电极/溶液界面单分子吸附层的统计力学处理 III. 汞电极上水-甲醇混合溶剂化层结构

按文[11]模型, 对金属/(混合溶剂)溶液界面, 导出单分子层混合吸附等温方程并拟合计算汞/(水+甲醇)溶液界面的内层微分电容对表面电荷密度依赖关系。结果表明, 对不同组成的(H_2O+CH_3OH)混合溶剂, 计算的C_1～σ曲线与实验值甚一致。而其对应的吸附等温线则表现为在汞电极上, 由水-甲醇形成的溶剂化层几乎为甲醇分子复盖满。     

Inertial and bias effects in the rotational brownian motion of rodlike molecules in a uniaxial potential

Inertial effects in the rotational brownian motion in space of a rigid dipolar rotator (needle) in a uniaxial potential biased by an external field giving rise to asymmetry are treated via the infinite hierarchy of differential-recurrence relations for the statistical moments (orientational correlation functions) obtained by averaging the Euler-Langevin equation over its realizations in phase space. The solutions of this infinite hierarchy for the dipole correlation function and its characteristic times are obtained using matrix continued fractions showing that the model simultaneously predicts both slow overbarrier (or interwell) relaxation at low frequencies accompanied by intermediate frequency Debye relaxation due to fast near-degenerate motion in the wells of the potential (intrawell relaxation) as well as the high frequency resonance (Poley) absorption due to librations of the dipole moments. It is further shown that the escape rate of a brownian particle from a potential well as extended to the Kramers turnover problem via the depopulation factor yields a close approximation to the longest (overbarrier) relaxation time of the system. For zero and small values of the bias field parameter h, both the dipole moment correlation time and the longest relaxation time have Arrhenius behavior (exponential increase with increasing barrier height). While at values of h in excess of a critical value however far less than that required to achieve nucleation, the Arrhenius behavior of the correlation time disappears.     

Energy transfer in collisions of alkali-metal clusters and ions

The electronic energy loss of a positively charged projectile passing through a sodium cluster is calculated by the semiclassical Vlasov method. The electron density before impact is obtained within the localdensity approximation in the jellium model. For the dynamical response, the Vlasov equations are solved by test particle discretization. The model gives the correct breathing mode and dipole Mie frequencies. The energy deposit is calculated as a function of projectile velocity, charge and impact parameter and is compared with theoretical and tabulated values for bulk sodium.     

Estimation of absorption band contribution for overtones and combined frequencies in the fingerprint region for alkanes, nitriles, amines, and nitroalkanes

In the anharmonic approximation the intensities of fundamental, overtone, and combined absorption bands are calculated in the range from 100 cm^-1 to 4000 cm^-1 for a series of alkanes, nitriles, amines, and nitroalkanes. The first and second derivatives of the molecular dipole moment are calculated by quantum chemical methods in the ab initio approximation using the MP2/6-31G(1d) basis set. Overtones and combined frequencies are found to make a significant contribution (5–20%) to the total absorption. The spectral distribution of this contribution depends on the molecular structure. For nitriles and amines significant absorption of the overtones and combined frequencies is observed in the same regions where fundamental absorption bands characteristic of these compounds are located.     

On the quantum properties of adsorbed particles within the model of a hydrogen atom near a hard wall

A hydrogen molecule near a hard wall was considered within the Heiteler–London method using the atomic orbits in the presence of the wall. Rapidly converging expansions of the atomic wave functions were proposed. The asymptotics were obtained and studied. The atomic dipole moment induced by the surface gave rise to a long-range interatomic interaction. The covalent bond length increased near the wall. Depending on the orientation relative to the wall, the potential well can vanish. It is interpreted as an interaction between the atomic dipole moments. An approximation of the interatomic potential was proposed.     

