Saturated hydraulic conductivity of a volcanic ash soil affected by repulsive potential energy in a multivalent anionic system

Acid rain is supposed to influence soil structures, because soils have pH-dependent charges. The adverse effects of acid rain on soils must be assessed. Although repulsive potential energy among soil clay particles generates swelling and dispersion, thereby changing the soil’s hydraulic conductivity, the relationship between hydraulic conductivity and repulsive potential energy has not been evaluated. Moreover, research into repulsive potential energy in multivalent counterionic systems has been rare. In this paper, repulsive potential energies for a volcanic ash soil (allophanic Andisol), which is characterized by a number of pH-dependent charges, were evaluated in a multivalent counterionic system. Changes in saturated hydraulic conductivity (K) of volcanic ash soil during dilute acid leaching and their relationship with the repulsive potential energies were examined. When nitric acid at pH 3 or 4 was leached, K decreased rapidly. On the other hand, the decrease in K attenuated as the proportion of sulfate in the dilute acid increased. Electrophoretic mobilities were measured and the zeta potentials were estimated. From the zeta potentials and the calculation of repulsive potential energies between the clay particles in the NO₃–SO₄ system, we concluded that the decrease in K for an acid solution with a high proportion of nitrate was due to swelling and dispersion of the soil induced by electrostatic repulsive potential energy. Because sulfate formed complexes on the clay surface, the repulsive potential energy decreased as the proportion of sulfate in the dilute acid increased. Then, the flocculation of the soil was maintained, thereby inhibiting the decrease in K.     

Mean potential phase space theory of chemical reactions

A nonconventional application of phase space theory to the insertion reactions A+H(2), with A=C((1)D) and S((1)D), is presented. Instead of approximating the potential energies of interaction between separated fragments by their isotropic long-range contributions, as in the original theory, the latter are replaced by the accurate potential energies averaged with respect to Jacobi angles. The integral and differential cross sections obtained from this mean potential phase space theory (MPPST) turn out to be in very satisfying agreement with the benchmark predictions of the time-independent and time-dependent statistical quantum methods. The formal and numerical simplicity of MPPST with respect to any approach combining statistical assumptions and dynamical calculations makes it a promising tool for studying indirect polyatomic reactions.     

The Ne-Ne interatomic potential revisited

The neon-neon interaction has been re-examined in the light of new data, both theoretical and experimental. Because of recent interest in solids at very high pressures, a new accurate potential is presented with particular interest paid to its highly repulsive region. A potential in HFD-B form incorporating the most recent dispersion coefficients was fitted to accurate viscosity data and high-energy scattering beam data. The potential is able to predict a wide range of macroscopic (second virial coefficients, viscosity, thermal conductivity, diffusion and 0 K binding energy) and microscopic properties (spectroscopic differential and high-energy total cross sections). The potential is extended to very short range by extrapolating to united atom perturbation results.     

Basis set effects on the intermolecular interaction of the H2-H2 system obtained using ab initio molecular orbital calculations with the Møller-Plesset perturbation correction

The intermolecular interaction potential of the H₂-H₂ system was calculated by an ab initio molecular orbital method using several basis sets (up to 6-31 lG(3pd)) with inclusion of the electron correlation correction of the Møller-Plesset perturbation method and the basis set superposition error (BSSE) correction of the counterpoise method in order to evaluate the basis set effect. The calculated interaction energies depend strongly on the basis set used. Whereas the interaction energies of the repulsive and coulombic energy components calculated at the Hartree-Fock level are not affected by a change of basis set, the dispersion energy component depends strongly on the basis set used. Parameters of an exp-6-1 type non-bonding interaction potential were optimized on the basis of the MP4(SDTQ)/6-311G(3p) level intermolecular interaction energies of the H₂-H₂ system.     

Monoenergetic neutron sources below 100 MeV

Monoenergetic neutron sources are essential for fundamental studies in radiobiology and dosimetry, for measurement of cross sections and kerma coefficients, for calibration of detectors, for activation analysis and for fusion research. Monoenergetic neutrons below energies of 20 MeV are most conveniently produced by reactions between the hydrogen isotopes or between protons and ⁷Li. By proper choice of reaction type monoenergetic neutrons up to 20 MeV can be produced with negligible secondary background radiation. These reactions cannot provide monoenergetic beams between about 8 and 14 MeV and in this “gap” region inverse reactions are most favourable. The most practical way of producing quasi-monoenergetic neutron beams in the energy range from 20 to 100 MeV is by the bombardment of light elements with protons. Because of the relative simplicity of manufacturing suitable isotopically-pure targets and the large 0° cross section, the ⁷Li(p,n)⁷Be reaction is a convenient source of quasi-monoenergetic neutrons over this range of energies, although the ⁹Be(p,n)⁹B reaction is also used.     

