An improved model of the fringing fields of a quadrupole mass filter

The fringing field region of a quadrupole mass filter is modelled using an iterative finite difference technique to solve Laplace's equation for the electrostatic potential. The results are used to formulate an expression f(z) such that the electrostatic potential of the fringing field, F(x, y, z, t) may be written in the form F(x, y, z, t) = f(z) φ(x, y, t), where φ(x, y, t) is the hyperbolic field of the quadrupole. The function f(z) is expressed in the form f(z) = 1 - exp {- az - bz²}), where a and b are constants and z is in units of r_O, where 2r_O is the spacing of the quadrupole rods. The effect of the distance from the quandrupole rods to the end plate of the quadrupole mass filter, d on f(z) is investigated and the results presented show that for d < 0.125 r_O the function f(z) does not alter significantly.     

A master equation theory of excitonic annihilation phenomena for finite latices of low dimensionality

A master equation is written for the equation of motion for the vector p(t), in which the nth component corresponds to the probability of observing n excitons on a lattice with L sites. The dependence of the properties of p(t) on the magnitude of L, the intensity of excitation, the lattice dimensionality, and the magnitude of the biexcitonic annihilation rate is discussed. The dependence of the biexcitonic annihilation rate on the exciton density is discussed in the context of a simple simulation of exciton annihilation. The applicability of these ideas to excitonic annihilation in polymers is discussed.     

Some difficulties in the application of the concept of density of states

The problem of defining N(E), the density of states, for molecular systems is discussed. It is shown that a numeric evaluation of N(E) is often only approximate, even for continuous data, and is not well defined for quantized systems. The application of the concept of density of states is discussed, particularly with regard to the RRKM theory of unimolecular reactions. The sum of states, W(E), and density of states curves are evaluated for several harmonic and anharmonic model systems and the results discussed in order to illustrate the foregoing considerations.     

Two-photon ionization of an inner shell electron of the Cl atom

We have calculated, using second order perturbation theory, the two photon ionization cross section of a K-shell electron of chlorine forE=1.6 keV incident photons. Two classes of intermediate states must be considered, those in which a 1s electron moves to an emptyp-orbital, and those in which an electron from an occupiedp-orbital moves into the continuum. The first class of intermediate states is followed by the ejection of ap-electron into the continuum. The second class of intermediate states is followed by the transfer of a 1s electron into an emptyp-state. The largest contribution comes from 3p →d-continuum followed by 1s → 3p transition. Our result is σ⁽²⁾/I=2.06×10⁴¹ cm⁴/W where σ⁽²⁾ is the two photon ionization cross-section andI is the light intensity.     

Experimental study of short- and long-time diffusion regimes of spherical particles in carboxymethylcellulose solutions

The behavior of latex particles in carboxymethylcellulose (CMC) solutions with different probe radii and solution concentrations is investigated. The diffusion processes of probe particles are studied by means of dynamic light scattering experiments. The existence of two diffusion regimes (a short-time regime and a long-time regime) is established for concentrations higher than the threshold value which changes for each probe dimension. Information is also provided about the relative weight of the probe population undergoing the two diffusion processes. This experimental evidence is discussed in connection with the existence of a defined volume (cage) in which short-time diffusion is the characteristic motion, while the long-time diffusion process dominates the motion over all the cages. Furthermore, there was found to be some deviation from Stokes-Einstein behavior.     

Compositions and structures of nanoparticles of tetrahedral symmetry

The formulas for calculation of the composition of nanoparticles with symmetry group T _d are reported. The numbers of atoms in the shells of tetrahedral symmetry are determined by three structurally invariant numbers and the “quantum number” of the group order n. Four classes of all possible nanostructures with symmetry group T _d are revealed: C _{? + 12z} , where z = 0, 1, 2, ..., and C _? is C ₄, C ₆, C ₈, C ₁₀, or C ₁₄. The sum rule for the coordination numbers of the shell sites located on the axes of symmetry is obtained. The six-shell periodicity of the number of occupied sites located on the symmetry axes and in symmetry planes is revealed.     

