Abstract: A number of different methods have been proposed for assigning partial charges to the atoms of a molecule, including both quantum chemical and empirical schemes. A reasonable expectation for any successful calculational scheme is that the atomic charges it produces should vary in a manner consistent with chemical intuition and, more specifically, that these variations should be correlated in a sensible way with experimental observations. Seven of the most popular atomic charge schemes (Bader's AIM charges QAIM, electrostatic potential charges QESP, GAPT charges QGAPT, Gasteiger π charges QGast ,L owdin chargesQLow, Mulliken charges QMul, and charges derived from natural population analysis QNPA) were tested for their ability to represent variations in the pKa's of 19 monosubstituted anilines and 19 monosubstituted phenols. In most cases the calculations were performed at the B3LYP/6-311G ∗∗ level of theory. For the substituted anilines, the amino nitrogen, anilinium proton, and total amino group charges were taken as representative regression parameters, and for the phenols, the phenolic hydrogen, phenoxide oxygen, and hydroxyl group charges were employed. Overall, QAIM, QLow, and QNPA yielded the most successful correlations with the pKa's of these compounds, although for the phenol series, QGAPT and QMul also yielded good results. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem 90: 445-458, 2002

Abstract: In an attempt to improve the understanding of the basic mechanisms on the degradation of pollutants in water by TiO2 photocatalysis, we discussed the primary degradation mechanism of three triazolidine derivatives, such as 1,2,4-triazolidine-3,5-dione (TRIANE), 4-hydroxy-1,2,4-triazolidine-3,5-dione (OH-TRIANE) and 4-phenyl-1,2,4-triazolidine-3,5-dione (Ph-TRIANE) and one triazole derivative of the 4-phenyl-1,2,4-triazole-2,5-dione (Ph-TRIOLE), on the basis of the experimental results together with molecular orbital (MO) calculation of frontier electron density and partial charge. The above four heterocycles were selected as molecular probe, principally because the highest frontier electron density was situated at different places of the molecule, while their structures were very similar, two major pathways have been revealed by LC/MS analysis for each heterocyclic compound. The pathway (a) corresponded to the hydroxylation of the atom bearing the highest electron density, via oxidation on nitrogen atom of azo group with respect to the photodegradation of TRIANE, OH-TRIANE and Ph-TRIOLE and opening of aromatic ring when Ph-TRIANE was used. This initial attack occurred with the preferential electrophilic attack of OH radicals. The pathway (b) was caused by the attack of active species on to carbon atom of carbonyl group. Considering the calculation of the relative electrophilic density of this carbon atom for the four heterocyclics and the evolution of large amount of N2 gas at the initial degradation step, it has been suggested that this step (b) was favored by adsorption of carbonyl group on TiO2 surface as theoretically determined by partial charge and confirmed by IR analysis. The participation of hole (h+) to form R–CO+ was envisaged in this step. The presence of 1,2,4,5-tetrazixane-3,6-dione also agrees with the participation of TiO2 surface. The nitrogen inorganic analysis (N2, NH4+ and NO3−) determined by gas chromatography and ion liquid chromatography, show that the hydrazo group were photoconverted mainly into N2 gas and partially to NH4+ ions. No nitrate ions were observed at the beginning of the degradation even when a OH substituent was present on one nitrogen atom (case of OH-TRIANE). However, the presence of –OH group increased the NO3−/NH4+ ratio observed after a few hours of irradiation. The presence of OH also improved the total mineralization of carbon atom into CO2, which was attributed to mesomeric effect of –OH group.

Abstract: It is known that SO42− is not electronically stable as an isolated species but can be rendered stable by solvation (e.g., by adding a few H2O molecules). Recently, our group introduced a Coulomb repulsion model that offers an approximation to the energy instability and lifetimes of such species. In order to achieve an independent and likely more reliable estimate of the instability of SO42−, we have undertaken a follow-up study of this dianion. Specifically, we apply a stabilization method to determine the vertical electronic energy difference between the metastable SO42− dianion and its SO4−1 daughter at several levels of theory. The particular variant of the stabilization method used here involves adding a partial positive charge to the central sulfur nucleus in order to confine the escaping electron. Our coupled-cluster data, which represent our highest level of theory, suggest that SO42− is unstable by 1.1 eV and has a lifetime with respect to electron loss of 1.6×10−10 s (our earlier estimates were 0...

