Probable Configuration Research Articles

Computer simulations of ordering in homologous series of p- n-alkoxybenzoic acids ( nOBAC) at nematic–isotropic transition temperature—role of dielectric medium

Computer simulations of molecular ordering in nematogenic p- n-alkoxybenzoic acids has been carried out with respect to translatory and orientational motions for the acids having 7 (7OBAC); 8 (8OBAC) and 9 (9OBAC) carbon atoms in the alkyl chain. The net atomic charge and atomic dipole components at each atomic centre of the molecule has been evaluated using the CNDO/2 method. The configurational energy has been computed using the Rayleigh–Schrodinger perturbation method. The total interaction energy values obtained by these computations were used to calculate the probability of each configuration in a dielectric medium (i.e., non-interacting and non-mesogenic solvent, benzene) at phase transition temperature using the Maxwell–Boltzmann formula. The flexibility of various configurations has been studied in terms of variation of probability due to small departures from the most probable configuration. The various possible geometrical arrangements between a molecular pair during the different modes of interactions have been considered. An attempt has been made to develop a new and interesting model of nematogens in dielectric medium.

CMEs Associated with Eruptive Prominences: How to Predict?

Solar prominences can be viewed as pre-eruptive states of coronal mass ejections (CMEs). Eruptive prominences are the phenomena most related to CMEs observed in the lower layers of the solar atmosphere. The most probable initial magnetic configuration of a CME is a flux rope consisting of twisted field lines which fills the whole volume of the dark cavity stretched in the corona along the photospheric polarity inversion line. Cold dense prominence matter accumulates in the lower parts of helical flux tubes, which serve as magnetic traps in the gravitation field. Coronal cavity is rather inconvenient feature for observation owing to reduced emission, so prominences and filaments are the best tracers of the flux ropes in the corona long before the beginning of eruption. Thus, the problem of the CME prediction can be reduced to the analysis of the filament equilibrium and estimation of the stability store. The height of a prominence (or a filament when observed against the disk) increases with its age and the death of a filament is usually an eruption which is followed by a CME. The filament height, then, can be a measure of its age and its readiness for eruption. In inverse-polarity models the equilibrium height of a filament is related to the value of the filament electric current. The stronger the electric current, the greater the height of the filament. However, the equilibrium and stability of a filament depend not only on its current but also on the characteristics of the external magnetic field. In order to estimate the probability of eruption, we should therefore compare the observed prominence height with a value characterizing the photospheric magnetic field. This value is the critical height, which can be found in the distribution of the magnetic field vertical gradient above the polarity inversion line. We had analyzed three dozens of filaments and found that eruptive prominences were near the limit of stability a few days before eruptions. We believe that the comparison of the real heights of prominences with the calculated critical heights could be a basis for predicting filament eruptions and following CMEs.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Ab initio structural study of M(mda)2 complexes (M = Be, Mg, Ca; mda = C3O2H3)

The structure of M(mda)2 (M = Be, Mg, Ca; mda = C3O2H3) bis-complexes was investigated by the ab initio Hartree-Fock method and by including electron correlation in terms of second order Moller-Plesset perturbation theory; for calculations we used triple-zeta valence basis sets complemented with polarization functions. Two most probable geometrical nuclear configurations (D2h and D2d) are considered for each molecule. The structure with two mutually orthogonal chelate ligands (D2d symmetry) corresponds to the potential energy surface (PES) minimum. The planar D2h configuration corresponds to the first order saddle point on PES; consequently, its relative energy determines the height of the barrier to the D2d→D2h→D2d′ intramolecular rearrangement. Correlation equations that relate the calculated values of equilibrium internuclear distances, force constants, and rearrangement barrier heights to the value of the ionic radius of the metal atom have been obtained. These correlations were employed to evaluate the molecular constants for Sr(mda)2 and Ba(mda)2. The theoretical data are compared with the available experimental literature data.

