Huckel Theory Research Articles

Effect of ions on confined near-critical binary aqueous mixture

Near-critical binary mixtures containing ions and confined between two charged and selective surfaces are studied within a Ginzburg–Landau theory extended to include electrostatic interactions. Charge density profiles and the effective interactions between the confining surfaces are calculated in the case of chemical preference of the ions for one of the solvent components. Close to the consolute point of the binary solvent, the preferential solubility of ions leads to the modification of the charge density profiles with respect to the ones obtained from Debye–Huckel theory. As a result, the electrostatic contribution to the effective potential between the charged surfaces can exhibit an attractive well. Our calculations are based on the approximation scheme that is valid if the bulk correlation length of a solvent is much larger than the Debye screening length; in this critical regime, the effect of charge on the concentration profiles of the solvent is subdominant. Such conditions are met in the recent measurements of the effective forces acting between a substrate and a spherical colloidal particle immersed in the near-critical water–lutidine mixture [Nature, 2008, 451, 172]. Our analytical results are in quantitative agreement with the experimental ones.

Ionic excesses and entropies in mean-field screening models

We examine how the mean-field ionic screening excess associated with a charged macromolecule depends upon the macromolecule's geometric conformation and net charge. Our approach involves expanding about Debye–Huckel theory, which allows us to derive a series of general results that establish explicit connections between the ionic excess, non-linear screening mechanisms, and the mobile ion entropy. We conclude with an analysis of the ionic screening entropy in familiar models for polyelectrolyte force-extension behavior, and we find the changes in this entropy are often quite substantial relative to the total free energy differences characterizing a polyelectrolyte's extended and compact states.

Evaluation of the Standard Potential of the Ag/AgCl Electrode in a 50 wt-% Water–Ethanol Mixture

This paper deals with the evaluation of the standard potential of the Ag/AgCl electrode in a water–ethanol mixture (50 wt-%). A potentiometric method was applied using a cell without liquid junction. Mean activity coefficients of HCl in the same mixture have been also determined. The measurements were performed in the HCl molality range from 0.005 to 0.1 mol⋅kg−1. The Debye–Huckel theory and Pitzer’s model, based on the interactions present in the solution, have been applied. Good agreement was found between the results obtained with the two approaches. Uncertainties of the Pitzer parameters and interionic forces are discussed based on the values found. The variation of the standard potential as a function of the temperature was used to calculate the transfer thermodynamic functions. The effects of the solvent composition on the thermodynamic properties of HCl allow to highlight structural changes in water–ethanol mixtures.

Theory of the β-Type Organic Superconductivity under Uniaxial Compression

We study theoretically the shift of the superconducting transition temperature (Tc) under uniaxial compression in beta-type organic superconductors, beta-(BEDT-TTF)2I3 and beta-(BDA-TTP)2X[X=SbF6,AsF6], in order to clarify the electron correlation, the spin frustration and the effect of dimerization. The transfer integrals are calculated by the extended Huckel method assuming the uniaxial strain and the superconducting state mediated by the spin fluctuation is solved using Eliashberg's equation with the fluctuation-exchange approximation. The calculation is carried out on both the dimerized (one-band) and nondimerized (two-band) Hubbard models. We have found that (i) the behavior of Tc in beta-(BEDT-TTF)2I3 with a stronger dimerization is well reproduced by the dimer model, while that in weakly dimerized beta-BDA-TTP salts is rather well reproduced by the two-band model, and (ii) the competition between the spin frustration and the effect induced by the fluctuation is important in these materials, which causes nonmonotonic shift of Tc against uniaxial compression.

Effect of doping by boron, carbon, and nitrogen atoms on the magnetic and photocatalytic properties of anatase

The effect of doping of titanium dioxide with the anatase structure by boron, carbon, and nitrogen atoms on the magnetic and optical properties and the electronic spectrum of this compound has been investigated using the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) band-structure method in the local spin density approximation explicitly including Coulomb correlations (LSDA + U) in combination with the semiempirical extended Huckel theory (EHT) method. The LSDA + U calculations of the electronic structure, the imaginary part of the dielectric function, the total magnetic moments, and the magnetic moments at the impurity atoms have been carried out. The diagrams of the molecular orbitals of the clusters Ti3 X (X = B, C, N) have been calculated and the pseudo-space images of the molecular orbitals of the clusters have been constructed. The effect of doping on the nature and origin of photocatalytic activity in the visible spectral range and the specific features of the generation of ferromagnetic interactions in doped anatase have been discussed based on the analysis of the obtained data. It has been shown that, in the sequence TiO2 − y N y → TiO2 − y C y → TiO2 − y B y (y = 1/16), the photocatalytic activity can increase with the generation of electronic excitations with the participation of impurity bands. The calculated magnetic moments for boron and nitrogen atoms are equal to 1 μB, whereas the impurity carbon atoms are nonmagnetic.