Influence of the cavity size of cyclodextrins on the photochromism of azoimidazoles

1-Alkyl-2-(arylazo)imidazole (RaaiR^/) exists in trans configuration about the –NN- bond. Upon optical excitation in UV region the trans-configuration isomerises to cis-RaaiR^/. The photochromism is very susceptible to internal substituents and external environment like solvent polarity, viscosity and presence of innocent foreign species. In this work, the trans-to-cis photoisomerisation of RaaiR^/has been studied in DMF solution of cyclodextrin (α/β/γ-CD). The rate of trans-to-cis isomerisation is decreased in presence of CD compared to native form of RaaiR^/. The quantum yield of the photoisomerisation is decreased by 35–55% in inclusion phase, [email protected]^/, than free photochrome and follows the rate sequence: free state > γ-cyclodextrin > β-cyclodextrin > α-cyclodextrin. The cis-to-trans isomerisation proceeds slowly in visible light irradiation while it is appreciably fast with increasing temperature. The activation energy (E_a) of cis → trans thermal isomerisation is also diminished compared to free state of photochrome. The absorption spectral studies have been used in case of Pai-C₁₈H₃₇ with β-CD and inclusion constant is K_b ?= ?0. 121 M^?1. The ¹H NMR spectral measurement also suggests interaction of aryl protons with cavity protons of β-CD. DFT computation is also attempted to examine the inclusion of RaaiR^/with CD and the negative values of binding energy show that the process of inclusion is spontaneous and complexes formed are stable.     

Investigation of the exo and endo isomers of dioxolane-type benzylidene derivatives of carbohydrates by 13C NMR spectroscopy

The absolute configurations of nine 2,3-O-benzylidene-α-L-rhamno- and α-D-mannopyranoside diasteteomeric pairs were determined and the ¹³C NMR spectra of further thirteen α-L-rhamno- and α-D-mannopyranosides, having various substituents, were completely assigned.Four ¹³C shifts were found suitable for the determination of the absolute configuration of the dioxolane skeleton. (1) The chemical shift of the acetal carbon in the endo isomers is between 103.9 and 104.7 ppm whereas for the exo isomers this region extends from 102.8 to 103.4 ppm; (2) The formation of the dioxolane ring causes a deshielding effect for the bridgehead carbons, in the exo isomers this effect is more pronounced for C-3 whereas in the endo isomers for C-2. For C-4, shielding effect was found in the exo isomers and deshielding effect in the endo ones; (3) The chemical shift of the quaternary carbon of the phenyl group is greater in the exo isomers than in the endo ones; (4) The difference between the shift of the acetal carbon and that of the quaternary carbon of the phenyl group in the exo isomers is greater than 35.4 ppm, in the endo isomers is less than 33.7 ppm.     

On the effect of the porosities of microporous membranes on their ionic selectivities

In order to help clarify the correlation between the structure and the transport parameters in the microporous membranes mentioned in this paper, the effect of porosity upon ionic selectivity is examined for Nucleopore®, Millipore® and Pall® membranes. These membranes separate two NaCl solutions of concentrations c₁ (constant for all the experiment,) and c₂ (fixed for each experiment, c₂ >c₁). The concentrations are small enough to take the place of the activities in the transport equations. The temperature is 298.0 ± 0.1 K and the stirring speed used to homogenize the solutions is 50 rpm.The ionic selectivity is characterized, in the steady state, by an experimental parameter K, which is obtained from measurements of the membrane potential as a function of the relative saline concentration n_c = c₂/c₁ (with c₁ being constant). This parameter K makes it possible to obtain the relative ionic permeability n_p = P_-/P₊ and the transport numbers. The relation between K, n_p and t₊ or t_- is obtained by integrating the Nernst-Planck equations and making use of the Goldman hypothesis of constant electric field. The porosity θ, expressed in percents of the empty volume over the total volume of the membrane, is measured by means of a pycnometric method which is explained in this paper. From the values of K and θ, obtained for the membranes and experimental conditions of these experiments, it can be concluded that ionic selectivity increases linearly with porosity.     

Melting behaviour of a series of monoamides

The melting behaviour of the monoamides of general formula (n-C_pH_2p+1)CONH-(n-C_qH_2p+1) is investigated by differential scanning calorimetry (DSC) and infrared spectroscopy. The lower values of melting entropies, compared to those for linear hydrocarbons with the same number of conformationally flexible chain bonds, is attributed to a reduction in the number of conformations available to the hydrocarbon portion of the molecule, because of the large amount of hydrogen bonding maintained in the melt. The melting behaviour of the monoamides is compared with that of the diamides discussed in a previous paper [1]. The persistence of a network of hydrogen bonds in the melt of the diamides reduces the conformational freedom of the chain segments more than for the monoamides.     

