Magnitude Of Dipole Moment Research Articles

Novel unsymmetrically substituted push–pull phthalocyanines for second-order nonlinear optics

The γ(3 ω; ω, ω, ω) and γ(2 ω; ω, ω, 0) hyperpolarizabilities of some novel unsymmetrically substituted phthalocyanines (Pcs) have been measured at λ=1.34 μm and λ=1.9 μm, respectively. The EFISH values are positive as expected from a strong orientational contribution. By using a two-level model including the ground and the Q-excited levels, the off-resonant dipolar β v(0) component of the β tensor has been evaluated. Values are quite significant and reach 46.7×10 −30 esu for the free molecule with an extended conjugated path to the NO 2 acceptor group. The molecules containing a transition metal yield similar values that increase with the sequence Co, Ni, Cu in correlation with the magnitude of the ground state dipole moment. THG susceptibilities are negative and much larger than those measured for similar symmetric compounds. This has been related to a dominant asymmetric contribution associated to the Q-band which is resonant at the second-harmonic frequency.

Modulation of phospholipase A2 by electrostatic fields and dipole potential of glycosphingolipids in monolayers

Phospholipase A2 activity against mixed monolayers of dilauroylphosphatidic acid or dilauroylphosphatidylcholine with glycosphingolipids can be reversibly modulated by external constant electrostatic fields. The changes of enzymatic activity are correlated to the depolarization or hyperpolarization of the film caused by specific dipolar properties of glycosphingolipids. Hyperpolarizing fields enhance the enzymatic activity against pure dilauroylphosphatidic acid while depolarizing fields induce a decrease of activity. Compared to the pure substrate, the interface of mixed films containing neutral glycosphingolipids or gangliosides is already partially depolarized and the magnitude of activation induced by an external hyperpolarizing field is decreased; conversely, depolarizing fields cause an increased inhibition of activity. Differing from gangliosides, sulfatides bring about a hyperpolarization of the mixed lipid monolayer and external hyperpolarizing or depolarizing fields cause enhanced activation and reduced inhibition, respectively. The effects of glycosphingolipids depend on their relative proportion in the monolayer. Results were similar with dilauroylphosphosphatidylcholine but the field effects were less than half of those found with dilauroylphosphatidic acid. Our work shows that the activity of phospholipase A2 in addition to responding reversibly to external electrostatic fields, is directly modulated by the polarity and magnitude of the lipid polar head group dipole moments.—Maggio, B. Modulation of phospholipase A2 by electrostatic fields and dipole potential of glycosphingolipids in monolayers. J. Lipid Res. 1999. 40: 930–939.

Mixed Self-Assembled Monolayers of Highly Polar Rigid Biphenyl Thiols

Mixed self-assembled monolayers (SAMs) of 4‘-nitro-4-mercaptobiphenyl (I) and 4‘-(dimethylamino)-4-mercaptobiphenyl (III) have been compared with that of 4‘-nitro-4-mercaptobiphenyl (I) and 4‘-methylmercapto-4-mercaptobiphenyl (II). The composition of the mixed SAMs was determined by external reflection Fourier transform infrared (ER−FTIR) spectroscopy and showed that both the magnitude and direction of dipole moment determined the final composition of mixed SAMs. In toluene, with a dielectric constant of 2.4, a significant enhancement dipolar interaction is observed. The composition of mixed SAMs of I and II showed a plateau at 40% surface concentration of I. These results can be explained by simple calculation based on the Hammett equation. The experimental results indicate that the equilibrium concentration of the two components in the mixed SAMI/II, in a nonpolar solvent, is driven by the formation of a two-dimensional assembly with zero net dipole moment.

Conformation-dependent dipoles of liquid crystal molecules and fragments from first principles

We determine accurate molecular dipole moments for mesogenic fragments and liquid crystal molecules from quantum mechanical computer simulations adapted from large-scale electronic structure calculations on periodic solids. We employ density functional theory and use ab initio pseudopotentials for the interaction between valence electrons and ions and the generalized gradient approximation to account for the many-body effects of exchange and correlation. Periodic boundary conditions are enforced so that the molecular electronic wave function can be expanded in terms of a plane wave basis set. We test our method on several small molecules and then apply it to determine the direction, position and dipole moment magnitude for 4-4' pentyl-cyanobiphenyl (5CB) (and related fragments), phenyl benzoate, and 2-2'difluorobiphenyl. For the latter compound, we parametrize a torsional potential for rotation about the dihedral bond. We perform full structural optimization, and find that the torsional barrier heights for fully relaxed molecular structures are substantially reduced relative to nonoptimized geometries. We then demonstrate the influence of conformation and temperature on the molecular dipole moment. We also find that simple vector addition of dipole moments of fragments provides a good estimate of the total dipole moment of the complete molecule. We compare our results to experiment and to conventional quantum chemistry methods where data is available.