Abstract: We have investigated electronic properties of K-doped single-wall carbon nanotube bundles by the first principles calculations. The lattice expands up to 8% at K 0 . 1 C with negative binding energies, where distortion of tube walls is negligible up to K 0 . 2 5 C with full relaxation, contrary to the previous reports. Partial charge transfer occurs from potassium atom to the tube. The Fermi level and the amount of charge transfer increase with increasing doping concentration and saturate at large concentration, strongly indicating that the charge transfer is mainly responsible for conductivity increase of the tubes.

Abstract: The charge response kernel (CRK), ∂Q a /∂V b , was reformulated using a modified definition of the partial charge Q a . The modifications could effectively prevent numerical instability of the partial charges based on the electrostatic potential, and thus eliminate spurious components of CRK due to the fitting problem. The modified CRK was directly derived via the coupled-perturbed Hartree-Fock equations. The performance of this modified CRK model was examined by applying it to some test molecules including ethanol, DMSO, chloroform, and trimethylamine.

Abstract: We present a new class IV charge model. The model, called Charge Model 3 (CM3), is designed to be able to obtain accurate partial charges from hybrid density functional calculations with a variable amount of Hartree−Fock exchange and with or without diffuse functions in the basis. The model maps atomic partial charges obtained by Lowdin or redistributed Lowdin population analysis into improved (class IV) charges that reproduce accurate charge-dependent observables for molecules containing H, Li, C, N, O, F, Si, S, P, Cl, and Br. The hybrid density functional theory we use here is based on Adamo and Barone's modified Perdew−Wang (mPW) gradient-corrected exchange functional and the PW91 gradient corrected correlation functional. These parametrizations can be used with any arbitrary fraction of Hartree−Fock exchange in conjunction with any of the five basis sets, MIDI!, MIDI!6D, 6-31G*, 6-31+G*, and 6-31+G**. We also present two parametrizations for Hartree−Fock theory employing the MIDI!6D and 6-31G* basis ...

Abstract: An ion trap mass spectrometer uses electrospray ionization to introduce multiply-charged positive ions in an axial direction into a quadrupole ion trap and glow discharge ionization to introduce singly-charged negative ions in a radial direction into the ion trap Methods of controlling ion-to-ion charge transfer reactions include applying a combination of a dipolar DC voltage and a dipolar RF voltage across endcap electrodes to allow partial charge state neutralization reactions to occur between the positive and negative ions and then control suspension and resumption of further charge state neutralization reactions The remaining ions can be further processed and transformed and a mass spectrum created by scanning a quadrupolar RF field

Abstract: To explore the binding properties of benzene towards small molecules bearing C–H groups with different acidities, we have undertaken ab initio quantum-chemical calculations, including correlation effects through Density Functional Theory methods, on the benzene–CH3X (X = F, Cl, Br, I, CN, NO2) adducts. Benzene acts as a Lewis base and the CH3X molecule as a Lewis acid. The partial charge transferred from benzene to the Lewis acid is mainly confined on the X group and increases with the electron withdrawing character of X. The calculations performed on the various systems predict that two different stable structures for each adduct exist: one with C3v and the other with Cs symmetry, the latter being the most stable one. A simple HOMO–LUMO model suggests that the charge is transferred from the benzene HOMO to the CH3X LUMO and that this process is easier in the systems with Cs symmetry due to the better overlap between the frontier orbitals.

