Electronegativity Equalization Research Articles

K0.72Na1.71Ca5.79Si6O19 - the first oligosilicate based on [Si6O19]-hexamers and its stability compared to cyclosilicates.

Synthesis experiments were conducted in the quaternary system K2O-Na2O-CaO-SiO2, resulting in the formation of a previously unknown compound with the composition K0.72Na1.71Ca5.79Si6O19. Single crystals of sufficient size and quality were recovered from a starting mixture with a K2O:Na2O:CaO:SiO2 molar ratio of 1.5:0.5:2:3. The mixture was confined in a closed platinum tube and slowly cooled from 1150°C at a rate of 0.1°C min-1 to 700°C before being finally quenched in air. The structure has tetragonal symmetry and belongs to space group P4122 (No. 91), with a = 7.3659 (2), c = 32.2318 (18) Å, V = 1748.78 (12) Å3, and Z = 4. The silicate anion consists of highly puckered, unbranched six-membered oligomers with the composition [Si6O19] and point group symmetry 2 (C2). Although several thousands of natural and synthetic oxosilicates have been structurally characterized, this compound is the first representative of a catena-hexasilicate anion, to the best of our knowledge. Structural investigations were completed using Raman spectroscopy. The spectroscopic data was interpreted and the bands were assigned to certain vibrational species with the support of density functional theory at the HSEsol level of theory. To determine the stability properties of the novel oligosilicate compared to those of the chemically and structurally similar cyclosilicate combeite, we calculated the electronegativity of the respective structures using the electronegativity equalization method. The results showed that the molecular electronegativity of the cyclosilicate was significantly higher than that of the oligostructure due to the different connectivities of the oxygen atoms within the molecular units.

Interpretable AI and Machine Learning Classification for Identifying High-Efficiency Donor-Acceptor Pairs in Organic Solar Cells.

To enhance the efficiency of organic solar cells, accurately predicting the efficiency of new pairs of donor and acceptor materials is crucial. Presently, most machine learning studies rely on regression models, which often struggle to establish clear rules for distinguishing between high- and low-performing donor-acceptor pairs. This study proposes a novel approach by integrating interpretable AI, specifically using Shapely values, with four supervised machine learning classification models, namely, support vector machines, decision trees, random forest, and gradient boosting. These models aim to identify high-efficiency donor-acceptor pairs based solely on chemical structures and to extract important features that establish general design principles for distinguishing between high- and low-efficiency pairs. For validation purposes, an unsupervised machine learning algorithm utilizing loading vectors obtained from the principal component analysis is employed to identify crucial features associated with high-efficiency donor-acceptor pairs. Interestingly, the features identified by the supervised machine learning approach were found to be a subset of those identified by the unsupervised method. Noteworthy features include the van der Waals surface area, partial equalization of orbital electronegativity, Moreau-Broto autocorrelation, and molecular substructures. Leveraging these features, a backward-working model can be developed, facilitating exploration across a wide array of materials used in organic solar cells. This innovative approach will help navigate the vast chemical compound space of donor and acceptor materials essential in creating high-efficiency organic solar cells.

Origin of Enantioselectivity in Engineered Cytochrome c-Catalyzed Carbon-Radical FePP Hydrolysis Revealed Using QM/MM (ABEEM Polarizable Force Field) and MD Simulations.

The origin of highly efficient asymmetric aminohydroxylation of styrene catalyzed by engineered cytochrome c is investigated by the developed Atom-Bond Electronegativity Equalization Method polarizable force field (ABEEM PFF), which is a combined outcome of electronic and steric effects. Model molecules were used to establish the charge parameters of the ABEEM PFF, for which the bond-stretching and angle-bending parameters were obtained by using a combination of modified Seminario and scan methods. The interactions between carbon-radical Fe-porphyrin (FePP) and waters are simulated by molecular dynamics, which shows a clear preference for the pre-R over the pre-S. This preference is attributed to the hydrogen-bond between the mutated 100S and 101P residues as well as van der Waals interactions, enforcing a specific conformation of the carbon-radical FePP complex within the binding pocket. Meanwhile, the hydrogen-bond between water and the nitrogen atom in the active intermediate dictates the stereochemical outcome. Quantum mechanics/molecular mechanics (QM/MM (ABEEM PFF)) and free-energy perturbation calculations elucidate that the 3RTS is characterized by sandwich-like structure among adjacent amino acid residues, which exhibits greater stability than crowed arrangement in 3STS and enables the R enantiomer to form more favorably. Thus, this study provides mechanistic insight into the catalytic reaction of hemoproteins.

