Valence Force Field Research Articles

Driving factors for the peculiar bond length dependence and tetragonal distortion of (Ag,Cu)(In,Ga)Se2 and other chalcopyrites

Abstract The chalcopyrite alloy (Ag,Cu)(In,Ga)Se2 is a highly efficient thin film solar cell absorber, reaching record efficiencies above 23 %. Recently, a peculiar behavior in the bond length dependence of (Ag,Cu)GaSe2 was experimentally proven. The common cation bond length, namely Ga-Se, decreases with increasing Ag/(Ag+Cu) ratio even though the crystal lattice expands. This is opposite to the behavior observed for Cu(In,Ga)Se2, where all bond lengths increase with increasing lattice size. To better understand this peculiar bond length behavior, element-specific bond lengths of (Ag,Cu)InSe2 and Ag(In,Ga)Se2 alloys are determined using extended x-ray absorption fine structure spectroscopy. They show that the peculiar bond length dependence occurs only for (Ag,Cu) alloys, independent of the species of common cation (In or Ga). The bond lengths are used to determine the anion displacements and to estimate their contribution to the bandgap bowing. Again, both behaviors differ significantly depending on the type of alloyed cation. A valence force field approach, relaxing bond lengths and bond angles, is used to describe the structural distortion energy for a comprehensive set of I-III-VI2 and II-IV-V2 chalcopyrites. The model reveals bond angle distortions as main driving factor for the tetragonal distortion and reproduces the literature values with less than 10 % deviation. In contrast, the peculiar bond length dependence is not reproduced, demonstrating that it originates from electronic effects beyond the scope of this structural model. Thus, a fundamental understanding of bond length behavior and tetragonal distortion is achieved for chalcopyrite materials, benefiting their technological applications such as high efficiency thin film photovoltaics.

Bandgap tuning in ZnxCd1-xTe superlattices through variable atomic ordering.

We explore the entire search space of 32-layer ZnxCd1-xTe superlattices to find the structures that minimize and maximize the bandgap at each possible zinc concentration. The searching is accomplished through an accurate and efficient combination of valence force field dynamics, the empirical pseudopotential method, and the folded spectrum method. We also describe the use of an alternate preconditioner that improves the robustness and efficiency of the locally optimal preconditioned conjugate gradient's solutions to the folded spectrum method. The physical properties of these superlattices, such as their formation energies, bandgaps, densities of states, effective masses, and optical response functions, are investigated with density functional theory paired with hybrid functionals and compare well to available experimental measurements. It is revealed that the bandgap of ZnxCd1-xTe may change by up to 0.2eV depending on how the layers in the superlattice are ordered. Stacking order has a large, irregular effect on the effective masses, but optical response functions seem insensitive to it.

Comparative Studies of Phonon Frequency Spectrum of HTSCs GdBa2Cu3O7 and SmBa2Cu3O7 Through Normal Co-Ordinate Analysis

The prime application of Nano technology is High temperature superconductor. The Raman spectroscopy of HTSC explains that the role played by the phonons in the mechanism of superconductivity in the perovskite family of superconductors is not fully understood although a large series of experimental results has shown considerable coupling of some phonons to electronic excitation in these systems. Optical spectroscopy and especially Raman and Infrared spectroscopy revealed strong direct influences of the changes in the electronic states to the phonon at the center of the Brillouin. However, it is indispensable to have a precise knowledge of the zone –boundary phonons to investigate the contribution of electron-phonon coupling to Tc which can be achieved by neutron scattering techniques. The most commonly employed model is the valence force field model for computing phonon frequencies based on stretching bond and bending bond coordinates. Such models have the advantage of being well adopted to describe a covalent bonding but ignored the long–range Coulomb forces and wave vector dependence of the phonon spectrum. Even though a fairly good amount of literature is available on the vibrational spectra of high temperature superconductors, still some specific feature in the experimental vibration spectra could not be assigned reliable to a definite type of vibration. Hence a normal coordinate analysis (NCA) which is applicable to zero wave-vector normal-mode vibrations have been carried out for the high temperature superconductors and the assignment of specific features like phonon softening and hardening were looked in for the clear understanding of the superconducting mechanism in these new class materials. In the normal coordinate analysis, PED play an important role for characterization of the relative contributions from each internal coordinate to the total potential energy associated with particular normal coordinate of the molecule. The contribution to the potential energy from the individual diagonal elements give rise to a conceptual link between the empirical analysis of infrared spectra of complex molecules dealing with characteristic group frequencies and the theoretical approach from the computation of the normal modes. NCA gives complete assessment of all normal vibration modes of the system.