Energetics and structure of the hydrated gaseous halide anions

Energetics and geometries for the hydrated gaseous halide anions have been computed from a simple model in which the molecular dipole of water was composed of two parts, one due to a lone pair on oxygen (60%) and the rest to formal charges on the nuclei. The calculations were made for both the symmetric and nonsymmetric structures. A variety of structures were used to compute potential energies and distances with up to six water molecules. The total energy consisted of a sum of electrostatic, polarization, dispersion, and repulsion terms. Various sets of repulsive potential parameters, ranging from those determined from molecular beam experiments to those determined using experimental ion–water distances or energies, have been employed to compute repulsive interaction energies. It was found that the range parameters play a significant role in deciding the magnitudes of the distances and energies, as the latter are most sensitive to them. It was also shown that with a simple correlation scheme the consistency of the experimental energies and distances can be tested separately without using repulsive potential parameters from other sources. It also suggests that a range of parameters can be used to compute repulsion energies. Despite the fact that the model is greatly simplified, the agreement of both the predicted ion-oxygen distances and energies with both experiment and other calculations is excellent. A detailed analysis of our calculation suggests that the negative ion clusters with one to three water molecules contain symmetric orientation of water molecules, while those with more than three may contain asymmetric orientations of water molecules or a mixture of both. From the log–log plots of hydration energies versus (R + radius of water molecule), we have proposed empirical expressions of the type ΔE_n?1,n = 10·0^x (R + 1.38)^?y with both Pauling's and Ladd's radii for univalent ions with which stepwise hydration energies of the latter can be predicted if we know thier radii. The values predicted for the alkali cations are in excellent agreement with the experimental and theoretical values, indicating the consistency of the simple model.     

Ester decomposition as a two-center synchronous reaction

Experimental data on the molecular decomposition of esters with various structures into an olefin and the corresponding acid in the gas phase are analyzed in terms of the intersecting parabolas method. Enthalpies and kinetic parameters characterizing this decomposition have been calculated for 33 reactions. Ester decomposition is a concerted two-center reaction characterized by a very high classical potential barrier of thermoneutral reaction (148–206 kJ/mol). The totality of reactions examined is divided into eight classes. Activation energies and rate constants have been calculated for 38 reactions using the kinetic parameters obtained. The activation energies and rate constants of the reverse bimolecular reaction of acid addition to olefins have been calculated by the intersecting parabolas method. Factors in the activation energy of ester decomposition and formation reactions are discussed.     

Perturbation theory for discotic nematic liquid crystals of axially symmetric molecules: effect of dispersion interaction

We investigate the influence of dispersion interaction on a variety of thermodynamic properties of discotic nematic liquid crystals at the discotic nematic-isotropic transition. We report calculations for a hard oblate ellipsoidal system, superposed with an attractive interaction represented by dispersion interaction subjected to different external pressures ranging from 1 to 300 bar. We consider a model system (which simulates a discotic nematic liquid crystal) in which molecules are assumed to interact via a pair potential having both repulsive and attractive parts. The repulsion part is represented by a repulsion between hard oblate ellipsoids of revolution and is a short range, rapidly varying potential. The attractive potential, a function of centre of mass distance and relative orientation between two molecules, is represented by dispersion interaction. The properties of the reference system and first order perturbation term are evaluated using a decoupling approximation which decouples orientational from translational degrees of freedom. The inclusion of fourth and sixth rank orientational order parameters in the calculation slightly improves the result. The role of pressure on phase transition parameters has also been studied.     

Perturbation theory for discotic nematic liquid crystals of axially symmetric molecules: effect of dispersion interaction

We investigate the influence of dispersion interaction on a variety of thermodynamic properties of discotic nematic liquid crystals at the discotic nematic-isotropic transition. We report calculations for a hard oblate ellipsoidal system, superposed with an attractive interaction represented by dispersion interaction subjected to different external pressures ranging from 1 to 300 bar. We consider a model system (which simulates a discotic nematic liquid crystal) in which molecules are assumed to interact via a pair potential having both repulsive and attractive parts. The repulsion part is represented by a repulsion between hard oblate ellipsoids of revolution and is a short range, rapidly varying potential. The attractive potential, a function of centre of mass distance and relative orientation between two molecules, is represented by dispersion interaction. The properties of the reference system and first order perturbation term are evaluated using a decoupling approximation which decouples orientational from translational degrees of freedom. The inclusion of fourth and sixth rank orientational order parameters in the calculation slightly improves the result. The role of pressure on phase transition parameters has also been studied.     

