Showing papers in "Pure and Applied Chemistry in 1986"

The absolute electrode potential: an explanatory note (Recommendations 1986)

Abstract: The document begins with the illustration of the most widespread misunderstandings in the literature about the physical meaning of absolute electrode potential. The correct expression for this quantity is then de— rived by a thermodynamic analysis of the components of the emf of an elec— trochemical cell. It is shown that in principle three reference levels can be chosen to measure an absolute value of the electrode potential. Only one of these possesses all the requisites for a meaningful comparison on a con— mon energy scale between electrochemical and physical parameters. Such a comparison is the main problem for which the adoption of a correct scale for absolute electrode potentials is a prerequisites. The document ends with the recommendation of a critically evaluated value for the absolute potential of the standard hydrogen electrode in water and in a few other protic solvents. The \"electrode potential\" is often misinterpreted as the electric potential difference between a point in the bulk of the solid conductor and a point in the bulk of the electrolyte solution (L4) (Note a). In reality, the transfer of charged particles across the electrode/electrolyte solution interface is controlled by the difference in the energy levels of the species in the two phases (at constant T and p), which includes not only electrical (electric potential difference) but also chemical (Gibbs energy difference) contributions since the two phases are compositionally dissimilar (refs. 1,2). The value of the tjq of a \"single\" electrode, e.g. one consisting of an electronic conductor in contact with an ionic conductor, is not amenable of direct experimental determination. This is because the two metallic probes from the measuring instruments, both made of the same material, e.g. a metal M1, have to be put in contact with the bulk of these two phases to pick up the signal there. This creates two additional interfaces: a M1/solution interface, and a M1/electrode metal interface. The experimental set-up can be sketched as follows: M1 SIMIMI (1) where M is the metal of the electrode under measure, S is the electrolyte solution, M1 is the metal of the \"connections\" to the measuring instrument and the prime on M indicates that this terminal differs from the other one (M1) by the electrical state only. It is expedient to replace the M1/S interface with a more specific, reproducible and stable system known as the reference electrode. It ensues that an electrode potential can only be measured against a reference system. The measured quantity is thus a relative electrode potential. For the specific example of cell (1), the measured quantity E, the electrode potential of M relative to M1 (Note b), is conventionally split into two contributions, each pertaining to one of the electrodes: EEM_EM1 (2) EM and EM1 can be expressed in their own on a potential scale referred to another reference electrode. In this respect, the hydrogen electrode is conventionally taken as the universal Note a: This quantity, known as the Galvani potential difference between M and 5, has been defined in ref. 3. Note b: In accord with the IUPAC convention on the sign of electrode potentials, all electrode potentials in this document are to be intended as \"reduction potentials\", i.e. the electrode reaction is written in the direction of the reduction (refs. 3,4). 956 Absolute electrode potential (Recommendations 1986) 957 (for solutions in protic solvents) reference electrode for which, under standard conditions, E°(H/H2) = 0 at every temperature (Note c). Since EM as measured is a relative value, it appeals to many to know what the absolute value may be: viz. , the value of EM measured with respect to a universal reference system not including any additional metal/solution interface. Actually, for the vast majority of practical electrochenilcal problems, there is no need to bring in absolute potentials . The one outstanding example where this concept is useful is the matching of semiconductor energy levels and solution energy levels . However, from a fundamental point of view, this problem comes necessarily about in every case one wants to connect the \"relative\" electrode potential to the \"absolute\" physical quantities of the given system. On a customary basis, since the electrode potential is envisaged as the electric potential drop between M and S, the cell potential difference for system (1) is usually written as the electric potential difference between the two metallic terminals: EMi M1 (3) Since three interfaces are involved in cell (1), eqn.(3) can be rewritten as: E (M{ M) + (M S) + (S Mi) (4) Comparison of eqn. (4) with eqn. (2) shows that the identification of the absolute electrode potential with (M S) is not to be reconmended because it is conceptually misleading. Since M' and M are in electronic equilibrium, then (ref. 3): (4M ) = ('/F pr/F) (5) where the right hand side of eqn. (5) expresses the difference in chemical potential of electrons in the two electrode metals. Substitution of eqn.(5) into eqn.(4) gives: E = (p ii'/F) (E'q p'/F) (6) The two exoressions in brackets do not contain quantities pertaining to the other interfaces. They can thus be defined as single electrode potentials (Note d). Since eqn. (6) has been obtained with the two electrodes assembled into a cell, it is possible that terms common to both electrodes do not appear explicitly in eqn. (6) because they cancel out ultimately. The relationship between the truly absolute electrode potential and the single electrode potential in eqn.(6) can thus be written in the form (Note e) (ref. 5): EM(abs) = EM(r) + K (7) where K is a constant depending on the \"absolute\" reference system, and

Photochemistry of metal coordination complexes: metal to ligand charge transfer excited states

