Showing papers in "Pure and Applied Chemistry in 1971"

Photodimerization in the solid state

Abstract: Following a brief historical review of solid-state photochemistry current research is discussed under four parallel headings: first, analysis of the topochemical postulate according to which the course of the solid-state reaction and the stereochemistry of the photodimer (if any) can be predicted from the configuration and nearest-neighbour geometry of closest monomer molecules in the crystal lattice; secondly, the study of the lOCUS of the reaction, that is the dependence of the course of the reaction or (dimerization, cis — trans isomerization) on crystal texture (dislocation, grain and phase boundaries); thirdly, a better understanding of the packing principles of organic molecules in their crystal structure to enable us to construct photoreactive (or lightstable) crystal structures of any given monomer as well as mixed crystals of potentially codimerizable monomers for synthetic purposes or energy-transfer studies. Several approaches to this problem of 'crystal engineering' based on generalizations of packing modes of primary amides and of dichiorophenyl derivatives are outlined, and their application to a systematic photochemistry of the solid state discussed.

Effects of unshared pairs of electrons and their solvation on conformational equilibria

Abstract: The empirical rules for estimating molar rotations of carbohydrate structures proposed by Whiffen and elaborated by Brewster provided interpretations of solvent effects on optical rotation with changes in conformational equilibria which in certain cases were confirmed by n.m.r. Conversely, it was found that only slight changes in rotation occur on changing the solvent for conformationally rigid molecules. Thus, studies involving optical rotations measured at the D-line of sodium and n.m.r. spectra provided the experimental support for the following solvation phenomena which are related to the role of unshared pairs of electrons in conformational equilibria. (a) The reverse anomeric effect was substantiated by the effect of introducing a positive charge on the imidazole ring of certain N-glycosides of imidazole. (b) The magnitude of the anomeric effect was influenced as expected by changes in the polarity of the solvent but hydrogen bonding of the solvent with the acetal oxygen atoms had a more pronounced effect. (c) The orientation of oxygen atoms in gauche relationship appears particularly favourable with water as solvent. (d) An intramolecular hydrogen bond between two hydroxyl groups is strengthened by hydrogen bonding of the free hydrogen to a basic solvent. le) The non-bonded interaction between two opposing axial oxygen atoms is dependent on the nature of the substituents. The repulsion is substantially greater when the oxygen atoms are either bonded to methyl groups or hydrogen-bonded to the solvent than when attached to acetyl, benzoyl or methane-sulphonyl groups. Tim first statement that unshared pairs of electrons may play an important role in establishing conformational preferences was made by Edward' with reference to the apparent stability of that anomeric form for a glycosyl halide which has the halogen in axial orientation. The statement made was, as seen in Figure 1, that by going axial the polar C, to X bond avoids an interaction with the axially oriented orbital of the ring oxygen. Later this was discussed by Kabayama and Patterson2 who pointed out that the unfavourable interactions could, indeed, involve repulsions between the orbitals occupied by lone pairs of electrons in the aglycon X with those of the ring oxygen. Such interactions equivalent to syn-axial interactions, would be released on passing from the equatorial to axial anomer.

Magnetic effects on triplet exciton interactions

Abstract: The rates of many bimolecular reactions involving triplet excitons are magnetic field dependent at room temperature and for modest field strengths. The specific interactions discussed are the fusion of a pair of triplet excitons to yield either a single or triplet exciton, the interaction between a triplet exciton and a trapped triplet, the fission of a singlet exciton into a pair of triplets and the triplet exciton-free radical interaction. The magnetic field effects result from the influence of the field on the spin wavefunctions of the triplet exciton together with the existence of spin selection rules for the interactions. A simple theory of these effects is presented. Application of the theory to the experimental field dependence results yields new information about the mechanisms of these interactions.

Conformational mobility and fast bond shift in the annulenes

Abstract: After a general description of the configuration, of the conformation and of the fast and reversible dynamic processes characteristic of the annulenes, results of an experimental study of [8], [12], [14], [16] and [18]annulenes by N.M.R. spectroscopy are presented. Theoretical analysis of the N.M.R. line shape has allowed a determinati9n of the structures of these compounds and of the thermodynamic and kinetic parameters of their dynamic processes. Study of bond shift in cyclooctatetraene (COT) and of bond shift and ring inversion in alkoxy-COT's point to a small negatiVe resonance energy in the planar conformation of COT. Two configurations of [16]annulene in 3: 1 ratio, coded 85 and 91, have been found in solution. Both of them undergo isodynamical conformational mobility and bond shift and a fast exchange between both is also occurring. The newly synthesized [12]annulene of configuration 21 undergoes extremely fast conformational mobility but no bond shift. Conformational mobility is rather slow in [18]annulene and in the major isomer of [14]annulene, of configuration 43. It is very fast in [14]-21 annulene. Exchange between [14]-43and [14]-21-annulenes is very slow. The present results agree generally with the calculations of Dewar and Gleicher.4