Molecular photoionization cross sections in electron propagator theory: angular distributions beyond the dipole approximation

Corrections to dipole approximation results for angular distributions in photoionization of first-row hydrides have determined by using Dyson orbitals calculated with ab initio electron propagator theory and by considering the full multipole expansion for the incident photon representation. The relative importance of first-order corrections which consist of electric quadrupole and magnetic dipole terms and of higher-order terms has been estimated as a function of photon energy. Multipole corrections to the dipole approximation depend on photon energy and on the characteristics of the Dyson orbitals.     

Statistical correlation between configuration-dependent properties of polymer chains

A new methodology for formulating the averages of elements of generator matrices is presented. Incorporation of this technique to the Rotational Isomeric State (RIS) model allows the efficient generation of the higher and mixed moments of any conformation-dependent physical property of a polymer chain. The computational technique is based on symbolic manipulation of the generator matrix elements which are treated as character strings. Successful application of the technique is demonstrated by calculating the correlation coefficient between the dipole moment and the end-to-end distance of polymer chains with symmetric rotation potentials and in the independent bond rotation approximation. It is shown that the correlation heavily depends on the orientation of the bond dipole moment vector orientation.     

The Langmur-blodgett monolayer dipole potential: a smeared dipole model for a lipid array,and pulsing of the potential by direct subphase infusion of immunochemical and lecti/polysaccharide complexes

An important contribution to the surface potential of lipid bilayers and monolayers comes from the intrinsic dipole moment of the lipid molecules. A theoretical model of the monolayer which involves a smeared dipole sheet approximation is introduced. This model is used to explore the nature and origins of the surface potential. In addition, the potential associated with phosphatidyl choline/cholesterol monolayers compressed on a Langmuir-Blodgett trough was measured with a non-contacting electrostatic voltmeter. A trough infusion configuration was fabricated to perform dynamic subphase experiments with compressed films in place. The potential/time response of monolayers to selective bimolecular systems such as antibody-antigen and concanavalin A-saccharide pairs was examined. These reactions induce spontaneous transients in dipole potential of magnitude 20–80 mV and duration of less than 1 s. The potential transients are attributed to local perturbation of lipid orientation and introduction of protein dipole fields caused by the formation of aggregates at the monolayer/water interface.     

MECHANISM OF ORIENTATION AND LINEAR DICHROISM OF PHOTOSYNTHETIC PARTICLES IN ELECTRIC FIELDS: CHLOROPLASTS AND CHLOROPHYLL-PROTEIN COMPLEXES

Abstract— The mechanisms of orientation in pulsed and alternating electric fields of thylakoids (derived from the sonication of spinach chloroplasts) and of light-harvesting chlorophyll a/b-protein complexes (CPII) were investigated by utilizing linear dichroism techniques. Comparisons of the linear dichroism spectra of thylakoids and CPII particles suggest that the latter are oriented with their directions of largest electronic polarizabilities (and thus probably their largest dimensions) within the thylakoid membrane planes. At low electric field strengths (< 12 V cm^?1), and at low frequencies of alternating electric fields (< 0.25 Hz), thylakoid membranes tend to align with their normals parallel to the direction of the applied electric field; the mechanism of orientation involves a permanent dipole moment of the thylakoids which is oriented perpendicular to the planes of the membranes. However, at high field strengths and high frequencies of the applied alternating electric fields, the thylakoids tend to orient with their planes parallel to the applied field, thus exhibiting an inversion of the sign of the linear dichroism as the electric field strength is increased. At the higher frequencies and at higher field strengths, the orientation mechanisms of the thylakoids involve induced dipole moments related to anisotropies in the electronic polarizabilities. The polarizability is higher within the plane than along a normal to the plane, thus accounting for the inversion of the dichroism as the electric field strength is increased. The CPII particles align with their largest dimension parallel to the applied field at all field strength, indicating that the induced dipole moment dominates the orientation mechanisms in pulsed electric fields. The magnitude of the absolute linear dichroism of CPII suspensions increases with increasing dilution, indicating that aggregates of lower symmetry are formed at higher concentrations of the CPII complexes.     

