Valence- and dipole-bound anions of the thymine-water complex: ab initio characterization of the potential energy surfaces

The potential energy surfaces of the neutral and anionic thymine-water complexes are investigated using high-level ab initio calculations. Both dipole-bound (DB) and valence-bound (VB) anionic forms are considered. Four minima and three first-order stationary points are located, and binding energies are computed. All minima, for both anions, are found to be vertically and adiabatically stable. The binding energies are much higher for valence-bound than for dipole-bound anions. Adiabatic electron affinities are in the 66-287 meV range for VB anions and the 4-60 meV range for DB anions, and vertical detachment energies are in the 698-977 meV and 10-70 meV range for VB and DB anions, respectively. For cases where literature data are available, the computed values are in good agreement with previous experimental and theoretical studies. It is observed that electron attachment modifies the shape of the potential energy surfaces of the systems, especially for the valence-bound anions. Moreover, for both anions the size of the energy barrier between the two lowest energy minima is strongly reduced, rendering the coexistence of different structures more probable.     

Extension of semiempirical methods to simulation of surfaces

Various approaches for surface simulation are described. They are based on free, saturated, and embedded clusters, as well as periodic models. The features of semiempirical methods are reviewed and ideas for their improvement are discussed. Special features of the structure and stability of clusters are presented which are suitable for the transition to the solid state. Linear and nonlinear relationships for binding energies and bond lengths in dependence of the average coordination number are discussed. Finally, a systematic way for the simulation of adsorption at ion crystal surfaces by model clusters is suggested. © 1996 John Wiley & Sons, Inc.     

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.     

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.     

Two-dimensional liquid chromatography of diblock copolymers: Simulation at various adsorption interaction conditions

Diblock copolymers, which are heterogeneous in both molar mass and composition, can be fully characterized by using two-dimensional chromatography. Since the size-exclusion, the adsorption, and the critical interaction based modes of chromatography are possible for each of the polymers A and B, this leads to a variety of options for 2D-chromatography of copolymers AB. Using the theory of chromatography of block copolymers, 2D-chromatograms are simulated that correspond to the most interesting of these options. Orthogonal 2D-chromatograms are expected, if in the 1st dimension the critical condition is created for A, while in the 2nd dimension – for B. The situations, where A and B are both adsorbable, as well as those where the conditions of adsorption for A and SEC for B are created, are also considered. In particular, it is shown that the 2nd dimension combination of the critical condition for A and SEC – for B is preferable than that with SEC condition for both A and B. The simulated 2D-chromatograms of low- and high molar mass diblock copolymers, as well as of copolymers with one short block are compared with the reported real ones; it is concluded that the corresponding virtual and real 2D-chromatograms are qualitatively very similar.     

ThermoData Engine (TDE) software implementation of the dynamic data evaluation concept. 7. Ternary mixtures

ThermoData Engine (TDE) is the first full-scale software implementation of the dynamic data evaluation concept, as reported in this journal. The present paper describes the first application of this concept to the evaluation of thermophysical properties for ternary chemical systems. The method involves construction of Redlich-Kister type equations for individual properties (excess volume, thermal conductivity, viscosity, surface tension, and excess enthalpy) and activity coefficient models for phase equilibrium properties (vapor-liquid and liquid-liquid equilibrium). Constructed ternary models are based on those for the three pure component and three binary subsystems evaluated on demand through the TDE software algorithms. All models are described in detail, and extensions to the class structure of the program are provided. Reliable evaluation of properties for the binary subsystems is essential for successful property evaluations for ternary systems, and algorithms are described to aid appropriate parameter selection and fitting for the implemented activity coefficient models (NRTL, Wilson, Van Laar, Redlich-Kister, and UNIQUAC). Two activity coefficient models based on group contributions (original UNIFAC and NIST-KT-UNIFAC) are also implemented. Novel features of the user interface are shown, and directions for future enhancements are outlined.     

Extracellular vesicles based electrochemical biosensors for detection of cancer cells: A review

Extracellular vesicles (EVs) derived from cancer cells are considered as ideal biomarker for liquid biopsy in cancer diagnosis, and are stable and abundant. Electrochemical methods for the detection of EVs are preferred over conventional methods such as Western blotting and enzyme-linked immunosorbent assay for their high sensitivity and real-time detection. This article summaries studies proposing the electrochemical methods utilizing immunological and molecular methodologies for detecting EVs derived biomacromolecules such as miRNAs and transmembrane protein for cancer diagnosis. Moreover, the electrochemical detection methods are compared and future prospects for the development of electrochemical methods for EVs detection are concluded.     

