Complexation of Cd2+, Ni2+, and Ag+ metal ions with 4,13-didecyl-l,7,10,16-tetraoxa-4,13-diazacyclooctadecane in acetonitrile-ethylacetate binary mixtures

Conductometric titrations have been performed in acetonitrile-ethylacetate (AN-EtOAc) binary solutions at 288, 298, 308, and 318 K to obtain the stoichiometry, the complex stability constants and the standard thermodynamic parameters for the complexation of Cd²⁺, Ni²⁺, and Ag⁺ cations with 4,13-didecyl-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane (cryptand 22DD). The stability constants of the resulting 1: 1 complexes formed between the metal cations and the ligand were determined by computer fitting of the conductance-mole ratio data. There is a non-linear relationship between the logK _f values of complexes and the mole fraction of ethylacetate in the mixed solvent system. In addition, the conductometric data show that the stoichiometry of the complexes formed between the Cd²⁺, Ni²⁺, and Ag⁺ cations with the ligand changes with the nature of the solvent. The standard enthalpy and entropy values for the 1: 1 [ML] complexation reactions were evaluated from the temperature dependence of the formation constants. Thermodynamically, the complexation processes of the metal cations with the C22DD, is mainly entropy governed and the values of thermodynamic parameters are influenced by the nature and composition of the binary mixed solvent solutions.     

Effect of multiply charged paramagnetic metal cations on J-aggregation of thiacyanine dyes

J-Aggregation of thiacyanine dyes with a different length of the polymethine chain has been studied in aqueous solutions in the presence of paramagnetic metal cations. The introduction of multiply charged metal cations stimulates J-aggregation. The number of molecules involved in the composition of J-aggregates has been calculated to be four on the basis of analysis of the absorption spectra. The equilibrium constants and the thermodynamic parameters of J-aggregation have been determined. The effect of the dye structure and the metal cation nature on J-aggregation has been revealed.     

源自天然席夫碱的过渡金属配合物在分光光度法测天然水中的Fe(Ⅲ)的应用

描述了一种新颖、简便的合成含五齿配位基的大环席夫碱配体的方法,制备和表征了该席夫碱的1∶1包合物。用化学和光谱学方法测定了标题配合物的组成,认为在所有配合物中配位金属原子取八面体结构。数据表明:配体起O2N4六齿结构而每个环绕金属原子在八面体环境中。配合物的红外和1H NMR光谱符合中心金属原子的配位结果。用分光光度法测定了配合物的稳定常数。用共轭余量法(CR method)计算了在热分解的不同阶段配合物活化的动力学和热力学参数。此外,用抑菌圈直径法筛选了配体及其金属配合物抑制细菌和真菌的能力。用回收率试验研究了天然螯合配体在不同天然水体中对Fe(Ⅲ)离子配合作用的影响。     

NMR spectroscopy in coordination supramolecular chemistry: A unique and powerful methodology

Metal-mediated self-assembly is emerging as a very important strategy for the synthesis of supramolecular species. Still, a major challenge in coordination supramolecular chemistry continues to be the characterization of the self-assembled complexes and the investigation of their dynamic behaviour in solution. In this context, NMR spectroscopy appears as a unique and powerful methodology. This practical-oriented review describes the rich variety of NMR techniques which are applied to the investigation of different aspects of the structure and behaviour of supramolecular complexes. “Classic” 1D NMR spectra reflect characteristic chemical shifts due to metal–ligand interactions or encapsulation phenomena, as well as symmetry and chiral properties of host–guest assemblies. Mainstream ¹H, ¹³C, ¹⁹F and ³¹P spectra are eventually complemented by the use of NMR-active metal nuclides. Homo- and heteronuclear 2D correlation experiments are ubiquitous in the literature, providing through-bond and through-space connectivities. Increasingly, diffusion measurements are also gaining popularity in this field, being used to gain information about molecular size, intermolecular interactions and even association constants of supramolecular complexes. Knowledge about the thermodynamic properties and the dynamic behaviour of coordination supramolecular assemblies is essential for the development of their practical applications. The most frequently used NMR methodologies for the calculation of association constants (simple signal integration, NMR titration and diffusion measurements) and for the investigation of dynamic supramolecular equilibria (lineshape analysis, selective inversion recovery experiments and 2D EXSY spectra) are described, together with the use of variable-temperature investigations for the determination of the thermodynamic and activation parameters of self-assembly and encapsulation processes.     

