Showing papers in "Pure and Applied Chemistry in 1970"

Interfacial processes in mineral/water systems

Abstract: Factors that control the selective adsorption of both inorganic and organic ions at mineral/water interfaces are reviewed and summarized. In particular, the role of the electrical double layer in the adsorption of surface-active materials on oxides in aqueous media is stressed. The importance of interfacial processes to mineral chemistry is illustrated through the application of specific adsorption phenomena to the understanding of the physical chemistry of flotation systems.

Structure, bonding and reactivity of (stable) transition metal carbonyl carbene complexes

Abstract: ABSTRACT The generally applicable method for the preparation of stable metal carbonyl carbene complexes by nucleophilic addition of a carbanion from an organolithium reagent to a carbonyl group with subsequent methylation of the resulting acylcarbonyl metallate by diazomethane or trimethyloxoniumtetrafluoroborate is described. The i.r. and 1 H-n.m.r. spectra of new cyclopentadienyl carbonyl nitrosyl carbene complexes of chromium, molybden\lm and tungsten confir.m the idea ofthe bonding ofmethoxyorganylcarbene ligands as being predominantly a donation of electron density from the Iigand to the rest of the molecule, with only a very much weaker 7t-acceptance of electrons. Five types of reaction of carbonyl carbene complexes, CO substitution, carbene substitution, ester cleavage, addition and rearrangement, and H substitution are described with representative examples. The kinetic results on the aminolysis of phenylmethoxycarbene pentacarbonyl chromium, and the conclusions which can be drawn from the i.r. and 1 H-n.m.r. spectra of some o-, mand p-substituted derivatives and arene chromium dicarbonyl carbene complexes are discussed. Results obtained from the analogous iron carbonyl system are briefly given. Some of the general features of bonding of this type of compound are summarized by the x-ray results obtained for six carbene complexes. Finally, an account ofthe secondary reactions of cleaved carbene ligands with themselves and olefins close this survey of this new field of organometallic chemistry.

The angular overlap model of the ligand field: theory and applications

Abstract: The Angular Overlap Model, AOM, is historically reviewed. Solid and surface harmonics are discussed in relation to the model and chosen as real basis functions for the two-dimensional as well as the three-dimensional rotation group. The concepts of angular overlap integrals and group angular overlap integrals are described. Further the angular overlap operators and the surface angular overlap operators are developed. The AOM endeavours to parametrize the non-spherical part of the ligand field. The assumptions which are necessary for this parametrization are enumerated by considering it (1) as a perturbation model (2) as an overlap model. The two points of view give identical results, and for linearly ligating ligands the AOM is, in a formal parametrical sense, equivalent to the point charge electrostatic model, when all narameters of both models are taken into account. The expanded radial function model for parametrizing the interelectronic repulsion is discussed and the field strength series described. Finally the twodimensional spectrochemical series is used as an illustration of the application of AOM to experiment.

Diabatic and adiabatic processes in photochemistry

Abstract: Photochemical reactions may be classified as adiabatic or as diabatic whether the chemical change occurs on the same potential energy surface or not. In the adiabatic cases deexcitation occurs either in the reactant (I) or in the product (II) while in the diabatic case it occurs in between them (III). The potential energy surfaces of polyatomic systems are discussed in relation to photochemistry and the possible conditions for the different behaviour of photo-reactions are formulated. Experimental evidence for the appearance of excited products in photoreactions in solution is given. I. CLASSIHCATION OF PHOTOCHEMICAL REACTIONS Thermal reactions with rare exceptions occur adiabatically on the lowest potential energy surface. An equivalent statement for photochemical reactions is less easy. They start in one of the electronically excited states of a reactant but end with the products in their ground states. This corresponds to a transition from a higher potential surface of the system to the lowest one. So, in principle, all photoreactions include diabatic processes. If one regards, however, electronic excitation and deexcitation as physical rather than chemical processes one may classify a reaction as adiabatic or as diabatic whether the chemical change takes place on the same potential surface or not. This leads us to a distinction between three different classes of photoreactions in which deexcitation occurs (I) in the reactant(s), or (II) in the product(s), or (III) anywhere in between on the reaction path. Schematically, these classes may be characterized as follows:

Water and aqueous solutions at high pressures and temperatures

Abstract: A survey is given of recent results on properties of water and aqueous solutions at high pressures and high temperatures with emphasis on supercritical conditions. New PVT-data for water from static measurements are available to 1000°C and 10 kb. Dielectric constants and viscosity have been measured to 550°C and 5 kb. Infra-red and Raman spectra of OD-vibrations of HDO in F120 to 400°C and 5 kb give information about the extent of hydrogen bonded structure. Critical curves of binary aqueous systems with one inert component, for example argon, extending to 3 kb and 400°C are discussed. Absorption spectra of bivalent cobalt and nickel chlorides are measured to 500°C and 6 kb and conclusions about the stability of octahedral and tetrahedral complexes are drawn. Shock wave and static conductance measurements to 1000°C and more than 100 kb demonstrate the increase of the ion product of water by twelve orders of magnitude or more at these conditions.

The chemiluminescence of organic compounds

Abstract: The chemiluminescence of organic compounds in solution is briefly surveyed with some emphasis on the mechanisms which result in relatively efficient population of an excited state of a product molecule. This is done in terms of the relation = x 'E 1F with particular reference to peroxide compositions. iNTRODUCTION The study of chemiluminescence mechanism in a persistent and systematic fashion is an activity largely confined to the last decade. This is not surprising when one considers that the problems associated with ground—excited state interactions must be multiplied by the complexities of a vigorous oxidative reaction for a full description of a chemiluminescent reaction. A general survey of the phenomenon discloses that, with almost no exceptions, oxidation is involved, the mechanism ranging from the relative simplicity of an electron transfer reaction to the confusion of a many step, bond cleavage process. The excitation step in certain bioluminescent reactions is of particular interest as an example of the latter kind. Although luminescence in the gas phase has been comparatively well understood for a long time, there are various reasons why the problem of chemiluminescence in solution must be approached differently. All gas phase reactions emitting light involve dior tn-atomic molecules and often atom recombination. The restriction of the degrees of freedom in small molecules and the near absence of collision in the gas phase leads to a non-equilibrium distribution of the energy derived from the exothermic reaction. If one assumes that the reactants in an exothermic reaction in solution are, within the lifetime of an excited species, in equilibrium with the surrounding solvent, then it is obvious that in this case vibrational energy, no matter how great, is an unlikely source of electronic excitation. Thus of the various mechanisms for organic chemiluminescence, electron transfer is particularly attractive. Conversely, there is no well proven instance of the emission of light from a radical recombination reaction in solution, although the bonds formed in such a process usually release more than sufficient energy to populate an electronically excited state. A further consequence of the conditions under which most organic chemiluminescent reactions occur is the observation that emission is from the first excited singlet state, the triplet state being easily quenched. However, there

Recent advances in the chemistry of terpenoid tumor inhibitors

Abstract: A continuing search for tumor inhibitors from plant sources has yielded over two hundred extracts with reproducible growth-inhibitory activity. Systematic fractionation, guided by assay in cell culture and animal tumor systems, has led to the isolation of the active principles of more than sixty plants. Chemical studies of novel sesquiterpene lactones, diterpenoid quinone methides, steriod lactones and other terpenoid tumour inhibitors are discussed.

Structures and reactivities of organolithium compounds

Abstract: The characteristic species present in organolithium compounds under various conditions are reviewed. The effects of environment on the kinetics and energetics of exchange reactions are discussed. A summary is given of various pathways by which organolithium compounds might react. Kinetic and spectroscopic evidence for these processes under various conditions are reviewed. A discussion is presented of the significance of certain results obtained by chemically induced dynamic nuclear polarization (CIDNP).

