Calculated Charge Distributions Research Articles

Variational Monte Carlo calculation of two body properties in atoms: Importance sampling considerations

Accurate determination of atomic and molecular properties is a long held but elusive goal of many-body theories. Significant progress has been made for small atoms, but for heavier systems the calculations are very scarce due to the great computational difficulties involved in the calculation. In this work we have explored the efficiency of the Variational Monte Carlo (VMC) method to compute two body properties in atoms, which are specially difficult to calculate even in a Hartree–Fock framework. The VMC method allows us to calculate the radial moments of the electron pair density and, by using a finite approach for the delta function one is also able to evaluate up to a good accuracy the density itself. The major drawback of the method is loss of precision for both short and long interelectronic distances. To deal with this problem we have proposed and calculated with several functions for the importance sampling, that give rise to a better convergence of the results in a particular region of the domain of the interelectronic distance. Thus we have improved the description next to the origin where the determination by means of a pure MCV calculation is worse. The main conclusion of the study is that VMC can be used to get insight of the atomic and molecular structure by means of the calculation of the relative charge distribution. The importance sampling technique allows one to improve the results in those regions were the statistic is poorer. The accuracy of the results is enough to enable to study, for example, the effects of the electron correlation in these quantities.

Resonant and Nonresonant Vibrational and Electronic Excitations in Trimethylaluminum Adsorbed on GaAs

The vibrational and electronic excitations of trimethyaluminum (Me3Al) adsorbed on GaAs(100) have been studied using high-resolution electron energy loss spectroscopy (HREELS). The C−H vibrational modes of the adsorbed Me groups are excited by a resonance scattering mechanism involving the formation of a temporary negative ion. Strong vibrational mode enhancement occurs at 4, 8, and 14 eV with several overtone and combination bands observed at these energies. Reflectivity measurements of both the clean and adsorbate-covered surface are consistent with resonance scattering at these energies. The molecular orbitals involved in the resonant process are identifed through calculation of electron affinities and charge distributions for the free Me3Al molecule. HREEL spectra recorded at electron beam energies greater than 10 eV reveal new vibrational modes not associated with CH3 groups but characteristic of adsorbed CH2, a species also formed after annealing the Me3Al exposed surface to temperatures greater than 350 °C. Electron beam induced dissociation of the Me groups occurs with a threshold energy of 10 eV, and the dissociation cross section for CH2 production is very large, varying from 0.7 × 10-16 cm2 at 20 eV to 2.0 × 10-16 cm2 at 30 eV. The results are consistent with a nonresonant dissociation mechanism, and an electron impact ionization mechanism is proposed. The high cross sections reflect the relatively long lifetime of the dissociative excited states, which are not quenched efficiently for adsorption on semiconductor surfaces.

Structural transformations and water associate interactions in poly-N-vinylcaprolactam–water system

Abstract Structural transformations and water associate interactions were studied in poly-N-vinylcaprolactam (PVCL)–H 2 O system by various methods (IR-spectroscopy, quantum-chemical calculations, DSC and light microscopy). A number of features of PVCL macromolecules were first found: (i) the conformation type of a side seven-member ring deduced by quantum-chemical calculations is the “chair”; (ii) the main chain of PVCL prepared by usual radical polymerization has syndiotactic structure; (iii) the calculation of charge distribution on a CO oxygen atom in the analogue of PVCL chainlink gives the large electron density value being −0.362. The addition of water to PVCL changes greatly glass transition temperature ( T g ). The DSC method shows a lowering of T g from 147° (dry polymer) to −17°÷−38° at N (number of water molecules per unit) being 2.6÷8.0. Cold crystallization and subsequent melting of ice-like units formed of water associates in the presence of PVCL macromolecules were displayed at N =8 and 10. Crystallization under cooling happens only at the increased N ( N >10). Sharp decrease of melting heat of ice-like units (Δ H mt ) from 225 to 48 J/g (334 J/g of ice in bulk water) occurs as N diminishes from 20 up to 8. PVCL macromolecules in aqueous solution are the strongest modifier of water associate structures being affected by the polarization action of high polar amide CO groups and by specific configurational and conformational structures of the polymer.

The hexanuclear rhenium cluster ions Re6S8X64− (X=Cl, Br, I): Are these clusters luminescent?