Molecular ordering of 4O.2 compound in a dielectric medium at room temperature‐A theoretical approach

AbstractThe molecular ordering of N‐(4‐n‐butoxybenzylidene)‐4′‐ethyaniline [4O.2] in a dielectric medium (benzene) has been carried out on the basis of statistical mechanics and intermolecular forces. The atomic net charge and atomic dipole moment at each atomic centre has been evaluated using the CNDO/2 method. The modified Rayleigh‐Schrodinger perturbation theory with multicentered‐multipole expansion method has been employed to evaluate the long‐range intermolecular interactions, while ‘6‐exp’ potential function has been assumed for the short‐range interactions. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration in a dielectric medium at room temperature (300K) using the MB‐formula. The flexibility of various interacting configuration has been studied in terms of variation of probability due to departure from the most probable configuration. A comparative picture of the molecular parameters like total energy, binding energy and total dipole moment has been given. An attempt has been made to analyse the characteristic features of nematogenic 4O.2 compound in terms of its relative order. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular ordering of Fluoro-Nematogen in a dielectric medium at phase-transition temperature: a statistical–mechanical approach based on computer simulation

A molecular-ordering study of p-phenylene-4-methoxy benzoyl 4-trifluoromethylbenzoate (FLUORO) in a dielectric medium (benzene) has been carried out on the basis of statistical mechanics and computer simulation. The atomic net charge and atomic dipole moment at each atomic centre has been evaluated using the CNDO/2 method. The modified Rayleigh–Schrodinger perturbation theory with multicentered–multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while ‘6-exp’ potential function has been assumed for the short-range interactions. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration in a dielectric medium at the phase-transition temperature (506 K), using the MB-formula. The flexibility of various interacting configurations was studied in terms of variation of probability due to departure from the most probable configuration. It has been observed that in dielectric medium the energies/probabilities are redistributed, and there is considerable rise in the probability of interaction, although the order of preference remains the same. The results are discussed in the light of experimental observations.

Solvent effects on the electrochemistry and spectroelectrochemistry of diruthenium complexes. Studies of Ru2(L)4Cl where L=2-CH3ap, 2-Fap, and 2,4,6-F3ap, and ap is the 2-anilinopyridinate anion.

Three Ru2(5+) diruthenium complexes, (4,0) Ru2(2-CH3ap)4Cl, (3,1) Ru2(2-Fap)4Cl, and (3,1) Ru2(2,4,6-F3ap)4Cl where ap is the 2-anilinopyridinate anion, were examined as to their electrochemical and spectroelectrochemical properties in five different nonaqueous solvents (CH2Cl2, THF, PhCN, DMF, and DMSO). Each compound undergoes a single one-electron metal-centered oxidation in THF, DMF, and DMSO and two one-electron metal-centered oxidations in CH2Cl2 and PhCN. The three diruthenium complexes also undergo two reductions in each solvent except for CH2Cl2, and these electrode processes are assigned as Ru2(5+/4+) and Ru2(4+/3+). Each neutral, singly reduced, and singly oxidized species was characterized by UV-vis thin-layer spectroelectrochemistry, and the data are discussed in terms of the most probable electronic configuration of the compound in solution. The three neutral complexes contain three unpaired electrons as indicated by magnetic susceptibility measurements using the Evans method (3.91-3.95 muB), and the electronic configuration is assigned as sigma2pi4delta2pi(*2)delta, independent of the solvent. The three singly oxidized compounds have two unpaired electrons in CD2Cl2, DMSO-d6, or CD3CN (2.65-3.03 muB), and the electronic configuration is here assigned as sigma2pi4delta2pi(*2). The singly reduced compound also has two unpaired electrons (2.70-2.80 muB) in all three solvents, consistent with the electronic configuration sigma2pi4delta2pi(*2)delta(*2) or sigma2pi4delta2pi(*3)delta*. Finally, the overall effect of solvent on the number of observed redox processes is discussed in terms of solvent binding, and several formation constants were calculated.

Roles of non-aligned eigenvectors of strain-rate and subgrid-scale stress tensors in turbulence generation