A Statistical Model of the Fluctuations in the Geomagnetic Field from Paleosecular Variation to Reversal

The statistical characteristics of the local magnetic field of Earth during paleosecular variation, excursions, and reversals are described on the basis of a database that gathers the cleaned mean direction and average remanent intensity of 2741 lava flows that have erupted over the last 20 million years. A model consisting of a normally distributed axial dipole component plus an independent isotropic set of vectors with a Maxwellian distribution that simulates secular variation fits the range of geomagnetic fluctuations, in terms of both direction and intensity. This result suggests that the magnitude of secular variation vectors is independent of the magnitude of Earth's axial dipole moment and that the amplitude of secular variation is unchanged during reversals.

Role of proline residue in the channel-forming and catecholamine-releasing activities of the peptaibol, trichosporin-B-VIa

Trichosporin-B-VIa (TS-B-VIa) has a Pro 14-kinked helical structure which is considered to be important for the formation of peptaibol-type ion-channels in lipid bilayer membranes. TS-B-VIa and its analog [Aid 14]TS-B-VIa with Pro → Aib substitution at position 14, resulting in a straight helical structure, were tested for ion-channel-forming activity in planar lipid bilayer membranes and for ability to induce catecholamine secretion from cultured bovine adrenal chromaffin cells. Voltage-dependent multi-channel conductance, which is characteristic of TS-B-VIa, was also observed for [Aid 14]TS-B-VIa. In single-channel measurements, current fluctuations induced by [Aid 14]TS-B-VIa had a shorter life-time and showed fewer substates than those induced by TS-B-VIa. Catecholamine secretion induced by these peptides at low concentrations is completely Ca 2+-dependent. At high concentrations, TS-B-VIa-induced secretion was partly independent of external Ca 2+, but this was not the case for the analog. The differences of behavior can be explained in terms of the differences of hydrophobicity, amphiphilicity, and magnitude of dipole moment due to the conformational changes around position 14 and the C-terminal domain caused by the Pro → Aib substitution.

Stark effect spectroscopy of tryptophan

The change in permanent dipole moment (magnitude of delta mu) for the transition from the 1La state to the ground state of tryptophan is the key photophysical parameter for the interpretation of tryptophan fluorescence spectra in terms of static and dynamic dielectric properties of the surrounding medium. We report measurement of this parameter by means of electric field effect (Stark) spectroscopy for N-acetyl-L-tryptophanamide (NATA) in two solvents, the single tryptophan containing peptide melittin, and 5-methoxytryptophan. The values ranged from 5.9 to 6.2 +/- 0.4 Debye/f for NATA and melittin, where f represents the local field correction. The 1Lb magnitude of delta mu was much smaller. Application of Stark spectroscopy to these chromophores required decomposition of the near-UV absorption into the 1La and 1Lb bands by measurement of the fluorescence excitation anisotropy spectrum and represents an extension of the method to systems where band overlap would normally preclude quantitative analysis of the Stark spectrum. The results obtained for 5-methoxytryptophan point out limitations of this method of spectral decomposition. The relevance of these results to the interpretation of steady-state and time-resolved spectroscopy of tryptophan is discussed.

A simple approach to electronic charge induction in atomistic simulations

A scheme is presented for periodically updating atom-centered partial charges due to changing local electric fields that arise during the course of a molecular simulation. The effects of electrostatic induction, due to polarization of the local electron density of bonded atom pairs, are realized through the approximation of the bond polarizability tensor as being cylindrically symmetric and directed along the bond axis, with negligible lateral polarizability. This simplification allows the reduction of polarization effects to a scalar bond ‘polarization parameter’, and subsequent partitioning of the local electron density into charge modifiers located at atom centers. The induced charges assigned to each atom are determined self-consistently. This simplification permits a reasonable approximation of both the magnitude and direction of molecular dipole moments and of molecular polarizabilities in the case of small-molecule fluorocarbons. The model is extended to the calculation of atom-centered partial atomic charges for two conformations of poly(vinylidenefluoride). The proposed method fits well into the framework of established atomistic simulation techniques based on Coulombic nonbonded atomic interactions.

