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The veterinary 16-membered macrolide antibiotics tylosin (HTyl, 1a) and tilmicosin (HTilm, 1b) react with copper(II) ions in acetone at metal-to-ligand molar ratio of 1:2 to form blue (2) or green (3) metal(II) coordination species,... more
The veterinary 16-membered macrolide antibiotics tylosin (HTyl, 1a) and tilmicosin (HTilm, 1b) react with copper(II) ions in acetone at metal-to-ligand molar ratio of 1:2 to form blue (2) or green (3) metal(II) coordination species, containing nitrate or chloride anions, respectively. The complexation processes and the properties of 2–3 were studied by an assortment of physicochemical techniques (UV-Vis, EPR, NMR, FTIR, elemental analysis). The experimental data revealed that the main portion of copper(II) ions are bound as neutral EPR-silent dinuclear complexes of composition [Cu2(µ-NO3)2L2] (2a–b) and [Cu2(µ-Cl)2Cl2(HL)2] (3a–b), containing impurities of EPR-active mono-species [Cu(NO3)L] (2a’–b’) and [CuCl2(HL)] (3a’–b’). The possible structural variants of the dinuclear- and mono-complexes were modeled by the DFT method, and the computed spectroscopic parameters of the optimized constructs were compared to those measured experimentally. Using such a combined approach, the main c...
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All possible monohydroxycoumarin derivatives are modelled using DFT computations in order to investigate the role of the hydroxyl group position for the radical-scavenging and antioxidant activity of these compounds. Geometry optimization... more
All possible monohydroxycoumarin derivatives are modelled using DFT computations in order to investigate the role of the hydroxyl group position for the radical-scavenging and antioxidant activity of these compounds. Geometry optimization is performed using B3LYP functional with the 6-311++G(d,p) basis set. The enthalpy changes are assessed in gas phase and in implicit water using the polarized continuum method (PCM). Structure-reactivity patterns are delineated. The most reactive isomers as well as the most probable mechanism of interaction between monohydroxycoumarins and free radicals are outlined.
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The Institute for Nuclear Research and Nuclear Energy at the Bulgarian Academy of Sciences has recently started the construction of an accelerator laboratory in Sofia. The cornerstone of the laboratory is a TR24 cyclotron, which provides... more
The Institute for Nuclear Research and Nuclear Energy at the Bulgarian Academy of Sciences has recently started the construction of an accelerator laboratory in Sofia. The cornerstone of the laboratory is a TR24 cyclotron, which provides a proton beam with a variable energy between 15 and 24 MeV and current of up to 0.4 mA. The TR24 accelerator allows for the production of a large variety of radioisotopes for medical applications and development of radiopharmaceuticals. The new cyclotron facility will be used for research in radiopharmacy, radiochemistry, radiobiology, nuclear physics, solid-state physics, applied research, new materials and for education in all these fields including nuclear energy.
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To create both greener and high-power metal-ion batteries, it is of prime importance to invent an unprecedented electrode material that will be able to store a colossal amount of charge carriers by a redox mechanism. Employing periodic... more
To create both greener and high-power metal-ion batteries, it is of prime importance to invent an unprecedented electrode material that will be able to store a colossal amount of charge carriers by a redox mechanism. Employing periodic DFT calculations, we modeled a new metal-organic framework, which displays energy density exceeding that of conventional inorganic and organic electrodes, such as Li- and Na-rich oxides and anthraquinones. The designed MOF has a rhombohedral unit cell in which an Ni(II) node is coordinated by 2,5-dicyano-p-benzoquinone linkers in such a way that all components participate in the redox reaction upon lithiation, sodiation and magnesiation. The spatial and electronic changes occurring in the MOF after the interaction with Li, Na and Mg are discussed on the basis of calculated electrode potentials versus Li0/Li+, Na0/Na+ and Mg0/Mg2+, respectively. In addition, the specific capacities and energy densities are calculated and used as a measure for the elect...
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The number of scientific papers on the unique properties and the potential for various applications of compounds with a diradical character is growing constantly. The diradical character enhances and even engenders certain desired optical... more
The number of scientific papers on the unique properties and the potential for various applications of compounds with a diradical character is growing constantly. The diradical character enhances and even engenders certain desired optical properties and its modulation is a modern molecular design strategy. Nowadays, molecules with a non-zero diradical character are regarded as promising materials for new-generation and highly efficient solar cells and photonics devices. What is the price, however, of the unique properties of open-shell compounds? Alongside all the benefits, the diradical character is usually associated with low stability and high reactivity—unwanted molecular qualities for practical purposes. Thus, from a fundamental and applied point of view, it is important to investigate the correlation between the diradical character and laboratory stability, which is the goal of the present paper. Here, we report a combined quantum–chemical study (conceptual DFT and spin-projec...
