Debye Units Research Articles

An Alternative Description for the Electronegativity Difference in Binary Ionic Solids

We are introducing an alternative analytical expression to the electronegativity difference (∆χ) as a function of the charge ge, g is the charge factor, the fraction of the electronic charge devoted to the bond and e the elementary charge, the packing factor (p) and the effective atomic number (Zeff) of binary ionic solids, by using the very basic Coulomb interaction, modified by the introduction of p, and the relationship between the electric dipole moment (µ) and ∆χ in Debye units. When compared to the Pauling’s, Gordy’s, Allred-Rochow’s and Allen’s scales, our calculations deviate around 10% to all ionic crystals with such data available in the literature. A very simple expression with satisfactory estimates, with no need of numerical procedure, is announced. The values of g play the important role of indicating the character of the chemical bond. It opens up an alternative opportunity to understand the nature of ionic chemical bonds and is able to describe the character of the bonding in any ionic polyatomic system.

Excited State Dipole Moments in Solution: Comparison between State-Specific and Linear-Response TD-DFT Values.

We compare different response schemes for coupling continuum solvation models to time-dependent density functional theory (TD-DFT) for the determination of solvent effects on the excited state dipole moments of solvated molecules. In particular, linear-response (LR) and state-specific (SS) formalisms are compared. Using 20 low-lying electronic excitations, displaying both localized and charge-transfer character, this study highlights the importance of applying a SS model not only for the calculation of energies, as previously reported ( J. Chem. Theory Comput. , 2015 , 11 , 5782 , DOI: 10.1021/acs.jctc.5b00679 ), but also for the prediction of excited state properties. Generally, when a range-separated exchange-correlation functional is used, both LR and SS schemes provide very similar dipole moments for local transitions, whereas differences of a few Debye units with respect to LR values are observed for CT transitions. The delicate interplay between the response scheme and the exchange-correlation functional is discussed as well, and we show that using an inadequate functional in a SS framework can yield to dramatic overestimations of the dipole moments.

Erratum to: Correlations for the Dielectric Constants of $$\mathbf{H }_\mathbf{2 }\mathbf{S }$$ H 2 S , $$\mathbf{SO }_\mathbf{2 }$$ SO 2 , and $$\mathbf{SF }_\mathbf{6 }$$ SF 6

The original version of this article unfortunately contained an error. The conversion from Debye units for the dipole moment to SI units was printed incorrectly on page 4 and in the footnote to Table 3.

Hydrogen bonding in 1-fluoroacetic acid rotamers

Ab initio LCAO-MO-SCF calculations of the energies and dipolc moments of four possible 1-fluoroacetic acid rotamers have tentatively been performed on a 7s,3p basis contracted to 4s, 2p. In spite of the obvious limitations of the method, the results are qualitatively in agreement with earlier experiments. In order of increasing magnitude the total energies are - 326.10564 a.u., - 326.10231 a.u., - 326.09890 a.u., and - 326.08294 a.u. In the same sequence the calculated dipole moments in debye units are 3.32, 0.74, 3.26, and 5.40.

Electronic Excited State of Alizarin Dye Adsorbed on TiO2 Nanoparticles: A Study by Electroabsorption (Stark Effect) Spectroscopy

Alizarin is one of the dyes extensively investigated as an example of a molecule capable of serving as a light absorber and an electron donor in model systems designed for the new type solar cells. Using the Stark effect measurements for alizarin, both free in solution and adsorbed to TiO2 nanoparticles, the question has been addressed whether the excited-state orbital of adsorbed alizarin extends into the solid and involves the orbitals of the Ti atom or remains localized within the alizarin molecule. Because an important role can be played by the electric field at the charged surface of the nanoparticles, the field was modulated by changing the pH of the medium. The results reveal a substantial dipole moment change on the electronic excitation of the alizarin−TiO2 system, |Δμ| ≈ 10 Debye units or slightly more. The observed dependence of the absorption maximum and the measured |Δμ| on pH were used to distinguish between Δμ directed toward the nanoparticle surface and that corresponding to the intrinsic rearrangement of electrons within alizarin or, in reverse direction, corresponding to more- or less-complete electron transfer from alizarin onto the orbitals of Ti and adjacent atoms comprising a localized surface (or a delocalized conductive) state. The results qualitatively contradict a significant dye-to-solid charge-transfer character of the electronic transition. It is shown that they can be interpreted in a self-consistent way by considering, in a first approximation, the light absorption by alizarin monoanion subject to the electric field generated by the charged nanoparticle surface.