Abstract: Systanatic variations appear in the othphysical and photochnical properties of t4LC excited states whicth n be accounted for qualitatively or even quantitatively based on the properties of the nxlecules and of the surrounding rreditin. In 1959, Paris and Brandt first reported anission fran the cxinplex (RuUpy) (bpy is 2,2'— bipyridine) in solution (ref. 1). The excited state involved is sufficienty long-lived (-8OO ns in water at roan tanperature) that the possibility existed for using it as a thernical reagent and based on quanching studies using ccmplexes of Co(III), 1dainson and cazorkers suggested that the excited state could act as a reducing agent (ref. 2). Their suggestion was soon verified by additional quanching and flash ckiothlysis studies (ref. 3,4). It was also shawn that the excited state could act as an oxidizing agent (ref. 5), that its redox potentials could be estimated experisontally by a kinetic qunnching techniqun (ref. 6,7), and that it could undergo facile, bisolecular electron transfer with a varie of oxidants or reductants (ref. 8—10). For example, oxidative qcnching by paraquat (PQ ),

Halichondrins—antitumor polyether macrolides from a marine sponge

Abstract: New antitunor polyether macrolides were successfully isolated fron a marine sponge, Halichondria okadai Kadota. One of them, halichondrin B exhibited remarkable in vivo antitumor activity. Physiological properties and structures of these compounds are reported herein. The structures have been characterized by a long—straight carbon chain, a polyether macrolide, and a novel 2,6,9—trioxatricyclo[3.3.2.03'7]decane system which is the first example in natural products as far as we know.

Aluminophosphate Molecular Sieves and the Periodic Table

Abstract: New generations of crystalline microporous molecular sieve oxides have been discovered based on the novel aluminophosphate family by incorporating one or more of an additional thirteen elements from the Periodic Table into the AIPO4 framework. Elements incorporated include Li, Be, B, Mg, Si, Ga, Ge, As, Ti, Hn, Fe, Co, and Zn, spanning monovalent through pentavalent framework cationic species. The new materials comprise more than two dozen structures and two hundred compositions, including multi-element frameworks containing combinations of up to six framework cations. Pore sizes range from 0.3nm to 0.8nm encompassing small, intermediate and large pore structures. The new molecular sieves are synthesized by hydrothermal crystallization of reactive aluminophosphate gels containing the additional framework elements and an organic template. Proof of framework incorportion includes the formation of novel structures, the enhancement of catalytic activity, elemental analysis, and various spectroscopic evidence. The Bronsted acidity observed ranges from weakly to strongly acidic. This landmark discovery of new generations of molecular sieve materials represents a remarkable diversity in crystal structure and crystal chemistry, and offers a nearly unlimited number of design parameters to tailor adsorptive and catalytic properties.

π-Orbital conjugation and rehybridization in bridged annulenes and deformed molecules in general: π-orbital axis vector analysis

Abstract: The field of bridged annulene chemistry began twenty years ago with the synthesis of 1,6methano[10]annulene, and since that time a great many variations on this theme have been reported. By their very nature it is usually impossible for these systems to attain complete coplanarity, and irorbital misalignment often occurs in the periphery. In many cases these deformations have been confirmed by structural, spectroscopic and theoretical investigation, and it has often been noted that these compounds tolerate remarkably high ir-orbital misalignment (as measured by the peripheral dihedral (torsional) angles), without quenching of the cyclic delocalization and aromatic character. It is the purpose of this communication to point out that in nonplanar conjugated molecules: (i) the skeletal dihedral angle may be a poor index of ir-orbital alignment; (ii) rehybridization (from sp2) may have a significant effect on ir-orbital alignment; (iii) the ir-orbital alignment obtained in molecules such as the bridged annulenes is far better than hitherto realized. This process is accomplished by development of a general analytical method for the location of the ir-orbital axis vector (POAV) in nonplanar conjugated molecules. The field of bridged annulene chemistry began twenty years ago with the synthesis of 1,6-methanoL 10]annulene,' and since that time a great many variations on this theme have been reported.2 By their very nature it is usually impossible for these systems to attain complete coplanarity, and ir-orbital misalignment often occurs in the periphery. In many cases these deformations have been confirmed by structural, spectroscopic and theoretical investigation,328 and it has often been noted that these compounds tolerate remarkably high ir-orbital misalignment (as measured by the peripheral (skeletal) dihedral angles), without quenching of the cyclic delocalization and aromatic character.29 It is the purpose of this study to point out that in nonplanar conjugated molecules: (i) the skeletal dihedral angle may be a poor index of ir-orbital alignment; (ii) rehybridization (from sp2) may have a significant effect on ir-orbital alignment; (iii) the ir-orbital alignment obtained in molecules such as the bridged annulenes is far better than hitherto realized. This process is accomplished by development of a general analytical method for the location of the ir-orbital axis vector (POAV) in nonplanar conjugated molecules. We begin by noting that in order for the dihedral (torsional) angle to provide a unique and accurate picture of ir-orbital alignment, each of the bonded pair of atoms must (separately) lie in the same plane as its nearest neighbors. Thus Cl, C2, C3 and R2 (Figure 1) are required to be coplanar, as are C2, C3, C4 and R3 if the dihedral angle (usually taken as Cl, C2, C3, C4) is to provide a meaningful index of ir-orbital alignment. Clearly as the restriction on coplanarity is removed, the dihedral angle is no longer unique, and in general there exist four possible choices (Cl, C2, C3, C4; Cl, C2, C3, R3; R2, C2, C3, C4; R2, C2, C3, R3). The point really at issue of course, is the state of hybridization of C2 and C3, and a more fruitful approach is to pursue the ir-orbital axis vector (POAV) directly, which is the real quantity of interest in the present context. 137 138 ft C. HADDON and L. T. SCOTT X2 — + JPx) Figure 2. Hybrid orbitals Xi, • X for hybridization intermediate between sp2 and sp3. Xi is colinear with the ic-orbital axis vector and is constructed so as to make an equal inclination to the edges of the trihedral angle formed by Xi X3 and X4 and is defined to lie along the Z-axis. 0 is the angle of inclination made by X2, X3 and X4 to the X, Y plane. X3 — — jPx — .jPy) X4 = —=(s — + /fpy) and Xi — Fs + /Pz) X2 — }(s — *Pz + jfpx) 1 1 X3 — Pz — .JfPx — JPy) X4 --(s *PzJfPx +py) (pure sp2)

