Editorial

Stability analysis is suggested as a preliminary step in isothermal flash calculations, and a number of numerical methods for stability analysis based on Gibbs' tangent plane criterion are described. These methods, which are applicable for both single phase and multiphase systems, are developed mainly for Equation of State calculations using a single model for all fluid phases. Special adaptions ensuring convergence in critical regions are discussed.     

Detecting Reaction Pathways and Computing Reaction Rates in Condensed Phase

Methods for the computation of rate constants that characterize classical reactions occurring in the condensed phase are discussed. While microscopic expressions for these transport properties are well known, their computation presents challenges for simulation since reactive events often occur rarely, and the long time scales that are typical for reactive processes are not accessible using simple molecular dynamics methods. Furthermore, the underlying free energy surface is very complex with many saddle points that prevent sampling of possible reaction pathways. As a result, the reaction coordinate may be a complex many-body function of the system’s degrees of freedom. Since there is not an a priori way to define a “good” reaction coordinate, methods are being developed to assist in a systematic construction of a reaction coordinate. These methods are reviewed and examples of non-trivial reaction coordinates are presented.     

A climatology of solar erythema dose

Abstract —Semi-empirical formulas for the ultraviolet erythema dose derived in an earlier paper are used to deduce an ultraviolet photoclimatology. We calculate the climatology of daily erythema radiation doses for the northern hemisphere at 5d? latitude intervals. Similar dose calculations are also performed specifically for ten metropolitan areas. Effects of seasonal and geographic variations of ozone, turbidity, and cloudiness on the local erythema doses are also investigated. We present a simple approximate analytic formula for the annual erythema dose as a function of latitude, cloud cover, and ground albedo for use in connection with studies of the epidemiology of skin cancer. The implications of possible ozone depletion due to a future fleet of supersonic aircraft in the stratosphere are discussed. These calculations are made for a normal ozone thickness of 0.32 cm and for a 5, 10, 20, and 50 per cent ozone reduction.     

Solution of the kinetic equation for a semiconductor with a narrow conduction band

Kinetic coefficients are calculated for a rectangular lattice and for a planar face-centered one. Analytic expressions are derived for the kinetic transport coefficients, which become the usual expressions from the ordinary theory of semiconductors if the bands are broad. The effects of narrow bands are considered.     

Application of power expansion in the thermodynamics of compressible Markovian copolymers

The problem of constructing phase diagrams for a compressible melt of a binary Markovian copolymer is reduced to a set of nonlinear differential equations in partial derivatives with transcendental relationships. Using power expansions, the closed set of nonlinear differential equations is derived. This set allows its further analytical study. Eigenvalues of a linearized system are analyzed, and the boundaries of the thermodynamic stability of melts are defined. Nonlinear equations in normal coordinates are obtained; for symmetric melts, these equations are reduced to a single equation by adiabatic elimination of small-scale variables. Binodal curves are calculated for such solutions of this equation, which correspond to the free energy minimum of melts. Corrections reflecting the effect of melt nonsymmetry are found. The results are applied for copolymers, whose composition is similar to that of homopolymers, diblock copolymers, and random and regularly alternating copolymers. Spinodals and binodals corresponding to microphase separation are constructed.     

Studies on monomer reactivity ratios. I. An electronegativity model

A model based on an electronegativity scheme is proposed for treatment of monomer reactivity ratios in free-radical bulk copolymerization. Values for each monomer are assigned to three parameters: a relative localization (or resonance stabilization) energy, a radical electronegativity, and a monomer electronegativity. Parameters for 17 monomers are given and calculated reactivity ratios are tabulated for a large number of copolymerizations. Agreement with experiment is usually obtained to within experimental error except for systems involving acrylonitrile. Computed parameters are rationalized on the basis of molecular orbital theory.     