Insights into synthesis, structural, energetic, vibrational, anticancer activity and molecular characteristics of 2-((2-aminopyridin-3-yl)methylene)-N-phenylhydrazinecarbothioamide as evaluated using spectroscopic and DFT investigations

Synthesis of 2-((2-aminopyridin-3-yl)methylene)-N-phenylhydrazinecarbothioamide (APHB) was attempted. Elemental analysis and NMR spectra were used to ascertain its formation. Torsional potential energy scans, for all the rotating bonds were made to get approximate initial values of dihedral angles. FT-IR, FT Raman, and UV–Vis spectra were measured for APHB. Their anticancer activity was determined experimentally, for human carcinoma cell lines pertaining to liver, Colorectal, and lung. Barrier heights, around six rotating bonds in APHB were computed. Optimized structure parameters, general valence force field, harmonic vibrational fundamentals, potential energy distribution, infrared and Raman intensities, frontier molecular orbital (FMO) parameters, NLO behaviour, NBO characteristics and molecular electrostatic surface potential (MESP) were determined using DFT/B3LYP/6-311++G(d,p) level of theory. TD-DFT was used to compute absorption maxima (λmax) of electronic transitions for the molecule in DMSO‑d6 solvent. Frontier molecular orbitals were used to understand origin of UV–Vis spectrum and chemical reactivity of the molecule. Good agreement was found between measured and computed structure parameters, IR, Raman and UV–Vis spectra. The rms error between experimental and theoretical vibrational frequencies was 9.5 cm−1, for APHB. All vibrational fundamentals were assigned unambiguously. The computations demonstrated that the molecule was good for NLO applications, which was substantiated by NBO analysis. Existence of bifurcated intramolecular hydrogen bond was predicted for APHB.

Insights into structural and vibrational characteristics of 1-methoxy-4-[2-(phenylsulfonyl)vinyl]benzene: An application of experimental vibrational spectroscopy and density functional theory

Fourier transform infrared and Raman spectra, in the spectral range 4000- 400 and 4000–50 cm−1, respectively, were measured for 1‑methoxy-4-[2-(phenylsulfonyl)vinyl]benzene (MPB). Initial values of torsion angles around five flexible bonds C1-S, S-C15, C17-C19, C22-O and OC30 (see Fig. 1 for numbering), essential for starting geometry optimization, were determined using "bond-pairing method" proposed earlier by us. Optimized structure parameters, harmonic general valence force field, vibrational fundamentals, potential energy distribution and infrared and Raman band intensities were determined using density functional theory, employing B3LYP/ 6–311++G(d,p) formalism. Good agreement was found, between measured and computed quantities investigated here. The r.m.s error between experimental and theoretical vibrational wavenumbers was 8.6 cm−1, for MPB. With the help of PED and eigenvectors, all vibrational fundamentals of the molecule were assigned unambiguously for the first time.

DFT simulation of barrier heights, infrared and Raman spectra, and investigation of vibrational characteristics of 2-((2-aminopyridin-3-yl) methylene) hydrazinecarbothioamide and its N-methyl variant

