The agreement of the present calculated wavenumbers (i.e. peak positions) with the available experimental data is excellent, in that the mean absolute difference for the full set of data smaller than 8 cm−1. Missing peaks in experimental spectra were found to correspond to modes with low calculated intensities. Correspondence between TO and LO modes was established on the basis of the overlap between the eigenvectors of the two sets and similarity of isotopic shifts; as result, the so called LO‐TO splitting could be determined. Animation of the normal modes was employed to support the proposed pairing.

Ab initio study of the structural, electronic, and thermodynamic properties of linear perfluorooctane sulfonate (PFOS) and its branched isomers.

Structural, electronic, and thermodynamic properties of linear perfluorooctane sulfonate (PFOS) and its trifluoromethyl-branched isomers (i.e. 1-CF(3)- to 6-CF(3)-PFOS) were theoretically studied by means of ab initio density functional theory (DFT) calculations with the B3LYP functional and a 6-31++G(d,p) basis set. The anionic form of linear PFOS and its trifluromethyl-branched isomers were considered for the initial construction of the computational models; subsequently, H(+), Li(+), and Na(+) were added as counter-ions to study their effect on the properties under investigation. Insignificant changes with respect to the anions were observed in the structure of both the protonated and salt forms due to the presence of these counter-ions. However, important differences in the electrostatic potential maps as well as HOMO and LUMO molecular orbitals were observed for the various forms of PFOS. The linear and branched PFOS ions were identified as the most suitable compounds for interacting with charged species. Furthermore, in the linear anion, it was observed that the LUMO orbital is diffused along the whole fluoro-carbon chain, whereas it is localized to the region close to the ternary carbon in the 4-CF(3)-PFOS, 5-CF(3)-PFOS, and 6-CF(3)-PFOS isomers. The higher accessibility of the LUMO orbital in these branched anions suggests that they have a higher probability of reacting with free radicals when compared with the linear counterpart. This behavior is reflected in the experimental observation that only the branched PFOS isomers were susceptible to reductive defluorination by reduced vitamin B(12) as we previously reported. The relative stability of the linear and branched PFOS in their different forms computed by comparing their calculated Gibbs free energy showed that 1-CF(3)-, 6-CF(3)-, and linear PFOS are the most favorable structures from a thermodynamic point of view.

Physico-Chemical and Structural Interpretation of Discrete Derivative Indices on N-Tuples Atoms

This report examines the interpretation of the Graph Derivative Indices (GDIs) from three different perspectives (i.e., in structural, steric and electronic terms). It is found that the individual vertex frequencies may be expressed in terms of the geometrical and electronic reactivity of the atoms and bonds, respectively. On the other hand, it is demonstrated that the GDIs are sensitive to progressive structural modifications in terms of: size, ramifications, electronic richness, conjugation effects and molecular symmetry. Moreover, it is observed that the GDIs quantify the interaction capacity among molecules and codify information on the activation entropy. A structure property relationship study reveals that there exists a direct correspondence between the individual frequencies of atoms and Hückel’s Free Valence, as well as between the atomic GDIs and the chemical shift in NMR, which collectively validates the theory that these indices codify steric and electronic information of the atoms in a molecule. Taking in consideration the regularity and coherence found in experiments performed with the GDIs, it is possible to say that GDIs possess plausible interpretation in structural and physicochemical terms.

Ab Initio investigation of the interaction of H2 with lithium exchanged low-silica chabazites

The interaction of molecular hydrogen with the polarizing centers of lithium-exchanged chabazites with low Si/Al ratio (CHA-5/1 and CHA-3/1) was theoretically studied within a periodic approach at the B3LYP level of theory. The cation site preferences in both zeolites were determined and the H2 interaction was then studied by adding a molecule in the proximity of the different polarizing centers. The energetic features of the H2-Li2CHA-5/1 complex were also refined by using a cluster model cut out from the periodic structure in which the contribution of the dispersive forces to the interaction (absent at the B3LYP level of theory) were estimated at the MP2 level by means of an ONIOM2-like approach. Overall results show that the position of the polarizing center in the structure and its stability in the site is crucial for determining the sorption capacity of the zeolite. Lithium exchanged low-silica zeolites are shown to be only effective for hydrogen storage when the Al loading is at least 3:1. However, the highest predicted enthalpy of adsorption is around 10 kJ/mol and this indicates that they are far from fulfilling the conditions for practical applications.