On-line separation and preconcentration in flow injection analysis

Various improvements in flow-injection systems involving on-line separation and preconcentration by gas diffusion, ion-exchange and liquid—liquid extraction are reviewed and their merits are discussed. On-line preconcentration systems based on gas diffusion are described for the spectrophotometric determination of cyanide, fluoride and ammonia. Different gas/liquid separation devices for hydride-generation and cold-vapour atomic absorption spectrometry (a.a.s.) are compared. Systems are outlined for on-line column preconcentration of cobalt, nickel, vanadium, beryllium and cerium for inductively-coupled plasma/atomic emission spectrometry, and for combinations of column preconcentration with hydride-generation and cold-vapour a.a.s for the determination of ultratrace selenium and mercury. An on-line liquid—liquid extraction/flame a.a.s. system capable of achieving 60-fold enhancement for lead is reported; the limit of detection is 0.02 μg ml^?.     

Cyclization in reversible and irreversible step-growth polymerizations

Several aspects of cyclization reactions in step-growth polymerizations are considered. The Jacobson–Stockmayer ring-chain equilibrium theory provides the basis for understanding these systems, but there are circumstances where some modifications to the theory are needed. In particular, for ring expansion polymerizations, i.e. those for which there is never a significant fraction of open chains, the statistics are a bit different from those appropriate for ring-chain equilibrium. Also, if the reactions are irreversible the occurrence of rings is significantly different from the reversible case. Rings are more prevalent in irreversible reactions. The implications of these observations for the processing of polymers are briefly considered.     

Fluid dynamics of dilute suspensions and fouling of tubular membrane modules

The role of dilute suspensions in fouling a ultrafilter tubular membrane module is studied in detail for a wide range of wall permeation flux conditions. The inlet flow profiles are assumed to be either uniform (plug flow) or parabolic (fully developed) shape. It is assumed that the particles are neutrally buoyant and the concentrations are so low that it does not influence the fluid flow. Furthermore, the particle-particle interactions and the forces of interaction between the particle and the membrane wall are assumed to be unimportant. The governing equations of motion for the fluid are solved by a finite difference scheme. To compute the particulate fouling, the equations of motion for the particles are solved by the fourth-order Runge-Kutta-Gill method. Results are presented for both hydrodynamics and membrane fouling by dilute suspensions for conditions such as the effect of assumed inlet velocity profiles, and a wide range of wall permeation flux conditions.     

Micro‐thermal analysis of polymers: current capabilities and future prospects

The current state of development of micro‐thermal analysis (micro‐TA) and related techniques are briefly reviewed. Results for a PET/epoxy resin composite and a bilayer polymer film are given as illustrations. Details are given of a new interface that enables the micro‐TA unit to be placed inside a conventional FTIR spectrometer to carry out photothermal IR microscopy. New results are presented for a micro‐pyrolysis‐mass spectroscopy technique. The limitations of the current instrumentation are discussed in terms of the overriding problem being one of spatial resolution. Images obtained using pulsed force mode AFM with a high‐resolution heated tip indicate the scope for future development of this technique. The possibility of even higher spatial resolution with other forms of probe are discussed along with the potential for imaging micro‐pyrolysis time of flight mass spectroscopy and even tomography. It is concluded that these methods offer excellent prospects for characterising a wide range of polymer systems.     

Analysis of kinetic isotope effects for nonadiabatic reactions

Factors influencing the rates of quantum mechanical particle transfer reactions in many-body systems are discussed. The investigations are carried out on a simple model for a proton transfer reaction that captures generic features seen in more realistic models of condensed phase systems. The model involves a bistable quantum oscillator coupled to a one-dimensional double-well reaction coordinate, which is in turn coupled to a bath of harmonic oscillators. Reactive-flux correlation functions that involve quantum-classical Liouville dynamics for chemical species operators and quantum equilibrium sampling are used to estimate the reaction rates. Approximate analytical expressions for the quantum equilibrium structure are derived. Reaction rates are shown to be influenced significantly by both the quantum equilibrium structure and nonadiabatic dynamics. Nonadiabatic dynamical effects are found to play the major role in determining the magnitude of the kinetic isotope effect for the model transfer reaction.     