The nature of “unusual” electro-optical transients observed for DNA

Unusual electro-optical transients have been observed for many different polymers and colloidal systems. These effects provoked serious confusion, because a simple-minded interpretation can be completely misleading. The case of double helical DNA is of particular interest, because DNA has been studied in more detail than other systems and because of its biological function. DNA is subject to bending, which implies a loss of symmetry. Due to its high charge density, non-symmetric conformations must have a non-symmetric distribution of charges leading to a torque of considerable magnitude in the presence of external electric fields. The dipole moment describing this torque must be calculated in a coordinate system with its origin at the center of diffusion. The resulting dipole values are in the range of thousands of Debye units. Because the new dipole type is analogous to but not identical with permanent dipoles, the notation “quasi-permanent” dipole is suggested. Application of this concept, using commonly accepted parameters for DNA and established procedures for calculation of electro-optical transients, leads to “unusual” transients. Thus, these transients must be expected from well-known parameters of DNA double helices. The influence of the quasi-permanent dipole moment may be amplified considerably by hydrodynamic coupling. This effect has been demonstrated for the case of smoothly bent rods. Both model calculations and experiments illustrate the danger of getting data that may be completely misleading. For example, depending on pulse amplitudes and/or pulse lengths, electro-optical decays may be accelerated artificially due to superposition of decay components with opposite amplitudes. Experiments show that unusual transients and apparent acceleration effects disappear, when high frequency sine pulses are used for the electro-optical analysis of DNA. Electro-optical effects depend upon the internal dynamics of the object under investigation. In general, the dynamics of DNA bending was assumed to be fast compared to rotational diffusion. Because stacking rearrangements in single stranded nucleic acids are relatively slow and recently the dynamics of the B–A transition was observed in the time range >1 μs, it is likely that there are also relatively slow rearrangements between bending conformers. Bending transitions are expected to be relatively fast, when there are no activation barriers in the bending pathway, and may be slow, when activation barriers must be passed between bending conformers.

Mechanism for the conductivity changes caused by membrane electroporation of CHO cell-pellets

Electric field pulses, applied to densely packed pellets of Chinese hamster ovary (CHO) cells of mean radius āc = 7.5 ± 0.7 μm, cause major electric conductivity changes, described by three kinetic normal modes. The first mode reflects Wien effects of ionic atmosphere perturbations and ion pair dissociations (cell surfaces). Using Maxwell’s conductivity equation, the second and third mode are converted to the respective membrane conductivity modes. Electrothermodynamic analysis in terms of structural transitions from closed (C) to porated (P) membrane states of very different lifetimes, according to the scheme (C ⇌ C1) ⇌ (P2 ⇌ P3), yields the mean pore radii 2 = 1.00 ± 0.05 nm (P2) pores and 3 = 1.5 ± 0.1 nm (P3) at T = 293 K (20 °C). The relaxation time τ2 (P2-formation) reflects the rate limiting step (C ⇌ C1), associated with the activation dipole moment of Δm1 = 63 × 10−30 C m (or 19 Debye units), suggesting orientational changes of dipolar lipid head groups, in the solution membrane interfaces preceding the actual pore formations. Besides: the field-dependencies of the pore fractions f2 and f3 (order of 10−3), the field reduction factors fλ,i≤ 1 and the membrane voltage, we obtain the zero-field pore conductivities λ0p,2 = 1.7 × 10−2 mS cm−1 (P2) and λ0p,3 = 0.10 mS cm−1 (P3) and the membrane conductivity λm0 = 3.2 μS m−1. The post-field conductivity changes, due to the long-lived P3-pores, are analyzed in terms of time- (and field-) dependent efflux coefficients. The characteristic post-field pore resealing time τR = τ03 = 45 ± 3 s is independent of the field strength of the causative pulse and independent of the distance between the two electrodes. These results are an essential part for the optimization of the electrical pulse parameters, also for the clinical electrotransfer of bioactive substances into aggregated biological cells (tissue).