ABSTRACT Fourier transform infrared spectra, for 2-((2-aminopyridin-3-yl) methylene) hydrazinecarbothioamide (APHT) and 2((2-aminopyridin-3-yl) methylene)-N-methylhydrazinecarbothioamide (APMHT), were recorded in the spectral range 4000–400 cm−1. Their Raman counterparts were measured in the spectral region 4000–50 cm−1. Preliminary values of dihedral angles around five rotating bonds C–NH2 pyridyl C–CN, N–N, N–CS and C–NH2 aliphatic in APHT, required for initiating geometry optimization, were obtained by pairing successive bonds and evaluating torsional potential energy for various values of dihedral angle around these bonds in the entire conformational space spanning 0° to 360°. Barrier heights, around five rotating bonds in APHT and six rotating bonds in APMHT were computed, making torsional scans in conformational space from 0° to 360°. This indicated existence of two rotational isomers for APMHT. General valence force field, harmonic vibrational fundamentals, potential energy distribution (PED), along with infrared and Raman intensities were determined using DFT/B3LYP/6-311++G(d,p) level of theory for both the molecules. Good agreement was found between measured and simulated spectra, for APHT and APMHT. This was also true for corresponding Raman spectra. The rms error between experimental and theoretical vibrational frequencies was 9.00 and 6.70 cm−1, for APHT and APMHT, respectively. All vibrational fundamentals were assigned unambiguously, on the basis of computed PED, eigenvectors, and literature range for the first time. These assignments were further supported by comparing with attribution of corresponding bonds in the parent molecule pyridine and a related molecule 2-((2-aminopyridin-3-yl)methylene)-N-ethylhydrazinecarbothioamide, wherever possible.

High Order Ab Initio Valence Force Field with Chemical Pattern Based Parameter Assignment.

Bonded (or valence) interactions, which directly determine the local structures of the molecules, are fundamental parts of molecular mechanics force fields (FFs). Most popular classical FFs adopt the simple harmonic models for bond stretching and angle bending and ignore cross-coupling effects among the valence terms. This may lead to less accurate vibrational properties and configurations in molecular dynamics (MD) simulations. AMOEBA models utilize an MM3(MM4)-style bonded interaction model, in which the vibrational anharmonicity, the coupling effects among different energy terms, and the out-of-plane bending for sp2-hybridized atoms are considered. In this work, we report the development of bonded interaction parameters for a wide range of chemistry based on quantum mechanics (QM). About 270 atomic types defined by SMARTS strings were used to model the valence interactions. Our results indicate that the resulting valence parameters produce accurate vibrational frequencies (RMSD from QM is less than ~36.6 cm-1) over a large set of molecules with diverse functional groups (445 molecules). By contrast, the harmonic models usually give an RMS error greater than 60 cm-1. Meanwhile, this model accurately reflects the potential energy surface of the out-of-plane bending. Our model can generally be applied to the AMOEBA family and any MM3(MM4)-based molecular mechanics FFs.

Electromechanical field effects in InAs/GaAs quantum dots based on continuum [formula omitted] and atomistic tight-binding methods

A comparison between k→·p→ and tight-binding methods for the analysis of InAs/GaAs quantum dot bandstructures is presented based on a fully coupled computation of electromechanical effects. Electromechanical effects are addressed using a continuum elastic model for the k→·p→ method and a pre-conditioned Valence Force Field algorithm for the tight-binding atomistic calculations. The Valence Force Field method allows the direct identification of the impact of internal strain. Results to ensure model parameter consistency between the two methods are also given by comparing bulk and unstrained quantum-well dispersion relations.The quantum dot size dependence of the bandstructure is investigated based on the models including electromechanical fields. Additionally, the effect of the electromechanical fields is studied for a specific dot size by comparing results with and without electromechanical fields. Good agreement is found for the confined energy levels but model differences show up in the symmetry of probability densities mainly due to the underlying crystal structure details taken into account by the tight-binding method but lacking in the k→·p→ formalism. The latter follows from not including bulk inversion-asymmetry effects in the k→·p→ method. Inclusion of piezoelectric field effects in the k→·p→ method, however, restores the correct symmetry in the k→·p→ model (in agreement with the tight-binding symmetry).Results are also given for oscillator strengths where both quantitative and qualitative differences are found in the comparison of k→·p→ and tight-binding models.

