Point Charge Approximation Research Articles

Transition intensities in rare gas triatomic ions: DIM versus point-charge approximation

Diatomics-in-molecule (DIM) approach is compared with the point-charge approximation for transition moments in Rg 3 + and is shown to be its extension capable of accurately reproducing the direct ab initio results. Relevant polarization corrections to the point-charge approximation are proposed. The developed procedure is applied to construction of transition moment and intensity (oscillator strength) 3D-surfaces of Ne 3 +. They are used to evaluate the photoabsorption spectra of the bound species and of the Ne + Ne 2 + collisional system. The major absorption band of Ne 3 + is predicted to be red-shifted by 200 nm from that for the diatomic ion, and the absorption spectrum of Ne 2 + is found to be significantly affected in collisions with Ne.

Atomic charges in RBa2Cu4O8 lattices, determined by comparison of experimental and calculated nuclear quadrupole interaction parameters

Nuclear quadrupole interaction parameters have been determined for the rare-earth and copper sites of the RBa2Cu4O8 lattices (R = Sm, Y, Er) by 67Cu(67Zn) and 67Ga(67Zn) emission Mössbauer spectroscopy. The lattice electric field gradient tensors have been calculated for all sites of the above lattices using the point charge approximation. A comparison of the measured and calculated parameters has shown that in the RBa2Cu4O8 lattices holes are predominantly confined to the oxygen chain sublattices.

Atomic charges in RBa2Cu4O8 lattices derived from a comparison of experimental and calculated nuclear-quadrupole coupling parameters

The nuclear-quadrupole coupling parameters at the rare-earth metal and copper sites in the RBa2Cu4O8 lattices (R=Sm, Y, Er) have been determined by 67Cu(67Zn) and 67Ga(67Zn) Mossbauer emission spectroscopy. The crystal electric-field-gradient tensor has been calculated for all sites of the above lattices in the point-charge approximation. A comparison of the experimental with calculated parameters shows that holes in the RBa2Cu4O8 lattices are localized primarily at chain-oxygen sites.

Unusual distributed charge models of water's electric potential

Based on wave functions determined from high level quantum mechanical theory, we critically evaluate some distributed charge models for the electrical potential of a water molecule. In three cases we find that for optimal parameterization several charges coalesce to form a multipole, reducing to four site descriptions, each of three point charges and a multipolar site. In two cases, the multipole is located at the distributed charge; in the third it is sited at the oxygen atom. All three limiting models are considerably superior to simple four site point charge approximations. The description of water's electrical potential can be dramatically improved, even close to the H and O atoms, if distributed charges are described as diffuse spherical charge densities. Some models constructed in this way can admit of intuitive `chemical' interpretations.

Charge states of atoms in HgBa2CuO4 and HgBa2CaCu2O6 lattices

Mossbauer emission spectroscopy on the 67Cu(67Zn) isotope has been used to determine the parameters of the electric-field gradient tensor at the copper sites of the HgBa2Ca n−1CunO2n+2 lattices (n=1,2), as well as to calculate these parameters in the point-charge approximation. An analysis of the results, combined with available NQR literature data for the 63Cu isotope, has shown that the best fit of calculated to experimental data can be reached by assuming that the holes due to the presence of defects in the material localize primarily in the sublattice of the oxygen lying in the copper plane.

Long-Range Electrostatic Interaction between a Charged Wall and Two Similarly Charged Colloidal Spheres at Low Surface Potentials

The long-range electrostatic interaction between a pair of similarly charged colloidal spheres and a charged planar wall atlowsurface potentials is theoretically investigated. The linear Poisson–Boltzmann equation (PBE) and the point charge approximation of the charged sphere are used. The electrical potential distribution in the electrolyte solution is found from the PBE at theconstant surface potentialsusing the image charge method. The electrostatic forces acting on the spheres are then calculated. The results show that the repulsive interaction between a pair of similarly charged colloidal spheres clearly decreases when a charged wall appears nearby, but it is impossible for an attractive force to emerge at the scaled surface potentials less than 1. There is, however, an attractive force between the chargedwalland the similarly charged colloidalspheres,when the surface potential ζpon the wall is sufficiently higher than the surface potential ζson the spheres to make ζp> ζsexp(κh) (his the distance from the wall to the sphere center). In this case, there are negative surface charges on the spheres at positive surface potential ζs. It is these negative charges that produce the above attraction.