Electrostatic and non-electrostatic contributions to ion association in solution

Abstract: The coordination behaviour of d°-cations on the one hand and that of d10cations of low charge on the other hand is designated as Aand B-character respectively. For A-character interactions the charges and radii and for B-character interactions the ionization potentials and electronegativities of the combining atoms are decisive (electrovalent and covalent behaviour). Generally Aand B-character may be developed to any extent and seem to be mixed in a complicated manner. The thermodynamic functions of the association processes are often a good criterion, the formation constant of the adduct being large because of a dominant positive T AS in the case of A-character interactions and because of a dominant negative AH in the case of B-character interactions. The increase in entropy which is generally observed when the association process is due to electrostatic forces, is caused by the negative temperature coefficient of the dielectric constant (DK) of the solvent. However, the calculation leads to reasonable results only, if a smaller effective DK e is used in place of the DK of the bulk of the solvent & Furthermore, — &te/oT is considerably smaller than — &/T and 6(lfle)/(lflT) becomes less negative the more the electrostriction increases. This statement is substantiated experimentally with the aid of proton transfer reactions, which are controlled entirely by simple electrostatics. The greater the electrostriction around the species associating on account of purely electrostatic forces, the more exothermic the reaction will be. The structural changes exerted by a given metal cation on its solvation shell depend not only on its charge and radius, but also on its individuality. 1. COORDINATION SELECTIVITY AN examination of the large body of facts on the solution stability of metal complexes which has accumulated during the past 30 years1 reveals markedly different preferences of the various metal ions for the ligands offered to them in aqueous solution. The d°-cations react only with fluoride and with oxygen donors to an appreciable extent. Insoluble fluorides are precipitated with alkali fluoride and often can be dissolved in an excess of the reagent. The hydroxides, carbonates and phosphates of almost all of the multivalent cations are

Triplet photosensitization and the photobiology of thymine dimers in DNA

Abstract: The knowledge of the energy levels of the lowest triplet states of the DNA constituents obtained from photophysical experiments has allowed the design of a photochemical technique which introduces a specific change in the DNA structure, the dimerization of adjacent thymine residues, which, in turn, has led to important conclusions about the role of thymine dimers in the u.v. photobiology of T4 phage and insight into the mechanism of u.v. induced mutations.

On the interconversion between viscoelastic material functions

Abstract: The problem of numerical conversion of different viscoelastic material functions is reviewed. A number of approximation formulae are given, together with bounds for their errors. 1. STATEMENT OF THE PROBLEM It is well known"3 that the linear viscoelastic behaviour of materials obeying the superposition principle may be characterized by various material functions, for instance: (a) Creep compliance, J(t), defined as the strain as a function of time, t, produced by a unit step in stress at time zero; (b) Relaxation modulus, G(t), defined as the stress effected by a unit step in strain at time zero; (c) Storage compliance, J'fro), and loss compliance, J"(w), defined as functions of angular frequency co; these are the amplitudes of the in-phase component and the out-of-phase component of strain under conditions of steady state response to a harmonic stress of angular frequency co and unit amplitude; (d) Storage modulus, G'(w), and loss modulus, G"(w), defined as the amplitudes of the in-phase component and the out-of-phase component of stress under conditions of steady state response to a harmonic strain of angular frequency co and unit amplitude; (e) Retardation spectrum, f(r), as a function of retardation time, 'r; it is defined by the equation: J(t) = J0 + fir) {1 — exp(—t/r)} dt + t/ (1) where J0 is the limit of the creep compliance for t —+0 and l/tj is the limit of the rate of the creep compliance for t —* f(r) is assumed to be a nonnegative function of retardation time. This assumption is supported by overwhelming experimental evidnce; it manifests itself experimentally by the fact that the rate of creep is a completely monotonic function of time [J(t) 0; J(t) 0; J(t) 0; etc.]. 219

Thermodynamics of surface phenomena

Abstract: The thermodynamic treatment of the interfacial region corresponding to two fluid phases in contact is discussed. The features of this analysis which are reviewed include the classical treatment of Gibbs and the extensions to this treatment which are due to Buff. It is shown that these extensions are essential if the logical structure of the analysis is to be regarded as complete.

Some aspects of the chemistry of polynuclear metal carbonyl compounds

Abstract: The reactions of metal carbonyl clusters are classified in: (a) photolysis, pyrolysis and S N 1 substitutions, (b) oxidation and reactions with electrophilic reagents, (c) reduction and (d) nucleophilic attack. For a qualitative discussion of the electronic spectra and of some S N 1 reactions the presence of molecular orbitals having both metal–metal and metal–carbon character, which are delocalized over the whole cluster, is assumed. Such a delocalized situation, formally similar to that of organic aromatic compounds, also agrees with the reactivity of the carbonyl clusters toward electrophilic agents and with facile reduction by alkali metals. On the contrary some common examples of reactions with nucleophilic agents are discussed considering a localized attack on the carbon atom of a carbonyl group, analogous to that observed with organic carbonyl compounds.