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ABSTRACT Nontoxic drug delivery systems for efficient trans-membrane transport are central in the successful therapy of a number of diseases. Appropriate building blocks of reversible drug-carrying micelles are water-soluble surfactants,... more
ABSTRACT Nontoxic drug delivery systems for efficient trans-membrane transport are central in the successful therapy of a number of diseases. Appropriate building blocks of reversible drug-carrying micelles are water-soluble surfactants, e.g. pentaethylene glycol onododecyl ether (C12E5). The present study aims to derive from first principles calculations and to test molecular mechanics parameters for such ethers to be used in subsequent all-atom simulations of micelle formation. Two monomers and one dimer with two different types of periphery, which are short-chain prototypes of the amphiphilic surfactant C12E5, are used as model systems. The geometry of low-energy conformers is obtained from conformational analysis with a modified OPLS force field and optimized at PBE and MP2 levels, with aug-cc-pVTZ basis sets in vacuum and in implicit solvent. The quantum-chemical calculations provide detailed information on the structural flexibility of the surfactant models and can be used as reference for MD simulations. Weak dependence of the parameters sought on the length of the oligomers and higher sensitivity to the type of periphery is found. Validation of the derived molecular mechanics parameters is carried out through comparison of the density, molecular volume, enthalpy of solvation and vaporization obtained from molecular dynamics simulations (Amber99/NPT/300K) of diethyl ether to the existing еxperimental data. The two theoretical approaches yield similar results both at molecular level and as secondary thermodynamic output. Moreover, the derived set of molecular mechanics parameters is consistent with experiment and can be used for extensive molecular dynamics simulations of larger CxEy surfactant assemblies.
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The aim of this theoretical study is the investigation of hydrocarbon stability as a function of BN- and B-substitution degree. The targets are anthracene and phenanthrene because they resemble the zig-zag and armchair peripheries of... more
The aim of this theoretical study is the investigation of hydrocarbon stability as a function of BN- and B-substitution degree. The targets are anthracene and phenanthrene because they resemble the zig-zag and armchair peripheries of graphene sheets. The computations are made at the RHF and MP2 level with various basis sets. The conclusions drawn are based on the results from
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Atomistic modeling of insoluble monolayers is currently used to inspect their organization and electric characteristics, providing a link between theory and experiment. Extensive molecular dynamics simulations at 300 K were carried out... more
Atomistic modeling of insoluble monolayers is currently used to inspect their organization and electric characteristics, providing a link between theory and experiment. Extensive molecular dynamics simulations at 300 K were carried out for model films of the lipids dipalmitoylphosphatidylcholine (DPPC) and dicaprin (DC) at the air/water interface. Surface concentrations corresponding to a set of points along the surface pressure/area isotherms of the surfactants were considered. The models contained 25 or 81 lipid molecules in hexagonal arrangement and explicit aqueous media (TIP3P) treated in periodic boundary conditions. Molecular dynamics simulations based on a classical force field (CHARMM27) were carried out and key characteristics of the studied films were estimated. The dielectric properties of the films in normal and tangential direction were quantified by means of dipole moment magnitude and orientation analysis and by monolayer dielectric permittivity. The contributions of lipids and interfacial water to each component of the considered characteristics were assessed and their variations upon film compression were discussed and compared for the two monolayers and to earlier results. The dielectric permittivity tensors were analyzed. Electrostatic potential profiles across the layers and surface pressure values were used for more detailed clarification of experimental measurements. The results show dissimilar behavior of the two lipids at the air-water interface. While the average electric and dielectric properties of DPPC monolayers result from opposite surfactant and water contributions, the two subsystems are synergetic in the DC films. The anisotropy of the monolayer dipole moment and dielectric permittivity is explained by domination of a different subsystem in the various components. Tangential characteristics turn out to be more sensitive to the size of the model and to the degree of film compression.