Atomistic analysis of piezoelectric potential fluctuations in zinc-blende InGaN/GaN quantum wells: A Stillinger-Weber potential based analysis

In this paper we investigate strain and local polarization field effects in zinc-blende indium gallium nitride (InGaN) alloys and quantum wells. To do so we parametrize and establish a Stillinger-Weber potential with parameters fitted to hybrid functional density functional theory data. The developed model gives very good agreement with quantities to which it has not been fitted, such as Kleinman parameters of cubic III-N materials or the composition dependence of the lattice constant in InGaN alloys. Equipped with this model, we extract the composition dependence of elastic constants ${C}_{11}$ and ${C}_{12}$ in InGaN alloys, including bowing parameters for these quantities, which may form input for continuum-based calculations. Furthermore, applying this model to InGaN alloys and wells reveals that random alloy fluctuations can lead to strong local strain field fluctuations. Building on this information, we present a model that allows for the calculation of connected local built-in field fluctuations at the microscopic level, accounting for first- and second-order piezoelectric effects. The approach is general and can be applied to any zinc-blende III--V alloy or heterostructure investigated in the frame of semiempirical models (e.g., valence force field models) targeting strain fields on an atomistic level. Here, building on our Stillinger-Weber potential we show that local strain fluctuations in zinc-blende InGaN quantum wells can lead to strong piezoelectric built-in field fluctuations. This contribution has been widely overlooked in previous theoretical studies of these systems. Finally, we briefly discuss the impact of these polarization field fluctuations on carrier localization effects in such quantum well systems.

Exhaustive and informatics-aided search for fast Li-ion conductor with NASICON-type structure using material simulation and Bayesian optimization

Currently, NASICON-type LiZr2(PO4)3 (LZP)-related materials are attracting attention as solid electrolytes. There are experimental reports that Li-ion conductivity can be improved by doping a small amount of Ca or Y into stoichiometric LZP. In previous studies, doping with only one element having a narrow search space has been attempted, and thus, further improvement of the Li-ion conductivity is conceivable by using multi-element doping. When multi-element doping is attempted, because the search space becomes enormous, it is necessary to evaluate the Li-ion conductivity using a low-cost method. Here, force-field molecular dynamics using a bond valence force field (BVFF) approach was performed to evaluate the Li-ion conductivity. We confirmed that the Li-ion conductivity of stoichiometric LZP derived from BVFF (6.2 × 10−6 S/cm) has good agreement with the first principle calculation result (5.0 × 10−6 S/cm). Our results suggest that the Li-ion conductivity can be further improved by simultaneously doping LZP with Ca and Y [6.1 × 10−5 S/cm, Li35/32Ca1/32Y1/32Zr31/16(PO4)3]. In addition, Bayesian optimization, which is an informatics approach, was performed using exhaustively computed conduction property datasets in order to validate efficient materials search. The averages for Bayesian optimization over 1000 trials show that the optimal composition can be found about seven times faster than by random search.

RETRACTED ARTICLE: Modeling and molecular simulation of natural gas hydrate stabilizers

ABSTRACT Uncontrolled decomposition of natural gas hydrate may lead to serious marine geological disasters and air pollution. The model of natural gas hydrate and lecithin was established. The stability mechanism of lecithin to structure hydrate was studied by molecular dynamics simulation. The consistent valence force field (CVFF) and TIP3P potential models are used to define the interaction between CH4-CH4 and water-water species, respectively. The simulations are performed on a combination of a 2 × 2 × 4 unit cell of sI hydrate and a water liquid phase with lecithin. The results of the simulations indicate that lecithin molecules adsorb on the hydrate surface with their hydrocarbon chains crossing and forming a net structure, easily producing the hydrate memory effect, which will narrow the available space for hydrate methane and water movement. Compared to the pure water-hydrate model, the mean square displacement (MSD) values of hydrate methane and water molecules are much lower, indicating that the hydrate dissociates more slowly.