Electron Spin Polarizations of Phthalocyaninatosilicon Covalently Linked to One TEMPO Radical in the Excited Quartet and Doublet Ground States

Phthalocyaninatosilicon(IV) covalently linked to a 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radical, SiPc-TEMPO, has been studied by time-resolved electron paramagnetic resonance (TREPR). A TREPR spectrum at 20 K is assigned to the doublet ground (D0) state and the excited quartet (Q1) state consisting of the excited triplet SiPc and the doublet TEMPO radical. The excited quartet spectrum is simulated using zero field splitting parameters D = 4.3 × 10-3 and E = 3.3 × 10-4 cm-1. The D value observed is almost identical with that calculated under a point charge approximation (= 4.7 × 10-3 cm-1). The electron spin polarization of the Q1 state is reproduced by selective intersystem crossing (ISC) from the excited doublet states to the |±1/2> sublevels in the Q1 state. This selective ISC is explained by spin−orbit coupling between the Px and Py orbitals on oxygen atoms of axial-ligands. In the TREPR spectrum at 293 K, SiPc-TEMPO molecules in the Q1 and D0 states exhibit A and E polarizations, respectively,...

Low-energy optical transitions in organic metal DMTSA–BF 4

Low-energy absorption bands in the polarized reflection spectrum of metal-like DMTSA–BF 4 (2,3-dimethyltetraselenoanthracene–BF 4) were interpreted based on a one-dimensional band theory using a point charge approximation. Drude-like dispersion observed in the E∥ c reflection spectrum was assigned to the intra-band transition along the conducting axis. A broad absorption band at ca. 6900 cm −1 in the E⊥ c reflection spectrum was assigned to the inter-branch transition from the lower to the upper branch produced by folding the HOMO band at the zone boundary. The transition probability of this electronic transition originated from the zigzag stacking structure of DMTSA molecules in this crystal. The calculation of the conductivity spectra of this transition according to this proposal agrees well with the observed conductivity spectrum.

Charge states of atoms in the lattices of the high-temperature superconductors Tl2Ba2Ca n−1CunO2n+4 and Bi2Sr2Ca n−1CunO2n+4

To determine the charges of atoms in the lattices of the compounds Tl2Ba2Can−1CunO2n+4 and Bi2Sr2Can−1CunO2n+4 (n=1,2,3), the parameters of the electric field gradient tensor at the copper sites of the indicated lattices were found by emission Mossbauer spectroscopy on the isotopes 61Cu(61Ni) and 67Cu(67Zn), and a calculation of these parameters was performed in the point-charge approximation. A comparison of the resulting values and the published data on 63Cu nuclear quadrupole resonance showed that agreement between the experimental and computed values of the parameters obtains for models in which the holes resulting from a reduction in the valence of some of the thallium (bismuth) atoms are localized predominantly at oxygen sites located in the same plane as the copper atoms (for the compounds Tl2Ba2Ca2Cu3O10 and Bi1.6Pb0.4Sr2Ca2Cu3O10—at oxygen sites in the same plane as the Cu(2) atoms).

Ionic Size Induced Defects in Lead Titanate-Zirconate Perovskite Studied by TDPAC Method

Abstract Time differential perturbed angular correlation (TDPAC) measurements were carried-out on PbZr0.8Ti0.2 O3 as a function of temperature. A broad distribution of electric field gradient (EFG) was observed in all cases with a second site for 181Hf-181 Ta probes in the R3c phase. The results were analyzed in terms of a model which simulates an attraction of the ions by the smaller Ti ions. The EFG was calculated within the point charge approximation, by averaging over a large number of configurations generated by random distribution of the Ti ions in an ensemble of lattice cells. It was verified that the model reproduces qualitatively the broadening of the TDPAC frequencies as well as explains the second component EFG observed experimentally.