Structure and thermodynamics of lanthanide and actinide complexes in solution

Abstract: Several questions of importance in the study of lanthanide and actinide coordination compounds are reviewed. There is considerable evidence that in aqueous solution the primary coordination number is nine for the ions La(ni) through Nd(m) and eight for the ions heavier than Gd(m). While it seems that some degree of covalency exists in the metal—ligand bonding a model of electrostatic bonding seems satisfactory for explaining the structure and formation of the complexes. The dehydration of the lanthanide ions upon complexation largely determines the enthalpy and entropy data. However, there seems to be a compensation effect in the hydration parts of those terms such that the free energy changes seem to reflect the metal—ligand reaction unobscured by hydration factors. A number of individual inorganic and organic ligand systems are reviewed from these viewpoints. The lanthanide and actinide elements constitute two families of metals which often exhibit very similar chemical behaviour. This similarity is most easily observed with the cations in oxidation state iii, the most common one for all the elements of the lanthanide series and for the transpiutonium elements of the actinide series. This paper reviews the present state of understanding of the complexes formed in aqueous solution by the trivalent ions of these two families ofelements. The coordination chemistry of these elements has been studied neither as intensively nor as extensively as that of the transition metal ions. One reason for this is that these elements have been less available generally than the transition metal elements. Although they are fairly abundant, a lack of industrial application until a relatively few years ago caused many of the lanthanide elements to be fairly expensive. Increased interest in the use of these elements in phosphors and lasers and as catalysts as well as improvements in the methods of separation has caused a substantial reduction in their cost. The availability of the transuranium elements of the actinide series was limited to a few nuclear energy laboratories where greater emphasis was placed on research of either a more nuclear or more applied nature. This latter situation has changed as the transpiutonium programme of the US Atomic Energy Commission has resulted in production of relatively large amounts of the elements as heavy as californium (Z = 98). However, the 23

The theoretical design of novel stabilized systems

Abstract: Simple symmetry and bonding arguments, coupled with detailed molecular orbital calculations, are used in the design of three novel stabilized systems: (1) Spirarenes—allylic radicals coupled in a Spiro system. (2) Diradicals converted by heteroatom replacement or substitution into singlet ground state zwitterions. (3) The stabilization of planar tetracoordinate carbon. To devise novel types of aromaticity is not easy. There are a lot of very clever theoreticians around, likely to think of these things before us. More important there is Nature to contend with. If a system is indeed extremely stable there is a good chance that it occurs naturally Or if an unnatural act is needed to produce such a molecule it is probable that the compound has already serendipitously fallen out of one of the many pots that organic chemists keep boiling around the world. Nevertheless it has been our fortune to come upon some novel means of conferring stabilization upon a molecule. Though not yet experimentally achieved these cases are theoretically amusing, and I would like to recount the story of three such systems to you. THE SPIRARENES AND SPIROCONJUGATION The well known molecular orbitals of an ally! system are shown in Figure 1. If two allyl radicals are brought together in such a way as to produce little interaction among the energy levels then the ground state of the system of the two radicals is likely to be a triplet. If the radicals are brought together so that the levels interact appreciably then it is possible that the nonbonding levels may interact strongly enough so that the ground state of the six electron system is a stabilized singlet (Figure 4). Such stabilization has been effectively achieved in the various collapse products of two allyl radicals. e.g. 1, 2, 3, 4.

The mycobacterial cell wall.

Abstract: The Mycobacterial cell wall contains a peptidoglycan—glycolipid complex and various more or less defined glycolipid fractions. The detailed chemical structure of the peptidoglycan and the glycolipid (a mycolate of an arabinogalactan) is discussed. Figure 4 shows a tentative formula for a 'monomer' of the cell wall and Figure 6 depicts a possible 'decamer.' Then the chemistry of three glycolipids which seem closely associated with the cell wall is reviewed: (1) Wax D of human strains is a mucopeptide-containing mycolate of arabinogalactan and is probably derived from the cell wall by enzymatic degradation as shown in Figure 6; (2) Cord factor is a dimycolate of trehalose (formula 22); (3) A newly defined suipholipid is a 2-sulphate of a tetra-acyl trehalose (formula 26). Other possible components of the cell wall are mentioned, such as the mycosides, a glucan and lipopeptides. Mass spectrometry has been a very helpful tool for defining the chemistry of all these compounds. Finally the biological activities of mycobacterial cell walls and their components are considered, such as: specific immunization against tuberculosis, adjuvant activity and stimulation of non-specific resistance to infections. In this lecture, I shall try to summarize our present knowledge of the chemistry of a very complex and most fascinating natural macromolecule: the cell wall of Mycobacteria and related organisms. 'Very complex' because we shall see that it contains lipids, peptides and carbohydrates. 'Fascinating' because there is a great deal of novel structural chemistry involved and new biosynthetic pathways to explore, and because many of the pathogenic effects of tubercie bacilli and related organisms are due to constituents of the cell wall. We shall be concerned not only with the insoluble macromolecular, rigid cell wall and its covalent chemical structure, but also with a series of soluble lipid compounds which seem to be located in or on the outer part of the cell wall, such as wax D, cord factor and sulpholipids. We shall start with a chemical study of the various structures, and shall see that mass spectrometry has been a most helpful tool for bringing precision and order into this most complex field, and we shall end this lecture with a rapid survey of the biological properties of mycobacterial cell walls. Animal experiments show indeed that preparations derived from mycobacterial cell walls have interesting properties concerning adjuvant activity, 135 E. LEDERER and the stimulation of non-specific resistance, which can be used not only against bacterial and viral infections, but also against certain types of leukaemia and cancer. I. CHEMICAL STRUCTURE OF THE MYCOBACTERIAL CELL WALL: A PEPTIDOGLYCAN GLYCOLIPID COMPLEX The two major components of the mycobacterial cell wall are: a peptidoglycan and a glycolipid (Kotani et al.1, Takeya et al.2, Misaki et al.3, Kanetsuna4). (1) The peptidoglycan (mucopeptide or murein)t Here we can distinguish the glycan backbone consisting of a repeating disaccharide unit and a peptide moiety. (a) The disaccharide unit The classical work of Ghuysen5' 6, Jeanloz7, Park8, Salton9 and Strominger1° has shown that the cell wall of all bacteria analysed so far contains a peptidoglycan having a glycan backbone of repeating disaccharide units of structure 1, where N-acetyl-D-glucosamine is linked in 31 —+ 4 to N-acetyl-D-muramic acid. The only variations found so far were that N-acetyl muramic acid is 6-O-acetylated in Staphylococcus aureus cell wall'1 and that muramic acid forms a lactame in certain bacterial spores1 2

Stereochemical problems in macrolide antibiotics.