Catalysis by shape selective zeolites-science and technology

Abstract: Pores of uniform dimensions characterize zeolite catalysts. If the pores are small, the fate of reactants and the probability of forming products are determined by molecular dimensions and configurations as well as by the types of catalytically active sites present. Reactant shape selectivity occurs when some of the molecules in a reactant mixture are too large to diffuse into the zeolite pores. Product selectivity occurs when, among all the product molecules formed within the pores, only those with the proper dimensions can diffuse out and appear as products. In restricted transition state type selectivity, certain reactions are prevented because the corresponding transition state requires more space than is available. Most commercial applications of shape selectivity involve either (1) cracking of undesirable molecules to smaller, easily removable fragments, or (2) avoiding undesirable competing reactions such as coking and transalkylation. Applications discussed are distillate and lube oil dewaxing, the production of p— xylene, ethylbenzene, and pax-ethyltoluene, and the methanolto—gasoline, methanol—to—olefins, and olefins—to—gasoline—and— distillates processes. Most of these processes use the pentasil type ZSM—5 zeolite.

Fluorescence probes for polymer free-volume

Abstract: A series of fluorescence probes (p-(N,N-dialkylamino) benzylidene malononitriles) which belong to a class of organic compounds known as “molecular rotors” has been developed. The internal molecular rotation of these compounds can be slowed down by increasing the surrounding media rigidity, viscosity or decreasing the free-volume available for molecular relaxation. Inhibition of internal molecular rotation of the probe leads to a decrease in the non-radiative decay rate and consequently enhancement of fluorescence. This behavior can be used to study both the static and dynamic changes in free-volume of polymers as a function of polymerization reaction parameters, molecular weight, stereo regularity, crosslinking, polymer chain relaxation and flexibility. In addition, the dependence of the fluorescence emission maximum of these probes on media polarity allow continuous monitoring of the probes location in the polymer matrix. These fluorescence materials are capable of simultaneously probing the flexibility and polarity of the surrounding media.

Polymer-polymer miscibility

Abstract: The theoretical and practical aspects of polymer-polymer miscibility in the solid amorphous state are reviewed. The polymers include homopolymers and both random and block copolymers. Although present theoretical treatments of polymer-polymer miscibility all contain the random mixing hypothesis and are thus not applicable to mixtures that involve specific interactions between the components, most of the observed singlephase polymer-polymer mixtures involve hydrogen-bonding or other specific interactions between the components. Even in the absence of specific interactions, the composition of a random copolymer can often be tailored to provide miscibility with a particular homopolyner. Many polymer-polymer mixtures have lower critical solution temperatures, and a small number of such mixtures have given indications of upper critical solution temperatures. The special phenomena that may be observed when other polymers are mixed with block copolymers are discussed.

Recent studies in phthalocyanine chemistry

Abstract: : This paper summarizes research in phthalocyanine chemistry carried out in our laboratories during the last few years. Included herein are synthesis of binuclear and polynuclear phthalocyanines, oxygen reduction studies using phthalocyanine monolayers on graphite electrodes, emission characteristics of mono- and polynuclear phthalocyanines, electrochemical and spectro-electrochemical properties, aggregation behaviour, and consideration of the electronic coupling present in the polynuclear species. The paper finishes with a survey of recent important contributions from other laboratories.

Industrial applications of phase transfer catalysis (PTC): past, present and future

Abstract: The advantages of PTC for industrial processes are discussed and recent progress in catalysts, methodology and applications is critically reviewed. Various trends and new applications of commercial interest are emphasized.