Investigation of Structures, FTIR, FT-Raman, In Vivo Anti-Inflammatory, Molecular Docking and Molecular Characteristics of 2-Amino-3-Pyridine Carboxaldehyde and Its Copper(II) Complex Using Experimental and Theoretical Approach

In this work, the FT-Raman and FTIR spectra of synthesized molecules, 2-amino-3-pyridine carboxaldehyde (APC) and Cu(II) complex of 2-amino-3-pyridine carboxaldehyde (Cu(II)APC) were characterized by using density functional theory (DFT). The optimized structural parameters such as harmonic vibrational frequencies, general valence force field, infrared and Raman intensities, frontier molecular orbital parameters, NLO properties, NBO and MESP were calculated using B3LYP functional with 6-311++G(d,p) basis set. Employing MOLVIB 7.0 program, the scaling process was engaged to reach a good agreement between experimental and computed frequencies. The rms error between the scaled calculated and experimental frequencies was found at 9.1 and 10.3 cm– 1 . Unambiguous vibrational assignments were reported by using potential energy distribution (PED) and eigenvectors. APC and Cu(II)APC were evaluated for their anti-inflammatory activity using the carrageenan-induced paw oedema method. Cu(II)APC complex exhibited significant activity compared to standard drug. Molecular docking results support the anti-inflammatory activity of the title compounds against the target protein cyclooxygenase-2 (COX-2) possess the good binding energy values of –4.50 and –7.67 kcal/mol, respectively.

Fully analytic valence force field model for the elastic and inner elastic properties of diamond and zincblende crystals

Using a valence force field model based on that introduced by Martin, we present three related methods through which we analytically determine valence force field parameters. The methods introduced allow easy derivation of valence force field parameters in terms of the Kleinman parameter $\zeta$ and bulk properties of zincblende and diamond crystals. We start with a model suited for covalent and weakly ionic materials, where the valence force field parameters are derived in terms of $\zeta$ and the bulk elastic constants $C_{11}$, $C_{12}$, and $C_{44}$. We show that this model breaks down as the material becomes more ionic and specifically when the elastic anisotropy factor $A = 2C_{44}/(C_{11}-C_{12}) > 2$. The analytic model can be stabilised for ionic materials by including Martin's electrostatic terms with effective cation and anion charges in the valence force field model. Inclusion of effective charges determined via the optical phonon mode splitting provides a stable model for all but two of the materials considered (zincblende GaN and AlN). A stable model is obtained for all materials considered by also utilising the inner elastic constant $E_{11}$ to determine the magnitude of the effective charges used in the Coulomb interaction. Test calculations show that the models describe well structural relaxation in superlattices and alloys, and reproduce key phonon band structure features.

Optimized valence force field model for the lattice properties of non-ideal III-nitride wurtzite materials

It has been discussed the optimization of the valence force field model for non-ideal III-nitrides wurtzite materials. To describe the static and dynamical lattice properties of nitrides within the same model, the anisotropy of the wurtzite structure and additional second-nearest-neighbor atomic interaction terms are included in the lattice strain energy expression. The sets of the force constants have been optimized through an iterative scheme in which calculated lattice properties are fitted to experimentally reported data. The model has been applied to the binary wurtzite compounds (GaN, InN) and random InxGa1-xN alloys. The calculated phonon dispersions, elastic constants and internal strain parameters of GaN and InN are in reasonable agreement with the experimental/ab initio values.

Comparison of first principles and semi-empirical models of the structural and electronic properties of $$\hbox {Ge}_{1-x}\hbox {Sn}_{x}$$ alloys

We present and compare three distinct atomistic models—based on first principles and semi-empirical approaches—of the structural and electronic properties of $$\hbox {Ge}_{1-x}\hbox {Sn}_{x}$$ alloys. Density functional theory calculations incorporating Heyd–Scuseria–Ernzerhof (HSE), local density approximation (LDA) and modified Becke–Johnson (mBJ) exchange-correlation functionals are used to perform structural relaxation and electronic structure calculations for a series of $$\hbox {Ge}_{1-x}\hbox {Sn}_{x}$$ alloy supercells. Based on HSE calculations, a semi-empirical valence force field (VFF) potential and $$sp^{3}s^{*}$$ tight-binding (TB) Hamiltonian are parametrised. Comparing the HSE, LDA+mBJ and VFF+TB models, and using the HSE results as a benchmark, we demonstrate that: (1) LDA+mBJ calculations provide an accurate first principles description of the electronic structure at reduced computational cost, (2) the VFF potential is sufficiently accurate to circumvent the requirement to perform first principles structural relaxation, and (3) VFF+TB calculations provide a good quantitative description of the alloy electronic structure in the vicinity of the band edges. Our results also emphasise the importance of Sn-induced band mixing in determining the nature of the conduction band structure of $$\hbox {Ge}_{1-x}\hbox {Sn}_{x}$$ alloys. The theoretical models and benchmark calculations we present inform and enable predictive, computationally efficient and scalable atomistic calculations for disordered alloys and nanostructures. This provides a suitable platform to underpin further theoretical investigations of the properties of this emerging semiconductor alloy.