A calix(4)arene pyridine derivative and its monomeric component: structural and thermodynamic aspects of their complexation with metal cations

The interaction of a calix(4)arene derivative, namely 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra[2-(4-pyridyl)methoxy]calix(4)arene, 1a, and its monomeric component, p-tert-butylphenoxy-4-pyridine, 1b, with metal cations has been investigated in acetonitrile and methanol. (1)H NMR measurements carried out in CD(3)CN show the primary role played by the pyridyl nitrogens in their complexation with metal cations. Conductance measurements demonstrated that for all cations (except mercury) the composition of the metal ion complexes of 1a is 1:1 (ligand:metal cation). However, 1a hosts two mercury cations per unit of ligand. For the monomer 1b, complexes of 2:1 (ligand:metal cation) stoichiometries are formed with the exception of Pb(2+) (1:1 composition). The thermodynamics of complexation of these systems are reported in acetonitrile. Data in methanol are limited to stability constant values for mercury(II) and these ligands. This paper demonstrates for the first time that thermodynamic data for the complexation of the monomeric component of the ligand and metal cations contribute significantly to the interpretation of systems involving cation-calixarene interactions in solution.     

通过SAPO-34分子筛笼中引入金属物种提升甲醇制烯烃反应中乙烯选择性

低碳烯烃(乙烯、丙烯)是化学工业极其重要的基本原料.甲醇制烯烃(MTO)反应是重要的烯烃生产石油替代路线.其中,磷酸硅铝类SAPO-34分子筛在MTO反应中表现出优异的低碳烯烃选择性.与丙烯相比,乙烯具有更高的经济附加值,因此提升MTO反应中乙烯的选择性有着重要的意义.本文采用传统离子交换法(CIE)、模板辅助离子引入法(TII)和醇相离子交换法(AIE)对SAPO-34分子筛进行金属Zn、Cu改性,利用多种表征手段对金属Zn、Cu改性SAPO-34分子筛的物理结构、化学组成、金属物种状态与分布、酸性及扩散性质等进行表征.首先,对金属Zn、Cu改性SAPO-34分子筛的物理结构和化学组成进行分析.X射线衍射表明,相比AIE法,CIE法和TII法改性基本保持SAPO-34分子筛的结晶度.X射线荧光分析表明,相比Co、Ni,金属Zn、Cu更易引入SAPO-34分子筛.N2物理吸附-脱附表明,CIE法改性能够保持SAPO-34分子筛的BET比表面积和微孔孔容.其次,考察了金属Zn、Cu改性SAPO-34分子筛中金属物种的状态.氢气-程序升温还原(H2-TPR)和X射线光电子能谱(XPS)结果表明,Zn物种主要以孤立态的Zn2+阳离子形式存在.H2-TPR、XPS、紫外-可见光谱和电子顺磁共振谱结果表明,Cu物种主要以孤立态的Cu2+阳离子以及部分CuO形式存在.继而对金属Zn、Cu改性SAPO-34分子筛中金属物种的分布进行表征.XPS表明,Zn阳离子改性的SAPO-34表层富硅、富Zn,呈类核壳结构;XPS和扫描式电镜-能量色散X射线光谱结果表明,Cu物种在Cu改性SAPO-34分子筛中均匀分布.进一步研究了金属Zn、Cu改性SAPO-34分子筛中酸性的变化.氨气-程序升温脱附和核磁共振氢谱结果表明,Zn、Cu改性SAPO-34酸性位点的酸量降低.最后,对金属Zn、Cu改性SAPO-34分子筛的扩散性质进行分析.智能重量分析表明,Zn、Cu阳离子的引入降低探针分子(乙烷、丙烷)的扩散系数,推断Zn、Cu阳离子的引入增加对MTO反应产物的扩散限制.热重表明,Zn阳离子改性SAPO-34分子筛反应初期积炭量略微增加.综上所述,Zn阳离子改性SAPO-34催化剂表层富硅、富Zn,呈现类核壳结构.Zn阳离子的引入增加对MTO反应产物的扩散限制,而且Zn阳离子的引入促进MTO反应初始阶段的碳沉积.因此,Zn阳离子改性SAPO-34分子筛显著增加MTO反应产物的扩散限制,对分子尺寸较大的反应产物的扩散限制更为明显,从而提高MTO反应初始阶段的乙烯选择性,增大乙烯/丙烯比.     