Evaluation of elastic‐scattering cross sections for electrons and positrons over a wide energy range

Quantification of surface‐ and bulk‐analytical methods, e.g. Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy (XPS), electron‐probe microanalysis (EPMA), and analytical electron microscopy (AEM), requires knowledge of reliable elastic‐scattering cross sections for describing electron transport in solids. Cross sections for elastic scattering of electrons and positrons by atoms, ions, and molecules can be calculated with the recently developed code ELSEPA (Elastic Scattering of Electrons and Positrons by Atoms) for kinetic energies of the projectile from 10 eV to 50 eV. These calculations can be made after appropriate selection of the basic input parameters: electron‐density distribution, a model for the nuclear‐charge distribution, and a model for the electron‐exchange potential (the latter option applies only to scattering of electrons). Additionally, the correlation‐polarization potential and an imaginary absorption potential can be considered in the calculations. We report comparisons of calculated differential elastic‐scattering cross sections (DCSs) for silicon and gold at selected energies (500 eV, 5 keV, 30 keV) relevant to AES, XPS, EPMA, and AEM, and at 100 MeV as a limiting case. The DCSs for electrons and positrons differ considerably, particularly for medium‐ and high‐atomic‐number elements and for kinetic energies below about 5 keV. The DCSs for positrons are always monotonically decreasing functions of the scattering angle, while the DCSs for electrons have a diffraction‐like structure with several minima and maxima. A significant influence of the electron‐exchange correction is observed at 500 eV. The correlation‐polarization correction is significant for small scattering angles at 500 eV, while the absorption correction is important at energies below about 10 keV. Copyright © 2005 John Wiley & Sons, Ltd.     

Elastic constants of uniaxial nematic liquid crystals: A comparison between theory and experiment

Using the unified molecular theory developed in our earlier paper (1992, Phys. Rev. A, 45, 974) we study in detail the influence of molecular interactions on the fundamental elastic properties of uniaxial nematic liquid crystals composed of molecules of cylindrical symmetry. The expressions for the elastic moduli associated with 'splay', 'twist' and 'bend' modes of deformations are written in terms of order parameters characterizing the nature and amount of ordering in the phase and the structural parameters which involve the generalized spherical harmonic coefficients of the direct pair correlation function of an effective isotropic liquid. Numerical calculations are done for a model system, the molecules of which have prolate ellipsoid of revolution symmetry and interact via a pair potential having both repulsive and attractive parts. The repulsive interaction is represented by a repulsion between hard ellipsoids of revolution. The attractive potential is represented by the dispersion and electrostatic interactions. Results for the elastic constants are reported for a range of molecular length-width ratio, temperature, density and molecular parameters and are compared with the experimental values of p-azoxyanisole (PAA) and 4'-n-octyloxy-4-cyanobiphenyl (8OCB). It is found that the inclusion of electrostatic interactions reduces the values of the ratios K₂/K₁ and K₃/K₁. The absolute values of the elastic constants and their ratios are in good agreement with the experimental and computer simulation values. The temperature dependence of the elastic constants and their ratios is studied. It is observed that the twist elastic constant has a weak temperature dependence but a pronounced influence is observed on the bend moduli. We also observed a pronounced increase in the values of the twist and bend elastic constants on approaching the nematic-smectic A transition temperature.     

On the Stability of Cyclophane Derivates Using a Molecular Fragmentation Method

A molecular fragmentation method is used to study the stability of cyclophane derivates by decomposing the molecular energy into the molecular strain and intramolecular interaction energies. The molecular strain energies obtained by utilising the fragmentation method are in good agreement with existing experimental data. The intramolecular interaction energies calculated as the difference between the supermolecular energy and the bonded fragment energies are repulsive in the cyclophanes studied. The nature of this interaction is studied for groups of systematically extended doubled layered paracyclophane systems using the random‐phase approximation (RPA), two recently developed extensions to the RPA and standard density functional theory (DFT) methods including dispersion corrections. Upon a systematic increase in conjugation the strongly repulsive intramolecular interaction energy reduces and thus leads to an increase in the stability. Finally, existing experimental and theoretical estimates of the molecular strain are compared with the results of this work.     

Absolute balmer line and NH(c Π→b Σ, 0-0) vibrational band emission cross sections from NH3 molecules excited by electron impact

The absolute Balmer line emission cross sections are determined in the processes of the electron impact dissociative excitation of ammonia. The optical excitation functions measured for these lines were investigated in the energy range 50–500 eV and normalized by the He benchmark procedure. The molecular continuum contribution has been eliminated from the obtained data. After that, the measured data have been corrected with the collection efficiency factor F to compensate the loss of optical signal due to non-thermal energies of the H excited fragments. The results for kinetic energy distribution functions for the ammonia molecules have been used for the F determination. The optical emission cross sections are determined with the accuracy of ± 15%. The cross sections for the NH(c ¹Π→b ¹Σ⁺, 0-0) vibrational band have also been determined with an accuracy of ± 25%.     