Dirac molecular orbital calculations on the octahedral rhenium clusters Re6S8X64− (where X=Cl, Br, I) are reported. The calculated relativistic molecular orbitals indicate that the manifold of closely spaced unoccupied energy levels are mainly localized on the octahedral rhenium core with some contributions from the μ3-S ligands, while all the cluster highest occupied molecular orbitals are largely centered on the terminal halide ligands. All three hexanuclear rhenium cluster ions have singlet ground state (spanning the Γ6+ symmetry) and are therefore diamagnetic. They have the same ground state and excited state features as the intensely luminescent W6Cl142− cluster ion. The calculated charge distributions indicate that these clusters could be formulated as: Re6−0.21S8−0.16Cl6−0.24, Re6−0.23S8−0.17Br6−0.21 and Re6−0.26S8−0.26I6−0.12, respectively.

Classification of organic molecules to obtain electron affinities from half-wave reduction potentials: cytosine, uracil, thymine, guanine and adenine

A procedure for obtaining the adiabatic electron affinities (AEA) of organic molecules from half-wave reduction potentials in aprotic solvents is presented. Molecules are placed into groups according to their structure. Each group has a different solution energy difference. Calculations of AEA and charge distributions with AM1-multiconfiguration configuration interaction are used to support the intuitive classification of the molecules. The procedure is illustrated for Vitamins A and E, riboflavin, the azines, polyenes, hydroxy-pyrimidine, oxo-guanine, the hydrogen bonded cytosine-oxo-guanine as well as the AEA, and vertical EA (VEA) of Cytosine (C), Uracil (U), Thymine (T), Guanine (G) and Adenine (A). The latter values are: (VEA) G, 0.10; A, −0.49; U, 0.33; T, 0.31; C, −1.48 and (AEA) G,1.51±0.05; A, 0.95±0.05; U, 0.80±0.05; T, 0.79±0.05; C, 0.56±0.05 in eV.

Molecular modelling studies on the reactions of fluoroalkenes with sulfur trioxide

The structures, molecular energies and electronic properties of a series of substituted fluoroalkenes and the corresponding alkane sultones, derived from the addition of sulfur trioxide to the double bond, have been explored using a combination of molecular modelling and theoretical calculations. Both the calculated charge distributions at carbon and the calculated enthalpies of the addition process show a good correlation with the observed selectivity of electrophilic attack by the sulfur atom of sulfur trioxide, especially for unsymmetrical alkenes where there are two possible sites of addition. Mechanistically, the reactions of ethene and tetrafluoroethene with sulfur trioxide proceed via formation of stable π-complexes to give a transition state in the former case which is predicted to have a comparable energy to that found experimentally for the related reaction of octadec-1-ene.

Theoretical study of the basicity of alkyl amines in vacuo and in different solvents: a density functional theory approach

Abstract A study is presented on the basicities of alkyl-substituted amines both in the gas phase and in solution. In the first step, calculated group hardnesses are used to clarify the origin of the inversion of the alkyl amine basicity when passing from the gas phase to solution for a series (CH 3 ) n NH 3− n with n =0, 1, 2 and 3. The basicity in the gas phase is primarily determined by the intramolecular stabilization of the positive charge in the conjugate acid whereas, in aqueous solution, the solvent accounts for this effect. Next, Sanderson's electronegativity equalization principle is shown to be a useful tool in describing the charge distribution in the molecules RNH 2 (R=CH 3 , CH 2 CH 3 and CH(CH 3 ) 3 ) and their conjugate acids in solution. These calculated charge distributions are then used in a study on the inversion of the alkyl amine basicities from the gas phase to aqueous solution for these molecules.