Solvation and complex formation in strongly solvating solvents

Abstract: Earlier proposed concepts for estimation of donor properties of solvents are briefly discussed and a comparison between some of the concepts is made for 53 solvents. The kind of solvents which can be regarded as strongly solvating are proposed for the further discussion. A comparison is also made of the solvation of typically soft and hard acceptors for a number of solvents. The solvation of univalent and some divalent ions in methanol, acetonitrile, dimethylsulfoxide, pyridine, tetrahydrothiophene and liquid ammonia have been studied by means of transfer thermodynamics from water. Oxygen donor solvents and nitriles solvate in general hard acceptors well and soft ones poorly. Amines, sulfur and phosphorous donor solvents solvate soft acceptors strongly while on the other hand they solvate hard acceptors poorly. The stability of a complex is in general inversely proportional to the solvation of the metal ion or complex and the ligand. The complex formation will there— fore be weaker in solvents where the acceptor is strongly solvated. When the dielectric constant is lower than 10 the tendency to neutralization of charge through ion pair formation becomes important and the stabilities of neutral complexes will increase dramatically. CLASSIFICATION OF SOLVENTS Several auhtors have proposed concepts for a general systematizing of the donor properties of solvents. It is, however, doubtful if such a general systematizing is possible. It is plausible that several donor scales for estimation of solvation ability of solvents are necessary because of the very different acceptor properties of metal ions and complexes. The first concepts was originated by Gutman et.al. who introduced the donor numbers, DN, for the coordinating property of a solvent (ref. 1—4). The donor number is defined asthe —AH° value, in kcal mo11, of the formation of the 1:1 adduct between the donor solvent and the chosen reference electron acceptor antimony(V) chloride in dilute l,2—dichloroethane solution. Antimony(V) chloride is regarded as an acceptor on the border—line between hard and soft. In recent years alternative concepts to the donor numbers have been proposed (ref. 5—12) in order to simplify the measurements and to extend the number of solvents. The donor number can not be determined for all solvents by the original procedure since other chemical reactions take place beside the adduct formation (ref. 5,6,11,13). No donor numbers have been reported for sulfur and phosphorous donor solvents. These will certainly react immediately with antimony(V) chloride and direct measurements of donor numbers are therefore not possible. Indirect measurements of donor numbers are not always reliable (ref. 11) and such will not be further discussed in this paper. It has therefore been important to find a simple approach from which it is possible to estimate the donor properties of especially soft donor solvents. Mercury in mercuric bromide is regarded as a fairly soft electron acceptor and has been chosen as probe in the concept donor strength. Donor strength, , is defined as the difference between the symmetric Hg—Br stretching frequencies of the neutral mercuric bromide complex in gaseous phase and in a saturated solution of the studied solvent (ref. 6). Mercuric bromide is soluble and stable in all solvents studied except in liquid ammonia where it dissociates (ref. 14) and in isocyanates where it decomposes (ref. 6,15). Maria and Gal have used an approach very similar to the definition of the donor numbers (ref. 5). They have used boron trifluoride as acceptor instead of antimony(V) chloride and dichloromethane as solvent instead of 1,2—dichioroethane in order to reduce the number of side reactions. The values are given in kJmol-. There is of course a very good correlation between the and LHBF scales.

Invention of new reactions useful in the chemistry of natural products

Abstract: The design of new Radical chain reactions useful in the synthesis or modification of Natural Products is discussed. Such reactions can give good yields, and show a selectivity which complements perfectly the ionic reactions in more general use. A flexible system for the production of many kinds of carbon radicals has been invented. It should be applicable for the generation of other kinds of radicals based on elements other than carbon. In the design of these reactions a vital role is played by a disciplinary group (usually the thiocarbonyl group), an idea which is of general application in the invention of radical chain reactions. In a final section a new synthesis of pyrroles is described which is specially designed (by S.Z.Z) for intermediates in the preparation of porphyrins. New reactions useful in synthesis are usually discovered by accident. It is, however, possible to invent reactions by conception (ref. 1). Most of this article is devoted to the invention of new radical chain reactions to give good yields of products. This can be done with the aid of the disciplinary group concept (vide infra). Radical reactions have, of course, an enormous importance in the synthesis of polymers. However, we are concerned here with their use in the chemical synthesis of homogeneous, low molecular weight molecules. In this area of scientific research, radical reactions are not often used, because they are considered to be unselective and to give poor yields of products. It is the purpose of this article to show that well designed radical reactions can give high yields of single products and play an important role in organic synthesis. Radicals react with various functional groups at very different rates which can vary over many powers of ten. By a judicious choice of reagents, solvents etc.. • one can devise a system capable of effecting a highly selective transformation. Radical—radical interactions however (coupling, disproportionation etc...) are extremely fast processes and therefore much more difficult to control. If the sequence of radical reactions is conceived so as to constitute a linear chain process where the propagating steps are so fast that the concentration of radical species remains very small, radical—radical interactions can be largely avoided. Under such controlled conditions, clean high yielding reactions become possible. Moreover radical and radical chain reactions have several advantages over conventional ionic processes : neutral conditions; lower steric effects; lower polar effects; lower tendancy to unwanted elimination reactions; tolerance of many functional groups that have to be protected in ionic chemistry. In ionic reactions s—elimination is a serious problem in the carbohydrate and amino— glycoside antibiotic fields. These important compounds are heavily functionalised and selective deoxygenation or deamimation presented a challenging problem. We shall first consider the problem of deoxygenation. Many years ago, Van der Kerk (ref. 2) discovered accidentally the facile reduction of alkyl halides by stannanes. The mechanism involved is that of a typical radical chain reaction. The stannyl radicals, generated photochemically, thermally or by a chemical initiator (e.g. azoisobutyronitrile — A.I.B.N.) abstract the halogen to give a carbon radical. This latter abstracts a hydrogen atom from the hydride to give the alkane and a stannyl radical, thus propagating the chain (Scheme 1). This reaction has developed 675 676 D. H. R. BARTON AND S. Z. ZARD into a useful synthetic tool, not only for reducing halides but also for making carbon—carbon bonds by interception of the intermediate carbon radical.