Abstract: The bimolecular rate constants kqT for quenching of some substituted naphthalene triplet states by molecular oxygen (O2(3Σg−)) in acetonitrile and cyclohexane and the efficiencies, fΔT, with which singlet oxygen (O2*(1Δg)) is thereby produced are reported for naphthalene derivatives with a wider range of oxidation potentials than those previously measured. The magnitude of kqT and fΔT are inversely correlated, and both parameters exhibit pronounced sensitivity to the oxidation potential (EMOX) of the naphthalene derivative and to the solvent polarity. The modified charge transfer mediated mechanism of quenching based on singlet and triplet channels for oxygen quenching is invoked to discuss these results. In cyclohexane the maximum value for fΔT of one is observed for compounds with high oxidation potentials indicating no contribution from the triplet channel whilst in acetonitrile the limit for fΔT of 0.25 expected when singlet and triplet channels give equal contributions, when spin statistics is taken into account, is observed for 2,6-dimethoxynaphthalene, which is the derivative with the lowest oxidation potential. These results are combined with those previously reported by ourselves in cyclohexane in order to examine the dependence of the quenching rate constants due to energy transfer on the energy gap (ET–E1Σ). This allows the quenching rate constants due to energy transfer in cyclohexane to be separated into contributions with and without charge transfer assistance. The latter contribution shows a smooth dependence on (ET–E1Σ) and the same dependence on the electronic energy, which has to be converted into vibrational energy, probably applies when the solvent is acetonitrile since making this assumption results in similar dependences of the charge transfer contributions in both singlet and triplet channels in acetonitrile. The free energy of activation for charge transfer assisted quenching is shown to have a linear dependence on the free energy change for full charge transfer but the indications are that quenching is via singlet and triplet charge transfer complexes with only partial charge transfer character being 12.5% and 17% in acetonitrile and cyclohexane respectively.

Abstract: Absolute energies, geometric structures, harmonic vibrational frequencies, and partial charge distributions (NPA) are obtained from ab initio (HF/MP2) and density functional theory (DFT) calculations of stable conformers of neutral and cationic 2-phenylethanol and n-butylbenzene, model aromatic molecules with flexible side chains. We focus on exploring the conformational preferences of the cations and find that cationic conformational preference is more pronounced in the system with the strongly interacting side chain. The DFT calculations presented use the Perdew−Wang exchange and correlation functional, PW91. For the neutral conformers, PW91 performs extremely well compared to MP2, indicating that this functional will be highly useful for future computational studies of neutral aromatic molecules with flexible side chains. For the cationic conformers, PW91 again performs well compared to MP2 for n-butylbenzene, a system in which the side chain interacts only weakly with the aromatic ring. However, consi...

Abstract: The contribution of electrostatic forces to the interaction of Na,K-ATPase with adenine nucleotides was investigated by studying the effect of ionic strength on nucleotide binding. At pH 7.0 and 20 °C, there was a qualitative correlation between the equilibrium dissociation constant (Kd) values for ATP, ADP, and MgADP and their total charges. All Kd values increased with increasing ionic strength. According to the Debye−Huckel theory, this suggests that the nucleotide binding site and its ligands have “effective” charges of opposite signs. However, quantitative analysis of the dependence on ionic strength shows that the product of the effective electrostatic charges on the ligand and the binding site is the same for all nucleotides, and is therefore independent of the total charge of the nucleotide. The data suggest that association of nucleotides with Na,K-ATPase is governed by a partial charge rather than the total charge of the nucleotide. This charge, interacting with positive charges on the protein, ...

Abstract: We present a combined theoretical and experimental study of charge transfer and dissociation in collisions of slow Li31(2+) clusters with Cs atoms. We provide a direct quantitative comparison between theory and experiment and show that good agreement is found only when the exact experimental time of flight and initial cluster temperature are taken into account in the theoretical modeling. We demonstrate the validity of the simple physical image that consists in explaining evaporation as resulting from a collisional energy deposit due to cluster electronic excitation during charge transfer.