Release of bioactive volatiles from supramolecular hydrogels: influence of reversible acylhydrazone formation on gel stability and volatile compound evaporation

In the presence of alkali metal cations, guanosine-5'-hydrazide (1) forms stable supramolecular hydrogels by selective self-assembly into a G-quartet structure. Besides being physically trapped inside the gel structure, biologically active aldehydes or ketones can also reversibly react with the free hydrazide functions at the periphery of the G-quartet to form acylhydrazones. This particularity makes the hydrogels interesting as delivery systems for the slow release of bioactive carbonyl derivatives. Hydrogels formed from 1 were found to be significantly more stable than those obtained from guanosine. Both physical inclusion of bioactive volatiles and reversible hydrazone formation could be demonstrated by indirect methods. Gel stabilities were measured by oscillating disk rheology measurements, which showed that thermodynamic equilibration of the gel is slow and requires several cooling and heating cycles. Furthermore, combining the rheology data with dynamic headspace analysis of fragrance evaporation suggested that reversible hydrazone formation of some carbonyl compounds influences the release of volatiles, whereas the absolute stability of the gel seemed to have no influence on the evaporation rates.     

Modulation of a supramolecular bowl and pot by changing solvent systems and/or metal/ligand ratios

A resorcin[4]arene-based ligand 2 a with four pyrimidine substituents at the upper rim was synthesized, and the generation of different metal-mediated superstructures from the same ligand and metal ions utilizing the unfavorable incorporation of the third and fourth Pd(II) ions to ligand 2 a was investigated. The supramolecular bowl 3 a, which comes from a 1:2 combination of ligand 2 a and [Pd(en)(NO(3))(2)], was obtained in water even though excess of Pd(II) complexes were employed. By adding methanol, the supramolecular pot 4 gradually formed, which was the major product when the ratio of mixed solvent reached methanol/water=5:1 (v/v). Host-guest complexation phenomena of 3 a toward several aromatic carboxylates were demonstrated by isothermal titration calorimetry (ITC) and by (1)H NMR spectroscopy; both the enthalpy gain from electrostatic and hydrophobic interactions, and the entropy gain from desolvation cooperatively contribute to the binding of anionically charged guests. The crystal structure of supramolecular pot 4 shows direct evidence for the hydrogen bonding between water and the aromatic pi electrons in the solid state. The modulation between supramolecular bowl 3 a and pot 4 was also made possible by changing the metal/ligand ratios in aqueous methanol solution as well as by varying the water content of the mixed solvent.     

Neutral gold(I) metallosupramolecular compounds: synthesis and characterization, photophysical properties, and density functional theory studies

The reaction of the tris-indole InTREN ligand (L) with different gold phosphine fragments allows the construction of new gold(I) complexes with different geometries depending on the chosen phosphine. A metallodendrimeric structure is obtained when the gold atom is linked to a triphenylphosphine ligand, and neutral gold(I) metallocryptands are constructed when a triphosphine is used. Characterization of the compounds was accomplished by 31P{1H} and 1H NMR, IR, absorption, and fluorescence spectroscopies, electrospray ionization mass spectrometry (ESI-MS(+)), and elemental analysis, and their geometry was optimized using density functional theory (B3LYP). Time-dependent density functional theory (TD-DFT) calculations have been used to assign the lowest energy absorption bands to LMCT N(p, tertiary amine)-->Au transitions. Photophysical characterization of the complexes shows strong luminescence in the solid state. The formation of heterobimetallic species has been detected in solution in the presence of equimolar quantities of metal cations, and their structures have been identified by a combination of spectroscopic methods and mass spectrometry.     