Molecular modeling: a tool for predicting anthelmintic activity in vivo.

The structural and electronic features of the broad-spectrum benzimidazole anthelmintic mebendazole [MBZ, methyl 5-(benzoyl)-benzimidazole-2-carbamate] have been determined using a combination of quantum mechanics, molecular graphics, and molecular modeling techniques. Using conformational analyses and quantum mechanics, we found that the three-dimensional structure and electronic features of MBZ were consistent with those previously reported for highly active broad-spectrum benzimidazole anthelmintics and that in vivo drug efficacy against Hymenolepis diminuta depends upon the orientation of the benzoyl group at position 5 on the heterocyclic ring system, the magnitude of the molecular dipole moment, and the percentage of polar surface area. The chemotherapeutic actions of MBZ on H. diminuta in vivo were accompanied by marked changes in worm weight and chemical composition. Tapeworms recovered from rats that had received a therapeutically effective dose of MBZ 24 h earlier were significantly smaller and contained much less glycogen (as a percentage of the wet weight) than worms from untreated controls. In MBZ-treated worms, protein concentrations rose at a rate sufficient to offset the decline in glycogen concentration. Glycogen/protein ratios in MBZ-treated worms were considerably lower than the corresponding control values. Differences in the absolute amounts of glycogen between control and drug-treated worms were even more profound. Administration of a curative dose of MBZ to the rat host produced in H. diminuta another change, the onset of which coincided with the gross alterations in worm weight and chemical composition.(ABSTRACT TRUNCATED AT 250 WORDS)

Electric-field effects on hole spectra: method for studying dye–matrix interactions in doped polymers

The magnitude of matrix-induced electric dipole moments of centrosymmetric dye molecules increases from the blue to the red edge of the inhomogeneous absorption band in a number of dye–matrix systems. For the polymer polyvinyl butyral doped with a symmetrically substituted zinc tetrabenzoporphin this behavior is analyzed with a microscopic stochastic theory of dye–matrix interaction. Moreover, it is shown how electric-field measurements on hole spectra performed at different optical frequencies can be interpreted as a sort of two-dimensional optical spectroscopy. The two frequency scales are given by the inhomogeneous and the homogeneous linewidths. The technique is best suited for investigating amorphous systems, in which the two scales differ most strongly.

Influence of the substrate on the spectroscopic characteristics of the fluorescence of adsorbed dye molecules

UDC 535.37 It was previously shown that various electronic [i, 2] and even phase [3] transitions in a solid substrate can be sensitized with the aid of dye molecules adsorbed on the surface of the solid. On the other hand, the heterogeneity of the surface and the presence of charges have a strong influence on the spectroscopic characteristics of the luminescence of adsorbed dye molecules. Local nonuniform fields can cause displacement of the maxima in the fluorescence spectrum and changes in the coefficient of anisotropy of the fluorescence [i, 4], and they also have a strong influence on the vibrational levels of the molecule, causing changes in the form of the spectrum [5]. Inhomogeneous broadening is observed along with the homogeneous broadening of the spectra. In the general case the magnitude of the inhomogeneous broadening of a spectrum depends both on the heterogeneity of the surface and on the molecular environment of the probe under consideration, particularly on the solvation shell of the molecule being studied. It is ultimately determined by the local electric field in which the molecule is found at the moment when the quantum of light is emitted and thus depends on the depth of the surface potential well, the magnitude of the dipole moment of the photoexcited molecule, and the degree of restructuring of its environment at that moment in time. In the present work we investigated the influence of nonuniformity in the distribution of the local charged centers on the surface and the corresponding fluctuations in the surface electrostatic potential on the spectroscopic characteristics of the fluorescence of adsorbed dye molecules. Structures consisting of a semiconductor, a dielectric, and adsorbed dye molecules were investigated. Thermally oxidized crystals (iii) of high-resistance silicon (p ~ 1-2 k~'cm) and germanium (p ~ 30 ~.cm) served as the substrates. The thickness of the SiO 2 (GeO 2) layer on the surface, which was measured with the aid of an ellipsometer, was equal to 5 nm. On some of the Ge samples the oxide was created by means of the molecular stratification of a film of Cr203. Some of the Si samples were subjected to surface implantation by argon ions with an energy equal to 40 keV. The following dyes were used: erythrosine, Rhodamine B, Acridine Yellow, and Coumarin 47. The dye molecules were adsorbed from alcoholic solutions. The concentration of the adsorbed molecules was evaluated with the aid of a piezoelectric resonance balance, and it was equal to 2"10 ~s molecules/cm 2. The charge of the traps in the dielectric SiO 2 (GeO 2) was measured by determining the field effect on a large sinusoidal signal. The fluorescence spectra were recorded on a KSVU-12 automated spectrofluorometer with a thermostated vacuum measuring cell and on an Hitachi MPF-4 fluorometer. The fluorescence spectra of both samples which were evacuated at a pressure P = 10 -5 Pa and samples which were hydrated as a result of the admission of water vapor to a pressure P = 2400 Pa at T = 300 K were studied over a broad temperature range. The adsorption of all the dyes investigated was accompanied by a shift to a longer wavelength (-10-20 nm) in comparison to their solutions with a concentration C = i0 -s M. Variation of the temperature also caused appreciable displacement of the maximum in the fluorescence spectrum.