Abstract: We report on the energetics of C60–Si clusters. By means of ab initio calculations based on the local density approximation to the density functional theory, we have investigated stable and metastable structures of C60–Si, C60–Si–C60, and (C60–Si)2 clusters. In each case, we show that silicon preferentially binds to C60 over a carbon–carbon double bond, in accordance with calculations on the interaction of C60 with silicon surfaces. This bonding is characterized by a partial charge transfer from silicon to C60. We show that the interaction between C60 and silicon is local and not perturbed by the addition of more C60–Si clusters or C60 molecules. The binding energy for stable and metastable (C60–Si)n⩽2 systems is high enough (several eV) to open the prospect of synthesizing nanostructured films from the C60–Si unit. Furthermore, in all three cases, the silicon position on a fivefold symmetry axis is found to be a metastable position. The nature and structure of nanostructured films resulting from the depo...

Abstract: Cyclic voltammetry and impedance methods are employed for a quantitative study of normal-heptanol (n-HepOH) adsorption kinetics at the bismuth single-crystal plane/aqueous Na2SO4 solution interface. The results of nonlinear regression analysis show that the Frumkin–Melik-Gaikazyan (FMG) or Frumkin–Melik-Gaikazyan–Randles (FMGR) equivalent circuits can be used for the simulation of experimental impedance data. The dependences of adsorption capacitance (caused by the potential dependence of surface coverage), Warburg diffusion impedance, and adsorption resistance on the electrode potential and organic-compound concentration are established. Analysis of impedance data demonstrates that the adsorption of n-HepOH is mainly limited by the rate of diffusion of organic compound to the electrode surface. Small deviations toward mixed adsorption kinetics are established at very high frequencies. In the region of maximum adsorption in more concentrated n-HepOH solutions, the slow reorganization or two-dimensional association of adsorbed molecules is possible. However, the very low adsorption or partial charge transfer resistance values indicate that then-HepOH adsorption at Bi planes is a practically reversible process and thus there is no noticeable partial charge transfer between adsorbed n-HepOH molecules and Bi surface atoms.

Abstract: Various methods for deriving atomic partial charges from the quantum chemical electrostatic potential and moments have been tested for the sucrose molecule. We show that if no further information is used, the charges on some carbon atoms become large and charge patterns involving these atoms are badly determined and poorly transferable. Adding lone-pairs on the ether oxygen atoms or dividing the molecule into smaller fragments did not cure the instabilities. We develop a method, CHELP-BOW0, that restrains charges toward zero with different weights for different atoms. These harmonic restraints preserve the linear form of the least-squares equations, which are solved in a single step using singular-value decomposition. CHELP-BOW0 improves the chemical transferability of the charges compared to unrestrained methods, and slightly improves their conformational transferability. It introduces a modest degradation of the fit compared to unrestrained CHELP-BOW (mean average deviation of the potential 0.00016 vs. 0.00010 a.u.). A second new method, CHELP-BOWC, avoids the need for restraints by including several conformations in the fit, weighting each according to its estimated energy in solution. CHELP-BOWC charges are more transferable than CHELP-BOW or CHELP-BOW0 charges to conformations not included in the training set. Restraints to zero charge do not further improve transferability of the CHELP-BOWC charges. We, therefore, recommend CHELP-BOW charges for rigid molecules and CHELP-BOWC charges for flexible molecules.

Abstract: We consider isotropic static distributions of a given electric charge in a sphere and show that the minimal energy occurs for the uniform surface distribution, as in a charged spherical conductor in electrostatic equilibrium. Moreover, we show that the energy of such uniform distribution is minimal and stationary with respect to perturbations of the charge density, in particular dipole fluctuations. We argue that the use of the minimum energy principle to explain static distributions of electric charge provides interesting physical insights.

