Person:
García Baonza, Valentín

First Name

Valentín

Last Name

García Baonza

URI

https://hdl.handle.net/20.500.14352/76144

Affiliation

Universidad Complutense de Madrid

Faculty / Institute

Ciencias Químicas

Area

Química Física

Identifiers

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Now showing 1 - 8 of 8

Charge analysis in (RE)CrO4 scheelites by combined Raman spectroscopy and computer simulations
(Journal of solid state chemistry, 2022) García Baonza, Valentín; Lobato Fernández, Álvaro; Recio, J. Manuel; Taravillo Corralo, Mercedes
The quest for structure-property relationships in scheelite-type (RE)CrO4 compounds (where RE is a rare earth element) is a difficult task due to the number of exceptions found in RE empirical trends and the uncommon Cr(V) oxidation state. In this work, we experimentally and computationally analyse how the stretching vibrational frequencies ν1(Ag) and ν3(Eg) associated with the [CrO4] tetrahedral units evolve in the (RE)CrO4 crystal family (RE ¼ Nd, Gd, Dy, Ho, and Lu). Since previously reported Cr–O distances and volume changes along with the RE series are not sufficiently accurate to explain the monotonic decrease observed for the ν1(Ag) and ν3(Eg) frequencies, a deeper analysis was performed involving the well-known fact that the bond strength (force constant) decreases as the interatomic distance increases. Our results demonstrates that structural and spectroscopic parameters can be reconciled with classical solid state chemistry ideas when charge effects are considered. This analysis provides a new method for predicting chromium oxidation states from Raman spectroscopy that can be generalised to the study of other crystal families.
Infrared spectroscopic study of the formation of fossil resin analogs with temperature using trans-communic acid as precursor
(Microchemical Journal, 2018) Rodríguez Montoro, Óscar; Lobato Fernández, Álvaro; García Baonza, Valentín; Taravillo Corralo, Mercedes
For million years resin exudates have undergone chemical alterations by heat, pressure, radiation, water, oxygen, microorganisms, and have suffered processes of sedimentation and diagenesis. These agents have affected the organic functional groups present in the terpenes of the resins, giving rise to what we nowadays know as fossil resins. In this work, we try to get further insight in the chemical formation of fossil resins. As the simulation of the natural process is quite complex, we have focused on the temperature induced reactivity of the trans-communic acid, the main component of the Class I resins. Using this terpene derivate as a very basic model of a resin exudate, we have monitored their thermal changes by infrared spectroscopy, Differential Scanning Calorimetry and Thermogravimetric Analysis within the range of 25 to 340 °C. The temperature-induced transformation, both in presence and absence of inert atmosphere, is discussed on the basis of the reactivity of the conjugated double-bond, the exocyclic bond and the carboxylic acid group present in the trans-communic acid. The results obtained in these series of experiments agree with the maturation scheme accepted in the literature for natural resins, i.e. an initial cross-linked polymerization and a subsequent maturation reaction. From combined DSC/TGA and infrared spectroscopy results, we conclude that chemical changes produced in the trans-communic acid in the range 130–175 °C may mimic the initial polymerization-like process in the natural resins, whereas those produced between 180 and 340 °C seem to correspond to the maturation pathways described in the literature for fossil resins Class Ib. Spectral assignment of the most relevant infrared-active modes of the trans-communic acid at 25 °C is also provided with the aid of Density Functional Theory calculations.
Generalized Stress-Redox Equivalence: A Chemical Link between Pressure and Electronegativity in Inorganic Crystals
(Inorganic Chemistry, 2019) Lobato Fernández, Álvaro; Osman, Hussien ; Salvadó, Miguel ; Pertierra, Pilar; Vegas, Ángel; García Baonza, Valentín; Recio, José Manuel