Electric Field Dependence of the Competition Between Permanent and Induced Dipole Orientation in Suspensions of Large Anisotropic Particles

ABSTRACT Accurate orientational probability distribution functions are derived which describe the dynamical behavior of permanent and induced dipole moments in an external field. The method involves a transformation of the Smoluchowski equation into a Schrödinger–like equation with real eigenvalues. The method is applied here for the first time to the case of suspensions of large noninteracting anisotropic particles which are of interest as artificial Kerr materials for nonlinear electrooptical applications. A matrix presentation is made in which the effects of any combination of permanent and induced dipole moments are described by a single matrix. The eigenfunctions and eigenvalues of this matrix when specific material parameters are substituted provide a Fourier cosine series expansion for the angular distribution function under conditions of cylindrical symmetry which is valid for aligning fields of any magnitude. The only restriction is that larger fields require larger matrices for more accurate results. The method is used to analyze the competition between the aligning contributions of permanent and induced dipole moments and the effects of permanent dipole moment and field magnitude on alignment times are obtained.

Discussion: partial discharges in ellipsoidal and spheroidal voids

Some questions are raised regarding the model described by Crichton et al. (see ibid. vol.24, p.335-342, 1989) in which the entire volume of the void is considered to be involved in the discharge process. The commenter questions how it can be explained that an increase in the applied voltage results in an increase in the discharge magnitude. In response it is noted that the assumption was made to allow evaluation of the change in the void dipole moment. This assumption is open to criticism with respect to discharge development, but is is maintained that this does not necessarily imply that, with reference to the actual dipole moment magnitude, the assumption is unsound. To assess the validity of the assumption in this context, it is necessary to compare the dipole moments associated with other charge patterns.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement of surface electric fields by an excited-state atomic beam technique

Electric fields due to adsorbates on a metal surface are studied experimentally with highly excited atoms as probes. A beam containing Li Rydberg atoms, with principal quantum numbers in the range 20 ≲ n ≲ 80, is detected after passing through a metal mesh having 6 micro m square openings. Adsorbates at 10 −2 monolayer can produce 400 V/cm electric fields 0.6 micro m from the surface. Fields of this magnitude can ionize Rydberg atoms in the atomic beam, reducing the flux through the mesh. Measurements of the excited-state atomic beam flux can thus yield information about the presence of adsorbates, including sticking probability, orientation and magnitude of electric dipole moment, and desorption energy. In the analysis of the experimental calibration, a relationship is obtained for the mesh transmission as a function of the adsorbate dipole-moment density. The measured desorption energies are compared with values reported in the literature.

Synthesis and ferroelectric properties of terphenyl derivatives with various structures of chiral terminal substituents

Abstract A new series of optically active substances, chiral esters of 4″-n-octylterphenyl carboxylic acid has been prepared. Some of the compounds with chlorine-and cyano-substituent at the chiral center show a smectic C phase. The induced Ps value depends on the dipole moment magnitude and alkyl length chain of the substituent adjacent to the C atom.

