Electron-density-dependent fused-sphere surfaces derived from pseudopotential calculations

Fused-sphere surfaces can be used to mimic a molecular boundary associated with a constant value of the electron density. The simplest of such fused-sphere models are constructed by using the atomic radii for the spherical isodensity surfaces of individual atoms. In this work, we discuss the extension of this model to molecules containing atoms beyond the second row. In these many- electron systems, the computation of electron densities is usually simplified by adopting a pseudopotential (or effective-core potential) approach. Here, we discuss the performance of large- and small-core pseudo-potential calculations as a tool to derive atomic radii. Our results provide an optimum set of variable radii that can be used to build fused-sphere surfaces. This continuum of surfaces provides a simple approximation to the low-electron-density regions around molecules with heavy atoms.     

Promoting effect of sodium on catalytic and adsorption properties of Fe?Mn catalysts for CO hydrogenation

It has been established that alkali metal (sodium) additives affect both texture and adsorption properties of Fe–Mn catalysts for olefin synthesis thus promoting their catalytic properties.

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() , Fe–Mn , .

     

Effect of the nature of carrier on dinitrogen fixation by supported titanium complexes

Cyclopentadienyltitanium (IV) complexes supported on Al₂O₃, SiO₂ and MgO reduced with BuLi were tested in dinitrogen fixation at room temperature under normal pressure. Only the system supported on Al₂O₃ is active in this reaction.

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- (IV), Al₂O₃, SiO₂ MgO, BuLi, . Al₂O₃.

     

Theoretical studies of the mechanism of the action of the neurohypophyseal hormones. I. Molecular electrostatic potential (MEP) and molecular electrostatic field (MEF) maps of some vasopressin analogues

Summary Continuing our theoretical studies of the oxytocin and vasopressin analogues, we have analysed the molecular electrostatic potential (MEP) and the norm of the molecular electrostatic field (MEF) of [1--mercaptopropionic acid]-arginine-vasopressin ([Mpa¹]-AVP), [1-(-mercapto-,-cyclopentamethylene)propionic acid]-arginine-vasopressin ([Cpp]-AVP), and [1-thiosalicylic acid]-arginine-vasopressin ([Ths]-AVP) whose low-energy conformations were calculated in our previous work. These compounds are known from experiment to exhibit different biological activity. The scalar fields mentioned determine the energy of interaction with either charged (MEP) or polar (MEF) species, the energy being in the second case either optimal or Boltzmann-averaged over all the possible orientations of the dipole moment versus the electrostatic field. The electrostatic interactions slowly vanish with distance and can therefore be considered to be the factor determining the molecular shape at greater distances, which can help in both predicting the interactions with the receptor at the stage of remote recognition and in finding the preferred directions of solvation by a polar solvent. In the analysis of the fields three techniques have been used: (i) the construction of maps in certain planes; (ii) the construction of maps on spheres centered in the charge center of the molecule under study and of poles chosen according to the main axes of the quadrupole moment; and (iii) the construction of surfaces corresponding to a given value of potential. The results obtained show that the shapes of both MEP and MEF are similar in the case of [Mpa¹]-AVP and [Cpp¹-AVP (biologically active), while some differences emerge when comparing these compounds with [Ths¹]-AVP (inactive). It has also been found that both MEP and MEF depend even more strongly on conformation.     

Delocalization model of regioselectivity and reactivity of free radicals in reactions of addition to olefins

On the basis of the concept of polarity (philicity) of free radicals as proposed by the authors, within the framework of methods of qualitative surfaces of potential energy (linear combinations of configurations of fragments) and stabilization energy, an effective model has been developed for the regioselectivity and reactivity of radicals in processes of addition. A critical examination is made of certain key aspects of the change in regiochemistry and reactivity with changes in the electronic structure of the free radical and substrate. The dominant trends in regioselectivity and reactivity in processes of free-radical addition to olefins are controlled by electronic effects and can be predicted by analyzing interactions of diabatic potential energy surfaces or orbital interactions for a system consisting of a free radical and an unsaturated substrate.Presented at 3rd All-Union Symposium Dynamics of Elementary Atomic-Molecular Processes (Chernogolovka, June 1985) and at the 9th All-Union Conference on Quantum Chemistry (Ivanovo, June 1985).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 2, pp. 135–148, March–April, 1987.The authors are grateful to M. V. Bazilevskii for his constructive discussion of the results obtained in this work.     