The crystal structure of many inorganic compounds can be understood as a metallic matrix playing the role of a host lattice in which the nonmetallic atomic constituents are located, the Anions in Metallic Matrices (AMM) model stated. The power and utility of this model lie in its capacity to anticipate the actual positions of the guest atoms in inorganic crystals using only the information known from the metal lattice structure. As a pertinent test-bed for the AMM model, we choose a set of common metallic phases along with other nonconventional or more complex structures (face-centered cubic (fcc) and simple cubic Ca, CsCl-type BaSn, hP4-K, and fcc-Na) and perform density functional theory electronic structure calculations. Our topological analysis of the chemical pressure (CP) scalar field, easily derived from these standard first-principles electronic computations, reveals that CP minima appear just at the precise positions of the nonmetallic elements in typical inorganic crystals presenting the above metallic subarrays: CaF2, rock-salt and CsCl-type phases of CaX (X = O, S, Se, Te), BaSnO3, K2S, and NaX (X = F, Cl, Br, I). A theoretical basis for this correlation is provided by exploring the equivalence between hydrostatic pressure and the oxidation (or reduction) effect induced by the nonmetallic element on the metal structure. Indeed, our CP analysis leads us to propose a generalized stress-redox equivalence that is able to account for the two main observed phenomena in solid inorganic compounds upon crystal formation: (i) the expansion or contraction experienced by the metal structure after hosting the nonmetallic element while its topology is maintained and (ii) the increasing or decreasing of the effective charge associated with the anions in inorganic compounds with respect to the charge already present in the interstices of the metal network. We demonstrate that a rational explanation of this rich behavior is provided by means of Pearson-Parr’s electronegativity equalization principle.
Temperature effects on the friction-like mode of graphite
(Theoretical Chemical Accounts, 2017) Menéndez, Cesar; Lobato Fernández, Álvaro; García Baonza, Valentín; Recio, José Manuel
Since the anharmonicity of the vibrational mode related to the relative rigid motion of graphene layers plays a decisive role in the friction behavior of graphite, a quantitative account of the temperature dependence of the frequency of this E2G(1) mode is worth to be investigated. Starting with the solution of the Morse quantum-mechanical oscillator, a relationship between the populated averaged vibrational quantum number and temperature is proposed. This expression is applied to our previous computed Morse fittings describing the anharmonic potential of this mode (Menéndez et al. in Phys Rev B 93:144112-1–144112-9, 2016) with the aim at providing the available vibrational energy at different pressures and temperatures. We show that the average vibrational quantum number decreases under pressure but the available vibrational energy is almost independent on pressure at a given temperature. As a result, the calculated temperature coefficient shows that inter-layer friction in graphite lowers as temperature increases with a similar trend regardless the pressure applied.
Chemical pressure–chemical knowledge: squeezing bonds and lone pairs within the valence shell electron pair repulsion model
(Physical Chemistry Chemical Physics, 2019) Lobato Fernández, Álvaro; Osman, Hussien; Salvadó, Miguel; Taravillo Corralo, Mercedes; García Baonza, Valentín; Recio, José Manuel
The valence shell electron pair repulsion (VSEPR) model is a demanding testbed for modern chemical bonding formalisms. The challenge consists in providing reliable quantum mechanical interpretations of how chemical concepts such as bonds, lone pairs, electronegativity, or hypervalence influence (or modulate) molecular geometries. Several schemes have been developed thus far to visualize and characterize these effects; however, to the best of our knowledge, no scheme has yet incorporated the analysis of the premises derived from the ligand close-packing (LCP) extension of the VSEPR model. Within the LCP framework, the activity of the lone pairs of the central atom and ligand–ligand repulsions constitute the two key features necessary to explain certain controversial molecular geometries that do not conform to the VSEPR rules. Considering the dynamical picture obtained when electron local forces at different nuclear configurations are evaluated from first-principles calculations, we investigate the chemical pressure distributions in a variety of molecular systems, namely, electron-deficient molecules (BeH2, BH3, BF3), several AX3 series (A: N, P, As; X: H, F, Cl), SO2, ethylene, SF4, ClF3, XeF2, and nonequilibrium configurations of water and ammonia. Our chemical pressure maps clearly reveal space regions that are totally consistent with the molecular and electronic geometries predicted by VSEPR and provide a quantitative correlation between the lone pair activity of the central atom and electronegativity of ligands, which are in agreement with the LCP model. Moreover, the analysis of the kinetic and potential energy contributions to the chemical pressure allows us to provide simple explanations on the connection between ligand electronegativity and electrophilic/nucleophilic character of the molecules, with interesting implications in their potential reactivity. NH3, NF3, SO2, BF3, and the inversion barrier of AX3 molecules are selected to illustrate our findings.
