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We review the principles of optical and electrochemical sensors based on the use of the sol-gel technique, in particular their fabrication, working principles, and various configurations. We also report on potential applications, e.g. to environmental and clinical analysis, to gas sensing, and to bioprocess monitoring. Methods are critically reviewed for making such sensors, how to encapsulate organic, inorganic and biological matter, and how to control the properties of the resulting materials. Specifically, sensors for gases, ions, and organic as well as bioorganic molecules are discussed in some detail. [ABSTRACT FROM AUTHOR]

Over the last few years, considerable progress has been made with sol-gel coating films for optical and electronic applications. Nonlinear optical films and ferroelectric films are among those that have been studied most extensively. Optical and electronic coating films based on inorganic-organic hybrid or composite films have attracted much attention. These and other recent developments are stressed in this article which deals with a wide variety of optical and electronic coating films. [ABSTRACT FROM AUTHOR]

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. [ABSTRACT FROM AUTHOR]

Several tetrapyrroles are employed by Nature to carry out fundamental biological functions and synthetic models compounds have been extensively used in order to understand the mechanisms of action of natural systems. One of the structures present in many enzymes is the corrinoid structure. It consists of a tetrapyrrolic macrocycle, where a direct link between two pyrrole rings exists. Such a direct link and the different degrees of unsaturation that can be introduced into the macrocycle modulate its chelating properties towards metal ions and its reactivity. A most interesting example of the corrinoid structure is corrole, a macrocycle where an 18 electron aromatic π system analogous to that of a porphyrin is maintained. Corrole has been shown to be a versatile ligand capable of coordinating transition and main group metals without significant distortion of the macrocycle plane. The present article reviews the developments of the chemistry of corrole and its metal complexes considering the synthetic procedures that can be followed in order to prepare such compounds, their spectroscopic characterization and redox reactivity and demonstrates the peculiar ligand field effect of this macrocycle. The chemistry of other synthetic corrinoids is also reviewed with special attention to their preparation procedures and axial ligand binding reactions. [ABSTRACT FROM AUTHOR]

The noble metals ruthenium, osmium, rhodium, iridium, palladium, platinum, silver, and gold form a variety of complexes M(P) LL′ with tetrapyrrole ligands which are studied in many laboratories because their coordination chemistry and catalytic power is similar to that of the corresponding iron and cobalt tetrapyrroles forming the prosthetic groups in heme proteins and vitamin B12. Apart from the corrins and corroles which are treated in the second article of this volume, the main interest is devoted to noble metal complexes of porphyrins, but complexes of hydroporphyrins, porphyrin analogs, homologs, porphyrinoids, and phthalocyanines are also mentioned where appropriate. The treatise is organized into four main topics: metal insertion ("equatorial coordination chemistry", Sect. 2, formation of the M(P) entity), substitution and redox reactions ("axial coordination chemistry", Sect. 3, exchange of axial ligands L and L′), organometallic chemistry (a special case of axial coordination chemistry, Sect. 4), and notes on current special chemical topics like electrochemistry, catalysis, supramolecular chemistry, and biological applications. Where possible, the material is compressed in schemes and tables giving a synopsis of consecutive reactions of Ru/Os and Rh/Ir dyads. The two other dyads, Pd/Pt and Ag/Au, are so strongly governed by the square planar d8 configuration that they are practically devoid of axial coordination chemistry. The redox reactions of the Ru/Os dyad live on the switches between d6-d5 and d6-d4-d2 configurations mostly with octahedral geometry about the metal, those of the Rh/Ir dyad on the switches between the d6-d7-d8 configurations, giving rise to octahedral, square-pyramidal, and square planar geometries. The occurrence of the dinuclear or mononuclear Rh d7 systems give a special touch of radical chemistry to the organometallic chemistry of rhodium porphyrins which is expected to be fruitful to organometallic chemistry as a whole. Although a broad overview was intended, many entries are just given in keywords or tables in order to give the reader a chance to get more information. Nevertheless, out of nearly 500 papers scrutinized, only nearly 400 references appear in order to keep the volume and the writing time within certain limits, and any research group missing a paper in this review is asked for patience with the authors. [ABSTRACT FROM AUTHOR]

