Congested molecules. Where is the steric repulsion? An analysis of the electron density by the method of interacting quantum atoms

The computed electron density of several congested saturated hydrocarbons and halogenated derivatives has been analyzed by the method of interacting quantum atoms (IQA). For all the molecules studied, the calculations show the existence of a bond path between the congested atoms and which, according to the Quantum Theory of Atoms in Molecules, indicates that there is a stabilizing interaction between these atoms. The bond path is found to exist up to interatomic distances well‐beyond the sum of the van der Waals radii. The IQA results indicate that steric hindrance is not a repulsive force between the congested atoms but that is the result of an increase in the intra‐atomic or self‐energy of the congested atoms. This increase in self‐energy is caused by the deformation of the atomic basin of the congested atoms. © 2013 Wiley Periodicals, Inc.     

Evaluation of two sample treatment methodologies for large-scale pesticide residue analysis in olive oil by fast liquid chromatography–electrospray mass spectrometry

In this study, a comprehensive evaluation of two simple sample treatment methodologies has been carried out for the development of large-scale multi-residue methods for pesticide testing in olive oil. The proposed methodologies are based on (a) liquid–liquid partitioning with acetonitrile followed by dispersive solid-phase extraction clean-up using graphitized carbon black, primary-secondary amine and C₁₈ sorbents; (b) liquid partitioning with acetonitrile saturated with petroleum ether followed by matrix solid-phase dispersion (MSPD) using aminopropyl as sorbent material and a Florisil cartridge for final clean-up in the elution step. To evaluate the proposed sample treatment methodologies, 105 representative multi-class pesticides were studied using fast liquid chromatography–electrospray time-of-flight mass spectrometry (LC–TOFMS). For validation purposes, recoveries studies were carried out at 10 and 100 μg kg⁻¹ levels, yielding recovery rates in the range 70–130% for 72% of analytes using liquid–liquid procedure and for 57% analytes using MSPD procedure. The LC–MS method provided good linearity, precision and accuracy. The limits of detection obtained were lower than 10 μg kg⁻¹ for more than 85% analytes using both sample treatment methodologies. In addition, minor matrix effects (i.e. signal suppression or enhancement ≤20%) were observed in ca. 70% of the studied compounds. Data obtained shows that both sample treatment methodologies proposed can be successfully applied for large-scale pesticide testing in olive oil samples, showing the ability to quickly detect trace amount of over one hundred target species with different physicochemical properties, without requiring expensive instrumentation for sample treatment step and involving relatively low amounts of solvent consumption and waste generation.     

Direct observation of the “special” salt effect in the dynamics of transient ion pairs by time-resolved spectroscopy

Intimate ion pairs [A⁺,T⁻] are generated by specific excitation of the charge transfer band of the electron donor—acceptor complex of anthracene and tetranitromethane. Time-resolved spectroscopy in three distinct time regimes (ps, ns and μs) reveals the presence of a second “loose” or solvent-separated ion pair [A⁺//T⁻], the dynamic behavior of which is strongly modulated by added salts. The “special” salt effect is quantitatively evaluated, together with all the rate constants involved in ion-pair dynamics.     

A “phantom bubble” model for the distribution of free volume in polymers

A new approach to investigate free volume in atomistic simulation was devised. The atoms in the structure are represented by hard spheres. A phantom bubble is defined as an empty sphere, which contacts four or more hard spheres of atoms simultaneously in 3 dimensional space and does not overlap with any atom in the structure. Phantom bubbles are only allowed to overlap with other phantom bubbles. For the purpose of illustration, phantom bubbles were constructed in a fully atomistic structure. An amorphous polyethylene was prepared in a periodic box having the dimension of 28.2×28.2×28.2 Å at 293 K and a density of 0.83 g/cm³. There were two fully atomistic polymeric chains (each C₄₀₀H₈₀₂) in the box. All the atoms in the system were assumed to be hard spheres, usually with 89% of their van der Waals radii. Larger and smaller radii were also considered. The size distribution of phantom bubbles in this system was calculated and the bubbles had a median radius of 0.8 Å. Small changes in the radii used for the atoms have little effect on the shape of the distribution function.     