Effect of the experimental conditions on the thermooxidative behaviour of vegetable oils

The thermooxidative behaviour of sunflower and rapeseed oils has been investigated by means of a derivatograph, using both dynamic and static (isothermal) temperature programs. The aim was to find the optimum experimental conditions for studying the oxidative stability of edible oils, in order to determine their storability. A novel method has been developed for the rapid indication of stability by modelling the oxidative changes under isothermal conditions.

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Zusammenfassung Das thermooxidative Verhalten von Sonnenblumen- und Rapsöl wurde mittels eines Derivatographen unter Einsatz dynamischer und statischer (isothermer) Temperaturprogramme untersucht. Der Zweck der Arbeit war die optimalen Versuchsbedingungen zur Bestimmung der Oxidationsstabilität von Speiseölen zu finden um ihre Lagerfähigkeit zu bestimmen. Eine neue Methode zur schnellen Ermittlung der Stabilität wurde mittels Modellierung der oxidativen Änderungen unter isothermen Bedingungen erarbeitet.

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Résumé On a étudié l'oxydabilité thermique des huiles de tournesol et de colza, à l'aide d'unDerivatigraph et en utilisant des programmes de température dynamiques et statiques (isothermes). Le but du travail était de trouver les conditions d'expérience les mieux adaptées à l'étude de la résistance à l'oxydation des huiles alimentaires afin de déterminer les conditions de leur stockage. On a mis au point une nouvelle méthode qui indique rapidement la stabilité de l'huile en reproduisant les changements d'oxydation en conditions isothermes.

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, ( ) . , , . .

     

Intermolecular vibronic coupling and energy transfer in a flat molecular aggregate

Summary The migration of excitation within a small flat molecular aggregate composed of identical molecules is described using a Davidov-like model and a mechanism of excitation transfer of Förster type. We consider in this model the changes that take place in the equilibrium position of each molecule upon excitation and construct energy surfaces that describe paths, that is, conditions for excitation localization and transfer that govern, in first-order, the motion of excitation within the aggregate.Presented in part at XVIII Jornadas Chilenas de Química, Santiago, 1989     

A complete shape characterization for molecular charge densities represented by Gaussian-type functions

An algorithm for a detailed 3-D characterization of the shapes of molecular charge distributions is implemented, tested and applied for a family of AB₂ molecules. The characterization is performed by computing a number of topological invariants (“shape groups”) associated with a continuum of molecular surfaces: the complete family of all electronic isodensity contours for the given molecules. These shape groups (the homology groups of truncated surfaces derived from isodensity contours) depend continuously on two parameters: a density value defining the density contour, and a reference curvature value, to which the local curvatures of the isodensity contours are compared. The electronic charge distribution is modeled by means of Gaussian-type functions. The method employs an explicit form of the charge density function in order to compute the curvature properties for the molecular surfaces analytically, from which the shape groups are derived by the algorithm. No visual inspection is required for the characterization and comparison of shapes of molecular charge densities, as these are done algorithmically by the computer. However, visual inspection of the results of the shape analysis is a possible option. For a given molecule, in a given nuclear configuration, the technique provides a two-dimensional shape map, displaying the distribution of shape groups as a function of the local curvature and the level set value (the value of the charge density at the contour). The computer program GSHAPE performs the analysis of shape maps automatically. This feature makes it potentially useful in the context of computer-aided drug design, where unbiased, automated shape characterization methods are valuable tools. As examples, several two-dimensional shape maps for simple systems are discussed. The changes induced in these maps by a change in the nuclear geometry, as well as by the changes of the nuclear charge, are also analyzed. The method is applicable to large biomolecules of interest if charge density information is available.     

A study on universal Gaussian basis sets for first-row atoms

Analysis of various optimum and non-optimum Gaussian basis sets for firstrow elements have indicated that with a minimum increase of the basis set size and without loss of accuracy of the calculated total energy, a single universal Gaussian basis set may replace individually optimized Gaussian basis sets for a series of atoms. Such a universal Gaussian basis set may substantially reduce the computational work required for the calculation of molecular integrals in ab initio MO calculations.     