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ABSTRACT The interaction of Li atoms with BN-and B-substituted hydrocarbons was examined. A substantial num-ber of prescreened stable BN-and B-isomers of anthracene and phenanthrene at all possible degrees of doping were used as models... more
ABSTRACT The interaction of Li atoms with BN-and B-substituted hydrocarbons was examined. A substantial num-ber of prescreened stable BN-and B-isomers of anthracene and phenanthrene at all possible degrees of doping were used as models for adsorption of a Li-atom. The energy of the Li/substrate systems was cal-culated with the UMP2/6-31G * method. The total and the binding energies (BSSE and ZPE corrected) at each adsorption position of Li over the substituted hydrocarbons were evaluated. On the basis of the energy estimates and analysis of the molecular orbitals and electron density distribution, factors govern-ing the preferred locations of the Li-atom on BN-and B-substituted anthracenes and phenanthrenes with different degrees of doping were outlined. Li positioned closer and bonded tighter to the substituted than to the pristine hydrocarbons. Optimal parameters ensuring minimum irreversible capacity of the sub-strate were defined: medium binding of Li combined with maximum charge transfer – requirements met by BN-anthracenes and B-phenanthrenes at low degree of doping and B-anthracenes with higher B-contents, the two latter being more prospective. The results provide valuable information for the molecular design of anode materials for Li-ion batteries with improved performance.
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The structuring of water molecules at the water/vapour interface is an object of scientific interest for decades. After the first successful attempts to explore liquid water with the help of theoretical chemistry, the number of studies on... more
The structuring of water molecules at the water/vapour interface is an object of scientific interest for decades. After the first successful attempts to explore liquid water with the help of theoretical chemistry, the number of studies on this topic grows progressively. Most of them are focused on bulk water but there is still need of a more detailed research on surface water. In addition, interfaces with alkanes are interesting as being instructive from both biological and industrial perspectives. Since in both bio- and industrial applications water/air and water/oil interfaces are mediated by amphiphiles, the role of a surfactant monolayer on surface water structuring deserves more attention as well. In the present study several atomistic water models were chosen—non-polarisable (SPC, TIP3P, and TIP4P) and polarisable (SW-RIGID-ISO, SWM4-NDP, and COS/G2) and classical molecular dynamics simulations were carried out on bulk water, water/vapour and water/alkane (from pentane to nona...
ABSTRACT a b s t r a c t In this first part of our investigation the effect of –OH, –NH 2 , –NO 2 and other substituents on the planarity (and the level of delocalization) of a series of flavones and flavonols is assessed. This feature is... more
ABSTRACT a b s t r a c t In this first part of our investigation the effect of –OH, –NH 2 , –NO 2 and other substituents on the planarity (and the level of delocalization) of a series of flavones and flavonols is assessed. This feature is particu-larly important with respect to the biological activity of flavonoids. The values of the dihedral angles were obtained from the optimized in gas phase and in water geometries at the density functional theory B3LYP/6-31G(d,p) level. Ó 2013 Elsevier B.V. All rights reserved.
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Singlet fission, a multiple exciton generation process, can revolutionize existing solar cell technologies. Offering the possibility to double photocurrent, the process has become a focal point for physicists, chemists, software... more
Singlet fission, a multiple exciton generation process, can revolutionize existing solar cell technologies. Offering the possibility to double photocurrent, the process has become a focal point for physicists, chemists, software developers, and engineers. The following review is dedicated to the female investigators, predominantly theorists, who have contributed to the field of singlet fission. We highlight their most significant advances in the subject, from deciphering the mechanism of the process to designing coveted singlet fission materials.
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The construction of hybrid metal-ion batteries faces a plethora of challenges. A critical one is to unveil the solvation/desolvation processes at the molecular level in electrolytes that ensure efficient transfer of several types of... more
The construction of hybrid metal-ion batteries faces a plethora of challenges. A critical one is to unveil the solvation/desolvation processes at the molecular level in electrolytes that ensure efficient transfer of several types of charge carriers. This study reports first results on simulations of mixed ionic electrolytes. All combinations of homo- and hetero-binuclear complexes of Li+, Na+ and Mg2+, solvated with varying number of ethylene carbonate (EC) molecules are modeled in non-polar and polar environment by means of first principles calculations and compared to the mononuclear analogues in terms of stability, spatial organization, charge distribution and solvation/desolvation behavior. The used PF6¯ counterion is shown to have minor impact on the geometry of the complexes. The desolvation energy penalty of binuclear complexes can be lowered by the fluoride ions, emerging upon the PF6¯ decay. These model investigations could be extended to rationalize the solvation structure and ionic mobility in dual-ion electrolytes.