HXeSH, the First Example of a Xenon−Sulfur Bond

Quite recently, we have reported a number of novel rare-gas compounds of type HXY (X ) Xe or Kr, and Y is a fragment with a large electron affinity). The compounds obtained so far include HXeH, HXeCl, HXeBr, HXeI, HXeCN, HXeNC, HKrCl, and HKrCN.1-4 They are prepared in rare-gas matrices at low temperatures (<50 K) by photolyzing a suitable precursor HY, which yields isolated hydrogen atoms and Y-fragments. After photolysis, thermal mobilization of hydrogen atoms leads to their reactions with the rare-gas atoms surrounding the Y-fragments. The resulting novel HXY rare-gas compounds are detected by IR spectroscopy mainly via extremely intense X-H stretching absorptions. In this report we introduce a new member of this family, HXeSH. This molecule expands the number of elements capable of making chemical bonds with xenon and represents the first compound with a xenon-sulfur bond. The electronic structure calculations for HXeSH were carried out with the Gaussian 94 package of computer codes.5 The relativistic effective core potential (ECP) by LaJohn et al. was used on Xe.6 This ECP includes the d-subshell in the valence space resulting in 18 valence electrons and was used in a decontracted form. The standard split-valence 6-311++G(2d,2p) basis set was used for hydrogen and sulfur. The calculated structural parameters of HXeSH at different computational levels are given in Table 1. Like H2S, HXeSH is also a bent molecule with an H-S-Xe angle near 90 degrees. The calculated S-H bond distance of 1.34 A in HXeSH is essentially the same as the experimental value in H2S. The Xe-H distance (1.84 A at the CCSD(T)-level) is somewhat larger than the value of free XeH+ (1.60 A)8 but much smaller than the van der Waals distance of neutral Xe-H (3.95 A).9 The calculated length of the Xe-H bond compares well with XeH2 and HXeI molecules, which both have experimental Xe-H stretching absorptions in the 1200 cm-1 region of the spectrum.1,2,10 The calculated Xe-S bond distance of 2.77 A can be compared with the recently calculated values for the excited states of Xe-S. For the 11Σ+ state (Xe+ S(1D)) and the lowest chargetransfer (Xe+S-) state 23Π, 2.62 and 3.32 A were obtained for the Re-values, respectively.11 Also, our Xe-S bond distance is near the I-S bond distance of 2.67 A for the linear S-I-S unit in [(S7I)2I)]. The calculated charge distribution indicates significant ion-pair character as in all the other HXY molecules observed thus far.1-4 Xenon carries a relatively large positive charge, and the SH group is negatively charged.

Theoretical Study of the Solvent Effect on Functional Group Properties and on the Charge Distribution and Acidity of Alkyl-Substituted Alcohols

For a number of functional groups, the group electronegativity and hardness in the gas phase and in different solvents was calculated using the recently introduced self-consistent isodensity polarized continuum model (SCI-PCM). The results indicate that the groups become less electronegative and less hard with increasing dielectric constant. Using the calculated functional group properties and Sanderson's electronegativity equalization principle, charge distributions in the alkyl alcohols X−OH and their conjugated bases XO- (with X = −CH3, −CH2CH3, and −CH(CH3)2 are determined in solution. The calculated charge distributions are used in a study of the inversion of alkyl alcohol acidity from gas phase to aqueous solution. Relative acidities are calculated in the gas phase and in aqueous solution for methanol, ethanol, 2-propanol and tert-butanol. The experimentally observed inversion of the acidity sequences of these systems is reproduced, and special importance is assigned to the stabilization energy of the conjugate bases in both media.

Positive streamer propagation in large oil gaps: electrical properties of streamers

This paper presents experimental data and calculations concerning the electrical properties of positive streamers in mineral oil at large gaps and HV. The experiments concern the measurement of charge, electric field, and the determination of the potential drop along streamers, either in the liquid alone, or for streamers guided within insulating tubes. Calculations of charge and field distribution around streamers are carried out by charge simulation. To do this, streamers are represented by objects with simple shapes (spheres or cylinders) equivalent to their macroscopic aspect. These models lead to a correct agreement with measured streamer charge and field on the plane electrode. Qualitative correlations are established between calculated field distributions and streamer behavior such as velocity, transitions between propagation modes. It is also concluded that the potential drop in streamers and branching both act as regulating mechanisms that help to keep the streamer tip field, and hence the velocity, constant over a wide voltage range.