This article gives a review of complexes in which a dioxygen ligand is bonded to a transition metal. Three aspects of these complexes are discussed in detail: the structure, the electronic structure, and the reactivity. The structural section summarises the recent X-ray crystal structure determinations, and the structural data obtained by other methods. The electronic structure is first considered in qualitative terms which allow the rationalisation of the different structures observed, and this qualitative model is compared with the results of calculations and with spectroscopic data. The reactivity of the complexes is discussed separately for each structural class in terms of the electronic structure. An attempt is made to compare the results obtained in historically different areas of research. Our objective is to give a clear summary of current knowledge of these compounds for workers interested in their application to catalysis and in their rôle in biochemical systems. [ABSTRACT FROM AUTHOR]

The aim of the present article is to review several biomolecules, the spectroscopic and structural properties of which have been so far interpreted in the light of the Jahn-Teller effect. Indeed the importance of the vibronic coupling in the interpretation of physical properties of molecules and crystals is widely recognized now, but only a desultory attention has been paid to the role played by such a coupling in biological systems. After a brief outline of the Jahn-Teller effect, the whole problem is critically re-examined separating the systems for which the experimental evidence of the Jahn-Teller effect is clear, from those where its effectiveness is still debated. Furthermore some suggestions are given for a proper inclusion of the vibronic coupling in future analysis on biomolecules. [ABSTRACT FROM AUTHOR]

This review shows that the growing interest aroused by phospholes in coordination chemistry can be attributed to three main reasons: 1. The influence of the partial delocalization of the phosphorus lone pair upon the complexing ability of phospholes. 2. The discovery of phosphametallocenes. 3. The tremendous variety of complexing modes found for the phosphole nucleus. For a given phosphole, no less than ten different types of complexes can be obtained. [ABSTRACT FROM AUTHOR]

The ability of coordination polyhedra of different central atoms in the solid state to undergo distortions has often been discussed. These distortions are also correlated with the nature of the central atom. This paper deals with the conception of crystal structure non-rigidity for central atoms of some selected complexes by the using of statistical distributions of interatomic distances central atom-nearest ligand atom (M-L; M=Mn(II), Fe(II), Fe(III), Co(II), Co(III), Ni(II), Cu(II) and Zn(II)). Based on the vector equilibrium principle the dispersion of these empirical distributions has been introduced as a measure of non-rigidity of central atoms, decreasing in the order: Cu(II) > Fe(II)HS > Mn(II)HS > Ni(II)LS > Zn(II) > Fe(III)HS ≈ Co(II)HS > Ni(II)HS > Co(III)LS, where LS and HS denote low-spin and high-spin states, respectively. Starting from the distribution of M-L distances for certain types of central and ligand atoms, the contribution of atoms of the inner coordination sphere to the vector equilibrium of crystal structures is discussed. The structures of complexes with all M-L bond lengths of at least one symmetrically independent central atom in the interval of maximum frequency exhibit in the appropriate set the most stable inner coordination sphere. These compounds are classified both with respect to the geometry of their inner coordination sphere fulfilling the above condition and to their crystal structure aspect. The square-planar geometry is found to be the most stable for Cu(II) and Ni(II)LS complexes, tetragonal-pyramidal for Zn(II) complexes, while for Fe(II)HS, Mn(II)HS, Fe(III)HS, Co(II)HS, Ni(II)HS and Co(III)LS complexes it is the regular octahedral geometry. Except for one Mn(II)HS complex, in the discussed structures chromophores with the most stable geometry of the coordination polyhedron form part of the final relatively isolated structural unit (island) with expressively chemical bonds between atoms. These facts are illustrated by the stabilization effected by the crystal field and by the properties of the ligands. [ABSTRACT FROM AUTHOR]