Statistical mechanical theory for steady state systems. VIII. General theory for a Brownian particle driven by a time- and space-varying force

A Brownian particle subject to a time- and space-varying force is studied with the second entropy theory for nonequilibrium statistical mechanics. A fluctuation expression is obtained for the second entropy of the path, and this is maximized to obtain the most likely path of the particle. Two approaches are used, one based on the velocity correlation function and one based on the position correlation function. The approaches are a perturbation about the free particle result and are exact for weak external forces. They provide a particularly simple way of including memory effects in time-varying driven diffusion. The theories are tested against computer simulation data for a Brownian particle trapped in an oscillating parabolic well. They accurately predict the phase lag and amplitude as a function of drive frequency, and they account quantitatively for the memory effects that are important at high frequencies and that are missing in the simplest Langevin equation.     

Solid-phase microextraction in the analysis of food taints and off-flavors

Selected food taints and off flavors, for which solid phase microextraction (SPME) has been used as a method for volatiles isolation, are the subject of review. Compounds responsible for musty and earthy odor off-flavors and taints in foods are discussed. This group contains haloanisoles, geosmin, and methylisoborneol. Chlorophenols are discussed as precursors of chloroanisoles and compounds impairing the flavor of food. Also described are volatile phenolic compounds responsible for medicinal off flavors, mainly ethyl phenols and vinyl phenols. Sulfur compounds that contribute to off-flavor are also discussed. Finally, a group of volatile compounds being the products of lipid oxidation are summarized. A short review of the formation, occurrence, and information on odor properties of all of these groups of compounds is given. Examples of SPME use for the analysis of compounds belonging to all described groups are shown. Elaboration of method parameters, fiber selection, experimental conditions, and quantitation of compounds are subjects of interest. Also, applications of SPME as a method for introduction of volatiles in mechanical olfaction technologies are shortly outlined.     

Basis set construction for molecular electronic structure theory: natural orbital and Gauss-Slater basis for smooth pseudopotentials

A simple yet general method for constructing basis sets for molecular electronic structure calculations is presented. These basis sets consist of atomic natural orbitals from a multiconfigurational self-consistent field calculation supplemented with primitive functions, chosen such that the asymptotics are appropriate for the potential of the system. Primitives are optimized for the homonuclear diatomic molecule to produce a balanced basis set. Two general features that facilitate this basis construction are demonstrated. First, weak coupling exists between the optimal exponents of primitives with different angular momenta. Second, the optimal primitive exponents for a chosen system depend weakly on the particular level of theory employed for optimization. The explicit case considered here is a basis set appropriate for the Burkatzki-Filippi-Dolg pseudopotentials. Since these pseudopotentials are finite at nuclei and have a Coulomb tail, the recently proposed Gauss-Slater functions are the appropriate primitives. Double- and triple-zeta bases are developed for elements hydrogen through argon. These new bases offer significant gains over the corresponding Burkatzki-Filippi-Dolg bases at various levels of theory. Using a Gaussian expansion of the basis functions, these bases can be employed in any electronic structure method. Quantum Monte Carlo provides an added benefit: expansions are unnecessary since the integrals are evaluated numerically.     

Density functional calculations on electronic circular dichroism spectra of chiral transition metal complexes

Time-dependent density functional theory (TD-DFT) has for the first time been applied to the computation of circular dichroism (CD) spectra of transition metal complexes, and a detailed comparison with experimental spectra has been made. Absorption spectra are also reported. Various Co(III) complexes as well as [Rh(en)(3)](3+) are studied in this work. The resulting simulated CD spectra are generally in good agreement with experimental spectra after corrections for systematic errors in a few of the lowest excitation energies are applied. This allows for an interpretation and assignment of the spectra for the whole experimentally accessible energy range (UV/vis). Solvent effects on the excitations are estimated via inclusion of a continuum solvent model. This significantly improves the computed excitation energies for charge-transfer bands for complexes of charge +3, but has only a small effect on those for neutral or singly charged complexes. The energies of the weak d-to-d transitions of the Co complexes are systematically overestimated due to deficiencies of the density functionals. These errors are much smaller for the 4d metal complex. Taking these systematic errors and the effect of a solvent into consideration, TD-DFT computations are demonstrated to be a reliable tool in order to assist with the assignment and interpretation of CD spectra of chiral transition metal complexes.     