Evaluation of potential reaction mechanisms leading to the formation of coniferyl alcohol α-linkages in lignin: a density functional theory study

Five potential reaction mechanisms, each leading to the formation of an α-O-4-linked coniferyl alcohol dimer, and one scheme leading to the formation of a recently proposed free-radical coniferyl alcohol trimer were assessed using density functional theory (DFT) calculations. These potential reaction mechanisms were evaluated using both the calculated Gibbs free energies, to predict the spontaneity of the constituent reactions, and the electron-density mapped Fukui function, to determine the most reactive sites of each intermediate species. The results indicate that each reaction in one of the six mechanisms is thermodynamically favorable to those in the other mechanisms; what is more, the Fukui function for each free radical intermediate corroborates with the thermochemical results for this mechanism. This mechanism proceeds via the formation of two distinct free-radical intermediates, which then react to produce the four α-O-4 stereoisomers.     

Study of properties of catalysts for the oxidation of carbon black prepared by “ceramic” synthesis and by pyrolysis of polymeric salt compositions

Samples of catalysts of the composition La_1?x Cs _x VO_4?y were obtained by the standard “ceramic” technology and by pyrolysis of polymeric salt compositions. Comparative measurements of specific surface area and catalytic activity of the samples in reactions of carbon black and carbon oxide oxidation were carried out.     

An interlaboratory study of quantitation procedures for the analysis of water and wastewater for organo-chlorine pesticides by gas chromatography–electron capture detector

An interlaboratory study was conducted to assess two widely used procedures for estimating quantitation levels. Six laboratories participated in the analysis of artificially prepared water samples for organo-chlorine compounds by liquid-liquid extraction followed by gas chromatography–electron capture detector using USEPA Method 608. The study consisted of three phases, including six months of results from analyte free samples, the replicate analysis of fortified samples at a single concentration by the laboratory, and finally the analysis of blind fortified samples prepared by a third party. Estimated detection and quantitation limits (Currie's L_C and L_Q and USEPA's MDL and ML) were determined for each laboratory-method-analyte combination and then compared to the observed detection and quantitation limits. The overwhelming majority of analyte free samples had a reported value of zero. As a result, observed quantitation and detection limits were frequently zero. When they were not zero, the observed quantitation limits were sometimes less than the observed detection limits and when they were not, there was no observed fixed ratio between the quantitation and detection limits. The variability between days of analysis and the use of noise reducing techniques proved to be a significant source of the observed non-normal distribution of results from distilled water samples with a concentration of zero. Conventional procedures and their underlying analytical and statistical assumptions did not provide useful predictions of laboratory quantitation based upon the results of this study. Rather than one time statistical determinations, ongoing verification of quantitation limits may be a better approach.     

Contribution of microextraction in packed sorbent for the analysis of cotinine in human urine by GC–MS

A simple, rapid, sensitive, and non-consuming solvent method for the determination of cotinine in urine was developed, based on sample preparation by the relatively new technique microextraction in packed sorbent (MEPS) and analysis by GC–MS. This optimized method was compared with conventional solid-phase extraction/liquid–liquid extraction method used as reference. The wide linear range (5–5,000 ng/mL) and high sensitivity of the MEPS method (limit of detection 0.8 ng/mL) allow application to analysis of urine from smokers as well as non-smokers susceptible to passive smoking.     

The lp(O)...π-interactions in 1-[1-(methoxycarbonylmethylthio)methyl]-3,5-dimethylisocyanurate: topological analysis of the electron density distribution from X-ray diffraction data and quantum chemical calculations

The intra- and intermolecular interactions that make a major contribution to the formation of the crystal structure were detected and characterized in the crystal of 1-[1-(methoxycarbonylmethylthio)methyl]-3,5-dimethylisocyanurate. Topological analysis of the experimental and theoretical electron density distribution was used to reveal for the first time the role of the lp(O)...π interaction in the formation of a crystal of an isocyanurate derivative and to estimate its energy. Interactions of this type were found to occur in 74% of the fully N-substituted isocyanurate structures.     

