Calculated Charge Distributions Research Articles

Theoretical analysis of counterfoil influence over the physicochemical properties of aniline tetramers in several oxidation states

We present semiempirical quantum chemical calculations of geometric structures, charge distributions, energy levels, ionization potentials, and enthalpies of formation for aniline tetramers in different oxidation states using the semiempirical AM1 method. For tetraaniline radical cation the effect of these three counterions on the above-mentioned physicochemical parameters was analyzed. The ions studied included Cl−, HSQ4−, and CH3COO−. Chloride counterion showed a large charge transfer to the chain, as was shown in a preceding work [4]. HSO4− showed a strong charge stabilization without transfer. CH3COO− exhibits hydrogen bond formation and also displays a strong charge stabilization in a fragment of the chain, but does not transfer any charge. Also, Cl− was able to form a hydrogen bond, depending on the initial position that it occupies in relation to the tetrameric chain, which is then optimized. Charge transfer is present for the Cl− cases. For dication structures the effect of SO42− was analyzed. Our calculations showed that the distribution of the energy levels of tetraaniline radical cation can be comparable to the polyaniline ones by adding a counterion to the tetramer.

A theoretical study for methanol synthesis by CO 2 hydrogenation

Ab initio MO calculations were carried out using the density functional method for the reaction intermediate of methanol synthesis by CO 2 hydrogenation. The energy change of the bridging-formate due to changing the Cu-Cu interatomic distance showed that the formate intermediate was formed at the neighboring Cu atoms and no significant difference was found in the stability of the copper crystal surface. The stability of the methanol synthesis intermediates with the atomic charge of Cu was also calculated. The stability of the formate and the formaldehyde intermediates increased as the total charge increased. The stability of the methoxy species was almost constant between a total charge of 0 to + 0.4. There seems to be an appropriate value for the charge of the surface copper. On the other hand, the reverse water-gas shift reaction producing carbon monoxide occurred on the metallic copper. The calculation of the total atomic charge distributions on Cu CuO and Cu ZnO clusters showed that the cationic copper was located only on the boundary of Cu and the oxide. The number of active sites increased as much as the degree of dispersion of Cu and the oxide, and a high-performance catalyst is expected to be designed in such a manner.

Density Functional Calculations of Electronic Structure, Charge Distribution, and Spin Coupling in Manganese−Oxo Dimer Complexes

We have calculated the electronic structures of five different manganese-oxo dimer complexes using density functional methods combined with the broken symmetry and spin projection concepts. The number of carboxylate, oxo, and peroxo bridging ligands was varied, and the terminal ligands were triazacyclononane (TACN). The formal Mn oxidation states varied from Mn(III)(2) and Mn(III)Mn(IV) to Mn(IV)(2). These complexes have been synthesized and their X-ray structures and magnetic properties measured previously. We have calculated the Heisenberg spin coupling parameters J and resonance delocalization parameters B for all of these systems. Despite the very small energy differences involved, there is a good correspondence between calculated and experimental Heisenberg J parameters. We have analyzed potential changes in the calculated effective Heisenberg coupling J(eff) for the mixed-valence Mn(III)Mn(IV) complexes when partial or complete delocalization due to the B parameter is taken into account. These changes depend also on the energy of the relevant intervalence band. Surprisingly, in the two mixed-valence systems studied, the high spin S = (5)/(2) state lies below S = (7)/(2). This is consistent with spin coupling between Mn with site spins S(1) = 1, S(2) = (3)/(2), corresponding to intermediate spin Mn(I) and Mn(II) respectively, instead of the coupling expected from the formal oxidation states, S(1) = 2, S(2) = (3)/(2) from high spin Mn(III) and Mn(IV). The spin and charge distributions in the broken symmetry ground states are also consistent with intermediate spin S(1) = 1, S(2) = (3)/(2). The calculated charge distributions show strong metal-ligand covalency. In fact, as the formal oxidation states of the Mn sites increase, the net Mn charges generally show a slow decrease, consistent with a very strong ligand --> metal charge transfer, particularly from &mgr;-oxo or &mgr;-peroxo ligands. TACN is a better donor ligand than carboxylate, even when calculated on a per donor atom basis. The ligand atom charge transfer order is peroxo >/= oxo >> TACN > acetate. The TACN > acetate ordering is expected from the spectrochemical series, but the strong charge transfer and strong metal-ligand covalency of peroxo and oxo ligands with the Mn sites cannot be simply related to their positions in the spectrochemical series. In the Mn(IV)(2)(&mgr;-O)(2)(&mgr;-O(2))(TACN)(2), each peroxo oxygen has a small charge (-0.3), much less than found for each &mgr;-O atom (-0.7). The high-spin S = 3 state lies quite low in energy, 8 kcal/mol from our calculations and about 4 kcal/mol based on the experimental Heisenberg spin coupling parameters. Potential molecular oxygen dissociation pathways involving a spin S = 1 state are discussed. Effective ligand field diagrams are constructed from the calculated energy levels which display the competition between spin polarization splitting and the ligand field t(2g)-e(g) splitting and allow comparisons of electronic structure among different complexes. The electronic structure and spin coupling of these complexes was also compared to the corresponding "core-only" complexes where both TACN ligands were removed, yielding a far weaker ligand field. There is a strong ferromagnetic shift in the "core-only" complexes compared with the complete TACN complexes, also showing the effects of a weaker ligand field.