溶胶-凝胶法设计与制备金属及合金纳米材料的研究进展

溶胶-凝胶法是常见的制备金属氧化物的方法之一。在溶胶-凝胶法中,各种反应物能达到分子级的均匀混合,因此能制备成份复杂的氧化物材料。目前,溶胶-凝胶法也应用于设计与制备金属纳米材料,特别是合金纳米颗粒。例如,溶胶-凝胶法能应用于制备CoPt、FePt等磁性纳米合金材料以及CoCrCuNiAl高熵合金纳米材料,以及物相结构为有序相的Cu3Pt合金纳米材料。本文综述溶胶-凝胶法设计制备金属纳米材料的研究进展,包括溶胶-凝胶法实施的基本步骤、该方法在制备金属纳米材料方面的具体应用,并着重论述采用热力学计算设计金属及化合物的基本原理。该基本原理包括计算金属氧化物与还原性气体如氢气的还原反应的吉布斯自由能的变化量、金属氧化物的标准电极电位(不同于金属离子的标准电极电位)。最后探讨溶胶-凝胶法设计制备金属纳米材料存在的问题以及后续可能的发展方向。     

Synthesis and supramolecular features of hybrid POM/onium solid-state assemblies

Abstract

Polyoxomolybdate-based organic?inorganic hybrid architectures were synthesised and characterised by X-ray crystallography. The supramolecular assemblies present rows of metallic clusters H-bonded by ammonium cations, with a 1:2 molybdate/ammonium ratio. The organic moieties of the ammonium cations establish hydrophobic contact among them such as van der Waals, C–H?π and π?π interactions that stabilise the supramolecular architectures. In particular, for compound 5 the n-alkyl tails pack closely together giving a lipid-like bilayer. In compound 6, the aromatic phenyl rings of the organic cation allow the stabilisation of the supramolecular architecture by C–H?π and π?π interactions. Regarding the X-ray structure of the compound 11, the tetraanionic octa-molybdate [Mo₈O₂₆]^4? cluster is surrounded by four ethyl-triphenyl-phosphonium cations. Running along the b-axis open channels are occupied by DMF solvent molecules. Interestingly, a soaking experiment in n-pentane with the corresponding crystals of compound 11 afforded to a crystal structure very different from the native one.     

Lanthanide-mediated supramolecular cages and host-guest interactions

The structure and thermodynamic properties of lanthanide complexes with a new tripodal ligand L2 have been elucidated using different physicochemical methods. At stoichiometric ratios, the tetrahedral three-dimensional complexes with lanthanide cations are formed in acetonitrile with good stabilities. Despite minor structural changes comparing to previously investigated tripodal ligands, the resulting assembly exhibits different features revealed with the crystal structure of [Eu(4)L2(4)](OH)(ClO(4))(11) (orthorhombic, Pbcn). Interestingly, the highly charged edifice contains an inner cage encapsulating a perchlorate anion. Such lanthanide mediated cage-like assemblies are rare, and may be of interest for different sensing applications. Indeed, the anionic guest can be exchanged with different anions. The related host-guest equilibria were investigated with NMR techniques. Various aspects of these reactions are qualitatively discussed.     

Influence de la complexation sur le transfert des cations en osmose inverse

The influence of complexation by different ligands on the transfer of metallic cations in reverse osmosis has been studied by using membranes composed of aromatic polyamides. In spite of the large volume increase of the diffusing solute, the addition of a complexant can significantly improve the transfer of metallic cations through a reverse osmosis membrane. The most important factor in the improvement of the transfer is the solubility in the membrane of the complexes created. This solubility is linked to the possibility of creating hydrogen bonds between the membrane and the complex. Moreover, the addition of substances which strongly solvate both the membrane and the solute leads to an improvement of transfer which increases considerably with the increasing pressure. These results suggest the possibility of separating metal cations by specific complexation.     

Charge distribution in bis-dioxolene radical metal complexes. synthesis and DFT characterization of dinuclear Co(III) and Cr(III) complexes with a mixed-valent, S = 1/2 semiquinone-catecholate ligand

Bis-dioxolene bridged dinuclear metal complexes of general formula M2(CTH)2(diox-diox)(PF6)n (n = 2, 3; M = Co(III), Cr(III); CTH = tetraazamacrocycle) have been synthesized using the bis-bidentate ligand 5,5'-di-tert-butyl-3,3',4,4'-tetrahydroxybiphenyl. These complexes were characterized by means of ESR, UV-vis, temperature dependent magnetic susceptibility, and cyclic voltammetry. Our results unambiguously suggest that the tripositive dimetal cations can be described as containing a fully delocalized bis-dioxolene trinegative radical ligand (Cat-Sq) bridging two tripositive metal cations. In this frame the sextet electronic ground state characterizes the Cr2(CTH)2(Cat-SQ)3+ as a result of the antiferromagnetic coupling of the radical bridging ligand with the two equivalent paramagnetic metal centers. The electronic and geometrical structure and the magnetic properties of Cat-Sq and of its complexes have been studied with density functional theory.     

