Effective Atomic Charges Research Articles

Electronic states of lanthanide in the ternary thiogallate CaGa2S4

The electronic states of lanthanide (Ln) doped CaGa2S4 are investigated by the molecular orbital calculations for a spherical cluster of LnCa8Ga12S24 using the FORTRAN program DVSCAT on the basis of the Discrete Variational method with Xα potentials (DV-Xα). In view of the SCF convergence, the Ln-doped lattice should contract to 85–90% of the mother crystal around the Ln atom for the lightweight lanthanides from Ce to Sm. On the other hand, the lattice contraction is very small for the heavyweight lanthanides, especially for Er, Tm and Yb in contrast to the generally known lanthanide contraction for Ln3+ ions. This is probably attributed to the effective charges of Ln atoms calculated here to be less than +1 for all lanthanides contrary to the chemically accepted value of +3. The energy level scheme of 4f and 5d related molecular orbitals is proposed for each Ln substituting Ca in CaGa2S4, showing that the optical processes relating to the 5d→4f transition must be complicated especially for the lightweight Ln-doped CaGa2S4.

Influence of dipole interactions on the lattice dynamics of crystalline ice

The Born effective charges of component atoms and phonon spectra of tetrahedrally coordinated crystalline ice are calculated using the first principles method based on density functional theory within the generalized gradient approximation with the projected augmented wave method. The phonon dispersion relations in a 3× 1× 1 supercell were evaluated from Hellmann–Feynman forces with the direct method. This calculation is in addition to the direct method of calculating the phonon spectra, which does not take into account the polarization charges arising from the dipole interaction of molecules of water in the ice. The calculated Born effective polarization charges from linear response theory are supplied as correction terms to the dynamical matrix in order to further investigate the LO–TO splitting of the polar modes of ice crystals at k=0, which has long been speculated for this system, especially in the region between 28 and 37 meV, both in theoretical and experimental studies. Our results clearly show evidence of splitting of longitudinal and transverse optic modes at the k=0 point, in agreement with experimental findings.

Ab initio potential-energy surface for the reaction Ca+HCl→CaCl+H

The potential-energy surface of the ground electronic state of CaHCl has been obtained from 6400 ab initio points calculated at the multireference configuration-interaction level and represented by a global analytical fit. The Ca+HCl-->CaCl+H reaction is endothermic by 5100 cm(-1) with a barrier of 4470 cm(-1) at bent geometry, taking the zero energy in the Ca+HCl asymptote. On both sides of this barrier are potential wells at linear geometries, a shallow one due to van der Waals interactions in the entrance channel, and a deep one attributed to the H(-)Ca(++)Cl(-) ionic configuration. The accuracy of the van der Waals well depth, approximately 200 cm(-1), was checked by means of additional calculations at the coupled-cluster singles and doubles with perturbative triples level and it was concluded that previous empirical estimates are unrealistic. Also, the electric dipole function was calculated, analytically fitted in the regions of the two wells, and used to analyze the charge shifts along the reaction path. In the insertion well, 16,800 cm(-1) deep, the electric dipole function confirmed the ionic structure of the HCaCl complex and served to estimate effective atomic charges. Finally, bound rovibrational levels were computed both in the van der Waals well and in the insertion well, and the infrared-absorption spectrum of the insertion complex was simulated in order to facilitate its detection.

Theoretical Prediction of the Abraham Hydrogen Bond Acidity and Basicity Factors from a Reaction Field Method.

A new methodology for the theoretical evaluation of the hydrogen bond acidity Σ and basicity Σ Abraham descriptors is presented. The first step is a quantum mechanical calculation at the Hartree−Fock level using a moderate basis set, including the solute−solvent interaction through a Reaction Field method, namely the Polarizable Continuum Model (PCM). The density charge on the surface of the cavity surrounding the solute, which contains the signature of the specificity of the molecule, is then translated into effective atomic charges through a suitable algorithm. These atomic charges can be related to the acidity and basicity properties of the molecule by a proper parametrization of empirical atomic factors, which account for the specific H-bonding capabilities of the individual atoms and group of atoms. The Abraham descriptors can be then evaluated with a high degree of accuracy for a large number of classes of molecules. Calculations performed for a set of 55 compounds give a standard deviation of 0.029...