Interphases in systems of conducting phases (Recommendations 1985)

Abstract: The document is an Appendix to the Manual of Symbols and Termino logy for Physicochemical Quantities and Units. Together with the other ap— pendices notably that on Electrochemical Nomenclature, it expands the recom mendations for nomenclature in the region of interphases containing charged particles. After suggesting terms for the description of the interphase it self, there are sections outlining nomenclature for electric potentials at free surfaces (a condensed phase in contact with vacuum or dilute gas) and in interphases (the boundary between two dense phases) . These include a di scussion of the electronic work function and the Volta (or contact) poten— tial difference. Interphases between metals, semiconductors and electroly— tes are discussed. Nomenclature for the thermodynamic properties of the in terphase at equilibrium is recommended including capacitances and components of charge. Quantities related to the description of the structure of the in terphase arO discussed and terms consistent with the present knowledge of this structure are recommended for the situations of the adsorption of ions or molecules and the possibility of partial charge transfer. Some general remarks are made about adsorption isotherms. The report concludes with some recommendations for the nomenclature of the mechanical properties of solid surface.

Regulation of secondary metabolism in fungi

Abstract: Secondary metabolites (idiolites) are structurally diverse and unusual, generally are produced in mixtures with other members of the same chemical family, and usually are formed at low specific growth rates. In batch cultures, processes leading to the production of idiolites are often sequential; the cultures exhibit a distinct growth phase (trophophase) followed by a production phase (idiophase). However, trophophase and idiophase may overlap. Timing depends on the nutritional environment presented to the culture. Specific mechanisms regulating the onset of idiolite synthesis include repression by sources of carbon, nitrogen and phosphate and enzyme induction. Cessation of idiolite biosynthesis occurs via decay of idiolite synthetases as well as feedback inhibition and repression of these enzymes. With regard to control of mycotoxin biosynthesis, most studies have been done on ergot alkaloid, aflatoxin and patulin formation. Ergot alkaloid formation is mainly controlled by carbon source regulation (glucose), phosphate repression, induction (tryptophan), growth rate and feedback inhibition (agroclavine and elymoclavine). Aflatoxin biosynthesis is controlled by nitrogen source repression (nitrate) and induction (glucose); zinc stimulates production in an unknown manner. Patulin production is regulated by nitrogen source repression, induction (6—methylsalicylic acid), growth rate and synthetase decay.

Photochemistry of organic molecules in microscopic reactors

Abstract: The photochemistry of dibenzyl ketone and its derivatives has been employed to investigate the ability of zeolite molecular sieves to modify and to control the reaction channels available to organic molecules adsorbed on the internal and external zeolite surfaces. It is found that the observed photochemistry is very sensitive to the size/shape characteristics of the substrate ketones and of the pores and internal void space of the zeolites. Although unprecedented reactions of ketones have been found to be induced by absorption of the ketones on the zeolite surfaces, the reactions are consistent with expectations based on the topological structure of the zeolite surfaces and on the mechanism of ketone photolysis in homogeneous solution. ZEOLITE MOLECULAR SIEVES. DESIGNER MICROSCOPIC REACTORS Zeolites (fron the Greek words zeo, "to boil" and lithos "stone")1 are synthetic o natural minerals that often expel water so violently when heated that they appear to boil. Classical zeolites are crystalline aluxainosilicates whose internal porous structure contain channels and/or cages filled with exchangeable cations and which may also be filled with adsorbed water. The framework composition of zeolites consists of cations, aluminum, silicon, and oxygen. The framework constitution of zeolites consists of tetrahedral Al atoms and tetrahedral Si atoms linked by the sharing of 0 atoms (Figure 1). The porous structure of zeoM,(AlO2)(SiO2)mH2O COMPOSI110N Figure 1. Composition and constitution of a /0N O\ /°NO ,O'i' classical aluiainosilicate zeolite framework. Al Si Si Si CONSTITUTION The subscript x refers to the number of Al j: f j j J . atoms (negative charges) in the framework; the "0 C) 0 9 ( Q ( tj'.rs subscript y refers to the number of Si atoms in the framework;and the subscript n refers to HYDROPHILIC HYDROPHOBIC HYDROPHIUC the charge of cation M. lites results from the framework configuration, i.e., the three-dimensional geometric network of Al04 and 5i04 tetrahedra. The zeolite frameworks which are obtained from natural or synthetic preparations contain pores, channels, cages, and interconnected voids. These void spaces and the internal zeolite surface occur in periodic fashion because of the crystalline nature of the framework. The combination of the topology of the internal void space and the chemical characteristics of the internal framework structure provide chemists with "designer microscopic reactors" in which chemical reactions can be performed. It can be imagined that the size and shape of these microscopic reactors, in conjunction with the peculiarities of molecular diffusion in periodically repeating void spaces will result in unusual characteristics of chemical reactions which are performed on molecules adsorbed on zeolites. Indeed, the ability of zeolites to selectivity adsorb molecules based on size/shape selectivity rules has led to their designation as "molecular sieves". As a result of this "sieving" characteristic, the usual domination of substrate molecular structure in determining the course of chemical reactions might be replaced in certain circumstances by a domination of environmental structure in determining the course of chemical reactions for reactions conducted on zeolite nolecular sieves. In this report we show how these ideas can be given experimental realization by the judicious selection of zeolites, substrates, and photoreactions. Before describing the actual systems investigated, we shall review briefly some characteristics of important classes of zeolites whose internal surface and internal void space will drive the cheist's imagination in the selection of substrates and photoreactions. Then we shall review a photoreaction whose outstanding generality, mechanistic characteristics, engineering versatility, and convenience of execution and analysis will further define the selection of substrates for study.