Complex formation in proton—D2 collisions

Classical trajectory calculations are used to compute the formation cross section (suitably defined) for strongly interacting collision complexes formed in H⁺ + D₂ collisions in the kinetic energy range from 0.1 to 4 eV. This cross section corresponds to the usual Langevin cross section only if the kinetic energy is less than 0.2 eV, and provided that little initial excitation is present, while for higher kinetic energies it drops exponentially. It is in much better agreement with absolute integral cross sections observed experimentally than the latter. Further study shows that it is the contribution from large orbital anglular momenta, which the Langevin cross section overestimates. Orbiting complexes (of H⁺ around D₂) play a negligible role, and are very short-lived. The lifetime of strongly coupled complexes is estimated to be 450 E^?1.3 fs, where E is the total energy in eV. The use of trajectory data to improve Light's phase space theory is discussed.     

Non-empirical calculations on diatomic transition metals. I. An investigation of various orbital contraction schemes of gaussian basis sets on the SCF level for Zn2

Ab initio SCF LCAO MO calculations with various contracted primitive gaussian basis sets were performed using Zn₂ as an example. When improving the basis set, polarization functions are shown to be more important for a proper reproduction of the repulsive potential curve, than a complete relaxation of the orbital contraction. This is considered to be true for all diatomics although for Zn₂ at larger internuclear distances no varr der Waals minimum is obtained on the SCF level.     

Bimolecular reactions of vibrationally excited molecules. Roaming atom mechanism at low kinetic energies

Quasiclassical trajectory calculations have been performed for the H + H'X(v) → X + HH' abstraction and H + H'X(v) → XH + H' (X = Cl, F) exchange reactions of the vibrationally excited diatomic reactant at a wide collision energy range extending to ultracold temperatures. Vibrational excitation of the reactant increases the abstraction cross sections significantly. If the vibrational excitation is larger than the height of the potential barrier for reaction, the reactive cross sections diverge at very low collision energies, similarly to capture reactions. The divergence is quenched by rotational excitation but returns if the reactant rotates fast. The thermal rate coefficients for vibrationally excited reactants are very large, approach or exceed the gas kinetic limit because of the capture-type divergence at low collision energies. The Arrhenius activation energies assume small negative values at and below room temperature, if the vibrational quantum number is larger than 1 for HCl and larger than 3 for HF. The exchange reaction also exhibits capture-type divergence, but the rate coefficients are larger. Comparisons are presented between classical and quantum mechanical results at low collision energies. At low collision energies the importance of the exchange reaction is enhanced by a roaming atom mechanism, namely, collisions leading to H atom exchange but bypassing the exchange barrier. Such collisions probably have a large role under ultracold conditions. The calculations indicate that for roaming to occur, long-range attractive interaction and small relative kinetic energy in the chemical reaction at the first encounter are necessary, which ensures that the partners can not leave the attractive well. Large orbital angular momentum of the primary products (equivalent to large rotational excitation in a unimolecular reaction) is favorable for roaming.     

Hydrogen-atom production channels of acetaldehyde photodissociation: Direct DFT molecular dynamics study

Direct density-functional (DFT) molecular dynamics (MD) calculations have been carried out for the following two hydrogen-atom production channels in acetaldehyde photodissociation on the lowest triplet-state (T₁) potential energy surface (PES): CD₃CHO → CD₂CHO + D (1) and CD₃CHO → CD₃CO + H (2). The employed DFT method was B3LYP with the cc-pVDZ basis set. The average product hydrogen kinetic energies estimated from the results of the direct DFT MD calculations were 18.3 and 16.6 kcal mol⁻¹ for reactions 1 and 2, respectively, and these were half – two thirds of the previously measured values [T.Y. Kang, S.W. Kang, H.L. Kim, Chem. Phys. Lett. 434 (2007) 6]. This is because of the low reverse barrier heights predicted at the B3LYP/cc-pVDZ level. The present results for the product hydrogen kinetic energies, however, agree qualitatively with the experimental measurements and strongly supports the mechanisms taking place on the T₁ PES.     

棉秆催化热解特性及动力学建模研究

通过不同添加剂处理棉秆的热重实验,分析NaOH、Na₂CO₃、Na₂SiO₃、NaCl、TiO₂、HZSM-5六种添加剂催化棉秆热解动力学特性,结合原料的组分分析,建立三组分独立平行一级反应热解动力学模型对试样热失重行为进行模拟,采用非线性最小平方算法求解热解动力学参数。研究发现,添加剂的加入改变了三组分动力学参数,在碱性添加剂作用下,纤维素和半纤维素热解活化能都有较大程度降低,且碱性越强,纤维素热解活化能越低,而半纤维素热解活化能越高;中性添加剂NaCl对纤维素和半纤维素热解活化能的影响不大;酸性添加剂使纤维素和半纤维素的热解活化能有所增大,但所有添加剂对木质素热解活化能的影响不明显。