A closer look to the bispericyclic transition structure leading to 4 + 2 and 2 + 4 cycloadducts in the endo dimerization of cyclopentadiene

AbstractThis work delves into the study presented by Caramella and co‐workers (J. Am. Chem. Soc. 2002, 124, 7) on the cyclopentadiene dimerization. We have reperformed the calculations of this work using updated methodologies, obtaining the structures of the reactants and transition states, as well as new energy profiles. Additionally, we have obtained new insights that were not included in the previous work, such as bond structures from NBO analysis, QTAIM theory, and spin density. Reactivity indices of the principal NBOs responsible for the reaction have also been obtained using a recently developed model based on conceptual DFT and Sanderson's principle of electronegativity equalization and the new results obtained have allowed us to propose reaction mechanisms that satisfactorily justify all reaction stages. This work is a study of the mechanism of the cyclopentadiene dimerization reaction, but it is also an example of applying the new model for calculating reactivity indices for NBOs. Additionally, it demonstrates the combined use of these descriptors with other types of analyses that also employ NBOs or provide a description of the molecular bonding structure. As shown in this work, the collective use of all these methodologies generally provides an adequate description of the chemical reactivity of the studied processes.

Dense Neural Network for Calculating Solvation Free Energies from Electronegativity-Equalization Atomic Charges.

I propose a dense Neural Network for evaluation of solvation free energies ΔG°solv for molecules and ions in water and nonaqueous solvents, Easy Solvation Energy with Electronegativity Equalization charges and Dense Neural Network (ESE-EE-DNN). As input features, it uses the Conductor-like Screening Model (COSMO) electrostatic energy, atomic cavity surface areas, total cavity volume, and induced surface charges. For the COSMO calculation, electronegativity-equalization atomic charges are employed. ESE-EE-DNN exhibits fairly high accuracy, similar or even superior to that of mainstream density functional theory-based methods. For neutral solutes in water, polar protic, polar aprotic, and nonpolar solvents, ESE-EE-DNN yields a root-mean-square error (RMSE) of 1.25, 1.36, 0.70, and 0.71 kcal/mol, respectively. ESE-EE-DNN is particularly advantageous for ionic solutes, with an RMSE of 2.82 and 1.42 kcal/mol for aqueous and nonaqueous ion solutions, correspondingly. ESE-EE-DNN is very efficient due to a fast evaluation of the electronegativity-equalization charges.

Local reactivity descriptors of the important atoms in chelotropic reactions provide insight into their global variants along the reaction path

AbstractChelotropic reactions are widely used in organic synthesis. These reactions are of the pericyclic type where a π bond and a lone pair are transformed into a pair of sigma bonds; with both sigma bonds added to the same atom. This work presents an analysis of several representative chelotropic reactions. To study the reaction path, we have divided it into two parts, from the reactants to the transition state and from the transition state to the products which has allowed us to compare the behavior of the reactants in these two ranges of the path. To perform the analysis, global reactivity descriptors based on the conceptual‐density functional theory (DFT), such as electrophilicity and global softness, have been calculated. To analyze these descriptors, the corresponding local descriptors have been used. A model based on Sanderson's principle (electronegativity equalization principle), developed previously, has been used to calculate the local reactivity descriptors (atomic). The statistical methodology of multiple regression analysis has been used to determine which local variables are the most relevant for the global parameter under study (softness, and electrophilicity), also the principal component analysis has been used as a guide to estimate the number of variables to take into account in the analysis, finally this has led us to important correlations between global and local parameters which has allowed us to analyze the reactions of the study. Transition states of the reactions are aromatic as obtained from the nucleus independent chemical shift (NICS) and gauge including magnetically induced ring current (GIMIC) analysis.