Application of β-cyclodextrin for the analysis of estrogenic steroids in human urine by high-performance liquid chromatography

Summary A procedure is described for simultaneous determination of estriol, estrone and 17-estradiol in human urine. After acid hydrolysis of the sulphate conjugates, the estrogens were extracted into diethyl ether. The ether extracts were concentrated and applied directly to an HPLC column. Using a 25 cm C-18 column and acetonitrile-water modified by the addition of -cyclodextrin as mobile phase, the separation of estriol, estradiol and estrone was achieved within 20 minutes. The extraction of estrogens from the biological matrix is excellent. Estrogens were detected using a UV-detector (280 nm) or a spectrofluorimetric detector (_exc=280nm, _em=312 nm). The latter detection system has been used for the determination of estrogens in the urine of non-pregnant women. The procedure is simple and can be used in clinical practice.     

Kinetic study for the formation of microcrystalline “disks” of204T1I in gelatin medium

This paper presents a radiochemical study of the appearance of²⁰⁴T1I microcrystals and their development as disks resembling Liesegang rings, in gelatin, under different experimental conditions. Thus, kinetic diagrams have been traced, from which gel diffusion rate constants for radioactive ions were calculated, as well as reaction rate constants between reactant species:²⁰⁴T1⁺ and I^–, in colloidal medium, followed by the development of concentrated microcrystals under form of one or more disks. There is an interconnection between gelatin concentration of the colloidal medium, on one hand, and²⁰⁴T1I microcrystal disks number and dimensions, on the other hand, disks which appeared in gel as a result of the global process of nucleation.     

Evaluation of measurement uncertainty in the elemental analysis of sintered silicon carbide using laser ablation in liquid—inductively coupled plasma mass spectrometry with external calibration and isotope dilution

Accreditation and Quality Assurance - The goal of this study was to evaluate the uncertainty of elemental analytical methods that use laser ablation in liquid (LAL) as a pretreatment. After LAL...     

Statement of quantum problems for “hot” molecules and solution of the Schrödinger equation for electron states in the field of distributed nuclear charges

The issue concerning the basics of statement of the problem of determining the energy levels and wave functions of the “hot” (vibrationally excited) molecule and the states of electrons in the field of spatially distributed nuclear charges has been considered. It has been shown that such a problem in which nonadiabaticity is taken into accounted in part from the very beginning can be reduced to the conventional problem by changing the AO type.     

Theoretical study of formation mechanism for 4-methyl-3-phenylbenzonitrile in the course of the Hiyama and Suzuki–Miyaura reactions

A theoretical study on the mechanism of conversion of 3-bromo-4-methylbenzonitrile into 4-methyl-3-phenylbenzo-nitrile in the course of the Suzuki–Miyaura and Hiyama−Denmark cross-coupling reactions has been performed at Cam-B3LYP-D3 level of theory. With the use of Pd–NHC type complex as the catalyst, the Hiyama−Denmark cross-coupling is best suited for this process from both thermodynamic and kinetic aspects.     

On the electron temperatures and densities in plasmas produced by the “torche à injection axiale”

The electron temperature and the electron density of plasmas created by the “Torche à Injection Axiale” (TIA) are determined using Thomson scattering. In the plasma with helium as the main gas, temperatures of around 25 000 K and densities of between 0.64 and 5.1 × 10²⁰m⁻³ are found. In an argon plasma the electron temperature is lower and the electron density is higher: 17 000 K and around 10²¹ m⁻³ respectively. From these results it can be established that the ionisation rates of both plasmas are much larger than the recombination rates, which means that the plasmas are far from Saha equilibrium. However, deviations from a Maxwell electron energy distribution function, as reported for the “Microwave Plasma Torch” (MPT), are not found in the TIA. The excitation and ionisation power of the TIA appears to be stronger than that of the MPT.     