Monte Carlo and molecular orbital study of solvent effect on the electronic structure and hyperfine coupling constants of the (CH 3) 2NO radical: the effect of electron transfer between the solute and solvent molecules

Ab initio UHF MIDI-4 calculations have been carried out for the dimethyl nitric oxide (DMNO) radical in the H 2O and CHCl 3 solutions. The configurations of the solvent molecules were selected from the snapshots in the Monte Carlo (MC) simulation for the solution, and the electronic structure and the hyperfine coupling constant ( hfcc) of DMNO were obtained by averaging the results of 300 configurations. The point charge approximation in which all solvent molecules were represented by point charges indicated that the electrostatic interaction polarizes the N–O bond and increases the spin density on the N atom. When some of the solvent molecules near DMNO were treated as a supermolecule together with DMNO and the remaining solvent molecules were approximated by point charges, the electron transfer occurs between DMNO and H 2O molecules in two directions, from DMNO to H 2O around the N–O group and from H 2O to DMNO around the CH 3 groups. These electron transfers polarize the N–O bond in the same direction as the electrostatic interaction and increase the hfcc of the N atom. The contribution of the electron transfer accounts for 40% of the solvent effect on hfcc of DMNO.

Atomic Charge States in Tl2Ba2Can?1CunO2n+4 as Determined by the61Cu(61Ni),67Cu(67Zn) and133Ba(133Cs) Emission M�ssbauer Spectroscopy

Parameters of the electric field gradient tensors have been found for the Cu and Ba sites of the Tl2Ba2Can—1CunO2n+4 lattices by the 61Cu(61Ni), 67Cu(67Zn) and 133Ba(133Cs) emission Mössbauer spectroscopy. The same parameters have been calculated using the point charge approximation. An analysis of the obtained quantities has been carried out taking into consideration literature data on the 63Cu nuclear quadrupole resonance. Agreement of the measured and calculated parameters could be reached assuming the holes resulting from lowering the valence of part of the Tl atoms to be placed mainly at oxygen sites of the Cu–O planes (especially, at oxygen sites of the Cu(2)–O planes for the Tl2Ba2Ca2Cu3O10 compound).

61Cu(61Ni) and 133Ba(133Cs) Mössbauer emission spectroscopy of Tl2Ba2Ca n−1CunO2n+4

The parameters of the electric-field gradient tensor at copper and barium sites in the Tl2Ba2Can−1CunO2n+4 (n=1,2,3) lattice have been determined by 61Cu(61Ni) and 133Ba(133Cs) Mossbauer emission spectroscopy, and calculated in the point-charge approximation. The calculated parameters can be reconciled with experiment if one assumes that the holes produced as the valence state of a part of thallium atoms is lowered are localized predominantly on the oxygen sublattice lying in the same plane with copper atoms [in the Cu(2) plane in the Tl2Ba2Ca2Cu3O10 lattice]. 133Ba(133Cs) Mossbauer emission spectroscopy data agree qualitatively with the proposed models of charge distribution in the Tl2Ba2Can−1CunO2n+4 lattices.

Structure and optical properties of Eu 3+-doped fluoroaluminate and fluorophosphate glasses

Emission spectra of Eu 3+ in fluoroaluminate and fluorophosphate glasses at 80 K were measured as a function of excitation energy by the fluorescence line narrowing (FLN) technique. The magnitudes of the splitting of the 7F 1 manifolds at lower excitation energies increased with increasing phosphate content. The relative intensity of 5D 0 → 7F 2 to 5D 0 → 7F 1 transitions increased by adding the phosphate compound. This increase decreased gradually with increasing excitation energy. The atomic structure around Eu 3+ in these glasses was simulated using molecular dynamics techniques. The Stark splitting and the transition probabilities between 5D 0 and 7F J energy states at each simulated Eu 3+ ion were calculated on the basis of the point-charge approximation. The predicted energy differences between 5D 0 and ( 7F 2, 7F 1 and 7F 0) energy levels agree within 100 cm −1 with the observed ones. The transition probabilities are discussed in terms of the crystal field potential and the configuration interaction parameters, Ξ( k, λ).

67Cu(67Zn) Mössbauer emission spectroscopy of Tl2Ba2CuO6 and Tl2Ba2CaCu2O8

The parameters of the crystal electric-field gradient at the copper sites of Tl2Ba2CuO6 and Tl2Ba2CaCu2O8 have been determined by Mossbauer emission spectroscopy using the 67Cu(67Zn) isotope, and calculated in the point-charge approximation. The results obtained are analyzed using the available 63Cu nuclear quadrupole resonance data. The experimental and calculated data can be brought in agreement if one assumes that the holes appearing after part of the thallium atoms lower their valency become localized primarily at the oxygen sites lying in the plane of the copper ions.