Abstract: Since 1950, well over fifty macrolides have been derived from various Actinomycetes in screening laboratories located at many corners of the world. Among the most intensely studied macrolides are those produced on a commercial scale (erythromycin, oleandomycin, tylosin, leucomycin and spiramycin) as well as examples of particular historical/scientific importance (Magnamycin, pikromycin, narbomycin, methymycin). As a result of chemical studies by numerous investigators, many macrolides are now structurally defined. Their overall constitutional structures reveal an unusual wealth of stereochemical features involving numerous asymmetric centres and conformational possibilities among 12-, 14or 16-membered lactone rings containing an array of substituents including one, two or three glycoside units. Fruitful approaches to the special macrolide sugar problems have followed classical carbohydrate stereochemistry while numerous aglycone asymmetric centres have been successfully defined through localization in a variety of diagnostic fragments. X-ray studies are now complete on three macrolide derivatives. Total absolute configurations are ascribed to oleandomycin, erythromycin, Magnamycins, leucomycins, spiramycins, methymycin and pikromycin. Moreover, considerable configurational data are available on the aglycones of narbomycin, neomethymycin, lankamycin and chalcomycin. Recent structural revisions have greatly simplified the overall stereochemical problem by bringing 'into line' certain biogenetically 'out-of-step' proposals that have involved unusual 17-, 18and 22-membered rings as well as misplaced sugars. Growing stereochemical knowledge has largely confirmed an earlier biogenetically-based hypothesis that all antibacterial macrolides can be viewed as following the same configurational model regardless of ring size or degree of carbon branching in the aglycone chain. Application of the model to specific macrolides in need of further configurational definition affords likely insight to the chirality at many experimentally unprobed asymmetric centres. Recent conformational analyses of the 14-membered ring system in erythromycin aglycones using 100 MHz nuclear magnetic resonance and circular dichroism techniques indicate that the shape of the molecule in solution is relatively stable and similar to that in the crystal. The conformation most evident for erythronolide B is a modification of the diamond-lattice section model, taken from the geometry of cyclotetradecane. Available evidence suggests that 'isosteric' macrolides (erythromycin, lankamycin and oleandomycin) have similar conformations. Although the configurational and conformational models for macrolide antibiotics are still undergoing further refinements, they have proven useful during the entire course of their development in testing certain biogenetic and physico-chemical theories. At present, they are finding increasing applica-

Natural and artificial bleomycins: chemistry and antitumour activities

Abstract: The bleomycins are a group of antibiotics produced by Streptomyces verticillus and their therapeutic effect against the squamous cell carcinoma has been proved by clinical studies. In our laboratory, eight products, viz. bleomycin A1, demethyl-A2, A-a, A-b, B2, A5 and B4 have been isolated in the pure state. Structures of the various bleomycins have been elucidated and it has been observed that they differ from each other in the amine moiety. The addition of an amine, especially diamines and triamines, during fermentation, induced the production of the bleomycin containing the amine added and suppressed the production of other bleomycins. Thus, 42 artificial bleomycins were synthesized and their biological activities studied. The selective effect against squamous cell carcinoma and the mechanism of the selective activity that was shown by an enzymatic inactivation will be discussed. A partial purification of the inactivating enzyme has been achieved. In the study started from phleomycin1'2 we found a group of antibiotics which were produced by Strepto,nyces verticillus and were differentiated from phleomycin by paper chromatography and stability in aqueous solution. Phleomycin caused irreversible renal damage but this antibiotic did not, and we named this antibiotic bleomycin3' '. Bleomycin has been confirmed to be effective against squamous cell carcinoma and it is used for treatment of this type of human cancer. Bleomycins which are obtained from the culture filtrate by cation exchange resin process, carbon chromatography and alumina chromatography are in their copper-chelated forms. Both the copper-chelated and the copper-free bleomycins are equally active and copper-free bleomycin mixture is used clinically. In the second paper, we reported bleomycins A1, A2, A3, A4, A5, A6, B1, B2, B3, B4 and B5. Later, we found demethyl-A2, A'-a, A'2-b, B'1 and B6. These bleomycins were separated by CM-Sephadex C-25 column chromatography raising the concentration of ammonium formate from 0.05M to 1.OM. An example of the chromatography is shown in Figure 1. All bleomycins have the maximum at 292 mt and the absorption of the eluate at this wavelength was determined. A1 and demethyl-A2 which appeared in peaks I and II respectively were separated by column chromatography of Dowex 50 resin treated with 02M pyridine acetate buffer at pH 4.4. The elution was carried out by raising the pH of the same buffer from pH 4.4 to 5.5. The separation of A-a and A-b, both of which appeared in the peak V, was accomplished by repetition of CM-Sephadex C-25 chromatography, raising the concentration of 665

Thermal degradation of polyvinyl chloride

Abstract: A survey is given of the recent advances in the study of and the present knowledge of the thermal degradation of polyvinyl chloride. The sites for initiation of the thermal degradation, the mechanism of the dehydrochiorination, the discolouration of PVC by heat and the influence of plasticizers on the rate of degradation are discussed.