Lattice energies of calcite-structure metal carbonates II. Results for CaCO 3, CdCO 3, FeCO 3, MgCO 3, and MnCO 3

Lattice energies ( ΔU) of five calcite-structure metal carbonates (i.e., CaCO3 3(s), CdCO 3(s), FeCO 3(s), MgCO 3(s), and MnCO 3(s)) have been calculated theoretically by separating ΔU into the following three energy components: electrostatic, polarization, and repulsive. Experimental crystal structure data and previously determined carbonate ion charge distributions (Mandell, G. K. and Rock, P. A., Lattice energies of calcitestructure metal carbonates I. Calculation of carbonate ion charge distributions from oxygen polarizabilities. J. Phys. Chem. Solids, 1998, 59, 695) serve as the primary input data to the model. The electrostatic portion of the lattice energy ( ΔU electrostatic ) is evaluated by a point charge approach, while the polarization energy contribution ( ΔUpolarization) is computed via a self-consistent method, which considers both dipole/dipole and ion/dipole interactions. Lastly, the repulsive energy segment ( ΔU repulsive ) is evaluated by means of the Slater potential (Slater, J. C., Introduction to Chemical Physics, Ch. 13. McGraw-Hill, New York, 1939). The ΔU values obtained from this theoretical approach compare favorably with the corresponding experimentally based values.

3-Methylthio-1,2,4-triazine: a Comparison of Experimental and Theoretical Structures

The theoretical equilibrium and X-ray crystallographic structures of 3-methyl-1,2,4-triazine, C4H5N3S, show a high level of agreement. The calculated charge distributions are analysed for each of four conformers; that with lowest energy is present both in the solid and in mesitylene solution, as shown by dipole-moment measurements.

Charge distribution from a simple molecular orbital type calculation and non-bonding interaction terms in the force field MAB.

A simple and fast method to calculate charge distributions in organic molecules is presented. The method is based on charge shifts within the saturated sigma-system, driven by orbital electronegativities, coupled to a modified Hückel treatment of the unsaturated pi-systems. Experimental molecular dipole moments of a set of 119 molecules are reproduced with a root mean square deviation of 0.36 Debye units. Furthermore, the obtained charge distribution is used to describe hydration free energies in terms of hydrogen-bonding donor and acceptor strengths of polar groups. Least square fitting to experimental data of 281 compounds leads to values for these strengths with accuracy limits of +/- 4.3% and +/- 2.5%, respectively. Properly normalized values are taken to parametrize the hydrogen bonding terms in our MAB force field. The method is sufficiently fast to be used in the preparatory phase of interactive force-field calculations.

Bipolar ion-exchange resins based on functional acidic tetrazolium groups — their synthesis, structure and properties

The synthesis of a novel kind of bipolar ion exchangers with a well-defined structure based on functional acidic 5-thio-2 H-tetrazolium groups is described. A macroporous resin with ortho-carboxylic acid groups ( m-T1), a gel-type resin with ortho-sulfonic acid substituted tetrazolium groups ( g-T4) and surface modified fibres with ortho-, meta- or para-carboxyl-substituted tetrazolium cations ( f-T1, f-T2 and f-T3) were prepared. Immobilisation of appropriate acidic dithizone derivatives onto the different chloromethylated polymeric supports and subsequent oxidation yielded the heterocyclic functionalities. The proposed structure was confirmed by synthesis of free model compounds and comparative IR-spectroscopic investigations. Theoretical calculations of conformation and charge distribution were carried out by molecular modeling. The theoretical capacities of the resins calculated from acid-base titrations are distincly lower than the data obtained from elementary analysis. The results of sorption experiments with electrolytes and conductivity measurements do not confirm the expectation of a simultaneous sorption of cations and anions similar to snake-cage polymers. These facts can be explained by the formation of an inner salt structure of the functional groups comparable with free betaines in aqueous solution. As a consequence, weak adsorptive dipole-induced-dipole interactions result in a specific uptake of Au(III) species from dilute hydrochloric acid media.

Non-LTE superconfiguration collisional radiative model

We have developed a non-LTE collisional radiative model which takes into account the overwhelming multiplicity of excited levels of heavy elements in hot and dense plasmas. The model, ‘SCROLL’ (Super Configuration Radiative cOLLisional), treats superconfigurations as effective levels and reproduces detailed configuration accounting results by an iterative procedure which splits superconfiguration until convergence. The fundamental assumption in the model is that within a superconfiguration levels are populated according to Boltzmann statistics. This assumption is effectively relaxed by the convergence procedure. The populations of the superconfigurations are obtained by solving the set of collisional radiative rate equations. For this purpose we have derived analytical formulae for the average transition rates between superconfigurations for collisional excitation and ionization, radiative transitions, photoionization and autoionization. These analytical formulae are obtained using the ‘Partition Function Algebra’ developed in our previous LTE STA model. The calculation of the average rates are performed in the distorted wave approximation framework. Using the resulting superconfiguration populations we can then calculate charge distribution, opacities and emissivities. The model is applicable to any atom. In the present work it was applied to investigate non-LTE thin plasmas of Al and Au for several temperatures and densities. Comparison with detailed calculations when these are possible gave excellent agreement. Both theory and results are addressed.