High-performance liquid chromatographic analysis of saponin compounds in Bupleurum falcatum

A mixture of saponin compounds (saikosaponin c, a, and d) in the 70% ethanol extract of a powdered sample of Bupleuri radix are analyzed by an Inertsil ODS-3 C(18) column at a flow rate of 1.0 mL/min and detection wavelength of 203 nm. Well resolved chromatograms of saikosaponin c, a, and d are obtained with a gradient elution of acetonitrile-water from 40:60 (v/v) to 50:50 (v/v). The total time required for a single analysis is approximately 20 min. Calibration curves for saikosaponin c, a, and d are linear up to 2.5 mg/mL. The coefficient of variability values for saikosaponins in the extract are below 4%, and the recoveries for saikosaponin c, a, and d are 95.2 +/- 1.1, 96.5 +/- 0.9, and 96.2 +/- 1.0%, respectively. The changes in saikosaponin contents for a two-year growth of Bupleurum falcatum are measured by the established high-performance liquid chromatography method.     

Does Marcus theory apply to redox processes in ionic liquids? A simulation study

Simulation studies on a model system of a spherical ion with various charges in two imidazolium ionic liquids and in acetonitrile are compared. The average vertical ionisation potentials as a function of the charge on the ion are similar for all three systems. The Landau free energies of each system as a function of the vertical ionisation potential are computed and are close to being parabolic. Results are shown for the solvent reorganisation energies and for the activation free energies. The similarities of all these quantities are interpreted in terms of continuum models. However, the dynamics are likely to be very different in a polar liquid and in an ionic liquid as in the former case screening occurs by reorientation of molecules and in the latter case it occurs by translation of ions.     

Cyclofunctionalisation of unsaturated acids with benzeneselenenyl chloride : Kinetic and thermodynamic aspects of the rules for ring closure

Experimental procedures are described for the synthetically useful reaction by which olefinic acids are converted into lactones carrying a benzeneseleno-group. Data are presented to define some of the mechanistic details of this type of cyclofunctionalisation and kinetic and thermodynamic factors relevant to the Rules for Ring Closure are discussed. A nomenclature is introduced for a treatment of ring-fusion stereochemistry.     

Fluorescence optical sensors for continuous determination of near-neutral pH values

The preparation and performance of two types of optical sensors for continous measurement of near-neutral pH values are described. the sensors are based on glass-immobilized fluorescent pH indicators and allow the determination of pHs in the range 6.4 to 7.7 with a precision of ±0.01 units. Response times are of the order of 1 min for 99% of the total signal change. Adverse effects of ionic strength are almost completely eliminated by appropriate treatment of the glass surface, thereby creating a well-defined and highly charged environment for the indicator. When the sensing layers are attached to the end of a bifurcated fibre optical light guide, a device for remotely sensing pH values is obtained.     

Intralaboratory Quality Control of Chemical Analysis with Reference Materials at Disposal

In this problem-oriented article, regarding the author's experience, a review of the methods of intralaboratory quality control based on systematic analysis of reference samples (control materials) is given. The requirements imposed on reference samples are briefly considered. The principles of selection of an inspection plan and the requirements on the evaluation period of statistical characteristics are formulated. The techniques for graphically representing the results of intralaboratory control and the algorithms for detecting out-of-control analytical processes are considered. Control charts for average values and ranges or charts for cumulative sums and geometric (exponentially weighted) moving averages are recommended as the main or auxiliary graphical representation, respectively. Westgard rules and the simplest rules for ranges are recommended as the main algorithms for revealing out-of-control analytical processes. Multivariate statistical process control methods, which are recommended for use in the cases of a great number of simultaneously determined factors, are briefly discussed.