Extraction of Quinolizidine Alkaloids in Non Aqueous Basic Conditions: The Case of Spartium junceum Flowers

In this work the use of basic nonaqueous conditions is suggested as a general method for the extraction of quinolizidine alkaloids. The extraction of quinolizidine alkaloids from fresh flowers of Spartium junceum is performed in ethanolic basic conditions. The procedure is efficient and selective. The extract is essentially formed by N-formylcytisine, N-methylcytisine, cytisine and anagyrine. The method allows a high recovery of N-formylcytisine which becomes the predominant alkaloid in S. Junceum flowers.     

Effect of the solvent on the properties of solutions of polysulfone-polyethersulfone blends

The effect of the solvent on the properties of concentrated solutions of polysulfone-polyethersulfone blends, which can be used to produce film and fibrous materials, is studied. It is found that equiconcentrated solutions of polysulfone-polyethersulfone blends, at the same ratio of polymers in N,N-dimethylacetamide and N-methyl-2-pyrrolidone, significantly differ in structure and rheological and optical properties. Solutions in N,N-dimethylacetamide are polydisperse emulsions, while solutions in N-methyl-2-pyrrolidone are systems composed of closely sized interpenetrating fields of polysulfone and polyethersulfone solutions, in which the dispersed phase and the dispersion medium cannot be detected. The turbidity-spectrum method is used to show that, after the introduction of the second (incompatible) polymer, the average size of supramolecular particles significantly increases in N,N-dimethylacetamide and hardly changes in N-methyl-2-pyrrolidone relative to the particle sizes in solutions of the individual polymers. It is determined that these differences results from the different thermodynamic qualities of N,N-dimethylacetamide and N-methyl-2-pyrrolidone with respect to polysulfone.     

Interpretation of static and dynamic responses of a dissolved oxygen electrode in viscous broths

The response of an amperometric oxygen electrode is studied theoretically and experimentally for the case of significant liquid film resistance at the outer side of the membrane. The behaviour of the probe in a gas stream is predicted by using a transfer function which involves an electrode model including oxygen diffusion within the electrode. The effects of the liquid film which exists at the membrane surface when dissolved oxygen concentrations are measured, are taken into account by modifying the transfer function. The new expression obtained is used to model the step response of the probe in a 2-1 stirred tank fermentor filled with water or xanthane solutions at different concentrations. First, the results are used to correct automatically the steady-state voltage readings of the probe. Secondly, the probe transfer function is used to evaluate k₁a by dynamic measurements: the response to a step change in the gas concentration is transformed via the fast Fourier transform algorithm and k₁a is identified in the Fourier domain by a Gauss-Newton algorithm. Data acquisition, Fourier transform and k₁a identification are implemented on-line on a HP-87 computer. This method of obtaining k₁a values appears to be a generalized moment method. It is shown that it is necessary to consider the liquid film dynamics around the probe in the actual fermenting conditions to evaluate k₁a successfully.     

Evaluation of the Static Permittivity of Aqueous Electrolytes

The static (relative) permittivity of aqueous electrolyte solutions at 25 °C, ε(ω = 0) where ω is the frequency of the external field, has been obtained from the literature. The limiting dielectric decrements δ = ? lim(c → 0)dε/dc, where c is the electrolyte concentration (≤ 1 mol·dm^?3), are tabulated. The most appropriate additive individual ionic δ _i values are derived. The effect of ion pairing in the solutions is briefly discussed.     

The drag of the row of parallel chains of spherical particles in the Stokes flow

The drag of thin-layered porous deposit consisting of dendrites of identical spherical particles with respect to the flow of viscous incompressible liquid is calculated. The deposit is approximated by a model system, a row of parallel chains of particles oriented perpendicular to a flow direction. The expression is derived for the dimensionless drag force acting on the unit chain length as a function of the ratio of a particle radius to a half-distance between chain axes, a/h. It is shown that, at a/h < 0.5, the hydrodynamic equivalent of the chains is the smooth cylinder whose radius is 1.16 times smaller than the particle radius that agrees with the experiment. It is also shown that, at a/h = 1, the drag force of a particle contacting with four adjacent particles in the layer with square packing is equal to F = 44F _St, where F _St is the Stokes drag force of a spherical particle. The pressure drop in this single layer is by 3.5% higher than in the layer of spherical particles with cubic packing. At a/h = 2/√3, drag force F of the particle contacting with six particles in a single layer with hexagonal packing is equal to 340F _St.     