Applications of mass spectrometry in the steroid field

Abstract: Among the various physical methods enjoying wide use by organic chemists, mass spectrometry is the most recent and probably also the most spectacular addition. It is not surprising, therefore, that it has also found rapid acceptance in the steroid field. Indeed, many of the recent generalisations about mass spectral fragmentation mechanisms of organic molecules were first established by model studies in the steroid series.

Application of electrochemical methods in the organometallic chemistry of transition metal π-complexes

Abstract: Polarography on both the dropping mercury and solid electrodes in aprotic solvents, electrolysis at a controlled potential, oscillopolarography, potentiometry, etc., along with non-electrochemical methods (such as e.s.r., nuclear-γ-resonance, i.r., n.m.r. spectroscopies) of a great number of organometallic transition metal π-complexes has been covered in detail. Methods and procedures are considered which may be helpful in the solution of such problems as a localization of electronic changes in a complex which may occur (a) on metal orbitals; (b) on the π-ligand orbitals, and (c) on the antibonding orbital of the metal–σ-ligand bond. The application of electrochemical methods in the solution of such problems as the relative location of the highest occupied and lowest vacant orbitals for a series of structurally similar π-complexes is discussed. The relation between the reduction potential of the LnM—σ—R bond and the pKa RH has been found; pKa values for a number of metallocenes as a C—H-type acid have been determined.

A theoretical discussion on some aspects of atomic fluorescence spectroscopy in flames

Abstract: In Section 1 a general theoretical expression is derived for the gain in signal strength attainable by atomic fluorescence spectroscopy (AFS) when compared to flame emission spectroscopy (FES), if the same atomic line, flame and measuring instrument are used. In particular, the usefulness of the hot hydrogen-oxygen flame in AFS is discussed. In Section 2 the shape of the analytical curve in AFS is derived from the curve-of-growth theory, for a continuum as well as a narrow-line source and under idealized conditions. The effects of partial illumination of the observed flame cell, and of partial observation of the illuminated flame cell on the shape of this curve, are shown both theoretically and experimentally. Possible sources of deviation of practical curves from the calculated curves are mentioned. In Section 3 the advantages of non-resonance fluorescence are summarized. A general theoretical expression is derived for the ratio of fluorescence signals obtained when one or another atomic line is used as an exciting and fluorescent line, respectively. This is done for direct line fluorescence as well as stepwise line fluorescence.

Configurational and conformational studies in the metallocene field

Abstract: The special molecular geometry of metallocenes offers many stereochemical problems. From these, in the present review especially the optical activity (‘metallocene chirality’) is treated with its particular aspects, such as configurational correlations, absolute configurations, optical rotatory dispersion and circular dichroism as well as the relationship between the activity of the ‘metallocene chromophores’ and preferred conformations. In this context optically active [3] ferrocenophanes and biferrocenyls are discussed in some detail.

Divalent carbon insertions into Group IV hydrides and halides

Abstract: A summary, dealing in large part with the author's own work, is presented of reactions in which a methylene bridge (CH2, CHX, CX2, etc.) is introduced into a Group IV element-to-other element covalent bond In these reactions M is silicon, germanium, tin or lead; Y can be hydrogen, halogen, carbon, mercury and, in the case of tin, another tin. The reagents which effect such methylenations include diazoalkanes, carbenes and ‘carbenoid’ organometallics. Emphasis is placed on carbenes generated via phenyl(trihalomethyl)mercury compounds and on diazomethane. The scope and mechanism of these reactions are discussed.

Synthetic cardenolides and related products.

Abstract: Since the publication in 1785 of one of the great classics of clinical medicine, ‘An Account of the Foxglove and Some of its Medical Uses’ by William Withering 1 , the steroidal glycosides of digitalis have maintained a unique position among drugs. Their usefulness as life saving products is unchallenged and yet very few drugs are known to possess a narrower margin of safety. Their toxicity has been described as an extension of the pharmacologic activity which in turn has been related to structure in a ‘none or all’ fashion. Chemical modifications to their structure had not produced results of practical significance: digitalis materials continue to be essentially products of extraction, qualitatively all similar, and still a challenge to the synthetic medicinal chemist.