Toxins from cyanophytes belonging to the scytonemataceae

Abstract: Two highly cytotoxic substances, scytophycins A and B, have been isolated from cultured Scitonema pseudohofmanni. Gross structures are proposed for scytophycins A and B, mostly on the basis of nuclear magnetic resonance spectral studies. The scytophycins are structurally related to tolytoxin, a toxic lipid found in field-collected Tolypothrix conglutinata var. colorata, another blue-green alga belonging to the family Scytonemataceae. At sublethal doses the scytophycins display moderate activity against P-388 lymphocytic leukemia and Lewis lung carcinoma in mice.

Calixarenes as new functionalized host molecules

Abstract: Water—soluble calixarenes with various substituents have been synthesized for the first time. It was demonstrated that these water— soluble calixarenes serve as a new class of catalysts, surfactants, ligands, and host molecules.

Macrocycles and their selectivity for metal ions on the basis of size

Abstract: The metal ion size selectivity of the oxygen— and nitrogen— donor macrocycles is examined It is shown that the presence of the neutral oxygen donor in ligands leads to stronger complexation of large metal ions, with less favourable complexation of small, irrespective of whether the oxygen donor is part of a macrocyclic ring or not Molecular Mechanics calculations indicate that the size—dependence of the complexing of metal ions by the neutral oxygen donors is controlled by a balance between the steric strain produced by the group bearing the oxygen donor, and its inductive effects For tetraazamacrocycles the stability is controlled by the size of the chelate ring formed on complex—formation Larger chelate rings lead to greater complex stabilisation for small metal ions, while larger metal ions show progressively greater complex destabilisation with larger chelate rings This apparent paradox is also examined using molecular mechanics calculations The use of neutral oxygen donors and chelate ring size to control metal ion size selectivity in ligand design is discussed

High-spin polycarbenes as models for organic ferromagnets

Abstract: In response to the Mataga's prediction of ferromagnetic hydrocarbons (1968), we have taken up the study of two series of high—spin polycarbenes (1 and 2). The corresponding polydiazo compounds were prepared throuh a sries of unambiguous synthetic reactions and photolyzed in 2—methyltetrahydrofuran matrices and in single crystals of a benzophenone host at cryogenic temperatures. The ESR fine structures and magnetic susceptibilities were measured and analyzed to show that the highest spin states were generated as the electronic ground state of 1 and 2. Similarly, isomeric bis(diazo)—[2.2]paracyclophanes were prepared aid ph&Eolyzed to find, in good agreement with the McConnell's theory on the ferromagnetic intermolecular interaction between organic free radicals (1963), that the pseudoortho and pseudopara dicarbenes have the ground quintet state while the pseudometa isomer is in the ground singlet state. A strategy for increasing the spin ordering over the high—spin aromatic molecules by orienting the stacking mode was thus obtained. Relevance of these results to macroscopic ferromagnets is discussed.

Developments in the synthesis and reactivity of encapsulated metal ions

Abstract: The use of the template strategy to make larger and smaller cavity sizes of encapsulating ligands is explored and the effect of cavity size and stereochemistry on redox potentials and electron transfer reactions is examined. The cages have also been modified in a variety of ways by oxidation of the ligand to hydroxylamines, imines, amides and aromatic systems. The mechanisms of extrusion of metal ions from the cages are also discussed. Much of the interesting chemistry associated with the hexaamine metal ion cages has been related to the rapid redox changes they undergo and their unusual stability, both in a kinetic and thermodynamic sense. These factors make them useful as redox reagents of an innocent kind and the Stability allows experiments which mostly are not feasible with their tris(bidentate) analogues. The elaboration of the cages has been carried on with respect to these properties but there are basic issues still to be answered. For example, what happens if the cavity size is increased; is the complex destabilized? Are the redox rates altered dramatically? Is the coordinated ligand reactive and how does the metal ion influence that issue? How does the metal ion come out of the cage? These are all questions which need to be answered in order to understand and use the encapsulation chemistry effectively and this lecture addresses some of those matters. One obvious problem has been to increase the natural cavity size in the ligand in order to accommodate larger low oxidation state ions and also modulate the redox potentials of couples by this strategy. An obvious route to take was to use the broad capping strategy successful for the tris(1,2—ethanediamine) complexes (ref. 1) and apply it to either tris(1,3-propanediamine) complexes or to sexidentate complexes of type 1:

Catalytic and acidic properties of boron pentasil zeolites

Abstract: Pentasil-type zeolite with boron isoraorphously substituted for silicon in the pentasil zeolite framework and aluminum pentasil (ZSM-5 type) zeolite impregnated with a boron compound have been prepared and studied. Lattice boron is shown to present trigonal or tetrahedral environment depending upon ligand adsorbates and to induce a weak acid strength and subsequently negligible catalytic activity for acidic-type reactions as methanol conversion to hydrocarbons, toluene alkylation with methanol and toluene disproportionation. Some activity was observed only if lattice aluminum was present or if aluminum based binder was used. Lattice boron was shown not to modify appreciably shape selectivity. Boron compound impregnated on aluminum pentasil sample was shown to decrease the acidity and subsequently the catalytic activity for acid type reactions but to sharply enhance shape selectivity, i.e. the yield in the less bulky para isomers of aromatics with respect to the other isomers. Boron of the impregnated compound (H3BO3) was shown to partly isomorphously subsitute for Si or Al upon calcination at 773K while calcination at 1073K results in the partial formation of a glassy borosilicate compound at the surface of the zeolite particles. These findings are discussed in light of binder-zeolite interactions.

Photochemistry on surfaces

Abstract: — The photophysics and photochemistry of organic molecules (aro— matics , aza—aromatics , diphenylpolyenes , azobenzene , triphenylmethane dyes, thioindigo) adsorbed on metal oxides (alumina, silica, thoria, ti—tania) were investigated by steady—state and time—resolved diffuse ref lec—tance and luminescence spectroscopy.The fluorescence decay curves of almost all adsorbates are non—exponen—tial. This effect is discussed in terms of different polarizing surfacesites, aggregation, and interference between directly emitted and scat—tered fluorescence radiation.A method is elaborated to determine rate constants and quantum yields ofphotoreactions in strongly scattering rigid media by photometric measure—ments using the model of radiative transfer. The method is applied toquantify internal and translational mobilities of adsorbed photoexcitedmolecules, ring closure reactions, bimolecular photoprocesses, and chargetransfer between the adsorbent and the adsorbate. INTRODUCTION Photochemical reactions are conventionally studied in homogeneous phases and, as far as pos—sible, in ideally stirred media. These conditions, however, are not to be realized in manysystems of biological or technical interest. The medium can be strongly light—scattering andhighly viscous, and the reacting species can be very inhomogeneously distributed in it (e.g.in membranes or in the adsorbed state). Compared with the total activities in photochemicalresearch, the heterogeneous phenomena have received up to now only moderate attention. Butthe attention is strongly growing —

Scientific evaluation of Tian Hua Fen (THF) : History, chemistry and application

Abstract: Tian Hua Fen (THF) was first recorded as a medicine for reestablishing menstruation in \"Quian Jin Yi Fang\" (about 682 A.D.) and as an abortifacient in IvTai Ping Sheng Hui Fang\" (992 A.D.). THF, prepared from the root tuber of Trichosanthes kirilowii Maxim (Cucurbitaceae), had been used locally in admixture with several other herbal ingredients with severe side reactions. Through series screening, an abortion—inducing protein, trichosanthin (T), of p1 9.4 has been isolated from the root tubers in pure crystals. It is composed of 234(3) amino acid residues: Ala28Arg13Asn17Asp9Gln9Glu10Gly12His1Ile15— Leu27Lys8Met4Phe9ProgSer24Thr13Trp1Tyr13val12; M.W.: found, 24,000; calculated, 25,682. T is obtained as a mixture of two homologous polypeptides differing only at their C— terminals. The C—terminal of one of them is —Met—OH, and that of the other, —Met—Ala—OH. Cyanogen bromide degradation of T gave 4 peptide fragments (CB1—4) and some free alanine. T, N—maleyltrichosanthin (MT) and CB1—4 were furthur subjected to enzymic degradations. The primary structure of T has been elucidated by sequencing T and all degradation peptide fragments from the N—terminals by usual Edman degradation methods and from the respective C—terminals by using carboxypeptidases A, B and Y. Trp was located in the T—200 position. X—ray crystallography revealed that T crystals from barbiturate buffer belong to monoclinic system and show 8 o(,—helices and 13 t3—strands constituting 4 — pleated sheets. T is parenterally employed as a safe abortifacient in dose of 1.2 mg for woman of mid pregnancy and in combination with other medicines for early pregnancy. It is also effective in treating hydatidiform moles, ectopic pregnancy as well as choriocarcinoma.