Electric dipole moments of magnetite colloidal particles in liquid dielectrics

The effects of alternating and pulsed fields on the electric birefringence in a colloided system of magnetite particles suspended in kerosene are studied. The permanent dipole moment is found to substantially influence the orientation of magnetite particles in alternating fields with frequencies of higher than 15 Hz. The mechanism for the generation of the permanent dipole moments in magnetite colloidal particles suspended in low-conductivity liquids is discussed.     

The story of the ammonia molecule: Ten years of investigation of molecular inversion

The application of the Hougen-Bunker-Johns (HBJ) Hamiltonian to quasiplanar molecules is discussed with special emphasis on ammonia. The double-minimum potential functions of ammonia obtained in the approximation of the rigid invertor, “harmonic” nonrigid invertor, and “anharmonic” nonrigid invertor are compared from the point of view of their isotopic invariancy. The HBJ Hamiltonian can also be applied with advantage to the quantitative analysis of the transition frequencies obtained by high and very high resolution spectroscopy, as well as to the interpretation of the anomalous inversional dependence of the dipole moment function, quadrupole coupling or of the g-tensor function.The newly determined “true” barrier height for ammonia inversion by fitting the experimental data on vibration-inversion-rotation transitions is 1834 cm^?1.     

On Feasibility of “Spectroscopic” Calculations of XH Bond Dissociation Energies for Polyatomic Molecules from Fundamental Vibration Frequencies Using the Anharmonic Molecular Model

This paper discusses the applicability of the variational technique using a minimal Morse-harmonic basis set to calculations of the fundamental spectrum and the potential function parameters for polyatomic molecules. The potential function is assumed to be the sum of the Morse function for XH bonds and the harmonic function for the skeletal and deformation vibrations. The initial approximation for the potential function is found by ab initio calculations and refined by solving the inverse mechanical problem (selecting the scaling indices). The thus selected harmonic part of the potential function gives equally good agreement between the experimental and calculated transition frequencies in both harmonic and anharmonic approximations. The anharmonic (Morse) term of the potential function (bond dissociation energy) is selected by solving the inverse mechanical problem until the best agreement between the experimental and calculated CH bond stretching frequencies has been achieved. Problem solving ends with the construction of a transmission curve in the IR spectrum. Variations of the dipole moment of the molecule induced by vibrations are found by ab initio calculations.     

Measurement of the electronic transition dipole moment by Autler-Townes splitting: Comparison of three- and four-level excitation schemes for the Na2 A 1Sigma(u)+ - X 1Sigma(g)+ system

We present a fundamentally new approach for measuring the transition dipole moment of molecular transitions, which combines the benefits of quantum interference effects, such as the Autler-Townes splitting, with the familiar R-centroid approximation. This method is superior to other experimental methods for determining the absolute value of the R-dependent electronic transition dipole moment function mu(e)(R), since it requires only an accurate measurement of the coupling laser electric field amplitude and the determination of the Rabi frequency from an Autler-Townes split fluorescence spectral line. We illustrate this method by measuring the transition dipole moment matrix element for the Na2 A 1Sigma(u)+ (v' = 25, J' = 20e)-X 1Sigma(g)+ (v" = 38, J" = 21e) rovibronic transition and compare our experimental results with our ab initio calculations. We have compared the three-level (cascade) and four-level (extended Lambda) excitation schemes and found that the latter is preferable in this case for two reasons. First, this excitation scheme takes advantage of the fact that the coupling field lower level is outside the thermal population range. As a result vibrational levels with larger wave function amplitudes at the outer turning point of vibration lead to larger transition dipole moment matrix elements and Rabi frequencies than those accessible from the equilibrium internuclear distance of the thermal population distribution. Second, the coupling laser can be "tuned" to different rovibronic transitions in order to determine the internuclear distance dependence of the electronic transition dipole moment function in the region of the R-centroid of each coupling laser transition. Thus the internuclear distance dependence of the transition moment function mu(e)(R) can be determined at several very different values of the R centroid. The measured transition dipole moment matrix element for the Na2 A 1Sigma(u)+ (v' = 25, J' = 20e)-X 1Sigma(g)+ (v" = 38, J" = 21e) transition is 5.5+/-0.2 D compared to our ab initio value of 5.9 D. By using the R-centroid approximation for this transition the corresponding experimental electronic transition dipole moment is 9.72 D at Rc = 4.81 A, in good agreement with our ab initio value of 10.55 D.     