The compensation effect: A fact or a fiction

In this article, the facts and fictions surrounding the compensation effect is explored. False compensation effect occurs mainly due to: propagation of computational and experimental errors resulting in inaccurate estimates of the Arrhenius parameters; and, the natural compensation between InA andE. Since Arrhenius parameters are sensitive to errors in temperature; the errors due to uncertainty in temperature should be minimized to eliminate false compensation effect. Increasing the experimental temperature range is helpful in minimizing errors due to uncertainty in temperature. A point of concurrence in a plot of Ink and 1/T establishes the occurrence of true compensation effect. True compensation effect has been shown to be a useful tool in chemical research for: identifying the governing reaction mechanism; predicting the effects of various reaction parameters; and, correlating and reducing experimental data.

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Zusammenfassung In dieser Mitteilung werden Fakten und Vorstellungen betreffs des Kompensationseffektes untersucht. Falsche Kompensationseffekte werden hauptsächlich verursacht durch: eine ungenaue Ermittlung der Arrheniusparameter durch die Fortpflanzung von rechnerischen und experimentellen Fehlern sowie dem natürlichen Kompensationseffektes zwischen InA undE. Das Arrheniusparameter gegenüber Fehler im Temperaturwert sehr empfindlich sind, sollten Temperaturungenäuigkeitsfehler zur Vermeidung eines falschen Kompensationseffektes minimalisiert werden. Zur Verminderung der aus der Temperaturungenauigkeit resultierenden Fehler ist es von Nutzen, den Temperaturbereich des Experimentes zu erweitern. Ein wahrer Kompensationseffekt wird durch einen Schnittpunkt in einem Ink-1/T Diagramm angezeigt. Es wird gezeigt, daß der wahre Kompensationseffekt ein nutzvolles Mittel in der chemischen Forschung darstellt: zur Feststellung des dominierenden Reaktionsmechanismus, zur Vorhersage der einflüsse verschiedener Reaktionsparameter und zur Aufarbeitung und Schlußfolgerung von bzw. aus experimentellen Daten.

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, . , , lnA E. , , . . Ink-1/T . , , , .

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The author would like to thank the reviewer for his constructive criticisms.     

Aniline hydrogenation in the presence of alumina-supported rhodium under hydrogen pressure

Aniline hydrogenation over 5% Rh/Al₂O₃ in water and an universal queous buffer has been studied at temperatures of 323–363 K and pressures of 2.02–10.10 MPa.

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5% Rh/Al₂O₃ 323–363 K 0,5–10,10 .

     

Symmetry and periodicity of potential surfaces: a test for multicenter interactions

Symmetry and periodicity of potential energy surfaces of chemical reactions and conformational changes are determined by the symmetry properties of the nuclear frameworks of all possible nuclear configurations of the given overall stoichiometry. For example, a mirror plane of a nuclear configuration implies a mirror plane of the potential surface (or that of the potential energy hypersurface in higher dimensions), and a local rotational symmetry of substituents implies a translational symmetry, that is, periodicity of the potential surface, if the latter is defined in terms of the usual bond length/bond angle internal coordinates. Such symmetry relations on potential surfaces are rather trivial consequences of molecular symmetry properties; however, when taken collectively for entire domains of nuclear configurations, they lead to nontrivial conclusions. Whereas symmetry properties and energy contents of individual conformations can be studied locally within limited domains of the potential surface, a global analysis of the potential surface may reveal significantly more. In this note, some consequences of the above approach are explored, and a simple test is proposed for the detection and evaluation of the importance of multicenter interactions in conformers related to one another by bond rotations.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday     

Problem of ambivalence in organic reactions

A pure model of intermolecular ambivalence (nucleophilic-electrophilic dichotomy) in organic reactions was developed within the bounds of a qualitative analysis of interactions of diabatic potential energy surfaces and frontier MO for an attacking reagent-substrate system. The chemical aspects of this phenomenon and the limits of its occurrence, as well as the possible limitations of the model and the range of its potential applications are discussed. The concept of the immanent ambivalence of electronic systems has been introduced.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 3, pp. 274–281, May–June, 1988.The author wishes to express his gratitude to M. V. Basilevsky for his constructive remarks.     