Microporous membranes in membrane distillation

Abstract: When a microporous hydrophobic membrane separates two aqueous solutions at different temperatures, selective mass transfer across the membrane is obtained. The process can be carried out at the atmospheric pressure and at temperatures which might be much lower than the boiling point of the solution. At the optimal membrane microporosity, polymer hydrophobicity and thermal conductivity, only water vapor is transported in the membrane phase and condenses as liquid on the low temperature side of the membrane. The driving force is the vapor pressure difference between the liquids at the two solution-membrane interfaces. Various hydrophobic polymeric membranes are available, prepared from polypropylene, PVDF and Teflon, in the flat sheet or capillary configuration. The membranes prepared by thermal phase—separation technique are particularly interesting. INTRODUCTION Membrane Distillation is a new, rapidly increasing membrane technology, characterized by the possibility of overcoming some limits of other membrane processes, such as reverse osmosis (ref. 1,2). When compared to traditional evaporation, membrane distillation offers also the basic advantages of membrane separations: easy scaling up, simplicity of operations, possibility of high membrane surface/volume ratio, etc. Moreover, there exists the possibility of treating solutions with thermosensitive compounds and high level of suspended solids, at a temperature much lower than the boiling point and at the atmospheric pressure. Theoretical 100% rejections might be predicted for all electrolyte and non-electrolyte solutes. The possibility to reach a high solute concentration in the feed is of particular interest, considering the limits of RO due to the osmotic pressure increase with concentration. PROCESSMECHANISM When a microporous hydrophobic membrane separates two aqueous solutions at a different temperature, a net pure water flux from the warm side to the cold one is observed. The process can be described by the following steps: water evaporation at the solution-membrane warm interface, transport of the vapor phase through the microporous system, and condensation at the cold membrane-solution interface (see Fig. 1). The driving force for the vapor transport in this process is given by the vapor pressure difference between the two solution-membrane interfaces due to the existing temperature gradient. The hydrophobic properties of the polymeric material prevent the bulk liquid transport of the liquid phase across the membrane. The morphology of the polymeric membrane is, however, a critical parameter of the process. A maximum critical pore size exists at which the liquid penetrates the microporous hydrophobic phase. This value is given by the Kelvin law: P = 2y cos /r where y is the surface tension of the liquid; 0 is the contact angle between the liquid and the membrane; r is the radius of the pore; P is the applied pressure.

Design of cation selectivity into liquid membrane systems using macrocyclic carriers

Abstract: Liquid membranes are useful devices for the design of systems capable of separating selectively one solute from another. The systems considered are of the bulk, supported, and emulsion liquid types in which a macrocycle—containing membrane (organic liquid or porous polymer containing an organic liquid) separates aqueous source and receiving phases. Four general types of transport in these systems are discussed and illustrated with examples of actual metal separations involving competitive experiments. The ability to control metal selectivity in these systems by altering macrocycle type and the compositions of the source and receiving phases is described. Membranes are ubiquitous in nature and are responsible for the transport of tremendous quantitites of material in the animal and plant kingdoms. The selectivities these membranes show for certain solutes and their rejection of others has fascinated scientists for centuries. Recently, there has been increasing interest in the possible use of artificial membranes in specific separation procedures of industrial and academic importance. The objective of this paper is to present and discuss selected design criteria which could be useful in the development of membrane systems for use in such procedures. The systems chosen to illustrate the design criteria selected for discussion are taken from recent work in our laboratory. Our earlier work and the work of others in defining and investigating the parameters involved in liquid membrane transport have been reviewed (ref.1,2). All of the different membrane systems studied by us have the same general configuration consisting of aqueous source and receiving phases separated by a membrane which usually consists of an organic liquid. Appropriate modification of the constituents of each of the component parts of these systems allows one to approach a desired level of selectivity in' solute transport. In the interfacial regions which exist in our membrane systems, properties such as solvent dielectric constant are intermediate between those of the aqueous and organic membrane phases. The accomplishment of successful species separation involves the preferential extraction, usually by a carrier molecule such as a macrocycle, of one species into the organic membrane and the subsequent discharge of that species into the receiving aqueous phase. This process requires that a concentration gradient in the desired species (or a constituent thereof) be created and maintained in the organic membrane from the source phase side to the receiving phase side. The liquid membrane systems studied by us are of the bulk, supported, and emulsion types. These membrane types are illustrated in Fig. 1. The details of their operation are available (ref. 3,4,5). Each type offers certain advantages. The bulk system is extremely simple, inexpensive, and uses small amounts of material (e.g., mg. quantities of carrier). These advantages make this system desireable for screening carriers, which may be available only in small quantities, and many cation systems to learn which are sufficiently interesting to study further. The supported system has the advantage of fixed interfacial areas making possible the quantitative determination of transport mechanisms. Both aqueous phases are stirred in this system. There are two major advantages of the emulsion system over the other types. First, the transported species can be enriched up to ten fold since the receiving phase volume is one—tenth that of the source phase. Second, the rate of transport is rapid. Whereas an experiment in either the bulk or supported system requires 21 hours, a single emulsion experiment requires approximately 30 minutes and near 100% transport can occur in 5— 10 minutes. Transport mechanisms in membranes of the general types illustrated in Fig. 1 have been discussed (ref. 2). A knowledge of the parameters which affect the transport process provides the information necessary to design selectivity into membrane systems. An important objective of our research has been to study the effect on transport of systematically varying various parameters, particularly where competitive transport of metal ions or metal— containing species is involved. The parameters which will be illustrated and discussed here are (1) the solvation energies of the species involved in the transport process, (2) the