Modification of PSRK mixing rules and results for vapor–liquid equilibria, enthalpy of mixing and activity coefficients at infinite dilution

The predictive Soave–Redlich–Kwong (PSRK) equation of state (EOS) is a well-established method for the prediction of thermodynamic properties required in process simulation. But there are still some problems to be solved, e.g. the reliability for strong asymmetric mixtures of components which are very different in size. The following modifications are introduced in the PSRK mixing rules: the Flory–Huggins term in the mixing rule for the EOS parameter a, and the combinatorial part in the UNIFAC model are skipped simultaneously; a nonlinear mixing rule for the EOS parameterb, instead of the linear mixing rule, is proposed. With these two modifications better results are obtained for vapor–liquid equilibria and activity coefficients at infinite dilution for alkane–alkane systems, specially for asymmetric systems. In order to obtain better results for enthalpy of mixing, temperature-dependent parameters are used. Group interaction parameters have been fitted for several groups, and the results are compared with the Modified UNIFAC (Dortmund), and the PSRK methods.     

First-principles investigations of the polarizability of small-sized and intermediate-sized copper clusters

Density functional theory calculations are used to compute the dipole polarizabilities of copper clusters. Structures for the clusters are taken from the literature for n = 2-32 and several isomers are used for each cluster size for n < or = 10. The calculated polarizabilities are in good agreement with the prediction of a simple jellium model, but much smaller than experimental observations for n = 9-32 [M. B. Knickelbein, J. Chem. Phys., 120, 10450 (2004)]. To investigate this difference, the calculated polarizabilities are tested for the effects of basis set, electron correlation, and equilibrium geometry for small-size clusters (n = 2-10). These effects are too small to account for the theory-experiment gap. Temperature effects are also studied. Thermal expansion of the clusters leads to very small changes in polarizability. On the other hand, the presence of permanent dipoles in the clusters could account for the experimental observations if the rotational temperature of the clusters were sufficiently low. The potential importance of the cluster dipole moments implies that reliable ground-state structures and experimental temperatures are needed to find quantitative agreement between calculated and observed polarizabilities.     

Assessment of Contemporary Theoretical Methods for Bond Dissociation Enthalpies

The density functional theory (DFT) is the most popular method for evaluating bond dis-sociation enthalpies (BDEs) of most molecules. Thus, we are committed to looking for alternative methods that can balance the computational cost and higher precision to the best for large systems. The performance of DFT, double-hybrid DFT, and high-level com-posite methods are examined. The tested sets contain monocyclic and polycyclic aromatic molecules, branched hydrocarbons, small inorganic molecules, etc. The results show that the mPW2PLYP and G4MP2 methods achieve reasonable agreement with the benchmark values for most tested molecules, and the mean absolute deviations are 2.43 and 1.96 kcal/mol after excluding the BDEs of branched hydrocarbons. We recommend the G4MP2 is the most appropriate method for small systems (atoms number ≤20); the double-hybrid DFT methods are advised for large aromatic molecules in medium size (20 ≤atoms number ≤50), and the double-hybrid DFT methods with empirical dispersion correction are recommended for long-chain and branched hydrocarbons in the same size scope; the DFT methods are advised to apply for large systems (atoms number ≥50), and the M06-2X and B3P86 methods are also favorable. Moreover, the di erences of optimized geometry of different methods are discussed and the effects of basis sets for various methods are investigated.     

Quantitative infrared intensity studies of vapor-phase glyoxal, methylglyoxal, and 2,3-butanedione (diacetyl) with vibrational assignments

Glyoxal, methylglyoxal, and 2,3-butanedione (diacetyl) are all known biomass burning effluents and suspected aerosol precursors. Pressure-broadened quantitative infrared spectra of glyoxal, methylglyoxal, and diacetyl vapors covering the 520-6500 cm(-1) range are reported at 0.112 cm(-1) resolution, each with a composite spectrum derived from a minimum of 10 different sample pressures for the compound, representing some of the first quantitative intensity data for these analytes. Many vibrational assignments for methylglyoxal are reported for the first time, as are some near-IR and far-IR bands of glyoxal and diacetyl. To complete the vibrational assignments, the far-infrared spectra (25-600 cm(-1)) of all three vapors are also reported, those of methylglyoxal for the first time. Density functional theory and ab initio MP2 theory are used to help assign vibrational modes. Potential bands for atmospheric monitoring are discussed.     

Inorganic and methylmercury speciation in environmental samples

The different strategies for mercury species analysis in environmentally-related samples are reviewed. After consideration of the main different steps involved in the speciation of mercury, such steps are discussed with more extension for mercuric ion and methylmercury. The different approaches for preservation of these mercury species during the storage of samples are considered. Different ways for the extraction of mercury species from the several possible environmental compartments and the possibilities for preconcentration of such species after previous derivatization reactions are discussed. Mercuric ions and methylmercury chromatographic and non-chromatographic separations along with different techniques used for sensitive and selective detection of mercury are also critically reviewed. Ranges of published detection limits achievable for such species determination, by using hyphenated techniques between a chromatographic separation and a specific atomic detector are also given.     