Abstract: The hexacoordinated AB6-type main group molecules have long been thought to have sp3d2 hybridization on the central atom, accounting for their molecular geometry (octahedral). However, the s-p-d hybridization does not explain how an energetically unfavorable np → nd excitation in an atom of nonmetallic elements, such as sulfur and phosphorus, can be achieved. In this article, the author has re-examined bonding in SF6 and PF6− (O h symmetry) and proposed that the linear F—S—F and F—P—F bonds in both species are formed via the overlap of the 3p orbital on the central atom with terminal ligand orbitals, resulting in a three-center, four-electron bond. This alternative model, which does not involve d orbitals in bonding, is supported by a partial charge analysis using Allen’s electronegativity approach. SF6 or PF6− can be characterized by several ionic resonance structures containing a postulated SF42+ or PF4+ cation (octet on sulfur or phosphorus). The three-center, four-electron bond model can also be used to study bonding in hexacoordinated AB5E (e.g., halogen pentafluorides) and AB4E2 (e.g., xenon tetrafluoride) explaining well the molecular geometry. The author believes that all the results will be useful in updating chemistry texts.

Abstract: The dependence of the formation enthalpy (Δ)HEx* and the Gibbs energy (ΔGEx*) of exciplexes with partial charge transfer on the Gibbs energy of electron transfer ΔGet*, the parameters of the electronic structure of an exciplex (the difference in the energies of the charge transfer (CT) state and a locally excited state (LE) in a vacuum ( H220– H110), the matrix element of the electronic coupling of the CT and LE states H12, the dipole moment of the CT state, and the repulsion energy in an exciplex a"), and the polarity of the medium was analyzed. The consideration of the repulsion energy in the exciplex is necessary for correlation of the experimental values of ΔHEx* and the spectral shift of the exciplex emission with respect to the LE state. All of these parameters depend on the particular nature of the exciplex, which is the reason for the lack of the general dependence of ΔHEx* and ΔGEx* on ΔGet* for exciplexes with partial charge transfer.

Abstract: Exciplexes with partial charge transfer generated from 9-cyanophenanthrene and methoxybenzenes were studied in solvents of different polarity. The parameters of the electronic structure and the thermodynamic parameters of the exciplexes were determined from the spectral data and the temperature dependence of the fluorescence quantum yields. It was shown that more accurate description of the experimental data requires the repulsion energy between the molecules of the exciplex components to be taken into consideration in the model of correlated polarization of an exciplex and its environment. The comparison of the electronic structure and the properties of the exciplexes with partial and full charge transfer showed that, as the extent of charge transfer decreases, the contribution of exchange interaction into the exciplex stabilization increases and the role of medium polarity and intermolecular repulsion simultaneously decreases. Owing to these factors, the exciplex energy is substantially lower than the energy of a locally excited state even at a positive difference between the energies of the charge-transfer and locally excited states in a vacuum.

Abstract: Ab initio calculations at the MP2/6-31G(d) level of theory have been performed to determine the geometries and relative energies of many isomers of protonated octane (also known as octonium or octanium ions). Five of the 18 structural isomers of octane were considered for protonation, as they provided C−C bonds containing all possible combinations of carbon substitution types (quaternary−tertiary, quaternary−secondary, etc.). All resulting isomers of C8H19+ feature either a CHC or a CHH 3-center-2-electron (3c2e) bond, although barrierless dissociation into an ion−molecule complex was very common. Octonium ion properties such as relative energies, 3c2e bond geometries, Mulliken partial charges, and the frequency of the most intense infrared absorption, have also been calculated. Each property is correlated to the level of substitution of C atoms in the 3c2e bond. The proton affinities of individual bonds in octane range from 154 to 187 kcal mol-1 for C−C bonds and 139 to 150 kcal mol-1 for C−H bonds. Alka...

Abstract: The pentacoordinated AB5-type main group molecules have long been thought to have sp3d hybridization of the central atom, accounting for their oxidation state and molecular geometry; however, this does not explain the difference in axial and equatorial bond distances within these molecules. In addition, d-orbital participation in bonding for nonmetals is energetically unfavorable because it requires a np → nd excitation energy. In this paper, we have proposed that the sp2 hybrid orbitals are formed on the central atom, overlapping with ligand orbitals in the equatorial positions. The longer axial bonds are formed via the overlap of the unhybridized p orbital on the central atom and the two terminal ligand orbitals, resulting in a three-center, four-electron bond. Examining the partial charge on the central atom using relevant electronegativity values supports this model without a d component in bonding. The model explains well the structure and bonding in pentavalent molecules. We believe that the results will be useful in updating chemistry texts.