Microwave Spectrum and Dipole Moment of 1,3‐dioxolane in the Hindered‐Pseudorotation States v = 0, 1, 2, and 3

The microwave spectrum of 1,3‐dioxolane (C3H6O2) in the ground and first three excited states of hindered pseudorotation was studied. The transitions corresponding to the μb, μc, and μa components of the dipole moment were identified. The spectrum was analyzed using numerical diagonalization of the effective rotational Hamiltonian for four interacting states of hindered pseudorotation. The rotational constants, centrifugal distortion constants, the constants of interaction between general rotation and hindered pseudorotation, and the distances between the quasidegenerate vibrational levels Δ01 = 64840.5, Δ12 = 122231.7 and Δ23 = 119732.7 MHz are determined. From the Stark effect of microwave transitions, the dipole moment components (in Debye units) in the hindered‐pseudorotation states v = 0, 1, 2, and 3 are determined: 〈0|μb|0〉 = 〈1|μb|1〉 = 1.22, 〈2|μb|2〉 = 1.20, 〈3|μb|3〉 = 1.21, 〈0|μc|1〉 = 0.77, 〈2|μc3〉 = 0.66, and 〈1|μa|2〉 = 0.19.

Effect of the substituents on the solvatochromic behaviour of benzimidazole-based betaine dyes

The solvatochromic effect was studied in the family of benzimidazole-based zwitterionic betaine dye molecules. The ground-state dipole moments of the molecules were determined from dielectric measurements; depending on the substituents, they ranged from ca. 6 to over 13 Debye units. A pronounced shift of the lowest-lying absorption band was found depending on the polarity of solvents: the betaine dyes substituted with the dimethylamine group in the terminal phenyl ring exhibit a bathochromic shift whereas the dye unsubstituted or substituted with the nitro group exhibit a hypsochromic shift. The comparison of the experimental results with the quantum-chemical calculations seems to indicate that the reason for this difference rests in enhanced probability of specific interactions in the former betaines (possible formation of hydrogen bond to the imidazole nitrogen).

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.

Simulation of electrostatic and hydrodynamic properties of Serratia endonuclease.

We analyze the electrostatic and hydrodynamic properties of a nuclease from the pathogenic gram-negative bacterium Serratia marcescens using finite-difference Poisson-Boltzmann methods for electrostatic calculations and a bead-model approach for diffusion coefficient calculations. Electrostatic properties are analyzed for the enzyme in monomeric and dimeric forms and also in the context of DNA binding by the nuclease. Our preliminary results show that binding of a double-stranded DNA dodecamer by nuclease causes an overall shift in the charge of the protein by approximately three units of elementary charge per monomer, resulting in a positively charged protein at physiologic pH. In these calculations, the free enzyme was found to have a negative (-1 e) charge per monomer at pH 7. The most dramatic shift in pKa involves His 89 whose pKa increases by three pH units upon DNA binding. This shift leads to a protonated residue at pH 7, in contrast to the unprotonated form in the free enzyme. DNA binding also leads to a decrease in the energetic distances between the most stable protonation states of the enzyme. Dimerization has no significant effect on the electrostatic properties of each of the monomers for both free enzyme and that bound to DNA. Results of hydrodynamic calculations are consistent with the dimeric form of the enzyme in solution. The computed translational diffusion coefficient for the dimer model of the enzyme is in very good agreement with measurements from light scattering experiments. Preliminary electrooptical calculations indicate that the dimer should possess a large dipole moment (approximately 600 Debye units) as well as substantial optical anisotropy (limiting reduced linear electric dichroism of about 0.3). Therefore, this system may serve as a good model for investigation of electric and hydrodynamic properties by relaxation electrooptical experiments.

Dipole Moments and Second-Order Hyperpolarizabilities of Imidazole-Based Betaines Determined from the Solvatochromic Effect

Abstract We report on results of measurements of the solvatochromic effect in two benzimidazole-based betaines (hereafter referred to as IB1 and IB2). The solvatochromic shifts exceed 2000 cm−1 in both molecules; however, the directions of the shift were found opposite in spite of similar chemical structures of the molecules under study. The ground-state dipole moments in both betaines exceed 13 Debye units, whereas the excited-state moments, calculated from the solvatochromic shift, differ markedly amounting to ca. 3 D in IB1 and 18 D in IB2. Quantum-chemical calculations indicate that betaines enjoy some freedom of orientation of their fragments which may influence the energies and intensities of the electronic transitions. The second-order hyperpolarizabilities, estimated from the measurements amount to ca. 20×10−40 m4/V (4.8×10−30 esu) for IB1 and 55×10−40 m4/V (13.8×10−30 esu) for IB2.