Thermodynamic study of complex formation between Ni2+ and Co2+ cations and 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane (kryptofix 5) in binary mixed nonaqueous solutions

Conductometric titrations were performed in pure and binary solvent solutions of ethyl acetate (EtOAc), methyl acetate (MeOAc) and methanol (MeOH) with acetonitrile (AN) at 288, 298, 308, and 318 K to determine the stoichiometry, the complex stability constants and the standard thermodynamic parameters for the complexation of nickel(II) and cobalt(II) cations with 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane (kryptofix 5). The stability constants of the resulting 1:1 complexes formed between the metal cations and the ligand in different solvent mixtures were determined by computer fitting of the conductance-mole ratio data. The results revealed that the stability order of (kryptofix 5)-Ni²⁺ and (kryptofix 5)-Co²⁺ complexes changes with nature and composition of the solvent system. There is a non-linear relationship between the logK _f values of complexes and the mole fraction of acetonitrile in the mixed solvent system. In addition, the conductometric data show that the stoichiometry of the complexes formed between the nickel(II) and cobalt(II) cations with the acyclic ligand changes with the nature of the solvent. The enthalpy and entropy values for the 1: 1 [ML] complexation reactions were evaluated from the temperature dependence of the formation constants. Thermodynamically, in most of systems, the complexation processes of nickel(II) and cobalt(II) cations with kryptofix 5, are both enthalpy and entropy stabilized and the values of these parameters are influenced by the nature and composition of the binary mixed solvent solutions.     

The structure of calcium-ammonia solutions by neutron diffraction

The microscopic structures of calcium-ammonia solutions have been established by using neutron diffraction. Total structure factors measured at 230 K reveal immediately the evolution of an uncommonly intense diffraction prepeak in the metallic solutions. As concentration is increased from 4 mole percent metal to 10 mole percent metal (i.e., saturation), this feature intensifies and shifts from 0.6 to 0.9 A(-1). It is therefore evidence of well developed intermediate-range ordering among the solvated cations, and is a microstructural signature of the observed strong phase separation of metallic (concentrated) and nonmetallic (dilute) solutions. The technique of isotopic labelling of *N by 15N was then used in conjunction with difference analysis to focus on the solvent structure in metallic solutions at 4 and 10 mole percent metal. These nitrogen-centered functions are analyzed in conjunction with classical Monte Carlo computer simulation techniques, to provide us with detailed insight into the calcium solvation and the extent of hydrogen bonding. We find that calcium is solvated by approximately 6-7 ammonia molecules, with a Ca-N distance of around 2.45 A. There is evidence of hydrogen bonding among the solvent molecules, even in the saturated 10 mole percent metal solution.     

Complexation of common metal cations by cyanins: Binding affinity and molecular structure

The ability of diverse metal cations to form complexes with cyanin has been investigated by means of Density Functional Theory (DFT) and the Quantum Theory of Atoms in Molecules (QTAIM). The strongest preference is shown by trivalent metals which exceed that of Mg(II), indicating that ion replacement processes are suitable detoxification mechanisms for plants. Molecular structure analysis indicates that the larger the metal affinity of Cy⁻ the longer the C2-C1’ bond length and smaller ρ_b value. This is understood as upon metal complexation the Cy⁻ ligand molecular structure is more compatible with a dienolate-like structure rather than the 4′-keto-quinoidal-like structure. The weight of the former increases as stronger the binding. QTAIM charges indicate that the stronger the binding energy the larger the charge transfer from Cy⁻ to the metal, reducing its positive charge below the values indicated by the corresponding Lewis structure.     