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Abstract Compounds able to adsorb at the interface of two immiscible liquids are of high importance for numerous chemical, physical and biological processes. Deeper understanding of the interfacial phenomena in such systems can contribute... more
Abstract Compounds able to adsorb at the interface of two immiscible liquids are of high importance for numerous chemical, physical and biological processes. Deeper understanding of the interfacial phenomena in such systems can contribute significantly to the rational practical application of small amphiphiles in electrochemistry, extraction, stabilization of emulsions, and drug design. Particularly interesting are the oil-water formulations, where the aqueous and the hydrophobic phases tend to express different molecular structure and characteristics under identical conditions. Surfactant behaviour in such environments is known to differ distinctly from the one observed in bulk water or oil or at the gas-liquid interface. It has been demonstrated that the free energy of adsorption and transfer of amphiphiles across water-oil interfaces can be decomposed into specific contributions of molecular fragments. The models developed so far do not provide information about the specifics of the functional groups contributions. To elaborate further on this topic, classical molecular dynamics simulations, combined with umbrella sampling calculations and weighted histograms method analysis are applied to reconstruct the free energy profile for several water-oil-amphiphile models along a relocation coordinate normal to the interface. Normal pentane, hexane, and heptane are chosen as models of the hydrophobic liquids and three short-chained normal alcohols⿿as low-molecular-weight amphiphiles. Gibbs energies of transfer and adsorption, as well as the average contributions per unit of elongation of the alkyl chain, are estimated and compared to experimental data and empirical results. The experimental trends of all quantities are reproduced correctly. Some quantitative differences are discussed. A detailed analysis of the free energy change vs. position of alcohol molecules shows that the transfer process can be decomposed into several stages. Such more precise stratification of the interfacial region is employed to determine and rationalize the free energy contributions of the separate alkanol functional groups.
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Abstract Complexes of Tb(III) with the biscoumarin derivative 3,3′-[(4-hydroxyphenyl)methyl]bis-(4-hydroxy- 2H -chromen-2-one) are synthesized by using a solution of tetraethylammonium hydroxide, [(C 2 H 5 ) 4 N]OH, in water as... more
Abstract Complexes of Tb(III) with the biscoumarin derivative 3,3′-[(4-hydroxyphenyl)methyl]bis-(4-hydroxy- 2H -chromen-2-one) are synthesized by using a solution of tetraethylammonium hydroxide, [(C 2 H 5 ) 4 N]OH, in water as deprotonating base. Elemental analysis, IR- and fluorescence spectroscopy are used to characterize the samples obtained. The complexes have good fluorescent properties and show the characteristic emission bands of the Tb(III) ion. The triplet state of the mono-deprotonated ligand H 2 L − was determined. The optimised geometries of the ligand and the complexes were obtained by means of molecular modelling with first principles (DFT) methods.
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Anthraquinone anchored on graphene holds possibilities to design next generation battery combined electrodes and current collectors.
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Quantitative Structure Activity Relationships (QSAR or SAR) have helped scientists to establish mathematical relationships between molecular structures and their biological activities. In the present article, SAR studies have been carried... more
Quantitative Structure Activity Relationships (QSAR or SAR) have helped scientists to establish mathematical relationships between molecular structures and their biological activities. In the present article, SAR studies have been carried out on 89 tetrahydroimidazo[4,5,1-jk][1,4]benzodiazepine (TIBO) derivatives using different classifiers, such as support vector machines, artificial neural networks, random forests, and decision trees. The goal is to propose classification models that will be able to classify TIBO compounds into two groups: high and low inhibitors of HIV-1 reverse transcriptase. Each molecular structure was encoded by 10 descriptors. To check the validity of the established models, all of them were subjected to various validation tests: internal validation, Y-randomization, and external validation. The established classification models have been successful. The correct classification rates reached 100% and 90% in the learning and test sets, respectively. Finally, m...