Molecular scale modeling approach to evaluate stability and dissociation of methane and carbon dioxide hydrates

A comprehensive knowledge and precise estimation of the dynamic, structural, and thermodynamic characteristics of hydrates are needed to assess the stability of gas hydrates. Thermodynamic model and experimental studies can be utilized to compute the physical and dynamic properties of hydrate structures. The use of molecular dynamic (MD) simulation is a well-established approach in gas hydrate studies at the atomic level where the properties of interest are obtained from the numerical solution of Newtonian equations. The present work uses MD simulations by employing the constant temperature-constant pressure (NPT), constant temperature-constant volume (NVT) conditions, and the consistent valence force field (CVFF) to monitor the stability and decomposition of methane and carbon dioxide gas hydrates with different compositions. The effects of temperature and composition on the hydrate stability are investigated. In this study, we also compute the radial distribution function, mean square displacement, diffusion coefficient, lattice parameter, potential energy, dissociation enthalpy as well as the density of methane and carbon dioxide under various thermodynamic and process conditions. The formation of methane and carbon dioxide bubbles is studied to investigate bubble evolution during hydrate dissociation. The sizes of methane and carbon dioxide bubbles are not the same due to different solubility conditions of methane and carbon dioxide in liquid water. In addition, the influences of pressure and temperature on the lattice parameter and density of clathrate hydrates are discussed. The obtained results are consistent with previous theoretical and experimental findings, implying that the methodology followed in this work is reliable. The most stable arrangement of methane and carbon dioxide molecules in the gas hydrate is found. The insights/findings of this study might be useful to further understand detailed transport phenomena (e.g., molecular interactions, gas production rate, carbon dioxide replacement, and carbon dioxide capture) involved in the process of carbon dioxide injection into gas hydrate reservoirs.

Barrier potentials, molecular structure, force filed calculations and quantum chemical studies of some bipyridine di-carboxylic acids using the experimental and theoretical using (DFT, IVP) approach

ABSTRACTFT-IR and FT-Raman spectra of 2,2′-bipyridine-3,3′-dicarboxylic acid (B3DA), 2,2′-bipyridine-4,4′-dicarboxylic acid (B4DA) and 2,2′-bipyridine-5,5′-dicarboxylic acid (B5DA) were recorded and analysed. The quantum chemical calculations of the title compounds begin with barrier potentials at different rotation angles around the C–C′ and C–Cα bonds in order to arrive conformation of lowest energy using DFT employing B3LYP functional with 6-311++G(d,p) basis set. This confirmation was further optimised to get the global minimum geometry. The vibrational frequencies along with IR, Raman intensities were computed, the rms error between observed and calculated frequencies were 11.2 cm−1, 10.2 cm−1 and 12.2 cm−1 for B3DA, B4DA, and B5DA. An 87-element modified valence force field is derived by solving the inverse vibrational problem using Wilson’s GF matrix method. This force field is refined using 163 observed fundamentals employing in overlay least-squares technique. The average error between computed and experimental frequencies was found as 12.85 cm−1 using potential energy distribution (PED) and eigenvectors. By using the gauge-independent atomic orbital (GIAO) method calculate the 1H and 13C NMR chemical shifts of the molecules and compared with experimental results. The first-order hyperpolarisability, HOMO and LUMO energies, molecular electrostatic potential (MESP) and natural orbital analysis (NBO) of titled compounds were evaluated using DFT.