Application of the modified electrostatic model to diffusion of transition metals in nickel and γ-iron

The modified electrostatic model is applied to diffusion of 3d to 5d transition metals in nickel and γ-iron. The comparison of experimental and calculated temperature dependence of the diffusion coefficients D permits conclusions with respect to effective charges of the solute atoms. The comparison with the results of diffusion in copper permits the estimation of D( T) for systems where no reliable data are available.

Application of the modified electrostatic model to diffusion of transition metals in noble metals

The modified electrostatic model is applied to diffusion of 3d–5d transition metals in noble metals. The comparison of experimental and calculated temperature dependence of the diffusion coefficients permits conclusions with respect to effective charges of solute atoms.

Chemical bonding evolution on compression of crystals

The electronegativity concept has been used to develop a method of determining effective atomic charges in pressurized crystalline binary compounds from the experimental values of volume, compression modulus and thermochemical characteristics. The data obtained are in accord with available spectroscopic data on the chemical bonding.

Theoretical Prediction of the Abraham Hydrogen Bond Acidity and Basicity Factors from a Reaction Field Method

A new methodology for the theoretical evaluation of the hydrogen bond acidity SigmaalphaH2 and basicity SigmabetaH2 Abraham descriptors is presented. The first step is a quantum mechanical calculation at the Hartree-Fock level using a moderate basis set, including the solute-solvent interaction through a Reaction Field method, namely the Polarizable Continuum Model (PCM). The density charge on the surface of the cavity surrounding the solute, which contains the signature of the specificity of the molecule, is then translated into effective atomic charges through a suitable algorithm. These atomic charges can be related to the acidity and basicity properties of the molecule by a proper parametrization of empirical atomic factors, which account for the specific H-bonding capabilities of the individual atoms and group of atoms. The Abraham descriptors can be then evaluated with a high degree of accuracy for a large number of classes of molecules. Calculations performed for a set of 55 compounds give a standard deviation of 0.029 and 0.044 for SigmaalphaH2 and SigmabetaH2, respectively. The correlation coefficients are 0.994 and 0.974.

Computational study of hydrogen binding by metal-organic framework-5

We report the results of quantum chemistry calculations on H(2) binding by the metal-organic framework-5 (MOF)-5. Density functional theory calculations were used to calculate the atomic positions, lattice constant, and effective atomic charges from the electrostatic potential for the MOF-5 crystal structure. Second-order Møller-Plesset perturbation theory was used to calculate the binding energy of H(2) to benzene and H(2)-1,4-benzenedicarboxylate-H(2). To achieve the necessary accuracy, the large Dunning basis sets aug-cc-pVTZ, and aug-cc-pVQZ were used, and the results were extrapolated to the basis set limit. The binding energy results were 4.77 kJ/mol for benzene, 5.27 kJ/mol for H(2)-1,4-benzenedicarboxylate-H(2). We also estimate binding of 5.38 kJ/mol for Li-1,4-benzenedicarboxylate-Li and 6.86 kJ/mol at the zinc oxide corners using second-order Møller-Plesset perturbation theory. In order to compare our theoretical calculations to the experimental hydrogen storage results, grand canonical Monte Carlo calculations were performed. The Monte Carlo simulations identify a high energy binding site at the corners that quickly saturated with 1.27 H(2) molecules at 78 K. At 300 K, a broad range of binding sites are observed.

Basis Set Orbital Relaxation in Atomic and Molecular Hydrogen Systems

Orbital relaxation (OR) amounts to variation of the orbital exponents in hydrogen molecules and ions relative to the exponents of the isolated atom; it is represented as the sum of the one- and two-center contributions depending on the effective atomic charge and on the presence of other atoms in the molecule. The procedure for isolating the contributions of the exponent includes treatment of the OR of hydrogen in a special set of neutral and charged atoms and molecules with certain multiplicities of their electronic states. Within the framework of the spin-unrestricted Hartree-Fock method, we found and discussed the optimal values of the exponents of the basis orbitals of hydrogen atoms and molecules using the minimal split valence-shell basis set, the basis set that includes the polarization function, and the expanded set of grouped natural orbitals. A simple energy model is suggested for OR. Expressions are derived for evaluating the exponents of the relaxed orbitals in hydrogen-containing systems.