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A combined theoretical/experimental study of the photoreactivity of two flavylium-derived chalcones, 2,4,4'-trihydroxychalcone and 2,4'-dihydroxychalcone, at the multiconfigurational wavefunction level of theory (CASSCF//CASPT2)... more
A combined theoretical/experimental study of the photoreactivity of two flavylium-derived chalcones, 2,4,4'-trihydroxychalcone and 2,4'-dihydroxychalcone, at the multiconfigurational wavefunction level of theory (CASSCF//CASPT2) in vacuo and in an implicit solvent (water, treated as a polarisable continuum) and by means of linear absorption spectroscopy is presented. The photosensitivity of flavium salts is expressed in the ability of their chalcone form to undergo a cis-trans isomerisation which has found application in logical networks. Despite a considerable amount of experimental data documenting the dependence of the isomerisation on solvent, pH and temperature, the knowledge of how chalcones process energy under various conditions at the molecular level is still scarce. On the example of 2,4,4'-trihydroxychalcone we unravel the complex excited state deactivation mechanism in vacuo involving ultrafast decay through conical intersections, formation of twisted intramo...
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The search for targeted drug delivery systems requires the design of drug-carrier complexes, which could both reach the malignant cells and preserve the therapeutic substance activity. A promising strategy aimed at enhancing the uptake... more
The search for targeted drug delivery systems requires the design of drug-carrier complexes, which could both reach the malignant cells and preserve the therapeutic substance activity. A promising strategy aimed at enhancing the uptake and reducing the systemic toxicity is to bind covalently the drug to a cell-penetrating peptide. To understand the structure-activity relationship in such preparations, the chemotherapeutic drug doxorubicin was investigated by unrestrained molecular dynamics simulations, supported by NMR, which yielded its molecular geometry in aqueous environment. Furthermore, the structure and dynamics of a conjugate of the drug with a cell-penetrating peptide was obtained from molecular dynamics simulations in aqueous solution. The geometries of the unbound compounds were characterized at different temperatures, as well as the extent to which they change after covalent binding and whether/how they influence each other in the drug-peptide conjugate. The main structural fragments that affect the conformational ensemble of every molecule were found. The results show that the transitions between different substructures of the three compounds require a modest amount of energy. At increased temperature, either more conformations become populated as a result of the thermal fluctuations or the relative shares of the various conformers equalize at the nanosecond scale. These frequent structural interconversions suggest expressed conformational freedom of the molecules. Conjugation into the drug-peptide compound partially immobilizes the molecules of the parent compounds. Nevertheless, flexibility still exists, as well as an effective intra- and intermolecular hydrogen bonding that stabilizes the structures. We observe compact packing of the drug within the peptide that is also based on stacking interactions. All this outlines the drug-peptide conjugate as a prospective building block of a more complex drug-carrier system.
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Page 1. Influence of the Level of Protonation on the Geometry and the Electronic Structure of Emeraldine Oligomers Jasmina Petrova, Julia Romanova, Galia Madjarova, Anela Ivanova, Alia Tadjer, and Natalia Gospodinova ...
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We report a theoretical study on the geometry characteristics and the electronic properties of bipolaron defects introduced in oligomers (tetramer to hexadecamer) of emeraldine salt. The main goal of the present investigation is to... more
We report a theoretical study on the geometry characteristics and the electronic properties of bipolaron defects introduced in oligomers (tetramer to hexadecamer) of emeraldine salt. The main goal of the present investigation is to establish a quantum chemical method suitable for description of the target system and for determination of the oligomer segment perturbed by protonation. For the purpose, a
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Polyaniline is among the most intensely investigated polymers because of its exceptional properties affording its current and potential applications. The structure and energy spectra of isolated oligomers and infinite chains in different... more
Polyaniline is among the most intensely investigated polymers because of its exceptional properties affording its current and potential applications. The structure and energy spectra of isolated oligomers and infinite chains in different oxidation states and degrees of protonation have been discussed at length from experimental and theoretical perspectives. The reaction environment effect, however, has received less attention and, particularly, the influence of monomer excess has been completely neglected in theoretical studies. Experimental measurements show that residual aniline is always detected in emeraldine samples obtained at low pH. Upon addition of oxidant to emeraldine PANI samples, post-polymerization due to the presence of excess monomers occurs. This is an indication of the formation of aniline-PANI complexes in the reaction medium. The presence of aniline monomers should affect the PANI chain arrangement and optical/conducting characteristics. Therefore, model clusters of aniline with neutral or singly protonated emeraldine tetramers in explicit water medium and periodic boundary conditions are addressed in this paper using a Monte Carlo/AMBER96/AM1 computational protocol to simulate the absorption spectra. The monomer impact on the structure, energy characteristics, and UV/vis spectra of the polymer are discussed.