Progress in atomic absorption spectroscopy employing flame and graphite cuvette techniques

Abstract: A simplified theory of analytical signal formation for various methods of sample atomization is proposed which takes into account the nature of sample transfer through the analytical cell and instrumental distortions of the pulse shape. An analysis of the relationships obtained shows the essential advantages of the integral absorption recording method over conventional equilibrium and peak methods. These advantages were employed to simplify the standardization procedure in the determination of impurities in the cuvette (10 −5 to 10 −8 per cent) and to develop a method of analysing solid samples by the flame technique using electric heating. The proposed theory also permitted optimization of the conditions of cuvette measurement by the peak method, making it possible to achieve record low absolute detection limits (10 −14 to 10 −15 g) and to reduce random measurement error to two per cent. The ‘transparency’ of the graphite cuvette in the vacuum spectral region was used for direct determination by atomic absorption spectroscopy of sulphur, phosphorus and iodine by their resonance lines. A possibility was established of determining phosphorus and iodine by non-resonance lines P 2136 A and I 2062 A. A method is described for determining relative oscillator strengths f in the spectra of elements from the absorption sensitivities of various lines measured by the flame technique. A comparison of the f values calculated from published sensitivity data with the most reliable values confirms the validity of the method.

Cooperative determination of the melting point of alumina

Abstract: A task force on secondary temperature standards, ~ponsored by the Commission on High Temperatures and Refractories, International Union of Pure and Applied Chemistry, has undertaken a programme to investigate various inorganic non-metallic substances for use as high temperature reference materials. Aspart of this programme a cooperative determination of the melting point of AbOs (alumina) was conducted by the task force. In all, nine scientific groups representing seven countries contributed experimental data. All work was performed utilizing a common supply of AbOs of nominal 99·9 per cent purity. Experimental techniques varied depending upon the individual investigator. The value for the alumina point as recommended by the task force is 2054 ± 6°C (IPTS 1968).

Geometry changes in excited states

Abstract: A brief discussion of the experimental methods of obtaining geometrical information about the equilibrium geometry of molecules in their electronically excited states is followed by a sampling of geometrical information available at present. The significance of simple orbital and symmetry arguments exemplified by Walsh diagrams is stressed. The geometrical tendencies of alkane excited states are studied. We find an excited state of methane that is planar. The lowest excited state of ethane should have longer C—H bonds and a shorter C—C bond than the ground state, and should also prefer an eclipsed conformation. Excited cyclopropane breaks one C—C bond. The lowest n, it" state of benzophenone is computed to be more planar than the ground state, and apparently does not become pyramidal at the carbonyl group. The isocyanide—cyanide rearrangement potential surface cautions us not to assume necessarily that excited state reaction is facilitated by a geometry change bringing closer the geometry of reactant to that of product. Every state of a molecule may be represented by a potential energy surface. An excited state surface is one such surface, the ground state of an isomer another. The excited state surface potentially differs from that of the ground state as much as the latter does from the surface of an isomer of very different geometry. It is not surprising therefore that upon electronic excitation molecules may adopt equilibrium geometries very different from those in their ground states. I would like to report on some of these geometry changes here, with particular emphasis on the interaction between theory and experiment in this area. Our primary source of experimental information on geometry changes remains the elucidation of the rotational fine structure of electronic transitions. The theoretical procedures leading from the observed high resolution spectrum to the moments of inertia of the molecule are summarized in the classic work of Herzberg'. These procedures rank among the highest achievements of theoretical chemistry and physics. The kind of information that emerges from spectroscopic studies is illustrated by the equilibrium bond length, the only free geometrical parameter, in various states of C2, listed in Table 12 Each molecular orbital may be classified as bonding (o, ire) or antibonding (at, itt), and these tendencies may be graded. Occupation of bonding orbitals decreases the equilibrium internuclear separation, occupation of antibonding orbitals increases this separation. These arguments, now so

The application of light scattering to the investigation of preferential adsorption effects in quasi-ternary systems