Proton transfer and hydrogen bonding are two aspects of the chemistry of hydrogen that respectively govern the behaviour and structure of many molecules, both simple and complex, from water to DNA. The β-dicarbonyls exhibit both of these features, and in ways which have singled them out for detailed study for many years. They provide the best known examples of keto ⇆ enol tautomerism, with the advantage of slow proton transfer and high concentrations of the enol tautomers in most cases. These enols are stabilized by intramolecular OHO hydrogen bonds which at various times have been thought of as being centred, linear hydrogen bonds that are somehow incorporated into the delocalised π system to give "aromatic" systems. Research involving structural, spectroscopic and computational techniques has deepened our understanding of these compounds and changed our picture of them. The hydrogen bonding is surprisingly strong, surprising since it is neither centred, nor linear, nor involved in the ring's delocalized bonding, although it is certainly coupled to it. This review deals with controversies that have surrounded the β-dicarbonyls and discusses the current view that the enol tautomers are a double-minimum potential well with a low energy barrier. The review ends with a brief look at the β-thioxoketones, which provide an analogous system based on a heteronuclear SHO hydrogen bond in the enol or enethiol tautomer. [ABSTRACT FROM AUTHOR]

The Angular Overlap Model is applied to the calculation of the Jahn-Teller coupling constants for all possible ground states of ML6dx systems in Oh symmetry. In the weak field scheme the LSMLMS〉 basis is treated by an operator equivalent technique and results are also derived for the LSJMJ〉 basis to include the effects of spin-orbit coupling. In the strong field basis both high- and low-spin ground states are considered where these arise and in all cases the extent of quenching of Jahn-Teller activity due to spin-orbit coupling is evaluated. The derivation of the required angular overlap parameters and the general utility of the method are briefly considered. [ABSTRACT FROM AUTHOR]

As a consequence of the non-spherical symmetry of the copper(II) ion, d9 configuration, and of the influence of the Jahn-Teller, and pseudo Jahn-Teller effect on six-coordinate geometries, the stereochemistries of the copper(II) ion are characterized by non-rigid geometries (fluxional behaviour), and ranges of distorted geometries (Plasticity Effect). The latter may be connected by a series of Structural Pathways, which may be characterised by an ‘Electronic Criterion of Stereochemistry' for a related series of complexes, e.g. the [Cu(bipy)2X] [Y] complexes. [ABSTRACT FROM AUTHOR]

Metal Complexes of the allyl (C3H5) and cyclopentadienyl (C5H5) ligands have played a major role in the development of organometallic chemistry. Metal allyl complexes are well known for the many intricate organic transformations which they may mediate, yet often possess limited stability. Metal cyclopentadienyl complexes on the other hand possess very high stabilities but only very limited catalytic chemistry. The pentadienyl ligand (C5H7 as well as various methylated derivatives) has similarities to both the allyl and cyclopentadienyl ligands, and it has now been demonstrated that metal pentadienyl complexes possess not only thermal stability but also chemical and catalytic reactivities. This favorable combination then will allow for detailed correlations to be made between chemical reactivities and the physical natures (structural, spectroscopic, etc.) of these compounds. In this article the presently available information on metal-pentadienyl compounds is discussed in order to gain some understanding of their nature compared to the related allyl and cyclopentadienyl systems, and to anticipate future applications of metal-pentadienyls. Particular emphasis is given to the bis(pentadienyl)metal complexes, which have been referred to as the "open metallocenes". [ABSTRACT FROM AUTHOR]

A great number of active centers in proteins and enzymes contain transition metals. Each metal imposes specific catalytic properties on the protein which could not be achieved when employing a different metal. The special role and characteristic reactivity of icoordinated copper is illustrated by looking at the evolutionary aspects of copper proteins. The evolution of copper-binding sites is closely linked to that of copper proteins. Copper centers have evolved according to various principles, similar to the evolutionary development of proteins and enzymes. Three basic principles may be observed: the copper-binding centers of metallothioneins and type 1 copper centers developed in non-metal proteins; the transformation of the metal-binding centers of iron or manganese proteins led to the development of the type 2 copper centers; and the trinuclear copper-binding centers of the blue oxidases were formed by alterations and recombinations of type 1 copper centers. Simple, although as yet unknown, mononuclear copper centers gave rise to the type 3 copper-binding sites of the hemocyanins and tyrosinases, as well as to the CuA- and CuB-centers of the cytochrome oxidases and N2O-reductase. According to this assignment, copper proteins containing either type 1 copper, type 3 copper, CuA-centers, or CuB-centers share a common ancestor. They developed at least in part by divergent evolution. Type 2 copper proteins are the products of convergent evolution and, consequently, show little or no phylogenetic homology. [ABSTRACT FROM AUTHOR]