The Dipole Moment of the B'0+ State of IBr Determined through the Line Shape Analysis of the Stark Modulation Spectrum

The dipole moment of the B'0+ state of IBr has been determined from the line shape analysis of the Stark modulation spectra of the B'0+←X1Σ+ transition. It is -0.056±0.017 D at the v'=11 level of IBr81 and -0.033±0.034 D at the v'=15 level of IBr79. The magnitude of dipole moment of the B'0+ state is considerably smaller than that of the ground-state dipole moment (0.726 D), and its direction is opposite to that of the ground-state one.

Dipole moment of in vivo and isolated perfused rabbit hearts.

We have measured magnitude and location of heart dipole moment during QRS in 46 New Zealand white rabbits. The spatial magnitude curve had one to three peaks. Mean values were M1 = 80 +/- 10 microA-cm (N = 5) pointing to right anterior and caudal, M2 = 260 +/- 15 microA-cm (N = 42) directed slightly to left of due anterior and caudal, and M3 = 236 +/- 9 microA-cm (N = 43) pointing towards left posterior and cephalad. The mean thorax resistivity was 250 ohm-cm. For 23 rabbits, M2/M3 greater than 1 and for 16 rabbits, M2/M3 less than 1. Mean times of occurrence of the three peaks were 5.8, 11.2, and 19.6 ms, respectively. Spatial magnitude curves for hearts perfused at the center of a sphere showed usually one major peak at about 19 ms. Locuscardiograms of in vivo hearts were also measured. By comparing M values for in vivo and isolated hearts, we found that M1 values agreed closely but mean M2 measured from the hearts in vivo was 2.5 times that for the isolated hearts, and M3 for in vivo hearts was about two-thirds that for isolated hearts. We relate these differences to the effects of intracardiac blood and lungs on the measured dipole moment.

Excited state dipole moments in a polyatomic molecule determined by Stark quantum beat spectroscopy

High resolution Stark quantum beat spectroscopy is demonstrated to be a powerful method to determine magnitude and orientation of excited state dipole moments in asymmetric polyatomic molecules. We measured the dipole moments μ of the vibrationless S1 state of planar propynal and αD-propynal in a pulsed supersonic jet and determined both components of the μ vector in the molecular plane: μ′a=0.874(10)D, μb=1.06(2)D, ‖μ′‖=1.374(22) D for HC≡CCHO; μ′a=0.863(10)D, μb=1.04(1) D ‖μ′‖=1.351(14) D for HC≡CCDO. The results are related with recently reported microwave data of the ground state dipole moment μ(S0) and are interpreted in terms of electron delocalization from the carbonyl group to the ethinyl moiety. The dipole moments of S0 and S1 propynal are oriented essentially along the CO bond. The applicability of the Stark quantum beat technique to polyatomic molecules is discussed.

Heliomagnetic dipole moment and daily variation of cosmic rays underground

Since the discovery of Forbush decrease of cosmic-ray intensity, it has become clear that the intensity and its daily variation are subject to various kinds of solar modulation; these are due to the diffusion-convection1,2 of cosmic rays in interplanetary magnetic fields, which is controlled by various kinds of solar activity. We now demonstrate that the amplitude of diurnal variation of cosmic-ray intensity is intimately correlated with the magnitude of magnetic dipole moment of the Sun and that, contrary to the conventional concept of the modulation, such a correlation exists only in the cosmic-ray high-rigidity region.

Charge-Transfer Dipoles at the Si-SiO2 Interface and the Metal-Semiconductor Worfunction Difference In Mos Devices.

AbstractThe magnitude of the dipole moment at the Si-SiO2 interface resulting from partial charge transfer that takes place upon the formation of interface bonds has been calculated. The charge transfer occurs because of the difference in electronegativity between silicon atoms and SiO2 molecules which are present across the interface. Results obtained for (100) and (111) silicon substrates indicate that the magnitude of the interface dipole moment is dependent on substrate orientation and the interface chemistry. Dipole moments at the Si-SiO2 and gate-SiO2 interfaces should be included in the definition of the flatband voltage VFB of MOS structures. CV-based measurements of the metal-semiconductor workfunction difference φms on (100) and (111) silicon oxidized in dry oxygen and metallized with Al agree with the predictions of this model. Other types of interface dipoles and their processing dependence are briefly discussed.