WILLIAM LIPSCOMB DIES AT 91

WILLIAM N. LIPSCOMB JR., 91, an emeritus professor of chemistry at Harvard University who won the 1976 Nobel Prize in Chemistry, died in Cambridge, Mass., on April 14 of pneumonia and other complications from a fall. Born in Cleveland in 1919, Lipscomb earned a B.S. in chemistry at the University of Kentucky in 1941 and a Ph.D. in chemistry under Linus C. Pauling at California Institute of Technology in 1946. Lipscomb then joined the faculty at the University of Minnesota; he moved to Harvard as a professor of chemistry in 1959. He was appointed Abbott & James Lawrence Professor of Chemistry in 1971 and retired in 1990. Lipscomb’s Nobel Prize was awarded for his studies on the structure of boranes and the insight that that work provided into chemical bonding. His work on boranes and on carbon compounds formed the experimental basis for the extended Huckel theory, the first widely applicable use of molecular ...

Direct imaging of molecular orbitals of metal phthalocyanines on metal surfaces with an O2-functionalized tip of a scanning tunneling microscope

High-resolution scanning tunneling microscope images of iron phthalocyanine and zinc phthalocyanine molecules on Au(111) have been obtained using a functionalized tip of a scanning tunneling microscope (STM), and show rich intramolecular features that are not observed using clean tips. Ab initio density functional theory calculations and extended Huckel theory calculations revealed that the imaging of detailed electronic states is due specifically to the decoration of the STM tip with O2. The detailed structures are differentiated only when interacting with the highly directional orbitals of the oxygen molecules adsorbed on a truncated, [111]-oriented tungsten tip. Our results indicate a method for increasing the resolution in generic scans and thus, have potential applications in fundamental research based on high-resolution electronic states of molecules on metals, concerning, for example, chemical reactions, and catalysis mechanisms.

A DFT Study of the Reactivity Indexes of Ionic [4 + 2+] Diels-Alder Cycloaddition to Nitrilium and Immonium Ions

The global electrophilicity index, defined within the conceptual density functional theory (DFT), was used to classify the dienes and dienophiles currently used in Diels-Alder reactions on a unique scale of electrophilicity. The index, obtained within the Kohn-Sham scheme, is based on the HOMO and LUMO energies. A systematic study of the global reactivity indexes of the reagents involved in formal [4 + 2+] Diels-Alder cycloaddition reactions of nitrilium and immonium ions with isoprene is presented. Keywords: Diels-Alder, reactivity indexes, nitrilium, immonium, electrophilicity, nucleophilicity, Huckel molecular orbital (HMO) theory, electrofugality and nucleofugality, Berny analytical gradient, isoprene

Inverse patchy colloids: from microscopic description to mesoscopic coarse-graining

Typically, patchy systems are characterized by the formation of a small number of directional, possibly selective, bonds due to the presence of attractive regions on the surface of otherwise repulsive particles. Here, we consider a new type of particles with patterned surfaces and we refer to them as inverse patchy colloids because, in this case, the patches on the repulsive particles repel each other instead of attracting. Further, these patches attract the parts of the colloid that are free of patches. Specifically, we consider heterogeneously charged colloids consisting of negatively charged spherical particles carrying a small number of positively charged patches. Making use of the Debye–Huckel theory, we derive the effective interaction potential between a pair of inverse patchy colloids with two patches on opposite poles. We then design a simple coarse-grained model via a mapping with the analytical pair potential. The coarse-grained model quantitatively reproduces the features of its microscopic counterpart, while at the same time being characterized by a much higher degree of computational simplicity. Moreover, the mesoscopic model is generalizable to an arbitrary number of patches.

ChemInform Abstract: Silicon-Silicon Interaction in Bis(silylene)iron, Disilanyliron, and Bis(silyl)iron Complexes

Abstract 29Si NMR coupling constants in a disilanyliron complex CpFe(CO)2SiMe2SiMe(OtBu)2 (1: CP = η5C5H5), a bis(silylene)iron complex Cp(OC)Fe{SiMe2 ⋯ O(tBu) ⋯ SiMeOtBu)} (2), and a cis-bis(silyl)iron complex (OC)4Fe{SiMe2CH2CH2SiMePh} (3) were measured by the INEPT-INADE-QUATE technique. The coupling constant for 2 (28.6 Hz) is much smaller than that for 1 (128 Hz) with a direct Si-Si bond, but larger than that for 3 (2.7 Hz) where there is no direct Si-Si bond. Model calculations using the extended Huckel method indicate that there is no direct bond between the two silicon atoms in the bis(silylene)iron complex Cp(OC)Fe{SiH2 ⋯ O(H) ⋯ SiH2}. This means that the coupling in 2 can be described as the sum of the two geminal couplings, 2J(SiFeSi) and 2J(SiOSi).