Molecular dynamic simulations to evaluate dissociation of hydrate structure II in the presence of inhibitors: A mechanistic study

The present work aims to investigate stability and decomposition of hydrate structure II of methane, propane, and isobutane systems at various temperatures, pressures, and compositions in the absence and/or presence of inhibitor molecules. To assess the stability of gas hydrates, a comprehensive knowledge of the structural, thermodynamic, and dynamic properties of the hydrates is needed. The structure II of gas hydrates is embedded in a molecular dynamic (MD) approach and the simulations are carried out under constant temperature-constant volume (NVT) and constant temperature-constant pressure (NPT) conditions by employing the consistent valence force field (CVFF). In this work, first, the mean square displacement (MSD) and diffusion coefficient are evaluated to demonstrate the movement of host molecules. The radial displacement function is then utilized to display the characteristic configurations of structure II under different process and thermodynamic conditions. In addition, other vital properties including lattice parameter and potential energy are determined. The effect of inhibitors on stability and/or decomposition of hydrate structure II is investigated. The achieved results are in good agreement with previous theoretical and experimental outputs, confirming reliability and appropriateness of the simulation method. The inhibition capability of different inhibitors based on the simulation results has the following order: methanol > ethanol > glycerol. The findings of this study can help for better understanding of hydrate dissociation in molecular scale as well as for proper selection and design of effective inhibitors for hydrates with different structures and characteristics.

Morphology, Ordering, Stability, and Electronic Structure of Carbon‐Doped Hexagonal Boron Nitride

Theoretical studies of morphology, stability, and electronic structure of monolayer hexagonal CBN alloys with rich content of h‐BN and carbon concentration not exceeding 50% are presented. The studies are based on the bond order type of the valence force field to account for the interactions between atomic constituents and Monte Carlo method with Metropolis algorithm to establish equilibrium distribution of atoms over the lattice. It is found that the phase separation into graphene and h‐BN domains occurs in the majority of growth conditions. Only in N‐rich growth conditions, it is possible to obtain a quasi uniform distribution of carbon atoms over the boron sublattice. It is been predicted that the energy gap in stoichiometric Cx(BN)1−x alloys exhibits extremely strong bowing.

Synthesis, crystal and molecular structure, and characterization of 2-((2-aminopyridin-3-yl)methylene)-N-ethylhydrazinecarbothioamide using spectroscopic (1H and 13C NMR, FT-IR, FT-Raman, UV–Vis) and DFT methods and evaluation of its anticancer activity

2-((2-aminopyridin-3-yl)methylene)-N-ethylhydrazinecarbothioamide was synthesized. It was characterized by making elemental analysis, assisted by experimental 1H NMR, 13C NMR, FT- Raman (4000-50 cm−1), FT-IR (4000-400 cm−1), and UV–Vis (200–400 nm) spectra and evaluating its anticancer activity, for human carcinoma cell lines HeLa (cervical), IMR-32 (neuroblastoma) and A549 (lung). Crystal and molecular structure of the molecule was determined by means of X-ray diffractometry, which showed that it belongs to triclinic crystal system, with space group P-1, having two molecules per unit cell (Z = 2). The parameters of the unit cell are a = 6.0960 (6) Å, b = 7.4119 (8) Å, c = 11.9959 (13) Å, α = 82.1695 (4)°, β = 81.6407 (4)°, γ = 88.3283 (4)° at 100 K. Quantum chemical computations were made using density functional theory (DFT), B3LYP functional and 6–311++G (d,p) basis set in order to determine optimized structure parameters, general valence force field, harmonic vibrational frequencies, potential energy distribution, infrared and Raman intensities, NLO properties, frontier molecular orbital parameters and NBO characteristics. Its time-dependent variant (TD-DFT) was used to calculate the oscillator strengths and absorption maxima (λmax) in DMSO‑d6 as a solvent, of various electronic transitions. There was a good agreement between the theoretical and experimental parameters such as molecular structure parameters, IR, Raman and UV–Vis spectra. The rms error between measured and estimated vibrational frequencies was 6.9 cm−1. The calculations showed that the molecule under investigation was good for NLO applications, which was supported by NBO analysis.