Theoretical Base of Pseudo Chemical Potential (PCP) Method

Pseudo chemical potential (PCP) method is a novel one based on the thermodynamic formalization, and the purpose of this work is to clarify theoretical base of PCP method and to perfect PCP method into a novel method for molecular calculation. Therefore, it is very important to clarify validity of thermodynamic formalization that becomes principle and methodology of this method for achievement of its purpose. The thermodynamic formalization in molecular calculation is to achieve molecular calculation by using principle and methodology of thermodynamics. In order to apply the method of thermodynamic formalization in molecular calculation, it must be clarified that molecule (atom) electron system, computational object of PCP method, can be considered to be “thermodynamic system”. In this paper, we have clarified the temperature zero limit (TZL) state of a finite temperature thermal equilibrium system, as physical base of the pseudo chemical potential (PCP) method, having thermodynamic properties and therefore being the research object of thermodynamic formalism. Furthermore, we have proved an existence of energy minimization principle, as the theoretical base of PCP method, and from it derived the variation equation and defined the electron charge distribution equation, and on the basis of it analyzed theoretically the electronegativity equalization principle. Resultantly, herein we have demonstrated theoretical validity of PCP method, as a novel method for calculation of molecular energies and charge distributions.

Methods involved in the treatment of four representative pharmaceuticals in hospital wastewater using sonochemical and biological processes

A primary pollution source by pharmaceuticals is hospital wastewater (HWW). Herein, the methods involved in the action of a biological system (BS, aerobic activated sludge) or a sonochemical treatment (US, 375 kHz and 30.8 W), for degrading four relevant pharmaceuticals (azithromycin, ciprofloxacin, paracetamol, and valsartan) in HWW, are shown. Before treatment of HWW, the correct performance of BS was assessed using glucose as a reference substance, monitoring oxygen consumption, and organic carbon removal. Meanwhile, for US, a preliminary test using ciprofloxacin in distilled water was carried out. The determination of risk quotients (RQ) and theoretical analyses about reactive moieties on these target substances are also presented. For both, the degradation of the pharmaceuticals and the calculation of RQ, analyses were performed by LC-MS/MS. The BS action decreased the concentration of paracetamol and valsartan by ∼96 and 86%, respectively. However, a poor action on azithromycin (2% removal) was found, whereas ciprofloxacin concentration increased ∼20%; leading to an RQ value of 1.61 (high risk) for the pharmaceuticals mixture. The analyses using a biodegradation pathway predictor (EAWAG-BDD methodology) revealed that the amide group on paracetamol and alkyl moieties on valsartan could experience aerobic biotransformations. In turn, US action decreased the concentration of the four pharmaceuticals (removals > 60% for azithromycin, ciprofloxacin, and paracetamol), diminishing the environmental risk (RQ: 0.51 for the target pharmaceuticals mixture). Atomic charge analyses (based on the electronegativity equalization method) were performed, showing that the amino-sugar on azithromycin; piperazyl ring, and double bond close to the two carbonyls on ciprofloxacin, acetamide group on paracetamol, and the alkyl moieties bonded to the amide group of valsartan are the most susceptible moieties to attacks by sonogenerated radicals. The LC-MS/MS analytical methodology, RQ calculations, and theoretical analyses allowed for determining the degrading performance of BS and US toward the target pollutants in HWW.•Biological and sonochemical treatments as useful methods for degrading 4 representative pharmaceuticals are presented.•Sonochemical treatment had higher degrading action than the biological one on the target pharmaceuticals.•Methodologies for risk environmental calculation and identification of moieties on the pharmaceuticals susceptible to radical attacks are shown.

Atoms-In-Molecules' Faces of Chemical Hardness by Conceptual Density Functional Theory.

The chemical hardness concept and its realization within the conceptual density functional theory is approached with innovative perspectives, such as the electronegativity and hardness equalization of atoms in molecules connected with the softness kernel, in order to examine the structure-reactivity equalization ansatz between the electronic sharing index and the charge transfer either in the additive or geometrical mean picture of bonding. On the other hand, the maximum hardness principle presents a relation with the chemical stability of the hardness concept. In light of the inverse relation between hardness and polarizability, the minimum polarizability principle has been proposed. Additionally, this review includes important applications of the chemical hardness concept to solid-state chemistry. The mentioned applications support the validity of the electronic structure principles regarding chemical hardness and polarizability in solid-state chemistry.