Electron correlations in crystals and molecules

An expression for the Green’s function of a disordered crystal is obtained with allowance for the electron-electron interaction. The electron states of the system are described in the framework of a multiband tight-binding model. The possibility of using the proposed approach for describing the energy spectrum of molecules is demonstrated in the limit of an infinitely large primitive cell of the crystal. The energy spectrum and effective charges of atoms of a tricyanobutadienecarbazole molecule are calculated.

Pb(II) and Cd(II) Tetrafluoroborate Complexes with Nitrogen-Containing Organic Bases. HSAB Concept

Comparative analysis of the calculated values of the equalized electronegativity, polarity, hardness (softness) of the coordination bond, and the effective charges of the metal atoms in Pb and Cd tetrafluoroborate complexes with nitrogen-containing bases was performed. The above characteristics were shown to depend on the electron-donor properties of the starting organic ligands. It was shown that an increase in the organic base hardness brings about an increase in hardness of the metal–ligand bond in a complex.

Spectroscopic properties, catalytic activities and mechanism studies of [(Tp Ph)Co(X)(CH 3OH) m]· nCH 3OH: bicarbonate dehydration in the presence of inhibitors

Two inhibitor-containing ‘half-sandwich’ cobalt(II) complexes [(Tp Ph)Co(X)(CH 3OH) m ]· nCH 3OH ((Tp Ph)=hydrotris(3-phenylpyrazolyl)borate; 1: X −=N 3 −, m=1, n=2; 2: X −=NCS −, m=0, n=0) have been synthesized and used as the catalysts in the bicarbonate dehydration reaction. The structures of 1 and 2 were determined by X-ray diffraction analysis, which shows that N 3 − and NCS − coordinate to the Co(II) ions of 1 and 2, respectively, with the CoN bond lengths of 1.992(6) Å and 1.901(3) Å. The coordination geometries of the Co(II) complexes in solution are five-coordinated trigonal bipyramid as revealed by the spectroscopic measurements. The dehydration kinetic measurements of HCO 3 − are performed by the stopped-flow techniques at pH<7.9. The apparent dehydration rate constant k obs varies linearly with Co(II) complex and H + concentrations, respectively, and the catalytic activity of 2 is lower than that of 1. The aqua Co(II) complex must be the reactive catalytic species in the catalyzed dehydration reaction and the rate-determining step is the substitution of the labile water molecule by HCO 3 −. The k obs values increase with increasing reaction temperature, and the large negative entropy of activation also indicates the associative activation mode. The inhibition ability of NCS − is stronger than that of N 3 −, which can be rationalized by the decreases in the CoN(N 3 −/NCS −) bond lengths and effective atomic charges of the Co(II) ions based on the X-ray crystallographic data and theoretical calculations in this work.

Is the charge on the nitrile carbon atom a driving force of the nucleophilic addition to coordinated nitriles?

The availability of the electrostatic model for the interpretation of nucleophilic addition to nitriles—free and coordinated to platinum—has been discussed. The analysis of effective atomic charges calculated at the HF/6-31G*, B3LYP/6-31G* and MP2/6-31G* levels of theory using both Mulliken and Natural Bond Orbital (NBO) schemes was performed for the neutral and cationic dinitrile complexes of Pt II and Pt IV trans-[Pt IIL 2(NCCH 3) 2] 2+ and trans-[Pt IVCl 2L 2(NCCH 3) 2] 2+ (L=Cl −, NH 3, PH 3, NCCH 3). The oxidation state of the metal, overall charge of the complex moiety and nature of the ligand L only weakly affect the atomic charge on the C atom of the CN group of the bound nitrile (especially the Mulliken one). Meanwhile, the significant enhancement of the latter charge from the free to the coordinated ligand was found for the NBO method. The activation of the nitriles in the complexes compared to the free ligand towards the nucleophilic addition to the CN bond can be interpreted in terms of charge control, but the enhancement of the reactivity from the complexes of Pt II to those of Pt IV should be explained by other arguments (orbital control and the effect of the transition state). The nature of the chemical bonds was investigated using the AIM and NBO methods and the charge decomposition analysis (CDA). The nitrile ligands do not exhibit noticeable π-acceptor properties in the complexes, and the electrostatic interaction plays an important role in the formation of the coordination bond.