Alignment of the eigenvectors for strain-rate tensors and subgrid-scale (SGS) stress tensors in large-eddy simulation (LES) is studied in homogeneous isotropic turbulence. Non-alignment of these two eigenvectors was shown in Tao, Katz & Meneveau (2002). In the present study, the specific term in the decomposition of the SGS stress tensor, which is primarily responsible for causing this non-alignment, is identified using the nonlinear model. The bimodal behaviour of the alignment configuration reported in Tao et al. (2002) was eliminated by reordering the eigenvalues according to the degree of alignment of the corresponding eigenvectors with the vorticity vector. The preferred relative orientation of the eigenvectors was . The alignment trends were conditionally sampled based on the relative dominance of strain and vorticity. The effect of the identified term on the alignment was the largest in the region in which the magnitudes of strain and vorticity were comparable and large (flat sheet). The most probable alignment configuration in the flat-sheet region was different from those in the strain-dominated and vorticity-dominated regions. The relative orientation of the eigenvectors was dependent on the degree of resolution for the flat sheet region yielded on the LES mesh. When the alignment was conditionally sampled on the events with the backward scatter of the SGS energy into the grid scale, the interchange of the alignment of the eigenvectors took place. Relevance of the identified term for the generation of turbulence is investigated. It is shown that the identified term makes no contribution to the production of the total SGS energy, but contributes significantly to the generation of the SGS enstrophy. The identified term causes a time-lag in the evolution of the turbulent energy and enstrophy. It is shown that generation of vorticity is markedly attenuated when the magnitude of the identified term is modified, and the original nonlinear model yielded the results which are in the closest agreement with the direct numerical simulation data.

Ordering of Mesogenic Compounds in a Dielectric Medium at the Nematic-Isotropic Transition Temperature - A Comparative Study

A comparative study of molecular ordering of trans-trans-4′-alkylbicyclohexyl-4-carbonitriles (cyclohexylcyclohexanes, CCHs) with alkyl group: propyl (CCH3); pentyl (CCH5); and heptyl (CCH7) has been carried out with respect to translational and orientational motions. The CNDO/2 method has been employed to compute the net atomic charge and atomic dipole moment at each atomic centre. Configurational energy has been computed using the Rayleigh-Schrodinger perturbation method. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration in a dielectric medium (i.e. non-interacting and non-mesogenic solvent, benzene) at the phase transition temperature using the Maxwell-Boltzmann formula. Further, the flexibility of various configurations has been studied in terms of variation of probability due to departure from the most probable configurations. It is observed that, in a dielectric medium, the probabilities are redistributed, although the order of preference remains the same. An attempt has been made to explain the nematogenic behaviour of these mesogenic compounds, and thereby develop a molecular model for the liquid-crystallinity.

Ground-based interferometric gravitational-wave detectors in the LISA epoch

Ground-based interferometry for gravitational-wave detection is in a state of rapid evolution. First-generation systems (LIGO, GEO, TAMA, VIRGO) are now (summer 2002) starting or will soon start observations, and second-generation instruments are well into the planning phases. By the time LISA joins the network of gravitational-wave detectors, we will have a mixture of second- and third-generation ground-based instruments observing. We give a description of some probable configurations for the ground-based instruments in that epoch.

New methods for domain structure determination of proteins from solution scattering data

New approaches for domain structure determination of macromolecules from X-ray solution scattering are presented. An ab initio method for building structural models of proteins from the scattering data implemented in the computer program GASBOR employs simulated annealing to find a chain-like spatial distribution of dummy residues (DR), which fits the experimental scattering pattern up to a resolution of 0.5 nm. This method substantially improves the resolution and reliability of models derived from the scattering data. A modification of GASBOR fitting distance distribution data in real space substantially (by a factor of 3 to 7) speeds up the calculation of DR models. The DR approach is further extended to reconstruct missing domains in multisubunit proteins and fusion proteins and to find probable configurations of missing loops in crystallographic models. Four computer programs have been written to provide appropriate tools for different situations in which a high or low resolution structural model lacks a loop or domain. These methods permit solution scattering analysis to usefully complement the results obtained by high-resolution methods like X-ray crystallography and nuclear magnetic resonance spectroscopy. The efficiency of the presented approaches is illustrated by their application for structure analysis of several proteins, with known and unknown crystal structure, from experimental scattering data.