ELECTRON TRANSPORT INVESTIGATION OF METAL–MOLECULE–METAL INTERFACE FOR NANOELECTRONICS

Theoretical understanding of electron transport phenomena through single molecules is of great importance for the design of future devices and materials in molecular electronics. Nonequilibrium Green's function (NEGF) formalism combined with extended Huckel (EHT) theory is used to investigate the electron transport characteristics of Au –benzene– Au , Au –borazine– Au , and Au –BCN– Au systems with selenium (Se) as terminal group. It is observed that the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of borazine ring is higher than the benzene and BCN does. Our result shows that for the terminal group selenium borazine is the best core molecule than benzene and BCN systems.

ChemInform Abstract: Icosahedron Oligomerization and Condensation in Intermetallic Compounds. Bonding and Electronic Requirements

Icosahedron-based clustering has been found to be very common in intermetallics, particularly for group 13 and early p-block icosogen elements. Linking of the icosahedral building blocks depends on the valence electron concentrations. Vertex-, edge-, or face-sharing icosahedra occur as the structure compensates for electron deficiency. Some examples of icosahedron-based clusters have been selected for an analysis of the relationships between the structural features (icosahedron oligomerization, atomic defects, etc.) and the bonding and electronic requirements. The extended Huckel method has been used with either a molecular approach or an electronic band structure calculation to rationalize bonding in the intermetallic framework.

Calculation of images of orientedC60molecules using molecular orbital theory

Using Huckel molecular-orbital theory, images are created to represent the electron distributions expected for a C60 molecule adsorbed on a substrate. Three different orientations of the C60 molecule on the substrate are considered. The effect of the interaction of the molecule with the substrate is treated purely from the basis of symmetry using group theoretical methods. The resulting electron distributions are then used to generate idealized images which represent how the molecule may appear when observed in a scanning tunneling microscope (STM) experiment. Comparison is made with STM images appearing in the literature. It is found that the more complicated ab initio methods usually employed to simulate STM images are not required in order to match observed results. Furthermore, we find that an unequivocal identification of the orbitals responsible for a given STM image cannot be made from analysis of the STM image alone.

Orbital Control of the Conductance Photoswitching in Diarylethene

Diarylethenes are photosensitive π-conjugated molecules whose application to various molecular devices is expected. The molecular and electronic structures of diarylethenes are switchable upon photoirradiation with their reversible structural isomerization. Site-specific electron transport phenomena through a diarylethene molecule, 1,2-di(2-methyl-1-naphthyl) perfluorocyclopentene, are studied by using the nonequilibrium Green’s function method combined with the Huckel molecular orbital method (NEGF−HMO) and density functional theory (NEGF−DFT). On the basis of the orbital symmetry rule, the conductance of the diarylethene is predicted to be efficiently switchable when the C3 and C10 atoms are appropriately connected with electrodes. Transmission spectra, spatial distribution of the MPSH (molecular projected self-consistent Hamiltonian) states, and I−V curves are obtained from DFT calculations. These results obtained from the higher-level DFT calculations are consistent with the prediction based on the qu...

Phenomenological chemical reactivity theory for mobile electrons

We present herein a model to deal with the chemical reactivity, selectivity and site activation concepts of π electron systems derived by merging the classical Coulson–Longuet-Higgins response function theory based on the Huckel molecular orbital (HMO) theory and the conceptual density functional theory. HMO-like expressions for the electronic chemical potential, chemical hardness and softness, including their local counterparts, atomic and bond Fukui functions and non-local response functions are derived. It is shown that sophisticated non-local concepts as site activation may be cast into deeper physical grounds by introducing a simplified version of static response functions. In this way, useful quantities such as self and mutual polarizabilities originally defined through the HMO parameters can be redefined as self and mutual softnesses. The model is illustrated by discussing the classical Hammett free energy relationship describing inductive substituent effects on the reactivity of benzoic acids.