An FSGO based Electronegativity Scale invoking the Electrophilicity Index

AbstractElectronegativity is an important chemical construct that plays a pivotal role to explain several chemical, biochemical and physicochemical phenomena. The study of this periodic descriptor is still considered an active domain of research, and a number of scientists are involved to propose different scales of electronegativity based on experimental findings and theoretical concepts. In this study, we have proposed a model to compute atomic electronegativity values of 103 elements based on the Floating Spherical Gaussian Orbital approach invoking the atomic electrophilicity index as a descriptor. The computed electronegativity scale observes the periodic trend and justifies many chemical phenomena. Molecular electronegativity values have also been computed using the computed atomic electronegativity data and utilized to justify the Electronegativity Equalization Principle. In order to validate our proposed model, internuclear bond distances for some molecules have been deduced in terms of our computed atomic electronegativity data. A strong correlation with experimental counterparts proves the efficacy of our proposed model.

From Density Functional Theory to Conceptual Density Functional Theory and Biosystems.

The position of conceptual density functional theory (CDFT) in the history of density functional theory (DFT) is sketched followed by a chronological report on the introduction of the various DFT descriptors such as the electronegativity, hardness, softness, Fukui function, local version of softness and hardness, dual descriptor, linear response function, and softness kernel. Through a perturbational approach they can all be characterized as response functions, reflecting the intrinsic reactivity of an atom or molecule upon perturbation by a different system, including recent extensions by external fields. Derived descriptors such as the electrophilicity or generalized philicity, derived from the nature of the energy vs. N behavior, complete this picture. These descriptors can be used as such or in the context of principles such as Sanderson’s electronegativity equalization principle, Pearson’s hard and soft acids and bases principle, the maximum hardness, and more recently, the minimum electrophilicity principle. CDFT has known an ever-growing use in various subdisciplines of chemistry: from organic to inorganic chemistry, from polymer to materials chemistry, and from catalysis to nanotechnology. The increasing size of the systems under study has been coped with thanks to methodological evolutions but also through the impressive evolution in software and hardware. In this flow, biosystems entered the application portfolio in the past twenty years with studies varying (among others) from enzymatic catalysis to biological activity and/or the toxicity of organic molecules and to computational peptidology. On the basis of this evolution, one can expect that “the best is yet to come”.

Introducing a new model based on electronegativity equalization principle for the analysis of the natural bond orbital reactivity in the c‐DFT background

AbstractPrevious studies presented two independent models based on the equalization of electronegativity and on an alternative operative formula for the Fukui function. The current paper presents modifications to these models to obtain a new model for calculating the local energy derivatives (electronic chemical potential, local electronic hardness and local hypersoftness) and density (Fukui and dual descriptor indices) in natural bond orbitals for the study of chemical reactivity from the perspective of these orbitals (local electrophilicity and local maximum charge variation have also been deduced). We have radically changed the way of partitioning the electron density because the atomic model cannot distinguish between bonding and non‐bonding electron pairs, or between electrons with different types of bonds, however, this is possible with the model based on bond orbitals. In this respect, this model is ideal for organic reactions because of their correspondence with Lewis structures, which are simple but very useful for representing reaction mechanisms.

Electronegativity Equilibration.

Controlling the distribution of electrons in materials is the holy grail of chemistry and material science. Practical attempts at this feat are common but are often reliant on simplistic arguments based on electronegativity. One challenge is knowing when such arguments work, and which other factors may play a role. Ultimately, electrons move to equalize chemical potentials. In this work, we outline a theory in which chemical potentials of atoms and molecules are expressed in terms of reinterpretations of common chemical concepts and some physical quantities: electronegativity, chemical hardness, and the sensitivity of electronic repulsion and core levels with respect to changes in the electron density. At the zero-temperature limit, an expression of the Fermi level emerges that helps to connect several of these quantities to a plethora of material properties, theories and phenomena predominantly explored in condensed matter physics. Our theory runs counter to Sanderson’s postulate of electronegativity equalization and allows a perspective in which electronegativities of bonded atoms need not be equal. As chemical potentials equalize in this framework, electronegativities equilibrate.