外部电场下水分子间静电相互作用的分子动力学研究

本文利用分子动力学模拟探讨了不同外电场下,液态水的分子间作用及分子排布的变化. 在不同外电场下,O…O原子间的径向分布函数差别很小,但是单个水分子的偶极矩的取向变化却很大. 当外电场为0时,单个水分子偶极取向的范围很宽（30-150度）. 与此同时,本文给出了局域诱导电场随着位置的变化关系图. 当外加电场增强时,局域的诱导电场强度也随之增加. 由于电场下偶极矩有序性的增加,局域诱导的静电相互作用能显著增加. 计算结果表明,相对介电常数随着电场强度的增加而呈现指数衰减的变化形式. 这一变化趋势可以用来理解不同电化学环境下,静电相互作用和局域诱导电场的变化.     

The equilibrium structures, vibrational spectra, NLO and directional properties of transition dipole moments of diguanidinium arsenate monohydrate and diguanidinium phosphate monohydrate. The theoretical DFT calculations

For diguanidinium arsenate monohydrate and diguanidinium phosphate monohydrate the energies were minimized and the theoretical vibrational frequencies and potential energy distribution (PED) were calculated by density functional method. The 6-31++G(d,p) basis set was used. The assignment of the bands has been made on the basis of the calculated PED. For calculated equilibrium geometries two methods for determination of direction of transition dipole moments (TDM) were used. The oriented gas model was used for calculation of "static" TDM and from the other side the "dynamic" approximation of TDM by analysis of changes in internal coordination during characteristic vibrations was performed. The restricted Hartree-Fock (RHF) methods were used for calculation of the hyperpolarizability for both investigated molecules. The theoretical results are compared with experimental value of beta.     

On the performance of molecular polarization methods. I. Water and carbon tetrachloride close to a point charge

The three main methods to implement molecular polarization (point dipoles, fluctuating charges, and shell model) are tested against high level ab initio calculations for a molecule (water, carbon tetrachloride) close to a point charge (at the distance of a lithium or magnesium ion). The goal is to check whether an approximation (linear polarization) strictly valid at large intermolecular distances is sufficiently accurate for liquid state molecular dynamics simulations, where strong polarization effects are to be expected at short separations. The monitored observable is the molecular dipole moment as a function of the charge-molecule distance for selected molecular orientations. Analytic formulas are derived for the components of the molecular polarization tensor, facilitating the optimization of the performance for each polarization method as a function of its underlying parameters. Overall, the methods studied provide a remarkably good representation of the induced dipole, with no divergences appearing even at the shortest distances. For water close to a monovalent point charge the point dipole model, implemented with one or three dipoles, accurately reproduces the water dipole moment at all distances. Deficiencies appear as the molecular polarizability and/or charge increase: basically, the ab initio induced moments grow faster at intermediate distances than the linear increase characteristic of the phenomenological polarization methods, suggesting that nonlinear effects (hyperpolarizability) cannot be neglected in these cases. Regarding the capabilities of each method, the point dipole method is the one that performs best overall, with the shell model achieving acceptable results in most instances. The fluctuating charge method shows some noticeable limitations for implementations of comparable complexity (in terms of the number of sites required).     