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The theoretical study addresses the type and nature of the transitions in the absorption spectra of octamers, dodecamers, and hexadecamers of the emeraldine salt-the conducting form of polyaniline. Each of the fully protonated oligomers... more
The theoretical study addresses the type and nature of the transitions in the absorption spectra of octamers, dodecamers, and hexadecamers of the emeraldine salt-the conducting form of polyaniline. Each of the fully protonated oligomers is modeled in its lowest singlet (bipolaronic form) and highest possible multiplicity (polaronic form). Two configurations of the chloride counterions with respect to the oligomer chains are considered. All structures are optimized with BLYP/6-31G*/PCM, while the spectra are evaluated with CIS/6-31G*/PCM. The optical behavior of the bipolaronic and polaronic forms of the investigated systems is discussed and compared to relevant experimental data. The optical transitions at about 400 and 800 nm characteristic for the emeraldine salt are registered for all model structures. Weighed against experimental and earlier theoretical findings the results prove that CIS gives qualitatively correct electronic spectra of these conjugated species. While the two configurations have almost identical spectra in the highest multiplicity, the singlets' absorption conduct turns out to be sensitive to the counterions position. In all cases the most intensive absorption is the longest wavelength one in the near-IR region, but the number and oscillator strengths of the polaronic and bipolaronic bands are noticeably dissimilar. The bands of the low-spin oligomers are grouped, while those of the high-spin species cover the entire visible region. Each extension of the chain with one elementary unit contributes systematically a set of new bands to the spectrum. The possibility for a solvatochromic effect is estimated.
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Model clusters of surfactant prototypes with small number of water molecules are calculated at different levels of theory. All approaches used yield correct trends in the variation of the dipole moment upon tail elongation or polar... more
Model clusters of surfactant prototypes with small number of water molecules are calculated at different levels of theory. All approaches used yield correct trends in the variation of the dipole moment upon tail elongation or polar headgroup variation. Models including one, two, or more water molecules are optimized. The most stable structures are those with maximum number of atoms involved in hydrogen bonding. The normal components of the dipole moment prove to be less sensitive to the nature (aliphatic or aromatic) of the hydrophobic tail, in accord with findings from the phenomenological models. Values of the dipole moment approaching the experimental estimates required inclusion of sufficient aqueous environment (>20 water molecules per hydrophilic head) and of lateral intersurfactant interactions into the model.
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The magnetic properties of a series of manganese(II) diacetylacetonate and dihexafluoroacetylacetonate hybrid-spin complexes with neutral pyridine-based organic radicals were characterized theoretically by DFT calculations. Three stable... more
The magnetic properties of a series of manganese(II) diacetylacetonate and dihexafluoroacetylacetonate hybrid-spin complexes with neutral pyridine-based organic radicals were characterized theoretically by DFT calculations. Three stable radicals, in which a radical group is bound in either para or meta position with respect to the pyridine nitrogen atom, were considered. The correct stable structures and multiplets of the complexes were obtained by full geometry optimization starting from an ideal structure. A total of three important geometry descriptors of the complexes were monitored and related to their magnetic characteristics. These structural parameters are (i) the torsion angle governing the conjugation of the organic radical m-PyNO (anti versus gauche), (ii) the coordination geometry of the acetyl acetonate ligands around the metal ion (square versus rhombic), and (iii) the relative orientation of the organic radical with respect to the acetyl acetonate plane (parallel versus perpendicular). It was found that the magnetic properties are not sensitive to the orientation of the radicals with respect to the equatorial plane but do depend on the conformation of the organic radicals. Even a spin switch between the ferromagnetic (S = (7)/(2)) and antiferromagnetic (S = (3)/(2)) ground state was found to be feasible for one of the complexes upon variation of the organic radical geometry, namely, the dihedral angle between the organic radical moiety and the pyridine ring. The pattern of molecular orbital overlap was determined to be the key factor governing the exchange coupling in the modeled systems. Bonding π-type overlap provides antiferromagnetic coupling in all complexes of the para radicals. In the meta analogues, the spins are coupled through the σ orbitals. A low-spin ground state occurs whenever a continuous σ-overlap pathway is present in the complex. Ferromagnetic interaction requires σ-π orthogonality of the pyridine atomic orbitals and/or π-antibonding Mn-pyridine natural orbital overlap. Using an estimate of the donor-acceptor energy stabilization, the affinity of a given Mn(II) d-orbital to mix with the sp(2) orbital from pyridine can be predicted.