Charge distribution and transport in polymers

Experimental work on charge distributions and charge transport in polymer films is reviewed. For studying the charge distributions in the thickness dimension, thermal or acoustic techniques are used with resolutions down to /spl sim/1 /spl mu/m. The polymer samples usually are charged with corona, electron beam or thermal methods. Observation of the change of charge and polarization distributions with time allows one to draw conclusions about charge transport and charge injection phenomena. In some cases theoretical models have been established, based on excess-charge drift due to a finite carrier mobility and on charge compensation due to intrinsic or radiation-induced conductivity. Other models are concerned with the interaction of injected or intrinsic space charge with a dipole polarization. Comparisons of measured and calculated charge distributions and their change with time show the applicability of these models and shed new light on charge trapping and charge transport phenomena.

Crystal structures and charge distributions in 2-phenyl-imidazo[1,2-a]-pyridine and two related salts

Abstract X-ray structural investigations of 2-phenyl-imidazo[1,2-a]pyridine and two of its salts: 1-methyl-2-phenyl-imidazo[1,2-a]pyridinium iodide and 1-methyl-2-phenyl-6-nitro-imidazo[1,2-a]pyridinium iodide (R = 0.040, 0.032, 0.034 respectively), together with semiempirical calculations of charge distribution, have been carried out. The insertion of nitro-group does not cause any fundamental changes in geometry, but induces changes in the electron density distribution in the bicyclic unit. As a result rather short cation-anion contacts (I … C = 3.52 Å) are observed in structure III.

First-principles study on the Ga-rich GaAs(001)-(4 × 2) surface

Total energy calculations have been performed for the Ga-rich GaAs(001)-(4 × 2) surface using first-principles plane-wave pseudopotential techniques. There are two distinctly different structural models proposed for the Ga-rich (4 × 2) surface. The Ga model proposed by Biegelsen et al. consists of two Ga dimers on the top layer and another Ga dimer at the third layer, whereas the As model by Skala et al. consists of two As dimers on the top layer and two Ga dimers in the second layer. Calculated results show that the As model can be safely ruled out since the As model is energetically unstable with respect to the Ga model. The Ga model, on the other hand, has a problem that there is apparent discrepancy between the STM image and the surface geometry of the Ga model. It is found that the local density of states near the Fermi level are significantly affected by the existence of the third layer Ga dimers and that the Ga dimer atoms on the top layer asymmetrically contribute to the STM image. The calculated charge distributions for the Ga model can explain the observed STM image quite well.

Surface-defect formation on heavily doped InAs and GaAs layers studied by scanning tunneling microscopy.

The surface atomic structure of GaAs and InAs layers heavily doped with Si is studied by using scanning tunneling microscopy. For GaAs, the straightness of dimer-vacancy rows is degraded by doping. The density of kinks in the rows coincides with the surface state density needed to move the surface Fermi level to the midgap. On the other hand, with InAs, the kink density is much lower than the surface-state density needed to move the level to the midgap. Self-consistent calculations of charge distribution in the neighborhood of the surface suggest that kink formation is governed by the competition between two processes: energy loss by new kink formation and energy gain by the electron trap from the conduction band to the surface state formed by the newly created kink. Using this model, the low kink density observed for the InAs surface can be explained in terms of its small band gap of 0.36 eV. \textcopyright{} 1996 The American Physical Society.

Spectrometry and reactivity of benzanthrenyl and 1-hydroperylenyl anions

Charge distribution and reactivity of benzanthrenyl ( 1 −) and 1-methyl-1-hydroperylenyl anion ( 2 −) are examined by means of semiempirical calculations, NMR spectroscopy and reactions with electrophiles. Highest charge density and reactivity are located at position 7 of 1 − and the comparable position 12b of 2 −. A small degree of reactivity is located at positions 4 and 6 of 1 −, as shown by reactions. Generally, a good correlation between calculated charge distribution and charge distribution obtained from 13C NMR is observed.

