A calculation method to estimate the concentration and elemental composition of organic substances in natural Waters

The possibility of calculating the elemental composition and concentrations of organic substances in natural water based on the results of determining organic carbon and nitrogen and chemical oxygen demand is substantiated both theoretically and experimentally. Three types of substances differing in the electrochemical valence of carbon were among organic substances. The elemental composition of a number of water objects was analyzed. It was found that the concentrations of organic carbon, oxygen, hydrogen, and nitrogen are close to 50, 40, 4–5, and 2–5%, respectively.     

The method of introducing correction factors into parameters of atom-atom potentials of intermolecular interaction used for calculation of thermodynamic characteristics of adsorption

Different methods of introducing correction factors into parameters of atom-atom potentials of intermolecular interaction used for calculation of thermodynamic characteristics of adsorption by the semiempirical molecular statistical theory were compared. A method of isostructural fragments based on the application of the atom-atom potential corrected for the molecular fragment was suggested for introducing the correction factors. The advantages of this method were demonstrated for chlorobenzenes, chlorodioxines, and chlorobiphenyls. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 681–687, April, 2000.     

The geometry of thermodynamic potentials

In this paper, the performance of a diagrammatic method based on geometric and algebraic considerations is illustrated. This method, based on the particular symmetry of a thermodynamic diagram, allows to obtain the most important thermodynamic expressions of a simple system. In fact, these thermodynamic expressions can be developed with the application of various geometric patterns to the proposed diagram. The particular symmetry of the thermodynamic diagram allows to develop a matrix formulation of the different geometric patterns. This matrix formalism requires that the thermodynamic parameters of the diagram be recast in a vectorial form.On sabbatical leave from November 1995 until October 1996. Address during this period: Centro de Quimica Fisica Molecular, Univerdade Técnica de Lisboa, IST, Av. Rovisco Pais, 1096 Lisboa-Codex, Portugal.     

Stochastic ensembles of thermodynamic potentials

The provision of uncertainty estimates along with measurement results or values computed thereof is metrologically mandatory. This is in particular true for observational data related to climate change, and thermodynamic properties of geophysical substances derived thereof, such as of air, seawater or ice. The recent International Thermodynamic Equation of Seawater 2010 (TEOS-10) provides such properties in a comprehensive and highly accurate way, derived from empirical thermodynamic potentials released by the International Association for the Properties of Water and Steam (IAPWS). Currently, there are no generally recognised algorithms available for a systematic and comprehensive estimation of uncertainties for arbitrary properties derived from those potentials at arbitrary input values, based on the experimental uncertainties of the laboratory data that were used originally for the correlations during the construction process. In particular, standard formulas for the uncertainty propagation which do not account for mutual uncertainty correlations between different coefficients tend to systematically and significantly overestimate the uncertainties of derived quantities, which may lead to practically useless results. In this paper, stochastic ensembles of thermodynamic potentials, derived from randomly modified input data, are considered statistically to provide analytical formulas for the computation of the covariance matrix of the related regression coefficients, from which in turn uncertainty estimates for any derived property can be computed a posteriori. For illustration purposes, simple analytical application examples of the general formalism are briefly discussed in greater detail.     

Nanosecond redox equilibrium method for determining oxidation potentials in organic media

A general, nanosecond equilibrium method is described for determining thermodynamically meaningful oxidation potentials in organic media for compounds that form highly reactive cation radicals upon one-electron oxidation. The method provides oxidation potentials with unusually high precision and accuracy. Redox ladders have been constructed of appropriate reference compounds in dichloromethane and in acetonitrile that can be used to set up electron-transfer equilibria with compounds with unknown oxidation potentials. The method has been successfully applied to determining equilibrium oxidation potentials for a series of aryl-alkylcyclopropanes, whose oxidation potentials were imprecisely known previously. Structure-property trends for oxidation potentials of the cyclopropanes are discussed.     