Ordering of fluoro-mesogens: a statistical approach based on quantum mechanics and computer simulation

Computational analysis of the molecular ordering of nematic p-phenylene-4-methoxy benzoyl 4-trifluoromethylbenzoate (FLUORO1) and smectic 4-propyloxyphenyl 4-(4-trifluoromethylbenzoyloxy) benzoate (FLUORO2) mesogens has been carried out with respect to translatory and orientational motions. The net atomic charge and atomic dipole components at each atomic centre of the molecule have been evaluated using the CNDO/2 method. Rayleigh-Schrodinger perturbation theory, along with the multicentred-multipole expansion method, has been employed to evaluate long range intermolecular interactions while a ‘6-exp’ potential function has been assumed for short range interactions. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration at the phase transition temperature using the Maxwell-Boltzmann formula. The flexibility of various configurations has been studied in terms of the variation of probability due to small departures from the most probable configuration. The results obtained enable the determination of the peculiarities of the molecular ordering, as well as the construction of models of the structures of FLUORO1 and FLUORO2 in different modes of interaction. The mesophase nature has been correlated with the parameters introduced in this paper.

DETECTION OF TRACKS IN AERIAL PHOTOS BY THE GIBBS SAMPLER

A problem of automatic detection of tracks in aerial photos is considered. We adopt a Bayesian approach and base our inference on an a priori knowledge of the structure of tracks. The probability of a pixel to belong to a track depends on how the pixel gray level differs from the gray levels of pixels in the neighborhood and on additional prior information. Several suggestions on how to formalize the prior knowledge about the shape of the tracks are made. The Gibbs sampler is used to construct the most probable configuration of tracks in the area. The method is applied to aerial photos with cell size of 1 sq. m. Even for detection of trails of width comparable with or smaller than the cell size, positive results can be achieved.

Ordering of smectogenic compounds--a statistical approach based on quantum mechanics and computer simulation

A statistical analysis of molecular ordering of smectogenic compounds, 3-[2-(perflurooctyl) ethoxy] nitrobenzene (FLUORO1) and ethyl 4-[2-(perfluorooctyl) ethoxy] benzoate (FLUORO2), have been carried out with respect to translatory and orientational motions. The net atomic charge and atomic dipole components at each atomic center of the molecule has been evaluated using the CNDO/2 method. The Rayleigh-Schrodinger perturbation theory, along with the multicentered-multipole expansion method, has been employed to evaluate the long-range intermolecular interactions while "6-exp" potential function is assumed for short-range interactions. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration at its phase transition temperature using the Maxwell-Boltzmann formula. The flexibility of various configurations has been studied in terms of variation of probability due to small departures from the most probable configuration. The results obtained have been correlated with the parameters introduced in this article.

COMPUTER SIMULATIONS OF MOLECULAR ORDERING IN DISUBSTITUTED BIPHENYLCYCLOHEXANES (BCHs) AT PHASE TRANSITION TEMPERATURE

A comparative computer simulation on two liquid crystalline disubstituted biphenylcyclohexanes (BCHs) of general formula R-C6H10-C6H4-X with R: C3H7; X: H (BCH30) and R: C5H11; X: CN (BCH5CN) has been carried out on the basis of quantum mechanics and intermolecular forces. The net atomic charge and atomic dipole moment at each atomic center has been evaluated using the CNDO/2 method. The configurational energy has been computed using the Rayleigh-Schrodinger perturbation method. The obtained energies were used to calculate the probability of each configuration at phase transition temperature, using the Maxwell-Boltzmann's formula. The flexibility of various configurations has been studied in terms of variation of probability due to small departures from the most probable configurations. Further, the various possible geometrical arrangements between a molecular pair during the different modes of interactions have been considered. An attempt has been made to explain the mesogenic behavior of these compounds in terms of their relative order.

Molecular ordering of a cyano compound at a displacive transition temperature: a statistical analysis based on quantum mechanics and computer simulations

A statistical analysis has been carried out to determine the configurational preference of a pair of 4-cyanophenyl 4-n-pentylbenzoate (CPPB) molecules with respect to translatory and orientational motions. The CNDO/2 method has been employed to evaluate the net atomic charge and atomic dipole components at each atomic centre of the molecule. Configurational energy has been computed using the Rayleigh—Schodinger perturbation method. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration at the phase transition temperature using the Maxwell—Boltzmann formula. An attempt has been made to identify the most probable configuration at the phase transition temperature. Further, the flexibility of various configurations has been studied in terms of variation of probability due to small departures from the most probable configuration. On the basis of stacking, in-plane and terminal interaction energy calculations, all possible geometrical arrangements of the molecular pair have been considered. The results are discussed in the light of experimental as well as theoretical observations. The nature of the mesophase has been correlated with the parameter introduced in this paper.