A theoretical study on the interactions of diorganodichlorosilanes with zinc oxide

Density functional theory B3LYP calculations of the interaction of diorganochlorosilanes, R 2SiCl 2, with zinc oxide have been performed. Two hypothetical schemes of the siloxanes [R 2Si–O] n formation, which involved initial insertion of ZnO at the Si–Cl bond to give zincsiloxanes, R 2ClSi–O–ZnCl, were considered: (i) interaction of the intermediate zincsiloxane molecules with each other or with molecules of the starting diorganodichlorosilane molecules, which leads to the appearance of the Si–O–Si bonds; (ii) evolution of the zincsiloxane into a transient organosilanone coordinated with zinc chloride and subsequent rearrangement of the latter into the end siloxane compound. The geometry parameters, total energy, Mulliken effective atomic charges, dipole moments, HOMO and LUMO energies, vibrational frequencies and infrared intensities for the starting, intermediate and end organosilicon molecules were computed. The analysis of the energetics of both pathways to formation of the siloxane bond has revealed that the first one is thermodynamically more preferable.

Experimental and theoretical studies of the dehydration kinetics of two inhibitor-containing half-sandwich cobalt(II) complexes

Two inhibitor-containing half-sandwich cobalt(II) complexes [TpMe]CoX ([TpMe] = hydrotris(3,5-dimethylpyrazolyl)borate; X−: N3− (1), NCS− (2)) have been synthesized and characterized. The structure of 2 was determined by X-ray crystallographic analysis. The dehydration kinetic measurements of HCO3− catalyzed by the cobalt(II) complexes are performed by the stopped-flow techniques at pH <7.9. The five-coordinated aqua complex must be the reactive catalytic species in the catalyzed dehydration reaction and the rate-determining step is the substitution of the labile water molecule by HCO3−. The acid dissociation constants (expressed as pKa values) for 1 and 2 are 7.9 and 7.8, respectively, which are obtained by pH titration. The dehydration rate constant k of the rate-determining step of 2 (95.9 M−1 s−1) is lower than that of 1 (129 M−1 s−1), which can be correlated with a higher activation energy barrier, this being the origin of the variation in the inhibition ability between N3− and NCS−. The inhibition ability of NCS− is strong than that of N3−, which is also confirmed by the decrease in effective atomic charges of the Co(II) ions as revealed by the theoretical calculations.

Theoretical Characterization of Oxoanion, XOmn-, Solvation

We describe a new cavity definition protocol that yields accurate solvation energies and electrode potentials for selected oxoanions, XOmn-, including some for which other cavity protocols do not perform well enough. In this new definition scheme with cavities made of interlocked atomic spheres, the radii are given by simple empirically based expressions involving effective atomic charges of the solute atoms that fit the solute molecular electrostatic potential and a bond length-dependent factor to account for atomic size and hybridization. The scheme shows substantial qualitative differences from other previously proposed schemes, for example, by assigning a large radius to the central atom of the oxoanions. This difference is put on a firm theoretical basis in the case of NO3- through an analysis of the molecular electrostatic potential of the nitrate ion and an analysis of its interaction with a “solvent” water molecule. Despite a large positive partial charge assigned to nitrogen in the nitrate ion, t...

Kinetics and mechanism of the bicarbonate dehydration of the half-sandwich zinc(II) complexes [TpPh]ZnX ([TpPh] = hydrotris(3-phenylpyrazolyl)borate; X− = OH−, N3−, NCS−)

The preparation and characterization of the half-sandwich zinc complexes [TpPh]ZnX ([TpPh] = hydrotris(3-phenylpyrazolyl)borate; X− = OH− (1), N3− (2), NCS− (3)) have been described. The bicarbonate dehydration kinetic measurements are performed by the stopped-flow techniques at pH < 7.9. The apparent dehydration rate constant kobsd varies linearly with the total Zn(II) concentration, and the catalytic activity of the model complexes decreases in the order 1>2>3. The catalytic activity decreases with increasing pH, which indicates that the aqua model complex must be the reactive catalytic species in the catalyzed dehydration reaction and the rate-determining step is the substitution of the labile water molecule by HCO3−. The kobsd values increase with increasing reaction temperature, and the apparent activation energies of the model complexes with small inorganic ions are remarkably higher, this being the origin of inhibition. The large negative entropy of activation also indicates an associative mode of activation in the rate-determining step. The inhibition ability of NCS− is strong than that of N3−, which can be rationalized by the decrease in effective atomic charges of the Zn(II) ions as revealed by the theoretical calculations.