ELECTRONIC STRUCTURE AND STM IMAGE OF C60–C70 ADSORBED ON THE Cu(111) SURFACE

A recent scanning tunneling microscopy (STM) experiment has revealed that C 60–C70 mixture on the Cu (111)–(1×1) substrate shows specific bias-voltage-dependent images which reflect intramolecular structure. In order to understand this observation, we perform a band-structure calculation. In this study, we adopt a simple model assuming a two-dimensional (2D) molecular crystal and a proper adsorption geometry. By comparing the calculated charge distributions of the lowest unoccupied molecular orbital (LUMO)-induced bands and the highest occupied molecular orbital (HOMO)-induced bands with the observed STM images, we find that the shapes of the intramolecular structures which change with the bias voltage strongly reflect the positions of pentagonal and hexagonal rings.

Photoelectrochemical Investigation of an Azopyridinium Compound ((CH3)2NC6H4NNC5H4NC18H37I) Modified SnO2Electrodes

The steady photoelectric response of an azopyridinium compound, (CH3)2NC6H4NNC5H4NC18H37I (AI), monolayer modified SnO2 electrodes prepared by the Langmuir−Blodgett technique has been investigated using a conventional photoelectrochemical cell. Under ambient condition, a cathodic photocurrent is observed when the AI−SnO2 electrode is illuminated by white light (the IR was filtered). The action spectrum is coincident with the absorption of the electrode in the region around 450 nm, indicating the AI aggregates in LB film are responsible for the photocurrent. Dependencies of the photocurrent on some factors have been studied, which may change the magnitude of the observed photocurrent, namely, the light intensity of the irradiation, the concentration of the donor or acceptor in electrolyte solution, bias voltage, and the saturation degree of O2 or N2. Based on the changes of absorption spectra of AI in benzene solvent before and after irradiation, the electrochemical behaviors of AI in LB films are studied and the calculations of bond length, charge distribution, and total energy of different state for an analogue of AI, (CH3)2NC6H4NNC5H4N+CH3 (AM), were performed by using MINDO/3 method. The photocurrent generation is believed to result from photoinduced tautomerism upon irradiation.

Deuterium isotope effect on 13C NMR spectra of ortho Mannich bases

The deuterium isotope effect upon 13C NMR spectra was studied for ortho Mannich bases of varying ΔpK a value [ΔpK a = pK a(NH +) − pK a(OH)]. The Δ values [Δ = δ 13C(OH) − δ 13C(OD)] for different positions in the phenyl ring are consistent with calculations of charge distribution by means of MNDO — PM3 method and assuming that deuteration leads to a weakening of the hydrogen bonding. In the critical range of ΔpK a, where the proton transfer appears, the inversion of the sign of the isotope effect for the carbon atom in position 1 takes place. Strong dependence of the Δ value on temperature was revealed for 2-(N,N-diethylaminomethyl)-3,4,5,6-tetrachlorophenol showing the proton transfer equilibrium.

A Dirac molecular orbital study for encapsulated heavy transition metals within ytrium cluster iodides

Dirac molecular orbital calculations (DSW-Xα) on the octahedral RuY 6I 12 2− and OsY 6I 12 2− clusters are reported. The calculated valence relativistic density of states clearly indicates that spin-orbit interaction is strongly acting on each constituent atom of both clusters, thus broadening the cluster valence bands. The interstitial Ru and Os atoms acquire and provide electrons and orbitals for bonding interactions with the cubooctahedral Y 6I 12 cage. The calculated charge distributions indicate that these clusters could be formulated as Ru −0.968Y 6 +0.458I 12 −0.315 and Os −1.028Y 6 +0.524I 12 −0.343, respectively.

Electronic structures of C60 and C70 adsorbed on the Cu(111) surface and intramolecular STM images.

Recent scanning tunneling microscopy (STM) experiments have revealed that ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ adsorbed on the Cu(111)-(1\ifmmode\times\else\texttimes\fi{}1) substrate show specific bias-voltage-dependent images that seem to reflect individual cage structures. In order to understand this experimental observation, band structures and partial charge-density distributions of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ are calculated by means of an all-electron mixed-basis approach. In this paper we adopt two-dimensional molecular models without substrates and proper orientational adsorption geometries of fullerenes. By comparing the calculated charge distributions of the lowest-unoccupied-molecular-orbital bands and the highest-occupied-molecular-orbital bands with the STM images, we find that the intramolecular patterns that change with the bias voltage strongly reflect the positions of five-membered and six-membered rings.