Acoustic method of measuring critical properties of thermally unstable substances

An acoustic method of measuring critical temperatures and pressures of thermally unstable substances is described. In the cell with the liquid under investigation there is a wire probe, 0.02 mm in diameter, which is heated by electric-current pulses with a duration of 0.025 to 0.25 ms. At the moment of liquid boiling-up in the layer that surrounds the probe, one can observe an acoustic wave, which is registered by a ceramic piezoelectric element on a base (PbZrO₃+PbTiO₃). The probe temperature at the moment of boiling-up is determined from its resistance. With an increase of the pressure in the cell with the substance under investigation the amplitude of an acoustic signal decreases. The cessation of boiling-up and disappearance of the acoustic signal show that the critical state is reached. A more precise critical pressure is found by linear extrapolation of the amplitude of the acoustic signal to zero in an amplitude–log (pressure) plot. The paper reports the experimental critical temperatures T_c and the critical pressures p_c of reference substances: n-hexane, n-heptane, n-decane, benzene, and of thermally unstable n-alkanes: docosane and tetracosane, and also the critical curve of the system n-hexadecane–benzene. The critical constants of reference substances were measured by this acoustic method with an error that does not exceed 0.015 T_c and 0.015 p_c.     

Magnetic quenching of positronium in organic solutions

The magnetic quenching of ortho-positronium (o-Ps) in some pure nonpolar liquids (n-hexane, cyclohexane and benzene) and solutions (nitrobenzene in n-hexane, cyclohexane and benzene; and carbon tetrachloride and biphenyl in n-hexane) is examined for steady magnetic fields up to 14 kG by the positron annihilation lifetime technique. The long lifetime of o-Ps is very sensitive to the strength of the external magnetic field, decreasing as the field strength increases. This effect follows from a well-known principle of atomic physics, the quadratic Zeeman effect. With one exception, all the liquids studied here appear to be normal in this regard. The exception is nitrobenzene in n-hexane, which shows a very marked enhancement of quenching beyond the Zeeman effect at low fields.     

Integration of the rate constant and linearization of the kinetic equations in non-isothermal reaction kinetics

The theory of reaction rates gives approximations of the type A T^b e^?E/RT for rate constants. In thermal analysis the case where b = 0 is used in kinetic calculations. However, if b is regarded as a variable, treatment of the non-isothermal kinetic equations is not more complicated than in the special case of b = 0. In this paper Padé approximations, described in the literature of the special functions, and Legendre's continued fractions are proposed for evaluation of the A∫T^b e^?E/RT dT integrals. The Coats—Redfern types methods for the determination of the kinetic parameters are discussed analysing the errors of approximation involved in their deduction. On this basis a modified parameter estimation scheme is proposed.     

Fine structure and transition probabilities of the 2p 5 nl bound excited states of Ne atom

We present a semi-empirical calculational procedure for thep ⁵ nl bound excited states of rare gas atoms based on the use of an LS-dependent orbital for an excitednl electron outside a frozen Hartree-Fockp ⁵ core. The spin-orbit interaction is accounted for approximately using a localized Hartree potential. The contribution from the long-range core polarization is estimated by using a parametrized potential. A model potential is also introduced to represent partially the short-range multi-electron interactions for thep ⁵ np levels. The energy levels are calculated by diagonalizing the Hamiltonian matrix following anLS →jl transformation. The calculated fine structure of the Nep ⁵ ns,p ⁵ np,p ⁵ nd, andp ⁵ nf ¦ (jl)KJ〉 levels are in close agreement with the observed level splittings. The transition probabilities are also in agreement with earlier theoretical and experimental results.     

Kinetics and mechanism of the liquid-phase oxidation of tert-butyl phenylacetate

The oxidation of tert-butyl phenylacetate in ortho-dichlorobenzene at 140°C occurs with short chains. The primary nonperoxide reaction products (tert-butyl α-hydroxyphenylacetate, tert-butyl α-oxophenylacetate, and benzaldehyde) are formed by the decomposition of a hydroperoxide (tert-butyl α-hydroperoxyphenylacetate) and (or) by the recombination of peroxy radicals with and without chain termination. Benzaldehyde and tert-butyl α-hydroxyphenylacetate undergo radical chain oxidation in a reaction medium to result in benzoic acid and tert-butyl α-oxophenylacetate. Homolytic hydroperoxide decomposition is responsible for process autoacceleration and results in benzaldehyde, which is also formed from hydroperoxide by a nonradical mechanism, probably, via a dioxetane intermediate. Both of the reactions are catalyzed by benzoic acid. Benzoic acid has no effect on hydroperoxide conversion into tert-butyl α-oxophenylacetate, which most likely occurs as a result of hydroperoxide decomposition induced by peroxy radicals. The rate constants of the main steps of the process and kinetic parameters have been calculated by solving an inverse kinetic problem.