Antibiotics and other naturally occurring hydroxamic acids and hydroxamates.

Abstract: Naturally occurring hydroxamic acids and hydroxamates are reviewed and classified according to the chemistry of the hydroxamic acid functions. The identity of the Fe(m) trihydroxamate albomycin o2 with antibiotic Ro 5-2667 is established. TABLE OF CONTENTS 1. Monohydroxamic acids 1.1. Linear hydroxamic acids 1.1.1. Primary hydroxamic acids 1.1.2. Secondary hydroxamic acids 1.2. Cyclic hydroxamic acids 1.2.1. Pyrazine-type skeleton

Mechanism of addition of singlet oxygen to olefins and other substrates

Abstract: Mechanisms of addition of singlet oxygen to various classes of substrates are surveyed. The experimental data for the allylic hydroperoxidation are best fit by a concerted, ene-type' mechanism; the addition to dienes is probably a Diels—Alder reaction. 1 ,2-Cycloadditions to electron-rich olefins may proceed by a 1,4-dipolar mechanism, although there is as yet little evidence, while the addition to suiphides may go by a very similar nucleophilic addition to oxygen.

Total synthesis of the gene for an alanine transfer ribonucleic acid from yeast.

Abstract: The methodology developed for the synthesis of bihelical DNA consists of the following three steps. (1) Chemical synthesis of deoxyribopolynucleotide segments containing 8 to 12 nucleotide units. These should represent the entire two strands of the intended DNA and those belonging to the complementary strands should have an overlap of 4 to 5 nucleotides. (2) The phosphorylation ofthe 5' -hydroxyl group with ATP carrying a suitable label in the y-phosphoryl group using the T4 polynucleotide kinase. (3) The head-to-tail joining of the appropriate segments when they are aligned to form bihelical complexes using the T4 polynucleotide ligase. U sing this methodology total synthesis of yeast alanine transfer RNA structural gene has been achieved. Methods have been developed in recent years for the chemical synthesis of deoxyribopolynucleotides of defined nucleotide sequence • • However, in practical terms, there is a severe Iimit on the length of the polynucleotide chains which can be assembled unambiguously by purely chemical methods. On the other band, for biological studies of the nucleic acids, it is often the high molecular weight nucleic acids which are the most useful. Therefore, it is desirable or even necessary to couple methods of organic chemistry, which alone can afford oligonucleotides of predetermined sequence, with other concepts or methods in order to prepare nucleic acids of defined nucleotide sequences. In earlier work reported from this laboratory, it was possible to prepare double-stranded DNA-like polymers of known repeating sequences by using short synthetic deoxypolynucleotides as templates for the DNA polymerases. The availability of the resulting polymers permitted extensive studies of the cell-free protein synthesis and of the genetic code • 1 . Clearly, a major aim of future synthetic work in the field of nucleic acids must be the development of methods for the synthesis of bihelical DNAs with specific nucleotide sequences. Towards this goal, the centrat concept which we have wanted to exploit is the inherent ability of polynucleotide chains to form ordered bihelical complexes by virtue of base-pairing. Thus, it was hoped to join relatively short chemically synthesized deoxyribopolynucleotides while * Present address: Departments of Chemistry and Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.

Organic chemical applications of magnetic circular dichroism

Abstract: Recent applications of magnetic circular dichroism spectroscopy (the Faraday effect) in organic chemistry are reviewed. Magnetic optical activity is a universal property of all matter and consequently can be applied to optically active as well as optically inactive molecules. It is emphasized, however, that natural and magnetic optical activity have separate physical origins and the information provided will necessarily be different. A brief theoretical background is given in order to explain the three types (A, B and C) of MCD curves which may be encountered in practice and the deductions that may be reached from their occurrence. Spectroscopic and analytical applications of MCD are illustrated for a wide variety of molecular systems such as metalloporphyrins, chlorins, corrins, aromatic hydrocarbons, carbonyl compounds, purine and pyrimidine bases and their nucleoside derivatives.

Structure and reactivity of polycyclic cross-conjugated π-electron systems

Abstract: In contrast to the well-known monocyclic conjugated systems with (4n + 2) and 4nit-electrons, non-benzenoid polycyclic conjugated it-electron system which should not obey Huckel's rule contain the element of cross-conjugation. In bicyclic and linear annelated polycyclic conjugated systems this crossconjugation is not associated with branching of the it-electron system, but this will however occur in pericondensed tnand polycyclic compounds. The participation of the element of cross-conjugation in the it-electron systems of such polycycles should affect their properties and result in characteristic differences in bonding character and reactivity compared with monocyclic conjugated compounds with the same number of it-electrons. To obtain experimental support for these theoretical predictions, several non-benzenoid bicyclic as well as tnand tetracyclic pericondensed it-electron systems were synthesized and studied with respect to the magnetic criteria of aromaticity and the connection between structure and reactivity. The successful preparation of most of the described hydrocarbons has centred on a single basic and rational synthetic principle.