Local and solvation pressures in aqueous solutions of ethylenediamine probed by Raman spectroscopy
(Physical Chemistry Chemical Physics, 2016) Cáceres Alonso, María Mercedes; Lobato Fernández, Álvaro; Mendoza, Nubia; Jimenez Bonales, Laura; García Baonza, Valentín
Raman spectra of 1,2-ethylenediamine (EDA) in aqueous solutions are used to demonstrate that EDA molecules experience an anti–gauche conformational change resulting from the interactions with water. The observed Raman shift reveals a compressive (hydrophobic) effect of water on both methylene and amino groups of EDA. Raman spectra of EDA at high pressures are used as reference to quantify the intermolecular EDA–H2O interactions in terms of local pressures. These results are compared with macroscopic solvation pressures calculated from the cohesive energy parameter. We compare and discuss all our observations with available computational and experimental studies.
Anharmonicity effects in the frictionlike mode of graphite
(Physical Review B, 2016) Menéndez, Cesar; Lobato Fernández, Álvaro; Abbasi-Pérez, David; Fernández-Núñez, José; Recio, Jose Manuel; García Baonza, Valentín
Graphite is a prototypical solid lubricant demanding a thorough understanding of its low-friction behavior. The 𝐸2⁢𝑔(1) Raman active vibrational mode of graphite is associated with the rigid-layer relative movement of its graphene sheets. Thus, this mode can provide a good means of exploring the low resistance of graphene layers to slip with respect to each other. To take advantage of this fact, the anharmonicity of the 𝐸2⁢𝑔(1) mode has to be carefully characterized and evaluated since the atomic arrangement of carbon atoms in the ambient condition ABA stacking of graphite evidences potential asymmetry. The calculated one-dimensional energetic profile of the 𝐸2⁢𝑔(1) mode reveals this local anisotropy around the energy minima and can be microscopically interpreted in terms of electron density interactions. Morse-type potentials accurately fit the energetic profiles at different interlayer separations, and provide simple analytical expressions for evaluating harmonic and anharmonic contributions to the Γ-point 𝐸2⁢𝑔(1) frequency 𝜔𝐸2⁢𝑔⁡(1) under a perturbative algebraic treatment. We quantify how the anharmonic contribution increases with the available energy (𝐸) at zero pressure, and how this contribution decreases as hydrostatic pressure (𝑝) or uniaxial stress is applied for a given available energy. The calculated 𝜔𝐸2⁢𝑔⁡(1)−𝑝 and 𝜔𝐸2⁢𝑔⁡(1)−𝐸 trends indicate an increasing (decreasing) of frictional forces in graphite with pressure (temperature). Our conclusions are supported by the good agreement of the calculated frequencies with existing Raman experiments under hydrostatic pressure conditions.
The self-absorption phenomenon in quantitative Raman spectroscopy and how to correct its effects
(Microchemical Journal, 2018) Uriarte, Lucas Martinez; Bonales, Laura; Dubessy, Jean; Lobato Fernández, Álvaro; García Baonza, Valentín; Cáceres Alonso, María Mercedes
When the wavelength of the Raman scattered light coincides with the absorption spectrum of the sample, a phenomenon known as Raman self-absorption occurs. If the absorption wavelength range matches completely with the Raman wavenumber range for a given excitation line (high absorption), this process causes spectra with poor signal-to-noise-ratio, making the acquisition nearly impossible. When the absorption of the sample and the Raman scattered are partially coincident (low absorption), smaller modifications in the band-profile of the Raman spectrum are expected difficult to be detected. If a different excitation wavelength is available in order to avoid or minimize this phenomenon, a correction method is customary. The correction methods developed so far in quantitative analysis are still very complex and require the knowledge of several sampling parameters. In this work, we present a very simple but reliable method based on the Beer-Lambert law to correct low self-absorption effects. This method uses simple concepts, a straightforward methodology and the concentration of the Raman active component in the liquid matrix is the only parameter required to perform the correction. This method may be extremely useful in different fields in which Raman spectroscopy is used to obtain molecular and structural information from a band profile analysis. As an example, the method is applied here to successfully correct the Raman spectra of different CuSO4 aqueous solutions excited at 532nm.