Efficient optimization and development of two methods for the determination of acrylamide in deep-frying oil by liquid chromatography–tandem mass spectrometry: Application of multifactor analysis assessment strategy

A new multifactor analysis assessment strategy was developed for evaluating, optimizing, and comparing analytical techniques for acrylamide in frying oils. Based on five indices (absolute recovery, absolute matrix effect, the intensity of the full ion scan, and the precursor ion scan to m/z 184 and m/z 241), the proposed strategy was performed with radar analysis, relative contribution analysis, and the entropy-weighted technique for order performance by similarity to ideal solution analysis. Two novel methods based on quick, easy, cheap, effective, rugged, and safe extraction methodology and gel permeation chromatography–liquid–liquid extraction followed by liquid chromatography–tandem mass spectrometry have been developed for the analysis of acrylamide in frying oils. Two methods were suitable for rapid and sensitive analysis of acrylamide in oils in different laboratories, with a limit of quantitation at 2 μg/kg, and the average recovery ranging from 92.5% to 107.8%, with relative standard deviations below 10%. When considering automation efficiency and matrix effects, gel permeation chromatography is the most efficient method, whereas the other method has an advantage when analyzing large samples. The developed methods were used in a pilot study to analyze frying oils with acrylamide content below 9.82 μg/kg, showing that the repeated frying process did not produce significant content of acrylamide in oils.     

The analysis of structural defects in piezo and ferroelectric materials by the “heat pulse” method

We consider the “heat pulse” technique for the analysis of structural imperfections. This method is based on the propagation of a pulse or packet of pulses of non-equilibrium acoustic phonons with 10¹¹–10¹² Hz frequencies.     

The role of 4-amino-5-phenyl-4H-1,2,4-triazole-3-thiol in the inhibition of nickel–aluminum bronze alloy corrosion: electrochemical and DFT studies

Inhibition of corrosion of nickel–aluminum bronze alloy (NAB) by use of 4-amino-5-phenyl-4H-1,2,4-triazole-3-thiol (APTT) was investigated by monitoring weight loss and open-circuit potentials (OCP), and by use of electrochemical impedance spectroscopy (EIS). Quantum chemical data were calculated by use of the density functional theory (DFT) model. Results showed that APTT inhibits corrosion of NAB alloy fairly well. The efficiency of inhibition increases with increasing concentration of APTT, and adsorption of the inhibitor obeys the Langmuir adsorption isotherm model. Negative values of adsorption free energy indicate chemical adsorption is spontaneous. The theoretical study showed that sulfur and nitrogen atoms (S, N7 and N8) were the reactive sites.     

Molecular structure and electron density distribution in the crystals of 3,6-dimethoxy-1,2,4,5-tetrazine and 3-phenyl-1,2,4,5-tetrazine based on X-ray diffraction data at 120 and 150 K. “Bent” bonds in the six-membered heterocycle

Characteristic features of the molecular geometry and electron density distribution in the crystals of 3,6-dimethoxy-1,2,4,5-tetrazine and 3-phenyl-1,2,4,5-tetrazine have been studied by X-ray structural analysis and quantum-chemical (RHF and MP2) and molecular mechanics (MM3) calculations. An unusual shift of the maxima of the deformation electron density from the N-N bonds toward the center of the heterocycles was found, which may be interpreted as a bending of the corresponding bonds. This bending was confirmed by calculations of characteristics of the electron density distribution within the multipole model.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2443–2450, December, 1995.The work was supported by the International Science Foundation (Grant No. M1F 000) and the Russian Foundation for Basic Research (Project No. 94-03-08338). We thank Dr. P. Mallinson (Chemistry Department, University of Glasgow, Scotland) for help in performance of multipole calculations.     

The role of disclinations in two phase changes induced by temperature and pressure in crystalline solids: melting and the brittle–ductile transition

It has long been known that the melting temperature T _m of close-packed metals correlates well with the mono-vacancy formation energy. However, with the possible exception of the face-centered-cubic metals, there is a prior phase transition from a mechanically brittle solid phase to a ductile phase. Here the likely role of disclinations in the brittle-ductile phase change is stressed. The present picture may help to understand the brittle–ductile transition not only in crystalline materials but also in amorphous phases. The structure of such phases can probably be characterized in terms of a disordered disclination network. As examples of elemental crystalline solids, Si and graphite are finally discussed, with the melting under pressure of graphite being quantified.