The electrochemistry of organic compounds in aprotic solvents - methods and applications

Abstract: The importance of aprotic solvents in studies of the electrochemistry of organic compounds is discussed and a general mechanism for the oxidation and reduction of aromatic hydrocarbons and related substances is presented. The effect of molecular structure on the electrochemical behaviour of organic compounds is illustrated by discussing the relationship between conformations of aryl-substituted anthracenes and dimethylaminophenyl-substituted ethylenes and their electrochemical properties. Techniques and methodology for electrochemical studies in nonaqueous solvents, including solution preparation and cell and electrode design, are briefly reviewed. INTRODUCTION The use of anhydrous aprotic solvents, such as N,N-dimethylformamide (DMF), acetonitrile (MeCN), and dichloromethane (CH2C12) as solvents for electrochemical studies started about fifteen years ago. Although many studies of the electrochemistry of organic compounds had been reported up to that time, most were carried out in partially or totally aqueous media. Indeed the second edition of Koithoff and Lingane's Polarography has a chapter of only six pages on nonaqueous solvents and in a discussion of the polarography of organic compounds characterizes most reductions of these as irreversible. Although the earliest applications of aprotic solvents were aimed at increasing the solubility of the organic compounds, pioneering studies by Wawzonek, Hoytink, Peover, and their co-workers2 demonstrated that the use of these solvents simplified the electrochemical behaviour of organic substances by removing or decreasing the extent of chemical reactions coupled to the electron transfer step. Under these conditions many organic electrode reactions are electrochemically reversible so that correlations of electrode potentials with spectroscopic and molecular orbital (MO) data became possible and a molecular basis of electrochemical reactions of aromatic hydrocarbons was established. Moreover, aprotic solvents provided reduction and oxidation limits beyond those obtainable in aqueous solutions, so that a wider range of potentials became available for electrochemical studies. 379

Molecular orbital theory for π-donor and π-acceptor complexes

Abstract: The capabilities and limitations of approximate methods for molecular orbital (MO) ~alculations are discussed. In particular, the apparent ability of approximate methods to obtain agreement with experimental absorption spectra is critically examined both from the point of view of theoretical considerations and comparison of eigenvectors obtained by different investigators on the same complexes. Applications of an approximate MO method to the interpretation of photoelectron spectra and carbonyl force constants are illustrated. These latter calculations suggest that changes in both the sigma and pi antibonding orbitals of the carbonyls are necessary for interpreting the trends in force constants. While the first attempt to apply molecular orbital theory to transition metal complexes was reported by Wolfsberg and Helmholz 1 in 1952, intensive study of the method did not occur until the 1960s. The eight year delay is quite understandable when one recalls that it was in this period that 'crystal field theory' and its modification 'Iigand field theory' were at the peak of their popularity. The initial success of the latter theories in the interpretation of the weak absorption baJ?.ds in the visible and ultra-violet regions resulted in a flurry of activity by both the theorists and the experimentalists. The pioneering work of Orgel, McClure, Jergensen, Liehr, Tanabe, Sugano and >Ballhausen comes quickly to mind2• While their sturlies gave the basic understanding of such spectra, at the same time they showed the fundamental limitations of the models. The reduction of the electron repulsion integrals (F 2 and F 4 in Slater-Cortdon notation, or Band C in Racah's terminology) from free ion to complex not only led_to a thorough study ofwhat Jergensenb termed the 'nephelauxetic effect', but it also required that these quantities be treated as variables to be estimated with the cubic field parameter, L1 or lODq, and the spin-orbit coupling term, T. In order to treat systems whose symmetry was lower than cubic, additional parameters, Ds and Dt, badtobe included. This proliferation of variables made band assignment less unambiguous. Another shortcoming of the model was its inability to treat the intense charge transfer bands which have their origins in electronic transitions either from the ligands to the metal or vice versa. Thus, while Iigand field theory was and is an excellent starting point for many considerations involving absorption spectra and is still used, its utility is more restricted. An alternative approach was sought within the framework of molecular orbital theory.

Cyclic cross-conjugated π-systems: α,ω-cycloaddition reactions

Abstract: In five chapters (A-E) new results concerning the cyclic cross-conjugated 1t-systems calicene (1), pentafulvalene (2), sesquifulvalene (3), fidecene (4) and pentaphenafulvalene (5) are presented. These hydrocarbons, or simple yet sufficiently stable derivatives, have become of interest as participants in cx, co-cycloaddition reactions-opening up attractive new pathways towards the bridged annulene-type systems 6-10. In preliminary experimentssuch additions with TCNE as a 'polyenophile' have been observed: [8 + 2] in the case of alkylated calicenes, [12 + 2] with the parent sesquifulvalene and several alkyl derivatives, and [16 + 2] with di-t-butyl-fidecene (see note on page 35). The same polyenes, however, add methylacetylenedicarboxylate in a formal [6 + 2] way. The isolation of two Stereoisomers starting out from C-10 (C-27a; C-27b) makes a two-step mechanism probable.

Chemistry of phospholipids in relation to biological membranes.

Abstract: The quantitative determination of molecular species of natural phospholipids gave new information about the pairing of the fatty acid chains in a given lipid class. Cells appear to be equipped with enzymes which control the composition and pairing of hydrocarbon chains of phospholipids and display regulatory mechanism(s) which allow for adaptation of physical properties of membrane lipids to alteration in environmental conditions. Phospholipids containing various types of fatty acid combinations encountered in membranes have been prepared by chemical synthesis. Examination of these compounds in artificial membrane systems demonstrated that chain-length, degree of unsaturation, and the type of pairing of hydrocarbon chains determine the rate of diffusion of non-electrolytes and efficiency of carrier mediated transport across the hydrocarbon barrier. Comparison with natural membranes of different lipid composition revealed a close similarity with the model systems. This endorses the conclusion that the detailed chemical make-up of the lipid dictates the permeability behaviour of the region of the biological interface. The diversity of polar headgroups of phospholipids is demonstrated by the polyglycerol phospholipids of bacterial membranes, Detailed information about the structure of amino acyl and glucosamine derivatives of phosphatidyl glycerol has been obtained by combination of chemical synthesis and enzymatic methods.