A Polarizable Cationic Dummy Metal Ion Model.

A novel locally polarizable multisite model based on the original cation dummy atom (CDA) model is described for molecular dynamics simulations of ions in condensed phases. Polarization effects are introduced by the electronegativity equalization model (EEM) method where charges on the metal ion and its dummy atoms can fluctuate to respond to the environment. This model includes explicit polarization and ion-induced interactions and can be coupled with nonpolarizable or polarizable water models, making it more transferable to simpler force fields. This approach allows us to enhance the original fixed charge CDA model where the charge distribution cannot adapt to the local solvent structure. To illustrate the new CDApol model, we examined properties of the Zn2+, Al3+, and Zr4+ ions in aqueous solution. The polarizable model and Lennard-Jones parameters were refined for octahedrally coordinated Zn2+, Al3+, and Zr4+ CDAs to reproduce thermodynamic and geometrical properties. Using this locally polarizable model, we were able to obtain the experimental hydration free energy, ion-oxygen distance, and coordination number coupled with the standard 12-6 Lennard-Jones model. This model can be used in myriad additional applications where local polarization and charge transfer is important.

A quick solvation energy estimator based on electronegativity equalization.

ESE-EE (Easy Solvation Estimation with Electronegativity equalization) is a quick method for estimation of solvation-free energies ΔGºsolv , which uses a thoroughly fitted electronegativity equalization (EE) scheme to obtain atomic charges, which are further employed in a scaled noniterative COSMO-like calculation to evaluate the electrostatic component of ΔGºsolv . Nonelectrostatic corrections including adjustable parameters are also added. For neutral solutes, ESE-EE yields a mean absolute error (MAE) in ΔGsolv ° of 1.5kcal/mol for aqueous solutions; 1.0kcal/mol for nonaqueous polar protic solvents; 0.9kcal/mol for polar aprotic solvents; and about 0.6kcal/mol for nonpolar solvents. Since ESE-EE only requires a molecular geometry as input for a ΔGºsolv prediction, it can be utilized for a rapid screening of ΔGºsolv for large neutral molecules. However, for ionic solutes, ESE-EE yields larger errors (typically several kcal/mol) and is recommendable for preliminary estimations only. Upon a special refitting, ESE-EE is able to yield partition coefficients with a good accuracy.

Recent Development and Applications of the ABEEM/MM Polarizable Force Field

In this paper, we review both development and applications of the atom-bond electronegativity equalization method fused into molecular mechanics, i.e., ABEEM/MM polarizable force field (FF). We will focus on the applications of the ABEEM/MM in pure water systems, chemical and biomolecular ion-containing systems, small molecules and biomolecules, etc. The results show that the performance of ABEEM/MM is generally better than that of the commonly used nonpolarizable force fields.

Can van der Waals constants be used in the chemical reactivity analysis? A new approach as a support to minimum magnetizability principle

AbstractOne of the most important purposes of theoretical chemists is to derive new, useful, and reliable equations to compute the well‐known parameters like hardness, polarizability, magnetizability, and electrophilicity providing enough hints for the reactivity analysis of the chemical systems. In the derivation of such equations, our research team widely considers the popular electronic structure rules like electronegativity equalization, maximum hardness, minimum polarizability, minimum magnetizability, and minimum electrophilicity principles. Recently, in the light of maximum entropy and minimum magnetizability principles, Kaya and Chattaraj (2021) presented a simple way to calculate their standard absolute entropies (S0298) based on molar diamagnetic susceptibilities (χm) of inorganic ionic systems. In the present paper, a new and useful molar diamagnetic susceptibility calculation method including the use of van der Waals constants (a and b) is introduced. Here, we named this method as Kaya–Şimşek approach. Through the relations between molar diamagnetic susceptibility and van der Waals constants, we can easily predict the magnetic susceptibility of molecules that their magnetic susceptibilities are unknown. The results of the equations derived are quite close to experimentally reported data. The analyses made proved that van der Waals constants can be considered as chemical reactivity descriptors. We propose that in stable states, van der Waals constants are minimized. The validity of minimum magnetizability principle is supported with solid evidences.