Quantum-chemical study of the spectral properties of stilbene isomers

Calculations of absorption spectra of cis-and trans-forms of stilbene by the quantum-chemical method of intermediate neglect of differential overlap with spectroscopy parametrization were carried out. The electron structure of a stilbene molecule was studied and energy-level diagrams were drawn and analyzed. Rate constants of different photophysical processes occurring in a stilbene molecule after absorption of a photon were calculated in relation to the molecule conformation. On the basis of the obtained results, possible configurations of photoisomer molecules were considered and the most probable configurations of excited stilbene molecules were determined. It was shown how the change in the configurations of cis-and trans-forms of stilbene affects its spectral properties.

Partial abductive inference in Bayesian belief networks - an evolutionary computation approach by using problem-specific genetic operators

Abductive inference in Bayesian belief networks (BBNs) is intended as the process of generating the K most probable configurations given observed evidence. When we are interested only in a subset of the network's variables, this problem is called partial abductive inference. Both problems are NP-hard, and so exact computation is not always possible. In this paper, a genetic algorithm is used to perform partial abductive inference in BBNs. The main contribution is the introduction of new genetic operators designed specifically for this problem. By using these genetic operators, we try to take advantage of the calculations previously carried out, when a new individual is evaluated. The algorithm is tested using a widely-used Bayesian network and a randomly generated one, and then compared with a previous genetic algorithm based on classical genetic operators. From the experimental results, we conclude that the new genetic operators preserve the accuracy of the previous algorithm and also reduce the number of operations performed during the evaluation of individuals. The performance of the genetic algorithm is thus improved.

Statistical Study of Molecular Ordering in a Nematogenic Compound - A Computational Analysis

A computational analysis has been carried out to determine the configurational preference of a pair of a nematogen, 4,4'-azodiphenetole (C 2 H 5 -O-C 6 H 4 -N=N-C 6 H 4 -O-C 2 H 5 ) [AZO] molecules with respect to translatory and orientational motions. The CNDO/2 method has been employed to compute the atomic charge and atomic dipole at each atomic centre. Configurational energy has been computed using modified Rayleigh-Schrodinger perturbation method. The interaction energy values obtained through these computations were used to calculate the probability of each configuration at phase transition temperature using Maxwell-Boltzmann formula. Further, the flexibility of various configurations has been studied in terms of variation of probability due to small departures from the most probable configuration. Molecular parameters like total energy, binding energy and total dipole moment have been given. Results have been discussed in the light of experimental as well as other theoretical observations.

Statistical Study of Molecular Ordering in a Nematogenic Compound – A Computational Analysis

A computational Analysis has been carried out to determine the configurational preference of a pair of a nematogen, 4,4′-azodiphenetole (C2H5-O-C6H4-N=N-C6H4-O-C2H5) [AZO] molecules with respect to translatory and orientational motions. The CNDO/2 method has been employed to compute the atomic charge and atomic dipole at each atomic centre. Configurational energy has been computed using modified Rayleigh-Schrodinger perturbation method. The interaction energy values obtained through these computations were used to calculate the probability of each configuration at phase transition temperature using Maxwell-Boltzmann formula. Further, the flexibility of various configurations has been studied in terms of variation of probability due to small departures from the most probable configuration. Molecular parameters like total energy, binding energy and total dipole moment have been given. Results have been discussed in the light of experimental as well as other theoretical observations.

In K-edge extended X-ray absorption fine structure of InGaN epilayers and quantum boxes

Extended X-ray absorption fine structure (EXAFS) above the In K-edge of luminescent InGaN heterostructures provides a unique probe of local structure on an atomic length scale. Through a process of fitting the experimental spectrum, we can refine a model of the probable configuration (the atom type, co-ordination number and radial separation) of the first few shells of neighbouring atoms in the vicinity of a selected probe atom. We present here, for the first time, the In K-edge EXAFS spectrum of an ultrathin, uncapped ‘quantum box’ (QB) sample and compare it to results we have obtained previously on thick luminescent InGaN epilayers with a range of composition. While the epilayers resemble simple alloys, more or less, the QB sample reveals itself to consist of a two-phase mixture of InN and dilute InGaN alloy. The use of EXAFS to distinguish the local In environments of different InGaN-based heterostructures is likely to provide key information to unlock the puzzle of the origin of luminescence in these important commercial semiconductors.