A finite expansion method for the calculation and interpretation of molecular electrostatic potentials

Because it is useful to have the molecular electrostatic potential as an element in a complex scheme to assess the toxicity of large molecules, efficient and reliable methods are needed for the calculation and characterization of these potentials. A multicenter multipole expansion of the molecular electron charge density calculated with a limited Gaussian basis set is shown here to have only a finite number of nonzero terms from which the molecular electrostatic potential can be calculated. The discrete contributions to the electrostatic potentials from the terms of this expansion provide a physically meaningful decomposition of the potential and a means for its characterization. With pyrrole as an example, the electrostatic potential calculated from this finite expansion of the electron density is compared to that obtained from exact calculations from the same wave function. Good agreement is obtained at distances greater than 1.5 A from any atom in the molecule. In contrast, rearrangement of the terms into an expansion corresponding only to Mulliken atomic charges and dipoles yields a decomposition that produces electrostatic potentials which agree less well with the exact potential. This discrepancy is attributable to the neglect of terms due to higher moments.     

Automatic coulometric method for the determination of carbon and hydrogen in organic substances

Conclusions A coulometric method is proposed for the microdetermination of carbon and hydrogen in organic substances.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 700–701, March, 1972.     

Open-Shell coupled-cluster method: Variational and nonvariational calculation of ionization potentials

A hermitian, variational open-shell coupled-cluster method is described and applied to the calculation of H₂O and N₂ ionization potentials in the _T ≈ T₂ approximation. A nonvariational calculation is also carried out, with the inclusion of T₁ and T₃ in addition to T₂. Both methods give fair agreement with experiment when only T₂ is taken into account. T₃, which is included at present in the nonvariational scheme only, has a considerable effect on the results and gives good agreement with experiment.     

Empirical procedure for the calculation of ionization constants of organic compounds in water from their molecular volume

An empirical procedure was used to calculate 278 ionization constants of 271 organic compounds, including phenols, NH acids, benzoic acid derivatives, and mono- and dihydric carboxylic acids, in water. The examined compounds were divided into 11 structural groups. The ionization constants for compounds belonging to a single group were calculated from constant empirical coefficients, molecular volumes, and formulas with an average error of less than 3.5%, the maximal error not exceeding 9.9%.     

Spectral-isotopic method of determination of gas-forming elements in organic and inorganic substances

A review is given of the latest papers on the application of the spectral-isotopic method of determination of gas-forming impurities in organic and inorganic materials. Main attention is paid to a new kind of investigation, i.e. the development of spectroscopic methods for isotopic analyses of nitrogen, carbon, oxygen, and hydrogen in biological objects for use in analytical monitoring in biological experiments with “tracers”. A universal method is described for the preparation of natural samples, for the separation of the gases to be analyzed from the sample, and for their purification from the impurities. The main characteristics are given of the methods for individual determination of the above elements and for joint analyses of some elements (N¹⁵-C¹³-H²) in one analytical cycle (spectrum excitation conditions, isotope concentration calculations, etc.). The characteristics of the methods have been compared with the results of other researches and the scope of our method for applications in biological investigations with tracers has been defined. The main results of spectral-isotopic method development in the traditional direction, i.e. for quantitative analysis of impurities in solid inorganic materials and for gas analysis, are produced.     

Density functional calculation of intermolecular potentials

Calculations of intermolecular potentials following the density functional theory (DFT) turn out to be very complicated without using some appropriate approximations. Most often the following three approximations have been considered. In one approximation the disturbed charge distributions during collisions are reduced to sums of undisturbed charge distributions from the colliding species. In another approximation, the so-called local density approximation (LDA), one neglects the fact that the intermolecular potentials that depend on charge densities also depend on gradients in the densities. In a third approximation one assumes that the intermolecular potential can be considered as a sum of two terms: a term for the long-range geometry and a term for the short-range geometry. In this Article the three approximations mentioned will be discussed for numerical accuracy for calculations of potentials between inert gas atoms and for calculations of potentials between surfaces and inert gas atoms. In the discussion a few other approximations will be mentioned too.     