Insight into generalized local softness based on ABEEM‐σπ method: Electrophilic additions to alkenes

AbstractAccording to the local hard‐soft acid – base (HSAB) principle, the generalized local softness has been investigated based on the electrophilic reactions of H2O, HF, HCl and HBr to alkenes, and the effect of the softness on the chemical reactivity was studied in terms of the ab initio and atom‐bond electronegativity equalization method plus σπ (ABEEM‐σπ) method. Studies show that the finite difference approximation with ab initio method cannot always be used to predict the regioselectivity of the investigated reactions in terms of the local HSAB principle. But ABEEM‐σπ model could well describe chemical reactivity according to the local HSAB principle. A significant characteristic of the ABEEM‐σπ method is that the double bond is explicitly considered and partitioned. The square sums of differences of the local softness of the reaction center atoms, that is, Δs, or ΔSG with the generalized local softness and with the electronegativity, have been studied on the considered electrophilic additions. Studies indicate that ΔSG can be utilized not only to predict regioselectivity rules but also to rationalize the chemical reactivity of these addition reactions, and the generalized local softness is more suitable for the investigation of chemical reactivity between two chemical species.

Generalized Method for Charge-Transfer Equilibration in Reactive Molecular Dynamics.

Variable charge models (e.g., electronegativity equalization method (EEM), charge equilibration (QEq), electrostatic plus (ES+)) used in reactive molecular dynamics simulations often inherently impose a global charge transfer between atoms (approximating each system as an ideal metal). Consequently, most surface processes (e.g., adsorption, desorption, deposition, sputtering) are affected, potentially causing dubious dynamics. This issue has been addressed by certain split charge variants (i.e., split charge equilibration (SQE), redoxSQE) through a distance-dependent bond hardness, by the atomic charge ACKS2 and QTPIE models, which are based on the Kohn-Sham density functional theory, as well as by an electronegativity screening extension to the QEq model (approximating each system as an ideal insulator). In a brief review of the QEq and the QTPIE model, their applicability for studying surface interactions is assessed in this work. Following this evaluation, a revised generalization of the QEq and QTPIE models is proposed and formulated, called the charge-transfer equilibration model or in short the QTE model. This method is based on the equilibration of charge-transfer variables, which locally constrain the split charge transfer per unit time (i.e., due to overlapping orbitals) without any kind of bond hardness specification. Furthermore, a formalism relying solely on atomic charges is obtained by a respective transformation, employing an extended Lagrangian method. We moreover propose a mirror boundary condition and its implementation to accelerate surface investigations. The models proposed in this work facilitate reactive molecular dynamics simulations, which describe various materials and surface phenomena appropriately.

Structural and Compositional Characteristics of Ball-Milled Lepidocrocite Alkali Titanate and the Correlation to Its Surface Acidic-Basic Properties.

The studies on mechanical treatments of layered alkali metal oxides are limited despite their diverse compositions/structures and potential for property tuning. In this work, we vibratory mill Cs0.7Zn0.35Ti1.65O4, K0.8Zn0.4Ti1.6O4, and Cs2Ti6O13 for up to 4 h, during which the lepidocrocite-type structure and the plate-like morphology are well preserved. X-ray diffraction (XRD) indicates a tiny (≤0.6 Å) interlayer expansion accompanied by the enhancement of the preferred orientation along the stacking direction. Chemical analyses across multiple length scales suggest Cs deintercalation, elemental redistributions, and bulk-to-surface (or crystal edge) Cs migration. This ball-milling-induced Cs-rich moiety partially blocks the surface acid sites, although the solids still show a dominating acidic character. The ball-milled samples Cs0.7-pZn0.35-qTi1.65O4-δ contain vacancies between the sheets (p) and at the sheets (q and δ). It is deduced from Sanderson's electronegativity equalization principle and experimentally verified by X-ray photoelectron spectroscopy (XPS) that ball milling increases (decreases) the partial charge at the surface acidic Ti4+/Zn2+ (basic O2-) sites. These nonporous solids (≤20 m2·g-1) contain water sorbed on the external surface as high as 1.1 mol·mol-1, which is comparable to that in a water-intercalated sample. Our work expands the current understanding of the reactivity vs robustness in layered alkali titanates under physically demanding conditions, complementing knowledge gathered via the soft chemistry approach.