Abstract: A partial charge database was constructed for 117 molecules. The actual database contains Mulliken, natural population analysis, and stockholder atomic charges calculated from the HF/6-31G(d) or HF/6-311+G(2d,p) wavefuntions. The MP2/6-31G(d), B3LYP/6-31G(d), or HF/6-31G(d) equilibrium geometries were used for these calculations. The database also contains the corresponding total energies calculated at HF/6-31G(d) or HF/6-311+G(2d,p) levels of theory. Gaussian 3 or G3SX [Curtiss L.A.; Redfern, P.C.; Raghavachari, K.; Pople, J.A. J.Chem. Phys. 114 (2001) 108.] composite enegies with experimental, zero point vibration energy, or thermal corrections are included. The database is used to obtain atomic correction parameters of Rapid Estimation of Basis set Error and Correlation Energy from Partial charges method [S. Kristyan, A. Ruzsinszky, G.I. Csonka, Theoret. Chem. Acc. 106 (2001) 319 and 404]. An XML database structure is also suggested.

Abstract: We study a two-dimensional $t\ensuremath{-}J$ model close to the Ising limit in which charge inhomogeneity is stabilized by an on-site potential ${e}_{s},$ by using diagonalization in a restricted Hilbert space and finite temperature quantum Monte Carlo techniques. Both site- and bond-centered stripes are considered and their similitudes and differences are analyzed. The amplitude of charge inhomogeneity is studied as ${e}_{s}\ensuremath{\rightarrow}0.$ Moreover, we show that the antiphase domain ordering occurs at a much lower temperature than the formation of charge inhomogeneities and charge localization. Hole-hole correlations indicate a metallic behavior of the stripes with no signs of hole attraction. Kinetic energies and current susceptibilities are computed and indications of charge localization are discussed. The study of the doping dependence in the range $0.083l~xl~0.167$ suggests that these features are characteristic of the whole underdoped region.

Abstract: Theoretical molecular solid crystal structure optimization presents many computational challenges. Calculation of both intramolecular interactions and intermolecular interactions are major obstacles. In this study, the intramolecular interactions are treated quantum mechanically and the intermolecular interactions are approximated by a force field, which is in part determined by a partial charge analysis of the quantum treatment. This combined approach, called the quantum coupled unit cell description (QCUCD) method, treats short-range and long-range intermolecular interactions with convergence-accelerated lattice sum techniques. QCUCD finds the internal molecular geometry; then crystal parameters are optimized until a fully consistent solution between the unit cell description and the quantum chemistry of the molecular electronic structure is achieved. The single molecule experiences the periodically repeating potential at one molecular site of the crystal. The solution of the electronic structure of the...

Abstract: The photo-induced charge transfer followed by the bidirectional phase transition in Fe-Co Prussian blue compounds is theoretically discussed. The result of ab initio calculations indicates that both the forward and backward transitions are triggered by photo-induced charge transfer excitations between Co-dγ and Fe-de orbitals, although different kinds of Co sites are excited. It is concluded that the charge transfer proceeds by way of the spσ-pπ transition on the nitrogen atom, which can explain such a long range charge transfer between Co and Fe separated by about 5A.

Abstract: Experimental evidences of the charge ordering with the use of 13 C-NMR techniques in various organic conductors are reviewed. In the insulating states of two-dimensional BEDT-TTF salts, α-I 3 and θ-RbZn(SCN) 4 , the formation of charge stripes perpendicular to molecular stacks are confirmed. Charge disproportionation is estimated as 0.8:0.2 in the latter and more in the former. It is also observed that charge disproportionation already develops well above the MI transition temperature while no long-range charge order is stabilized. This seems to require a serious reconsideration of the transport properties in the 'metallic' state.

Abstract: Charge transfer reactions are fundamental processes present in almost all areas of physics, chemistry and biochemistry.[...]