Engineering a simple polarizable model for the molecular simulation of water applicable over wide ranges of state conditions

We perform a systematic analysis of the relationship between the molecular geometry, the force-field parameters, the magnitude of the induced dipoles, and the resulting site–site microstructure of a model for water consisting of simple point charges plus a self-consistent point dipole polarizability. We constrain the model to represent the experimental values of the pressure and the configurational internal energy of water at ambient conditions, while keeping a permanent dipole moment of 1.85 D. The resulting force fields are then used to perform additional simulations at high temperature to determine the effect of polarizabilities on the site–site structure, and to make contact with neutron scattering experiments as well as ab initio simulation results. We show that the parameterization of the model is possible for 0≤ROM≤0.25 Å, where ROM is the oxygen-to-negative charge distance along the bisectrix of the H–O–H angle, resulting in total dipole moments from 2.88 to 3.03 D, with polarization energies accounting for 40%–57% of the total configuration internal energy of water. These results, in conjunction with the behavior of the short range site–site correlation functions, highlight the shortcomings of the simple point charge approximation for the polarization behavior at short intermolecular distances, and give a meaningful reference from which we can attempt to overcome these defects.

Semiempirical calculations of the electronic spectra of organometallic compounds: Estimation of solvent and counterion effects

The electronic spectra of the [Ru(NH3)5pz]2+, [Ru(NH3)5pz]3+, and [Ru(CN)5pz]3− complexes are calculated by the CI INDO method. The effect of solvation on the spectra is considered in a point charge approximation and in terms of the parametric model of a charged sphere enveloping the metal fragment. Interrelation of these approaches is discussed. Data on the molecular electrostatic potentials created by the complexes are presented to substantiate the models.

FINITE-TEMPERATURE SIMULATION OF ABSORPTION SPECTRA IN SMALL ARGON-CLUSTER IONS ${\rm Ar}_n^ + $(n=3, 4, 8, 13, AND 19)

We report finite-temperature simulations of the absorption spectra of argon-cluster ions using a diatomics in molecules (DIM) Hamiltonian (including spin-orbit coupling) for the determination of the potential-energy surfaces (PES), a point-charge approximation for the dipole transition moments and a Metropolis Monte-Carlo algorithm for the nuclear motion. The dependency of the absorption spectrum on cluster size (3≤n≤19) and on temperature (100 K≤T≤500 K) is analyzed.

Vapor Phase Vibrational Spectra for Re(2)O(7) and the Infrared Spectrum of Gaseous HReO(4). Molecular Shapes of Mn(2)O(7), Tc(2)O(7), and Re(2)O(7).

The Raman and infrared spectra of gas phase Re(2)O(7) are reported. The experimental vibrational spectra of molecular Tc(2)O(7) and Re(2)O(7) are compared with calculated spectra. The results of these studies agree with a nonlinear M-O-M bridge for Tc(2)O(7) and Re(2)O(7). For infrared intensity calculations, the point charge approximation is used, while for the Raman calculations a combination of bond and atom polarizabilities is adopted. Pure Re(2)O(7) was prepared from rhenium wire, but attempts to prepare it from rhenium powder and oxygen always led to infrared spectra showing serious contamination from a species containing an -OH linkage. Detailed experiments identified this molecule as HReO(4), a unique transition metal analogue of the perhalic acids, and a partial infrared spectrum of this molecule is reported.

Structure and magnetism of dysprosium/zirconium multilayers

Dy/Zr bilayers and multilayers were grown on oxydized Si(100) substrates by evaporation under ultrahigh vacuum conditions. X-ray and electron diffraction shows that both Zr and Dy layers are hexagonal, strongly textured with the c-axis along the growth direction. The grains can extend over several layers and have an in-plane diameter of about 200 Å. The Dy lattice displays an in-plane compression of about 1.5%. Interface profiles determined from high-resolution electron microscopy studies are sharp, extending over less than 3 lattice spacings. Magnetization measurements and polarized neutron reflectometry reveal a strong enhancement of T C, accompanied with a suppression of the helical antiferromagnetic phase in the multilayer samples. This effect can be understood as the result of the change in the balance between exchange and magnetoelastic energies caused by in-plane epitaxial compression of the Dy lattice by Zr. A large out-of-plane magnetic anisotropy energy is observed when the Dy layers are made thinner than 15 Å. Volume and interface contributions to this anisotropy compensate each other for a Dy layer thickness of about 8 Å. Comparison is made between the results of crystal-field calculations based on the point-charge approximation and the measured anisotropy energy.