Preparation and physico-chemical study of inclusion complexes between idebenone and modifiedβ-cyclodextrins

The inclusion properties of modified-cyclodextrins (trimethyl--cyclodextrin, dimethyl--cyclodextrin and hydroxypropyl--cyclodextrin) towards idebenone were compared with natural-cyclodextrin. The inclusion complexes were prepared by different methods (coprecipitation, kneading, and freeze-drying) and characterized by differential scanning calorimetry, X-ray diffractometry, UV, CD and NMR spectroscopy. The results obtained by CD and NMR spectroscopy indicate a different orientation of idebenone in dimethyl--cyclodextrin with respect to other cyclodextrins. Stability constants of the complexes were determined in water at various temperatures and consequently thermodynamic parameters were obtained. All cyclodextrins are able to significantly increase the water solubility of idebenone, particularly dimethyl--cyclodextrin and hydroxypropyl--cyclodextrin, as a result of complexation. Consequently, they enhance the dissolution rate of the complexed drug compared to the free drug.     

Ammonia adsorption on the surface of Ni(II)-, Cu(II)-and Co(II)-phthalocyanine

Ammonia adsorption on the surface of Ni(II)–, Cu(II)- and Co(II)-phthalocyanine has been studied by means of reflection spectroscopy. Ammonia bonds to the metal ions in the surface layers of phthalocyanines preferably in octahedral coordination. In the case of Co(II)-phthalocyanine the trivalent state of cobalt is stabilized. Close similarity between coordination in liquid phase and adsorption on the surface is observed.

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- , , . , - . Co (II)- . .

     

Study of the general properties of the adiabatic potential energy of molecules using the scaling transformation

General relations derived from the invariance of the electronic energy of molecules relative to the scaling of the electronic coordinates using the multiplier (R/R_o), which depends on the internuclear distance, and relative to the translation of the nuclear core in the Hamiltonian were studied. Universal inequalities correct for the energy and its derivative over the internuclear distance R were obtained; the question of the allowed forms of the approximations for the adiabatic potentials of molecules was investigated; the problem of the determination of the optimal form of the electronic Hamiltonian for the calculation of the second derivative of the energy of molecules over R using the perturbation theory was formulated and solved. The use of the optimal Hamiltonian form makes it possible to decrease, by 6–9 times, the absolute value of the second-order relaxation contribution for light molecules (H₂ ⁺ and H₂). With increase in the nuclear charge and the number of electrons, this gain increases rapidly, and the optimal value approaches 1.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 3, pp. 257–267, May–June, 1987.     

Structural aspects of aqueous tetraalkylammonium salt solutions

Various thermodynamic and spectroscopic investigations on aqueous tetraalkylammonium salt solutions are reviewed from a structural viewpoint. At present, it is not yet clear what kind of water structure is formed around the alkyl groups of the large cations. However, the importance of cosphere overlap and ion-ion interaction (cation-anion, cation-cation, cation-anion-cation, etc.) in determining the solution properties are emerging more clearly. In this regard, model calculations based on the approach of Friedman and his coworkers are expected to be of considerable value.This paper was presented at the symposium, The Physical Chemistry of Aqueous Systems, held at the University of Pittsburgh, Pittsburgh, Pennsylvania, June 12–14, 1972, in honor of the 70th birthday of Professor H. S. Frank.     

Toward the Experimental Structure Determination of Larger Molecular Clusters: Application and Limitations of the Weak Interaction Model to the (C6H6)13B2u ← A1g00 0 Spectrum

The experimental B_2u A_1g0⁰ ₀ spectrum of (C₆H₆)₁₃ was analyzed within the weak-interaction model using minimum energy structures calculated from six different potential energy surfaces. The coexistence of two isomers—both of C₃ symmetry and with nearly equal populations—is supported. Structures predicted by two of the six potential energy surfaces are strongly favored. The transition dipole of benzene moieties within the cluster has a magnitude of 0.23 Debye. Weak transition dipole–dipole interactions fall between –1.95 and +2.24cm^–1 and site shifts of ligand molecules range from –160.3 to –153.8cm^–1. Although the weak-interaction transition dipole–dipole model falls short of unambiguously determining isomeric structures of benzene-13, it establishes a solid foundation on which modeling can be based for determining structures of larger, high-symmetry, molecular clusters.     

Temperature-programmed desorption of hydrogen from Fe?Ni/Al2O3 catalysts

The aim of this paper is to investigate temperature activated hydrogen adsorption on Fe–Ni/Al₂O₃ catalyst and for the sake of comparison also on monometallic systems Fe/Al₂O₃ and Ni/Al₂O₃.

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- Fe–Ni/Al₂O₃ , , Fe/Al₂O₃ Ni/Al₂O₃.