A robust in vitro Anticancer,Antioxidant and Antimicrobial Agents Based on New Metal-Azomethine Chelates Incorporating Ag(I), Pd (II) and VO (II) Cations: Probing the Aspects of DNA Interaction

A novel azomethine ligand (HNAP) [HNAP = 1-(Pyridin-3-yliminomethyl)-naphthalen-2-ol] and its Ag(I), Pd (II) and VO (II) chelates have been synthesized and structurally inspected using a wide range of spectroscopic and analytical tools, including infra-red (IR), ultraviolet-visible (UV-Vis) and ¹H NMR spectroscopy techniques, CHN analysis, molar conductance, magnetic susceptibility, and thermogravimetric analysis. The molar conductance measurements reveal that the chelates are non-electrolytes. The thermal behavior of the investigated metal chelates shows that the hydrated, coordinated water molecules and the anions are removed in successive steps followed immediately by decomposition of the ligand in the subsequent steps. The activation thermodynamic parameters are calculated from the TG curves and discussed. Complexes formation study via continuous variation m molar ratio has been investigated, and results were consistent to those found in the solid complexes with a ratio of (M:L) as (1:1) or 1:2 (M:L) molar ratio for all the monolithic and bi-valent metal complexes with square planar for Pd (II), and Ag(I) cations while, square pyramidal geometry for VO (II) cation. DFT calculations for the titled different metal-chelates have been studied and showed a good correlation with the experimental data. The prepared compounds had been checked In vitro towards numerous sorts of plant pathogenic fungi and bacteria to evaluate their antimicrobial properties and compared with some known antibiotics. Significantly, all the complexes show excellent antimicrobial activity against various strains of bacteria and fungi, including both Gram-negative and Gram-positive bacteria. Besides, the complexes exhibited high cytotoxicity against various carcinoma cell lines, including HCT-116, MCF-7, and HepG-2. Moreover, the effect of the new synthesized compounds as antioxidants was determined by reduction of 1,1-diphenyl-2-picryl hydrazyl (DPPH) and compared with that of Vitamin C. Furthermore, the binding interactions of the complexes with CT-DNA were explored using UV-Vis spectroscopy, viscosity and gel electrophoreses measurements. They cooperatively bind to DNA possibly through intercalations. The binding ability of the complexes was shown as HNAPAg > HNAPPd > HNAPVO complex.     

Solid-state chelation of metal ions by ethylenediaminetetraacetate intercalated in a layered double hydroxide

The solid-state chelation of transition metal ions (Co(2+), Ni(2+), and Cu(2+)) from aqueous solutions into the lithium aluminum layered double hydroxide ([LiAl(2)(OH)(6)]Cl x 0.5H(2)O or LDH) which has been pre-intercalated with EDTA (ethylenediaminetetraacetate) ligand has been investigated. The intercalated metal cations form [M(edta)](2)(-) complexes between the LDH layers as indicated by elemental analysis, powder X-ray diffraction, and IR and UV-vis spectroscopies. If metal chloride or nitrate salts are used in the reaction with the LDH then co-intercalation of either the Cl(-) or NO(3)(-) anions is observed. In the case of metal acetate salts the cations intercalate without the accompanying anion. This can be explained by the different intercalation selectivity of the anions in relation to the LDH. In the latter case the introduction of the positive charge into LDH structure was compensated for by the release from the solid of the equivalent quantity of lithium and hydrogen cations. Time-resolved in-situ X-ray diffraction measurements have revealed that the chelation/intercalation reactions proceed very quickly. The rate of the reaction found for nickel acetate depends on concentration as approximately k[Ni(Ac)(2)](3).     

Theoretical conformational analysis of cyclopendant phosphorus-organic complexes,derivatives of triazacyclononane

The method of molecular mechanics has been used to calculate the strain energy of complex formation of two new phosphorus-organic cyclopendant ligands, derivatives of 1,4,7-triazacyclononane. It is shown that lengthening the pendant groups by one CH₂ unit increases the volume of the ligand cavity in hexadentate coordination. However, this requires a large strain energy, which leads to a loss of stability of the complexes with metal cations. A decrease in the strain energy for penta- and tetradentate ligand profiles accounts for the appearance of selectivity towards cations with small radii. Modifying the structure of cyclopendants based on triazacyclononane by means of benzene rings does not produce a ligand profile with a definite structure, or selective binding of metal cations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1075–1079, May, 1990.