Quantitative studies on aromaticity and antiaromaticity

Abstract: The properties of the simplest aromatic compound, are fully consistent with expectations. Spectroscopic studies show that it is a symmetric cation with approximately the stability expected from simple quantum mechanical considerations. Various studies on cyclopropenyl anions reveal that they are undoubtedly resonance destabilized. Thus, the kinetic studies which have been done on rates of hydrogen exchange in this system can be shown to be consistent only with electronic destabilization in the antiaromatic cyclopropenyl anion. Direct thermodynamic determinations of the stability of cyclopropenyl anions can also be performed using a novel electrochemical procedure, and using this method very high PKaS are found for some of these anions, indicating great stability. Resonance destabilization in cyclobutadiene can also be demonstrated by examining the basicity of a cyclobutadienocyclopentadiene anion and also by electrochemical studies on a fused naphthalenecyclobutadiene derivative. 1-Chloro-bicyclo{2,2,O]hexadiene has been prepared. On treatment with strong base it apparently eliminates HC1 to generate transient butalene (cyclobutadienocyclobutadiene) which is then trapped by the medium. Since the first synthesis of a cyclopropenyl cation, the triphenyl derivative synthesized in 19671, work in our laboratory and in a number of others has been devoted to a study o this simplest aromatic system. Of course, the description of the cyclopropenyl cation ring as 'aromatic' involves some assumptions about a definition of the term aromaticity, and I would like to focus first on this problem. There are a number of things which are special about the compounds which have been called aromatic, but the one which is certainly at the root of most of their unusual properties concerns their energies. Aromatic compounds are thermodynamically more stable, that is, they contain less energy than related non-aromatic reference compounds. We would like to focus on this particular definition of aromaticity, namely the possession by a cyclic conjugated system of less energy, because of its conjugation, than an open chain analogue. By logical extension, the property which we have referred to as antiaromaticity2 would be the possession by a cyclic system of more energy than would be expected by analogy with an open chain linear analogue. Thus the open chain compounds are considered to be normal, and the cyclic compounds are referred to them. This all seems so straightforward that it is hard to believe it requires

The stiffness of polymers in relation to their structure

Abstract: In the majority of applications of polymers, we are interested in one or more of three basic mechanical properties-stiffness, strength and toughness. To these can be added creep, which becomes important in many engineering applications. Stiffness represents resistance to deformation, and is a much simpler property than strength and toughness, which relate to failure. Strength is the ultimate stress which a material can withstand, before it fails, whether by fracture or by excessive deformation, whilst toughness represents the work required to fracture a material

Solvation of ions in pure and mixed solvents

Abstract: The problem of the influence of the solvent on activities of ions is discussed. Some proposals for the splitting of thermodynamic functions for electrolytes in solution into single ion values are reviewed. The effect of different solvation of ions on phenomena such as transfer of solvent in electrolysis, selective solvation in mixed solvent, phase boundary potential, surface tension and critical solution point in non-miscible solvents, and on solubilities of electrolytes in mixed solvents, may qualitatively and semiquantitatively be determined from the knowledge of single ion solvation behaviour. The most direct information on selective ion solvation in mixed solvent is provided from NMR measurements on, for example, Al3 ions in a mixture of two aprotic solvents. I. THERMODYNAMICS OF IONIC SOLVATION The standard free enthalpy of solvation of an electrolyte MX in a solvent L is defined by the free enthalpy of the process (Figure 1) M(gas) + X(gas) Z M(L) + X(L) (1) with the fugacities of the ions in the gasphase and the activities of the ions in solution being unity. This free enthalpy may be obtained with a Haber Born cycle and turns out to be of the order of from —150 kcal/mole (1-1 electrolytes) to more than — 1000 kcal/mole (e.g. for Al3 + salts). The uncertainty involved in the calculation of these energies amounts to at least 2 kcal/mole. However, the difference of free enthalpies of solvation of a given electrolyte in two different solvents, which we shall call the free enthalpy of transfer, is a smaller quantity ()LIGt 20 kcal/mole) and its experimental precision is much better, since data not precisely known, especially electron affinities of anions needed in the Haber Born cycle, cancel out in AGt. Process (1) is never realized in practice and is only of theoretical importance. Of much greater interest for the practical chemist is the process M(L1) + X(L1) M(L) + X(L2) (2) ¶ This review is not intended to be exhaustive and only covers work done in this laboratory 327

Small angle x-ray scattering studies of phase transitions in polymeric and oligomeric systems

Abstract: Small angle x-ray scattering (SAXS) is a valuable method for studying structure changes which take place during phase transitions in polymeric and oligomeric systems. Measurements of the dependence of SAXS on temperature yield information on the changes in morphology or of the structure on a supermolecular level. The technique can be applied to various problems: glass transition in highly crystalline polymers, solid state phase transitions in crystalline polymers, rotational transition in n-paraffins, premelting phenomena in crystals of chain molecules, and partial melting of homoand co-polymers. The glass transition temperature of the amorphous regions in highly crystalline polymers can be measured by SAXS since the scattered intensity depends on the thermal expansion coefficient of the two phases. This technique was applied to so-called single crystals grown from solution of polybutene-1, branched and linear polyethylene. The amorphous surface layer of linear polyethylene single crystals exhibits a 7, of — 125° ± 4°C. Solid state phase transitions of polymer crystals are often accompanied by characteristic changes in morphology. As examples, the annealing behaviour of trans-1,4-polybutadiene and isotactic polybutene-1 is described. The rotationa1' transition in paraffins studied with n-tritriacontane as an example also leads to a pronounced structure change of the surface of the paraffin lamellae, speaking in favour of a transition theory which is based on the assumption of a sudden change in the concentration of kinks within the crystals. With increasing temperature an increase of the roughness and depth of the surface region of paraffin crystals is observed. This phenomenon can be described as 'surface premelting of chain molecular crystals' and SAXS yields quantities suitable for a thermodynamic explanation of that process. In homoand co-polymer crystals partial melting during heating is observed. The SAXS studies show that this effect can be due to a 'boundary premelting of polymer crystals' which manifests itself by a reversible increase of the thickness of the intercrystalline layers. This phenomenon can be observed in many polymers, it can be described quantitatively by analysis either of the scattering curves or of the correlation functions y(x) obtained from the intensity distribution.