Coverage-dependent adsorption geometry of octithiophene on Au(111)

The adsorption behavior of α-octithiophene (8T) on the Au(111) surface as a function of 8T coverage has been studied with low-temperature scanning tunneling microscopy, high resolution electron energy loss spectroscopy as well as with angle-resolved two-photon photoemission and ultraviolet photoemission spectroscopy. In the sub-monolayer regime 8T adopts a flat-lying adsorption geometry. Upon reaching the monolayer coverage the orientation of 8T molecules changes towards a tilted configuration, with the long molecular axis parallel to the surface plane, facilitating attractive intermolecular π-π-interactions. The photoemission intensity from the highest occupied molecular orbitals (HOMO and HOMO - 1) possesses a strong dependence on the adsorption geometry due to the direction of the involved transition dipole moment for the respective photoemission process. The change in molecular orientation as a function of coverage in the first molecular layer mirrors the delicate balance between intermolecular and molecule/substrate interactions. Fine tuning of these interactions opens up the possibility to control the molecular structure and accordingly the desirable functionality.     

Orientation of pea chloroplast in an ac electric field

The orientation of pea chloroplast in an ac electric field was studied. The maximum linear dichroism signal at 681 nm was used as an indicator of orientation. It was found that the degrees of orientation depended on the frequency of the electric field. In the frequency range studied, there were two distinct dispersions at about 80 Hz and 750 kHz (α and β dispersion), and a maximum at about 100 kHz. The dependence of orientation on the electric field strength was used to estimate the polarizability. In agreement with the dispersion spectrum, there were stepwise decreases in polarizability at the two dispersions and an increase when moved into the region of the maximum. There was not permanent dipole detected.At a frequency below the α dispersion, an induced dipole arising from the oscillation of a counterion cloud, which is formed adjacent to the thylakoid membrane surface due to the presence of fixed surface charges, is believed to be a major cause of orientation. After the relaxation of this oscillation at higher frequency, the charge accumulation on both the inner and outer surfaces of the thylakoid membrane is believed to play a determinant role. Since the two interfacial polarizations have different dispersion frequencies, the polarizability of chloroplast increases after one dispersion (the maximum), which is then followed by a decrease at the second dispersion (the β dispersion).     

Molecular structure and spectroscopic properties of polyaromatic heterocycles by first principle calculations: spectroscopic shifts with the adsorption of thiophene on phyllosilicate surface

The molecular and electronic structure and spectroscopic properties of thiophene, benzothiophene, and dibenzothiophene molecules have been studied theoretically, using different levels of quantum-mechanical calculations based on Hartree–Fock and density functional theory (DFT) approximations. In all calculation levels used, the molecular structures, dipole moments, thermodynamic properties, and vibration normal modes agree well with the available experimental data and predict the non-available values. The calculated frequencies agree with experimental values within scale factors in the range of 0.94–1.0 for thiophene. Our calculations have refined the mode assignments previously reported. The adsorption of thiophene on a pyrophyllite surface along the 001 plane was also investigated by means of a quantum-mechanical method based on the DFT approximation in a periodical crystal lattice model. The adsorbed thiophene adopts a parallel orientation with respect to the phyllosilicate surface. The shifts observed experimentally in the IR bands of thiophene upon adsorption on pyrophyllite are well reproduced in the thiophene–phyllosilicate complex, finding explanations of this phenomenon and new effects with our calculations.     

Dielectrophoresis of cellulose nanocrystals and alignment in ultrathin films by electric field-assisted shear assembly

Ultrathin films of cellulose nanocrystals (CNCs) are obtained by using a convective assembly setup coupled with a low-strength external AC electric field. The orientation and degree of alignment of the rod-like nanoparticles are controlled by the applied field strength and frequency used during film formation. Calculated dipole moments and Clausius-Mossotti factors allowed the determination of the critical frequencies, the peak dielectrophoresis as well as the principal orientation of the CNCs in the ultrathin films. As a result of the combination of shear forces and low electric field highly ultrathin films with controlled, unprecedented CNC alignment are achieved.