Electrostatic effects on the C60 surface of alkyl-C60 radicals

The EPR spectra of alkyl- and fluoroalkyl-C60 radicals are described and discussed. These radicals are of the general type XYZC-C60, where X, Y and Z are CF3, F, H or CH3. It is shown that there is a competition among X, Y and Z for the pentagon position adjacent to the C–C60 bond, and that in any given radical this position is gained by the most electronegative of them. Thus, if X, Y, Z is the order of decreasing electronegativity in the above sequence, X will occupy the pentagon position. If Y = X, the equilibrium conformation is asymmetric, with one X over the pentagon and the other over one of the hexagons. This conformation exchanges with its enantiomer above ∼ 200 K. An explanation based on quantum-chemical calculations of charge distribution on the C60 surface indicates that the more electronegative ligands are attracted to regions of more positive charge over the pentagon.

Theoretical studies on the structure and electronic properties of 3-(4-tolyl)sydnone

The structure of 3-(4-tolyl)sydnone has been calculated using the semiempirical MNDO, AM1 and PM3 methods and at the ab initio STO-3G, 3–21 G and 6–31 G** levels and the resulting geometries compared and contrasted with the well resolved crystal structure. While the geometry of the phenyl ring is well reproduced particularly by the ab initio methods, none of the methods give a satisfactory account of the geometry of the sydnone ring. The calculated charge distributions of atoms in the heterocyclic ring vary considerably depending on both the molecular orbital method used and the analysis technique adopted. The calculated dipole moments are generally overestimated with the best results obtained with the MNDO method for the semiempirical procedures and the STO-3G basis set for the ab initio methods.

Experimental and theoretical studies of substituent effects in hydrogen bond based molecular recognition of a zwitterion by substituted arylureas

Electron withdrawing groups have a strong effect on hydrogen bonding to aryl ureas. The effect of para substituents modestly exceeds the effect of meta substituents. Among common substituent parameters, σ − ( ϱ = 1.77, r 2 = 0.988) is found to be the best predictor for the observed effects of para substituents in aryl ureas. Semi-empirical and ab initio methods are used to calculate charge distributions in substituted benzene derivatives and in these ureas. A comparison of experimental and predicted (AM1, STO3G, 321-G ∗, 631-G ∗∗) dipole moments of benzene derivatives is presented. It is shown that calculated surface electric potentials for these thioureas successfully predict the relative hydrogen bonded association energies.

Accurate First Principles Calculation of Molecular Charge Distributions and Solvation Energies from Ab Initio Quantum Mechanics and Continuum Dielectric Theory

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAccurate First Principles Calculation of Molecular Charge Distributions and Solvation Energies from Ab Initio Quantum Mechanics and Continuum Dielectric TheoryDavid J. Tannor, Bryan Marten, Robert Murphy, Richard A. Friesner, Doree Sitkoff, Anthony Nicholls, Barry Honig, Murco Ringnalda, and William A. Goddard IIICite this: J. Am. Chem. Soc. 1994, 116, 26, 11875–11882Publication Date (Print):December 1, 1994Publication History Published online1 May 2002Published inissue 1 December 1994https://doi.org/10.1021/ja00105a030RIGHTS & PERMISSIONSArticle Views2139Altmetric-Citations947LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (943 KB) Get e-Alerts Get e-Alerts

A theoretical study of the abstraction reactions of hydroxyl radicals with halogenated aldehydes

The semi-empirical quantum mechanical molecular orbital method PM3 has been used to investigate the nature of the transition states for the reactions of hydroxyl radicals with halogenated aldehydes. For each reaction, the geometric and electronic structures of the transition state corresponding to the abstraction of the aldehydic hydrogen atom were determined. The results indicate that the transition states are reactant-like and, from the calculation of electronic charge distributions, it can be concluded that changes in the reactivity along the series are mainly due to inductive effects in the transition states.

Isotopic differences of charge distributions of even isotopes of tin

Isotopic variation of rms charge radii of even isotopes of tin (112–124) has been calculated on the basis of an average one body potential. The potential parameters were optimized in the lead region after taking the effect of core polarisation into consideration. The optimized potential parameters were then interpolated in the tin region. Single particle (-hole) energies along with the orbit radius have also been calculated. Comparison with experimental results show very good agreement. The calculated charge distributions after the usual corrections were subjected to model independent Fourier-Bessel analysis from where Fourier-Bessel coefficients have been extracted. Results of rms charge radii show that this phenomenological approach gives a very good description of the experimental situation.

Proposed elemental charge distributions in finite-apertureSAW-device models

A fairly general and easy-to-implement concept allows the construction of a variety of novel elemental charge distributions for 3-D full-wave SAW-device modelling. The method relies on phased-periodic Green functions, the method of moments, and the superposition principle. 3-D calculation of elemental charge distributions is presented, accounting for the transversal end-effects.