Effective Coulomb interactions within BEDT-TTF dimers

We calculate the effective Coulomb interactions between holes in dimers of the organic molecule BEDT-TTF in vacuo. We use density functional theory (DFT) to parameterise Hubbard models for beta and kappa phase organic charge transfer salts. We focus on the intra-dimer Coulomb repulsion, U_d, and the inter-monomer Coulomb repulsion, V_m. We find that U_d = 3.22 \pm 0.09 eV and V_m = 2.71 \pm 0.10 eV for 23 experimental geometries taken from a range of materials in both polymorphs. The quoted error is one standard deviation over the set of conformations studied. We conclude that U_d and V_m are essentially the same for an isolated dimer with the geometries present in all of the compounds studied. We highlight the disagreement between our parameterisation of the Hubbard model and previous results from both DFT and Huckel methods and show that this is caused by the failure of an assumption made in previous calculations (that U_m >> V_m, where U_m is the effective intra-monomer Coulomb repulsion). We discuss the implications of our calculations for theories of the BEDT-TTF salts based on the Hubbard model on the 2D anisotropic triangular lattice and explore the role of conformational disorder in these materials.

Hückel MO studies on diuretics and carbonic anhydrase inhibitors

The electronic structures of sulfonamide compounds and thiazide derivatives are calculated by means of the Huckel molecular orbital method. The electron densities at nitrogen atom positions in the NCN group of thiazide derivatives are parallel to their diuretic potency. A good correlation is found between the formal charge or the frontier electron density of the lowest vacant level at the amide group position in the sulphonamide group of all these compounds and their inhibitory action against carbonic anhydrase.

Comparison for pyrrole and pyrazole of orbital energies and population analyses from ab-initio SCF, CNDO/2, INDO, extended hückel, and ARCANA calculations

The results of our recent ab-initio SCF calculations on pyrrole and pyrazole using large Gaussian basis sets are compared to those using less rigorous methods: CNDO/2, INDO, PCILO extended Huckel, and ARCANA (a type of iterative extended Huckel calculation). The ab-initio orbital energies are higher in energy than all those resulting from less rigorous methods. For the two highest occupied orbitals, both of π type, (pyrrole— 1a2 and 2b1; pyrazole—3a″ and 2a″) there was excellent agreement between the measured photoelectron spectroscopic ionization potentials and the Koopman's theorem values from the ab-initio results. Thus the orbital energies calculated by the less rigorous methods all lie too low in energy. The CNDO/2 and INDO orbital energies also span too large a width for the valence band compared to the ab-initio results. The gross atomic populations from the CNDO/2, INDO and PCILO methods show discrepancies from the ab-initio results for both the heteroatoms and the hydrogens. Total overlap populations calculated from the ab-initio SCF wave functions show large negative overlap populations between nonbonded ring atoms. Using the off-diagonal density matrix elements from CNDO/2 and INDO wave functions as if they were true off-diagonal elements from non-ZDO calculations, scaling them (shown previously to lead to reasonable values for TOPS between bonded atoms) led to TOPS between nonbonded ring atoms that were essentially zero. Wiberg bond indices, by definition only capable of giving positive numbers, also cannot reproduce these negative overlap populations. The extended Huckel and ARCANA methods, which are non-ZDO methods, both gave negative tops between nonbonded ring atoms, although not as large as those which resulted from ab-initio calculations.

Extended hückel β parameters applied to diatomic molecules

From international symposium on atomic, molecular, solidstate theory, and quantum statistics; Sanibel lsland, Florida, USA (20 Jan 1974). Atom-atom separations predicted by extended Huckel theory using four BETA parameter approximations are compared for 15 diatomic molecules. Equilibrium separations R/ sub e/ determined from a new proposed formula for the BETA parameter are compared with R/sub e/'s resulting from the Mulliken-Wolfsberg-Helmholtz, Cusachs, and Jug BETA formulas. Results show that the new BETA formula is best in its prediction of R/sub e/'s. The new formula has the required property of yielding rotationally invariant results. (auth)

Binding of O2, NO, and CO to model active sites in ferrous heme proteins: Ligand geometry, electronic structure, and quadrupole splittings

The electronic structures of model oxy-, nitrosyl-, and carbon monoxy ferroporphyrin compounds with N-methylimidazole as an axial ligand are calculated for a number of different conformations of the O2, NO, and CO ligands using the iterative extended Huckel method. For the dioxygen ligand a bent end-on (Pauling) geometry with a low-energy off-axis displacement is favored. The “anomalously large” electric field gradient at the iron nucleus can be accounted for by a paired iron(II)-dioxygen configuration. The carbon monoxide ligand is linear in model compounds but bent or tilted in protein. The reduced affinity observed for the intact protein is explained by these results. The nitrosyl ligand is bent both in model compounds and in the protein. The calculated electronic structure is consistent with the spin density observed in hyperfine splitting in electron spin resonance spectra and gives a quadrupole splitting in excellent agreement with experiment.