Enzyme electrode probes.

Abstract: Publisher Summary The enzyme electrode is a combination of an ion-selective electrode base sensor with an immobilized (insolubilized) enzyme, which provides a highly selective and sensitive method for the determination of a given substrate. In enzyme electrodes, the enzyme is usually immobilized, thus reducing the amount of material required to perform a routine analysis and eliminating the need for frequent assay of the enzyme preparation to obtain reproducible results. Moreover, the stability of the enzyme is often improved when it is incorporated in a suitable gel matrix. For example, an electrode for the determination of glucose prepared by covering a platinum electrode with chemically bound glucose oxidase is used for over 300 days. There are two methods to immobilize an enzyme: (1) the chemical modification of the molecules by the introduction of insolubilizing groups, and (2) the physical entrapment of the enzyme in an inert matrix such as starch or polyacrylamide. The technique of chemical immobilization is the best for making electrode probes.

Aromatic and nonaromatic 14π-electron systems

Abstract: The synthesis and aromaticity of 1,6-methano[IO]annulene and its analogues with a heteroatom bridge suggested the possibility that the homologous series of bridged (4n + 2)annulenes 1, 2, 3 etc., formally derived from the acene series, could be obtained. In tbe pursuit of this concept several bridged [14]annulenes with an anthracene perimeter differing in the conformation of the perimeter have been syntbesized. The pbysical and chemical properties of these [14]annulenes dramatically demonstrate the importance of a planar or near-planar 7t-electron system as a geometrical pre-requisite for aromaticity. As the lectures presented at this conference have amply demonstrated, the application of quantum-mechanical theories, in particular molecular orbital theory, to organic molecules and processes has become a major factor in promoting progress in organic chemistry. While, in these days, we witness with fascination the stimulus of W oodward and Hoffmann's concept of orbital symmetry conservation on the study of mechanistic pathways of organic reactions, the impact which the molecular orbital theory of benzene2, as expressed by Hückel's famous (4n + 2)-rule, has bad on the development of aromatic chemistry is almost taken for granted. The fruitful symbiosis between theory and _ experiment in the latter domain manifests itself impressively in the discovery of an entire realm of novel aromatic structures. Many of these structures, brought to light in the last two decades, have already found their way into textbooks of organic chemistry. lt may suffice to mention the tropylium ion , whose isolation first made the organic ehernist aware of the potential of molecular orbital theory, the homologaus cyclopropenyl cation , the cyclononatetraenyl anions ' 6 and finally the great variety of annulenes • The class of new aromatic and non-aromatic [14]annulenes that will be the subject ofthe present lecture is tobe regar:ded as yet another outgrowth of research having derived its inspiration from Hückel's pioneering work. The efforts at the Cologne laboratory in the field ofHückel-type aromatic compounds had as their initial goal the synthesis of [10]annulene in its various contigurations. If planar and not unduly strained, this type of species could be expected to be closer to benzene in its physical and chemical properties than any other member of the [ 4n + 2] annulene series, and would thus promise interesting chemistry. Unfortunately, the Stereoisomerie [lO]annulenes, regardless oftheir configuration, possess geometrical features presumed to be detrimental to aromaticity. In view of this handicap it is 355

A review of electrochemistry in non-aqueous solvents

Abstract: A review is given of the various extrathermodynamic assumptions made in the literature for the calculation of an ion distribution coefficient, for which the symbol K ion is used in this paper. An example is given of the different values of KAgt (w = water, AN = acetonitrile) obtained on the basis of these assumptions. From ApKA between aprotic solvents the value of KH can be found, but not from AKH+. Data on polarographic half-wave potentials published in the literature are critically discussed. The effect of small concentrations of hydrogen bond donors on the solvation of anions is best KH can be found, but not from AKH+. Data on polarographic half-wave as a solvent of the effect of homoconjugation on conductometric and potentiometric titration curves, on the solubility of slightly soluble salts and of small concentrations of water (hydrogen bond donor) on the pafl of a benzoatebenzoic acid mixture, Hydration constants of a number of cations and anions and of undissociated salts are presented in Tables 8 and 9 respectively and hydration and alcoholation constants of the proton in AN in Table 10. Solvation in Lewis acid-base and complexation reactions and stability of free radicals are briefly discussed. Finally, a comprehensive review of the importance of non-aqueous electrochemistry in various provinces of chemistry is presentcd.t ION DISTRIBUTION COEFFICIENTS To every chemist working in non-aqueous electrochemistry the most interesting and important property is the ion distribution coefficient between different solvents. The concept is not new. It was introduced by the famous Danish chemist Niels Bjerrum1, but there was a lapse of a quarter of a century before this distribution coefficient gradually became recognized as an important indicator of the extent of solvation of ions in different solvents, and before it allowed an interpretation of widely different kinetics of certain reactions in a variety of solvents2. The terminology for the distribution coefficient in the more recent literature varies; in Anglo-Saxon and Russian literature it is often referred to as the medium effect, the medium (solvent) activity coefficient or the degenerate activity coefficient, and in French literature more appropriately as coefficient de solvation' or coefficient de t To give in a single lecture a complete review of non-aqueous electrochemistry is an impossibility. For this reason I confine myself to some of the fundamentals. Several effects are illustrated by results obtained with Dr Chantooni in our laboratory.