Electronic Spectral Studies and Determination of the First Excited Singlet-State Dipole Moments of New Benzo[a]phenothiazine Derivatives

Abstract The room temperature (298 K) electronic absorption, and fluorescence excitation and emission spectra of seven new, pharmacologically-important benzo[a]phenothiazines (12H-benzo[a]phenothiazine ( 1 ), 9-methyl-12H-benzo[a] phenothiazine ( 2 ), 10-methyl-12H-benzo[a] phenothiazine ( 3 ), 11-methyl-12H-benzo[a]phenothiazine ( 4 ), 5-oxo-5H-benzo[a]phenothiazine ( 5 ), 6-hydroxy-5-oxo-5H-benzo[a]phenothiazine ( 6 ) and 6-methyl-5-oxo-5H-benzo[a] phenothiazine ( 7 ):) were measured in several solvents of different polarities and hydrogen bonding abilities. In combination with the ground state dipole moments of these benzo[a]phenothiazines, the spectral data were used to determine their first excited singlet-state dipole moments by means of the solvatochromic shift method. These excited singlet-state dipole moments were found to be significantly higher (1.9 to 2.5 Debye units) than their ground-state counterparts.

Electrostatics of hemoglobins from measurements of the electric dichroism and computer simulations

Hemoglobins from normal human cells, from sickle cells, and from horse were investigated by electrooptical methods in their oxy and deoxy forms. The reduced linear dichroism measured as a function of the electric field strength demonstrates the existence of permanent dipole moments in the range of 250-400 Debye units. The reduced limiting dichroism is relatively small (< or = 0.1); it is negative for hemoglobin from sickle cells and positive for the hemoglobins from normal human cells and from horse. The dichroism decay time constants are in the range from about 55 to 90 ns. Calculations of the electrooptical data from available crystal structures are given according to models of various complexity, including Monte Carlo simulations of proton fluctuations with energies evaluated by a finite difference Poisson-Boltzmann procedure. The experimental dipole moments are shown to be consistent with the results of the calculations. In the case of human deoxyhemoglobin, the root mean square dipole is higher than the mean dipole by a factor of about 4.5, indicating a particularly large relative contribution due to proton fluctuations. The ratio of the root mean square dipole to the mean dipole is much smaller (approximately 1.1 to approximately 1.5) for the other hemoglobin molecules. The calculations demonstrate that the dichroism decay time constants are not simply determined by the size/shape of the proteins, but are strongly influenced by the orientation of the dipole vector with respect to the axis of maximal absorbance. The comparison of experimental and calculated electrooptical data provides a useful test for the accuracy of electrostatic calculations and/or for the equivalence of structures in crystals and in solutions.

Stark spectra of chlorophylls and carotenoids in antenna pigment-proteins LHC-II and CP-II

The band structure of low-temperature absorption and Stark spectra of light-harvesting chlorophyll a/ b protein complexes LHC-II and CP-II is analysed. In the red spectral region, the Stark spectrum serves as an independent experimental reference allowing for the resolution of eight bands at 639, 642–3, 649, 656, 662, 665 671 and 678 nm, consistently with the gaussian deconvolution of the absorption spectrum. Different parts of the Stark spectrum are fitted with combinations of the derivatives of either the total absorption spectrum or of individual gaussian components. Two electronic transitions at 671 nm and 678 nm and two others at 639 nm and 642–3 nm are found to excite, respectively, localized chlorophyll a and chlorophyll b states. Increased changes in permanent dipole moment, Δμ ≅ 2.2 Debye units, corresponding to the transitions at 656 nm and at 665 nm, and an increase in polarizability ≅ 70 Å 3 corresponding to the first of them indicate that these transitions originate from strong exciton coupling. In the violet, the Stark spectra are dominated by the contribution from xanthophylls. A value of Δμ ≥ 10 D was estimated for xanthophyll bands at 484 nm and at 511–513 nm. The mechanism of the Stark effect in xanthophylls is briefly discussed.

Microwave Studies of Electron Scattering by Isolated Interstitial Oxygen and Oxygen Complex in Silicon

Results of microwave cyclotron resonance absorption are given for several Czochralski (CZ)-grown silicon crystals with different concentrations of isolated oxygen atoms. In the same way, results are also presented for some annealed samples with different concentrations of thermal donors (TD) and oxygen-nitrogen (O-N) complexes. We conclude that isolated oxygen atoms, though electrically inactive, contribute to electron scattering. We also give a theoretical prediction based on the electric dipole scattering model with an appropriate effective dipole moment, D=1.1 Debye units, for the Si-O-Si complex. Agreement between theory and experiment is quite satisfactory. In addition, experiments making use of annealed samples enable us to make a precise determination of the electron scattering rates by TD and O-N complexes.