Abstract: The study of preferential adsorption in quasi-ternary systems by means of light scattering is reviewed and a brief description of the theoretical treatment of multicomponent systems is given. The thermodynamic interpretation of preferential adsorption is then discussed and the effect this phenomenon has on the measurement of polymer molecular weight and coil size, and on the second virial coefficient of the solutions is outlined. Examples of systems exhibiting negative adsorption are mentioned and the application to biological systems is also illustrated. INTRODUCTION Errors, like straws upon the surface flow, He who seeks pearls must dive below". John Dryden. The study of binary systems solvent [1] polymer [3] by light scattering is a well established and extensively documented technique which has provided an impressive amount of information on the dilute solution properties and conformations of all kinds of macromolecules. It is convenient to develop the theoretical analysis of such systems from the fluctuation theory as suggested by Smoluchowski1 and Einstein2 who considered that the scattering of light within a small volume element in a liquid and a solution arises from local density and concentration fluctuations. An extension of this theory by Debye3 led to the well known light scattering equations which allow us to calculate molecular weights, chain dimensions and thermodynamic parameters relevant to the polymer-solvent system in question: H'[(ân/oc3)T,P]2 C 3 1 [ C 1 At M3P(O) [1 + = M3P(O) + 2A2 C3 (1) with the reciprocal particle scattering factor: P(0Y1 = 1 + [167z2/3L )2] sin2 O2 — .... (la) 355

Stereochemistry of tellurium(II) complexes and related compounds

Abstract: Linear three-centre systems of sixth-group atoms occur in the triselenocyanate ion and in a series of tellurium(ii) complexes. In the triselenocyanate ion, the Se—Se bonds are about 032 A longer than covalent single bonds. In centrosymmetric square-planar tellurium(n) complexes, the tellurium—ligand bonds are about 027 A longer than covalent single bonds. In tellurium(ii) complexes where the linear three-centre systems are not symmetrical, pronounced relative trans bond-lengthening effects of ligands are observed. The phenyl group has a particularly large trans bond-lengthening effect. The transition state in nucleophilic substitutions at divalent sulphur, selenium and tellurium is discussed. CHEMICAL bonds longer and weaker than covalent single bonds are known in various classes of compounds. Such bonds occur, for example, in the trihalide ions of the seventh Main Group of the Periodic Table. This article is concerned with similar aspects in the sixth Main Group; specifically, with linear three-atom systems centred on divalent selenium and tellurium, and also on divalent sulphur. Halogens add a halide ion to give linear trihalide ions. In the iodine molecule, in the gas phase, the iodine—iodine bond length1 is 267 A. In the triiodide ion, in the symmetrical case, the iodine—iodine bonds are 29O—293 A24, that is, &23—036 A longer than in molecular iodine. In the bromine molecule, the bromine—bromine bond length1 is 228 A; in the tribromide ion, in the symmetrical case, the bromine—bromine bonds are 254—255 A5 6, or 026—O27 A longer than in molecular bromine. Halogens and interhalogens also form linear adducts with electroneutral n donors like amines, suiphides and selenides7 In two iodine adducts where nitrogen is donor atom'° 12, the iodine—iodine bond is 2'83 A; in two adducts where sulphur is donor atom13 15 the iodine—iodine bond is 282 and 279 A; and in three adducts where selenium is donor atom 1618 the iodine—iodine bond is 287, 291 and 296 A. Thus, not only in the triiodide ion, the adduct of the halogen with its anion, but also in adducts with electroneutral n donors, the iodine—iodine bond is lengthened relative to molecular iodine.

The analytical chemistry of mixed ligand complexes

Abstract: Mixed ligand complexes are characterized by their extreme stability. The chemical individuality of the central atom displays itself clearly, and peculiarities in extractions with organic solvents may be observed. Such properties open new routes to the solution of various theoretical and practical problems in analytical chemistry; true mechanisms and kinetics of analytical reactions can be elucidated and the selectivity and sensitivity of analytical methods can be improved. The process of complex formation and the properties of mixed complex compounds containing inorganic ligands are considered with reference to the platinum group elements. The formation of mixed complexes containing organic and inorganic ligands sometimes determines the mechanism of the analytical reaction and the composition of the end product; examples of such interaction are given. The sensitivity and selectivity of analytical methods can be improved by means of the formation of complex compounds of the ionassociate type, and unusual methods of separation and group concentration can be achieved. The study of mixed chelates has developed in two directions: (a) studies of improved extractability (synergy), and coextraction of elements as mixed chelates; and (b) studies of changes in the physicochemical properties of systems, e.g. absorbance and fluorescence. Various new analytical procedures are discussed. INTRODUCTION Developments in the field of coordination chemistry, which is closely bound up with the study of mixed and mixed polynuclear complexes, have been very extensive in recent years. A study of mixed ligand complex formation is of extreme interest to analytical chemists for the following reasons. 1. Mixed ligand complexes are the most general and probable form of existence of the elements in solution. 2. Studies of mixed ligand complex formation make it possible to estimate the characteristics of the intermediate and final forms of the complexes, and therefore to comprehend the mechanism and kinetics of analytical reactions. 3. Certain peculiarities of elements, which are most pronounced in mixed ligand complexes, as well as the physical phenomena accompanying the process of mixed ligand complex formation, open new prospects for the development of selective and sensitive methods for the determination, separation and concentration of elements. The quantitative