On the Alternation Effect in Substituted Indolizines and Their Aza-analogs.

The SINDO1 method is used to calculate charge distribution and total energies in the series of isomeric nitro- and methoxy-substituted indolizines, azaindolizines, and N-methylazaindolizinium cations. The influence of donor and acceptor substituents on the total energy and charges may be treated according to the simple alternation rule. In the series of isomers (substituted or azaindolizines) the lower energies are observed for those molecules, where acceptor substituents are arranged consonant to the polarity of the chain N-C(5)-C(6)-C(7)-C(8)-C(9)-C(1)-C(2)-C(3) of the indolizine. The results are used to treat known peculiarities of ring opening/transformation of indolizine and its aza-derivatives. It is proved that the energies of C(3) and C(5) adducts of isomeric azaindolizines with hydroxyl anion follow the same alternation pattern.

Cost effective calculation of molecular charge distributions and gas phase deprotonation energies using density functional methods

Major advances have been made in density functional theory and linear scaling methods. However, routine application of these methods to larger systems possessing no symmetry still remains difficult. Moreover, no clear standard exists for the use of small basis sets in the calculation of atomic or molecular properties with density functional methods. In this work an evaluation is made of the performance of different density functional methods for small and often used basis sets. Deprotonation energies are calculated for a series of small compounds and compared with high-level quantum chemical calulations. Furthermore, atomic populations calculated with these methods are compared with high-level results. The results show that the use of density functional calculations with smaller basis sets is justified for calculations on larger systems, retaining relatively good accuracies.

Structural Aspects of Cyclopropyl Homoconjugation: Experimental Studies and Ab Initio Calculations

AbstractLow‐temperature crystal structural studies of a series of saturated and unsaturated bicyclo[2.2.1]heptadiene, heptene and heptane compounds with 7‐spirocyclopropyl substitution reveal significant differences in the bond lengths of the three‐membered rings and in the CC single bonds of the bicyclic fragment. A complex interplay of strain and different types of conjugation influence the molecular structure of the bicycloheptadiene derivative 1, where the difference in the length of the three‐membered ring bonds is 0.040 Å and all CC single bonds in the bicyclic fragment are lengthened significantly. Ab initio calculations at the HF/6‐31G(d) (to a minor extent) and MP2/6‐31G(d) levels are in good agreement with the experimental data. Calculated charge distributions and dipole moments further support the relevance of cyclopropyl homoconjugation in the investigated prototype of Walsh and through‐space π‐orbital interaction. Static difference electron density maps have been derived from the experimental data by multipole refinements which showed exocyclic shifts of electron density in the planes of the three‐membered rings and significant bond ellipticities at the CC single bonds in the unsaturated bicyclic units.

Effect of parasitics on electrochemical capacitance-voltage profiling of pseudomorphic high electron mobility transistor structures

The effect of parasitic series resistances on electrochemical capacitance-voltage (EC-V) profiling is simulated numerically for AlxGaj1-xAs/InyGa1-yAs pseudomorphic high electron mobility transistor (p-HEMT) structures. The actual EC-V measurement is simulated numerically by reconstructing the charge distribution from an intrinsic distribution calculated from a self-consistent k p model. The calculated charge distribution then forms the basis for examining the possible profiles an EC-V measurement would produce when parasitics are taken into account. From a simple lumped-circuit model, it is shown that parasitic resistances distort the shape of the charge distribution by changing the relative heights of the 5-layer and channel charges. Hysteresis effects may also occur and may be accompanied by a change in material type from n to p even though the material is known to be n-type over the entire range of measurement. Additionally, an undesirable dependence of the charge profile on the probe signal frequency is found.

Infrared spectroscopic studies on polarity and tautomerism of 3(2H)-pyridazinone derivatives

Lactam-lactim tautomeric structure of three hydroxypyridazine derivatives was investigated by infrared spectroscopy. The conclusion could be drawn that the molecules exist in oxo-enol tautomeric equilibrium in dioxane solution. The cyclic dimer intermolecular association of the molecules was supported by a charge distribution calculation.