Kinetic study of the effects of inhibitors on the catalyzed dehydration of HCO3- by copper(II) complexes [TpPh]CuX (X- = OH-, N3-, NCS-).

A series of half-sandwich copper(II) complexes [TpPh]CuX ([TpPh] = hydrotris(3-phenyl-pyrazolyl)borate; X- = OH- (1), N3- (2), NCS- (3)) have been synthesized as models for carbonic anhydrase. The structure of 3 was determined by X-ray diffraction analysis. Crystals of 3 (C37H30BCuN9S) are triclinic, space group P1 with a = 11.997(3) A, b = 12.116(3) A, c = 13.384(4) A, alpha = 81.088(5) degrees, beta = 79.289(6) degrees, gamma = 68.668(5) degrees, V = 1772.4(8) A3, and Z = 2. The dehydration kinetic measurements of HCO3- are performed by the stopped-flow techniques at pH < 7.9. The apparent dehydration rate constant kdobs varies linearly with total Cu(II) concentration, and the catalytic activity of the model complexes decreases in the order 1 > 2 > 3. The catalytic activity decreases with increasing pH indicating that the aqua model complex must be the reactive catalytic species in the catalyzed dehydration reaction and the rate-determining step is the substitution of the labile water molecule by HCO3-. The kdobs values increase with increasing reaction temperature, and the apparent activation energies of the model complexes with inhibitors are remarkably higher than that of the complex with no inhibitors, this being the origin of inhibition. The large negative entropy of activation also indicates an associative mode of activation in the rate-determining step. The inhibition ability of the inhibitor NCS- is stronger than that of the inhibitor N3-, which can be rationalized by the decrease in effective atomic charges of the Cu(II) ions as revealed by the theoretical calculations.

Metal-hydride bond activation and metal-metal interaction in dinuclear iron complexes with linking dinitriles: a synthetic, electrochemical, and theoretical study.

The dinuclear iron(II)-hydride complexes [[FeH(dppe)(2)](2)(mu-LL)][BF(4)](2) (LL = NCCH=CHCN (1a), NCC(6)H(4)CN (1b), NCCH(2)CH(2)CN (1c); dppe = Ph(2)PCH(2)CH(2)PPh(2)) and the corresponding mononuclear ones, trans-[FeH(LL)(dppe)(2)][BF(4)] (2a-c) were prepared by treatment of trans-[FeHCl(dppe)(2)], in tetrahydrofuran (thf) and in the presence of Tl[BF(4)], with the appropriate dinitrile (in molar deficiency or excess, respectively). Metal-metal interaction was detected by cyclic voltammetry for 1a, which, upon single-electron reversible oxidation, forms the mixed valent Fe(II)/Fe(III) 1a(+) complex. The latter either undergoes heterolytic Fe-H bond cleavage (loss of H(+)) or further oxidation, at a higher potential, also followed by hydride-proton evolution, according to ECECE or EECECEC mechanistic processes, respectively, which were established by digital simulation. Anodically induced Fe-H bond rupture was also observed for the other complexes and the detailed electrochemical behavior, as well as the metal-metal interaction (for 1a), were rationalized by ab initio calculations for model compounds and oxidized derivatives. These calculations were used to generate the structural parameters (full geometry optimization), the most stable isomeric forms, the ionization potentials, the effective atomic charges, and the molecular orbital diagrams, as well as to predict the nature of the other electron-transfer induced chemical steps, i.e. geometric isomerization and nucleophilic addition, by BF(4)(-), to the unsaturated iron center resulting from hydride-proton loss. From the values of the oxidation potential of the complexes, the electrochemical P(L) and E(L) ligand parameters were also estimated for the dinitrile ligands (LL) and for their mononuclear complexes 2 considered as ligands toward a second binding metal center.