Recent developments in valence theory

Abstract: In the early 1930s valence theory was confused by a conflict between molecular-orbital and valence-bond approximations. But during the l940s a reconciliation between the two methods was found; and from that time until now much the greater effort has been put into MO calculations. These calculations have been tremendously enriched in the 1960s by the widespread use of electronic computers, and package programmes. A study of the charge distribution calculated for covalent bonds shows that the early picture of a bond as being associated with a build-up of charge between the nuclei is still valid. However, if bonding electrons are drawn into this region, other electrons tend to get forced out of it; recent studies show the importance of these two effects. The early representation of these charge-clouds in terms of hybridization was particularly fruitful in the l940s, but is now recognized as too restrictive. The development of the experimental techniques of photoelectron spectroscopy and x-ray spectroscopy now enables a direct verification to be made of the early ideas of individual molecular orbitals. In this way Mulliken's original theoretical descriptions have been triumphantly justified. Another aspect of valence theory that has developed in recent years is that of the relation between a and it electrons. No longer may the it electrons be treated as if they were independent of the a electrons, but a close relationship exists between them. The account concludes with a description of those situations where the number of valence electrons is either too few (electron-deficient molecules) or too large (electron-rich molecules) to provide the normal complement of two electrons per bond. In each case the simple concept of a bond needs to be modified. Attention is drawn to the remarkable way in which theoretical concepts have recently received experimental verification. INTRODUCTION Valence theory is much older than the fifty years referred to in the title of this Symposium. We should at the very least go back to t86 when Odling, Crum Brown and others started using the horizontal line—-which has been the symbol for a chemical bond ever since; and we should think of Kekule only a few years later trying to use the symbol to indicate some sort of geometrical relationship between the various atoms of a polyatomic molecule: or Butlerov, with his early feeling for chemical structure. Moreover, for these last hundred years, as Mulliken and Van Vleck have shown in their

Chemical contributions to taxonomy and phylogeny in the genus artemisia

Abstract: The sesquiterpenic lactones of the family Compositae represent a group of compounds that appear to be of common origin and formed by closely allied biosynthetic processes. They provide an attractive means for further assessing the potential value of chemical characters in chemosystematics, and for examining presently accepted classifications within the family.

Photochemistry of alkenes in solution

Abstract: The direct and sensitized photochemical behaviour of cyclic and acyclic olefins is compared. Under either set of conditions cyclo-hexenes and -heptenes undergo photoprotonation in hydroxylic media, perhaps via a trans intermediate. By contrast acyclic, exocyclic, and larger-ring cyclic olefins fail to undergo photoprotonation on either direct or sensitized irradiation. However, both cyclohexenes and cycloheptenes, as well as larger-ring cyclic and acyclic olefins, undergo slow isomerization to positional and skeletal isomers on direct irradiation in either protic or aprotic media. Unique behaviour has been observed for tetraalkyl-substituted olefins, which exhibit behaviour on direct irradiation in alcoholic solvents suggesting the intervention of a cation radical intermediate. Mechanistic implications of these various types of photochemical behaviour are discussed. INTRODUCTION One of the most commonly occurring organic chromophores is that of the simple carbon—carbon double bond and yet, despite its ubiquity, the ethylene chromophore has received relatively little attention by photochemists. In a parallel sense, despite its apparent simplicity, this chromophore has confounded spectroscopists and theoreticians alike, and even today there exist many questions conce'rning the location and configurational assignments of the various electronically excited states of ethylene and -its simple alkyl derivatives'. In view of the common occurrence of the ethylene chromophore in organic systems, its photochemical behaviour offers the potential of new, unexplored reactions which should have broad synthetic utility, Moreover, a systematic study of its photochemical behaviour should shed additional light on the nature and location of its various excited states. It was with these thoughts in mind that several years ago we undertook such an investigation.