A simulation method for the calculation of chemical potentials in small, inhomogeneous, and dense systems

We present a modification of the gauge cell Monte Carlo simulation method [A. V. Neimark and A. Vishnyakov, Phys. Rev. E 62, 4611 (2000)] designed for chemical potential calculations in small confined inhomogeneous systems. To measure the chemical potential, the system under study is set in chemical equilibrium with the gauge cell, which represents a finite volume reservoir of ideal particles. The system and the gauge cell are immersed into the thermal bath of a given temperature. The size of the gauge cell controls the level of density fluctuations in the system. The chemical potential is rigorously calculated from the equilibrium distribution of particles between the system cell and the gauge cell and does not depend on the gauge cell size. This scheme, which we call a mesoscopic canonical ensemble, bridges the gap between the canonical and the grand canonical ensembles, which are known to be inconsistent for small systems. The ideal gas gauge cell method is illustrated with Monte Carlo simulations of Lennard-Jones fluid confined to spherical pores of different sizes. Special attention is paid to the case of extreme confinement of several molecular diameters in cross section where the inconsistency between the canonical ensemble and the grand canonical ensemble is most pronounced. For sufficiently large systems, the chemical potential can be reliably determined from the mean density in the gauge cell as it was implied in the original gauge cell method. The method is applied to study the transition from supercritical adsorption to subcritical capillary condensation, which is observed in nanoporous materials as the pore size increases.     

Empirical regularities of atomic ionization potentials

Regularities of the averaged ionization potentials for atoms and ions containing up to 18 electrons are studied in detail. It is shown that a two-variable function constructed from the averaged ionization potentials for each subshell is linear with respect to the degree of ionization q and the occupancy k of the s^k or p^k subshell. One linearity includes previous findings as a special case, and the other introduces a new regularity for atomic ionization potentials. Existing atomic ionization potentials and electron affinities are analyzed employing the regularities, and improved values of these quantities as well as term and fine structure separations in negative ions are derived.     

Density functional calculation of quinone electrode potentials

Here, we have applied density functional methods, in combination with free energy hydration calculations, to calculate two-electron electrode potentials for quinones and naphthoquinones. While we find that the free-energy perturbation method, implemented within a molecular dynamics framework, is superior to the PM 3—SM 3 continuum method for determining free energies of hydration, the computationally less expensive PM 3—SM 3 method does perform well when there is not an internal hydrogen bond. Generally, all the density functional approaches investigated gave good energetics when applied to this problem, but the Beck '88—Vosko—Wilk—Nusair combination of functionals for the exchange-correlation energy gave the best results. The density functional results are marginally better than the Møller-Plesset second-order perturbation results. Moreover, because the results are obtained using a thermodynamic cycle which involves taking differences in total energies, the results are not too dependent on the quadrature scheme used to calculate the exchange-correlation energy. By using semiempirically optimized geometries and the PM 3—SM 3 method for determining free energies of hydration, it has been possible to calculate electrode potentials for a series of large molecules (naphthoquinones) to within about 30 mV of experiment. This result is extremely encouraging and shows that density functional methods offer great promise in the design of redox-active molecules such as bioreductive anticancer agents. © 1995 John Wiley & Sons, Inc.     

Viabilty of atomistic potentials for thermodynamic properties of carbon dioxide at low temperatures

Investigation into volumetric and energetic properties of several atomistic models mimicking carbon dioxide geometry and quadrupole momentum covered the liquid-vapor coexistence curve. Thermodynamic integration over a polynomial and an exponential-polynomial path was used to calculate free energy. Computational results showed that model using GROMOS Lennard-Jones parameters was unsuitable for bulk CO(2) simulations. On the other hand, model with potential fitted to reproduce only correct density-pressure relationship in the supercritical region proved to yield correct enthalpy of vaporization and free energy of liquid CO(2) in the low-temperature region. Except for molar volume at the upper part of the vapor-liquid equilibrium line, the bulk properties of exp-6-1 parametrization of ab initio CO(2) potential were in a close agreement with the experimental results. Copyright 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1772-1781, 2001     

Approximate calculation of thermodynamic instability constants of complex compounds

A Debye-Hückel type equation is proposed for calculating thermodynamic instability constants from measurements at a single, sufficiently high, ionic strength, in solutions containing NaClO₄. The applicability of this equation to instability constants of complexes of different composition and charge is shown.