Solvent Effects upon the Electronic Absorption and Fluorescence Spectra of Pteridines and Riboflavin and Their Ground and First Excited Singlet State Dipole Moments

The electronic absorption. fluorescence excitation, and fluorescence emission spectra of a series of pteridines (lumazine, xanthopterin, isoxanthopterin, biopterin) and riboflavin (vitamin B2) were measured at room temperature (298 K) in a number of solvents covering a wide range of polarities (dioxane, ethyl ether, chloroform, ethyl acetate, 1-butanol. 2-propanol, ethanol, methanol, dimethylformamide, acetonitrile, and dimethyl sulfoxide). The effects of the solvent upon the spectral properties are discussed. Experimental groundstate dipole moments were measured for selected compounds and were used in combination with the spectral data to evaluate their first excited singlet-state dipole moments by means of the solvatochromic shift method (Bakhshiev's and Kawski-Chamma-Viallet's equations based on the variation of the Stokes shift with the solvent dielectric constant-refractive index term). The theoretical ground and excited singlet-state dipole moments for all pteridines and riboflavin were calculated as a vector sum of the π-component (obtained by the PPP method) and the a-component (obtained from a-bond moments). A second set of theoretical values was obtained by using the CNDO/2method. A good agreement was observed between the experimental and the theoretical values. Excited singlet-state dipole moments are higher than the ground state values by 1 to 6 Debye units for all the pteridines under study with the exception of xanthopterin.

Stark effect in P700, the primary electron donor of Photosystem I

Quadratic Stark effect in CP1 pigment-protein complex was examined at low temperatures in the red spectral region. The Stark spectra of samples containing P700 in reduced form exhibit a strong negative band at 704 nm, which disappears on chemical oxidation of P700. The change in permanent dipole moment, Δμ, of P700 on electronic excitation estimated from these spectra was found to be between 4.7 and 7.7 Debye units. It is suggested to reflect the charge-transfer contribution to the excited state of P700. For antenna chlorophyll, Δμ≌ 1 D was obtained in accordance with the data for monomeric chlorophyll.

Stark spectroscopy of the Rhodobacter sphaeroides reaction center heterodimer mutant.

The effect of an electric field has been measured on the absorption spectrum (Stark effect) of the heterodimer mutant (M)H202L of Rhodobacter sphaeroides reaction centers, where the primary electron donor consists of one bacteriochlorophyll alpha and one bacteriopheophytin alpha. The electronic absorption spectrum of the heterodimer mutant from 820-950 nm is relatively featureless in a poly(vinyl alcohol) film, but it exhibits some structure in a glycerol/water glass at 77 K. A feature is seen in the Stark effect spectrum of the heterodimer at 77 K centered at 927 and 936 nm in poly(vinyl alcohol) and a glycerol/water glass, respectively. This feature has approximately the same shape and width as the Stark effect for the primary electron donor of the wild type, which consists of a pair of bacteriochlorophyll alpha molecules. The angle zeta A between the transition moment at the frequency of absorption and the difference dipole delta muA is 36 +/- 2 degrees in the wild type and 32 +/- 2 degrees for that feature in the heterodimer. A range of values for [delta muA] = (13-17)/f Debye units (where f is the local field correction) is obtained for the 936-nm feature in glycerol/water, depending on analysis method. This feature is interpreted as arising from a transition to the lower exciton state of the heterodimer, which is more strongly mixed with a low-lying charge transfer transition than in the wild type.

Transition Phenomena in Poly(diacetylene) Solutions Studied by Electric Birefringence Measurements

AbstractAn external electric field induces strong birefringence in solutions of poly(diacetylenes) when they are in the red (low temperature) form. The transient behaviour and the field‐strength dependence of the steady‐state birefringence reveal that the orientation of particles is predominantly due to a permanent dipole moment rather than to the anisotropy of polarizability. The estimated permanent dipole moments are in the order of tenthousand Debye units, this order of magnitude being incompatible with single macromolecules. Thus the view that aggregated particles are formed at low temperatures is supported by our experiments.