Radiochemical determination of As,Cr, Mo,Sb and Se in foods

A radiochemical neutron activation analysis procedure was applied to the analysis of foods for As, Cr, Mo, Sb and Se. The elements are retained on an inorganic chromatographic resin of hydrated managanese dioxide while interfering elements are removed. Activities from Br, K, Na and P interferences are reduced by up to six orders of magnitude, and detection limits for food analysis are reduced by factors of 100–2000, compared with those normally found for instrumental neutron activation analysis. Detection limits for the foods analyzed are 0.1–0.3 ng/g for As and Sb and 1–10 ng/g for Cr, Mo and Se. Results are presented for various foods and several National Bureau of Standards Standard Reference Materials.     

A perturbative calculation of the rovibrational energy levels of methane

The rovibrational energy levels of methane are determined from a quartic ab initio potential energy force field where the expansion coordinates are the Morse coordinates for the stretches and extension coordinates for the bends. Energies are calculated using canonical Van Vleck perturbation theory. Results are obtained for both rotation-vibration Hamiltonians expressed as functions of curvilinear and rectilinear normal coordinates. Second, fourth, and sixth order curvilinear results are compared with experimental results, and fourth order results for the rectilinear and curvilinear Hamiltonian are compared to each other. The calculated rovibrational levels are in good agreement with the experimental values for low J levels. The calculated rotational level splittings are in even better agreement with the experiment. In particular, the ground state tetrahedral splittings, which are as small as 10(-4) cm(-1), are well reproduced by our calculations at sixth order.     

Strategies for detecting ecotoxicological effects of biodegradable polymers in agricultural applications

Biodegradable is the long awaited and highly accepted property for materials (polymers) used in agricultural applications. Methods for determining biodegradability and material disintegration are established and already in use for routine analysis. Methods for analysing ecotoxic effects caused by biodegradable materials are neither established nor in routine use. In the past, biotests have been developed and optimised solely for investigations of single chemicals. Such tests are applicable even if they are not validated for the detection of undesired ecotoxicological effects deriving from biodegradation of polymers in soil. Since the biodegradation process does have an influence on the physical, chemical and biological status of the soil matrix, both biotests and chemical analysis are required in many cases. Theory, background data from method development and some results are presented.     

Performance of the spin-flip and multireference methods for bond breaking in hydrocarbons: a benchmark study

Benchmark results for spin-flip (SF) coupled-cluster and multireference (MR) methods for bond-breaking in hydrocarbons are presented. The nonparallelity errors (NPEs), which are defined as an absolute value of the difference between the maximum and minimum values of the errors in the potential energy along bond-breaking curves, are analyzed for (i) the entire range of nuclear distortions from equilibrium to the dissociation limit and (ii) in the intermediate range (2.5-4.5 A), which is the most relevant for kinetics modeling. For methane, the spin-flip and MR results are compared against full configuration interaction (FCI). For the entire potential energy curves, the NPEs for the SF model with single and double substitutions (SF-CCSD) are slightly less than 3 kcal/mol. Inclusion of triple excitations reduces the NPEs to 0.32 kcal/mol. The corresponding NPEs for the MR-CI are less than 1 kcal/mol, while those of multireference perturbation theory are slightly larger (1.2 kcal/mol). The NPEs in the intermediate range are smaller for all of the methods. The largest errors of 0.35 kcal/mol are observed, surprisingly, for a spin-flip approach that includes triple excitations, while MR-CI, CASPT2, and SF-CCSD curves are very close to each other and are within 0.1-0.2 kcal/mol of FCI. For a larger basis set, the difference between MR-CI and CASPT2 is about 0.2 kcal/mol, while SF-CCSD is within 0.4 kcal/mol of MR-CI. For the C-C bond breaking in ethane, the results of the SF-CCSD are within 1 kcal/mol of MR-CI for the entire curve and within 0.4 kcal/mol in the intermediate region. The corresponding NPEs for CASPT2 are 1.8 and 0.4 kcal/mol, respectively. Including the effect of triples by energy-additivity schemes is found to be insignificant for the intermediate region. For the entire range of nuclear separations, sufficiently large basis sets are required to avoid artifacts at small internuclear separations.     

The optical properties of some alkali metal tungsten bronzes from 0.1 to 38 eV

The optical properties of the alkali metal tungsten bronzes are investigated. A review of existing data and the interpretation thereof is presented. Our absorptivity or reflectivity data for two cubic Na bronzes and for a tetragonal K bronze in the energy range 0.1 to 38 eV are displayed; the data are Kramers-Kronig analyzed and the optical constants are obtained. When feasible, comparisons are made with the existing data for the bronzes as well as for ReO₃, which is structurally similar. The interpretation of optical constants is based on an existing band structure for ReO₃ and modifications thereto are suggested.     

Incorporating the geometric phase effect in triatomic and tetraatomic hyperspherical harmonics

Hyperspherical harmonics in the democratic row-orthonormal hyperspherical coordinates are very appropriate basis sets for performing reactive scattering calculations for triatomic and tetraatomic systems. The mathematical conditions for incorporating the geometric phase effect in these harmonics are given. These conditions are implemented for triatomic systems, and their explicit analytical expressions in terms of Jacobi polynomials, in both the absence and presence of the geometric phase effect, are given.