Recent advances in the chemistry of troponoids and related compounds in Japan

Abstract: Extensively reviewed are recent contributions of Japanese chemists to the development of troponoid chemistry over the past five years. The contents are divided into three parts: (i) reactions of rnonocyclic troponoids and their benz-analogues (benzologues) towards electrophilic and nucleophilic reagents as well as photochemical and thermal reactions; (ii) syntheses and reactions of heptafulvenes, sesquifulvenes, sesquifulvalenes and azulenes in addition to syntheses of troponoids and azulenes fused with heteroaromatic ring systems; (iii) syntheses of tropone and its simple derivatives and naturally occurring tropolones and finally theoretical discussions concerning the physical properties of troponoids. It is shown that many troponoid compounds react as cyclic polyolefins but some can undergo reactions, clearly typical of aromatic compounds. It is also shown that there remain many interesting and important problems in this field that await further studies.

Dilute solution properties and molecular characterization of polyvinyl chloride

Abstract: Dilute solution properties of twenty-five PVC fractions prepared from commercial and experimental bulk-polymerized PVC samples have been investigated by means of light-scattering, viscometry and exclusion-chromatography The following Mark—Houwink—Sakadura relationships: []2YC = 1,63 10—2 M°76° x 1.017 c.gs. I 125t — 2 ir2 At 0.725 >< 1 02 Lflicyclohexanone — : ' c.g.s. have been found to be valid in a range of Mw-values between 6400 and 650000. The validity of various extrapolation procedures, proposed in order to calculate unperturbed dimensions and polymer-solvent interaction para meters from the empirical viscosity-molecular weight relationship, has been examined The Kurata—Stockmayer method applied to the measurements in tetrahydrofuran yields a Krvalue equal to 0.14 c.g.s. The polymer—solvent interaction parameter derived by the same method is not in agreement with the one deduced from second virial coefficient measurements. Molecular weight distributions determined in commercial PVC samples can be adequately represented, in a first approximation, by the usual exponential distribution function, first. proposed by Schulz. The chromatographic determination of MWD and its effect on the viscosity—molecular weight relationship are discussed. Finally, the possible occurrence of long chain branching in PVC has been investigated by means of an experimental study of the structure and properties of various polyethylene samples obtained by reduction with LiAIH4 of commercial PVC polymers. The melting temperatures and the intrinsic viscosity -molecular weight relationship for the polyethylene samples, so obtained, were found to be identical with those of High Density Polyethylene. It is concluded that most PVC polymers contain only very few long branches, if any INTRODUCTION Dilute solution properties of polymers may provide quantitative information on the size, shape and structure of the dissolved macromolecules as well as on the importance of polymer—polymer and polymer—solvent

The behaviour of macromolecules at phase boundaries

Abstract: Macromolecules can appear as new components at a pre-existing interface, produced by contact between phases composed of molecules of low molecular weight, or they themselves can be the components responsible for the phase separation. In either case an extensive interfacial zone of diffuse structure is built up. At a pre-existing interface strong energetic interactions are possible and a large measure of distortion of the macromolecule may be expected. On the other hand, where the interface occurs between a gel and a sol, or between two solutions of different macromolecular concentration or composition, small changes in configuration will occur. A number of cases arise but only some of these have been studied to any extent despite the importance of the results for biology and technology. The pertinent literature is reviewed and some points of general significance are discussed, why macromolecules tend to absorb so well even from dilute solutions, how we can interpret the thickness of the adsorbed layers, and whether there is adsorption in multilayers.

Potentiometric titration and conformation of synthetic and natural polyelectrolytes

Abstract: From the analysis of the potentiometric titration data of polyelectrolytes, we can obtain the electrostatic free energy of the macromolecules. Therefore, potentiometric titration is a useful experimental procedure to study the conformation and conformational change of macromolecules. A few examples of the application of potentiometric titration to the studies on polypeptides, protein and DNA are discussed. When the potentiometric titration method is not applicable, the numerical solution of the Poisson—Boltzmann equation without use of the Debye—Hflckel approximation may be employed to estimate the electrostatic potential at the surface of macromolecules. To confirm the reliability in the values thus estimated without experiments and in the values of the electrostatic free energy calculated from the surface potential, the calculated potentiometric titration data are compared with experimental data of various polyelectrolytes and proteins. The relationship between potentiometric titration data and conformations of stereoregular polyelectrolytes is also discussed. INTRODUCTION A carboxylic acid in aqueous solution dissociates as —COOH—COO + H (1) Its thermodynamic dissociation constant, K, is expressed by K = (—COO)(H)/(—COOH) (2) where parentheses denote the activity of each species. Often, however, the conventional dissociation constant, K0, defined by K0 = [—COO ](H)/[—COOH] (2) is used instead of K since we do not know the activities (in parentheses) of —COO and —COOH but only their analytical concentrations (in brackets). Therefore, K0 is not always a true constant, while K must be. Equation 2' can be written pH = pK0 log [(1 — (3) 519