Hemeproteins are widespread in all groups of living organisms, both as components of key biological processes as well as in very specific metabolic pathways. By variation of either the heme porphyrin structure or the heme protein environment, the function and behavior of the hemeproteins may vary drastically, allowing its role in the most diverse processes. In this short review, selected examples of hemeproteins from anaerobes will be presented, aiming to show the basic, common features of these family of proteins, as well as their diversity in terms of function and composition. [ABSTRACT FROM AUTHOR]

The terminal step in methane generation by several methanogenic organisms, of which the best studied is the archaeon Methanobacterium thermoautotrophicum, is catalyzed by the enzyme S-methyl coenzyme M reductase (methylreductase, EC 1.8.-.-). This enzyme contains a macrocyclic tetrapyrrole-derived cofactor, F430, at the active site coordinating Ni(II) in the resting state. A Ni(I) state (Ni1F430) has been proposed as the active form of the cofactor. Extensive mechanistic and spectroscopic studies have been performed on the holoenzyme, isolated cofactor, and various synthetic model compounds. These studies are summarized in the present review. [ABSTRACT FROM AUTHOR]

Nickel has so far been implicated as a cofactor in only a handful of biological reactions. These reactions are, however, central to many archae and bacteria as they provide reducing power from molecular hydrogen oxidation, readily available nitrogen from urease (also in plants and fungi) and acetyl-coenzyme A from CO and CO2. As new microbial proteins are studied, the role of nickel is likely to extent to several other metabolic processeses. One of the major issues concerning catalytic Ni is whether it can be redox active in the biological context. Recent results concerning hydrogenases and CODHs indicate that their catalyses are carried out by NiFe centers; thus the role of Ni in these enzymes may be essentially catalytic without necessarily involving redox chemistry. [ABSTRACT FROM AUTHOR]

Mixed valence has been considered with reference to three model systems: Fe3O4 (magnetite) is a representative of the ferrospinels and the iron oxides with spinel-related structures, Fe1−δO (wüstite) illustrates a system with random defects and clustering, CaFeO3 (a perovskite) illustrates the consequences of strong Fe4+-O-Fe4+ interactions. The influence of counter cations is particularly marked in systems like the spinels Fe2SnO4 and Fe2MoO4; more subtle effects are illustrated by a comparison of CaFeO3 with SrFeO3. The time τh for an electron transfer between cations of different valence has been considered relative to two times: the period ωR−1⋍10−12s of an optical-mode lattice vibration that traps a mobile electron and the time τn for a Mössbauer nuclear excited state to decay to its ground state. The full range of electron-transfer times τh<10−12s to τh>10−8s has been found for both Fe-Fe interactions across shared site edges or faces and Fe-O-Fe interactions across shared site corners. Perturbations of the periodic potential of an iron array by lattice defects or substitutional ions is shown to increase τh. Moreover, electron-lattice interactions were found to induce cooperative Jahn-Teller distortions in some compounds and charge-density waves (CDW) in others. In the mixed-valent compound Fe3O4, electrostatic interactions between mobile ions combine with electron-lattice interactions to produce, with increasing temperature, transitions from ionic ordering (or a static CDW) in the temperature interval 120

Sulphur containing amino acid residues are found among the structural and functional constituents of many proteins. The ease of oxidising sulphur in these biologically important polymers is well known. At present, there is no simple and fast technique available to monitor the respective oxidation states using chemical reactions. X-ray photoelectron spectroscopy has proven to be both convenient and efficient to follow these electron transport reactions. In the present survey a choice of some sulphur and selenium bearing proteins and metalloproteins will be examined upon excessive irradiation, treatment with hydrogen peroxide and cleavage of metal sulphur bonding. It will also be shown that the use of this technique in general will allow a quick decision on the rate of deterioration of a protein. The limitations of this spectrochemical approach will be debated. [ABSTRACT FROM AUTHOR]

The alternative approach to structures is an inversion of the traditional one: we concentrate on a packed array of cations, into the interstices of which the anions are inserted, rather than putting cations into a packed anion array. Even in conventional terms the logic of the new approach can be justified, since many structures have their own antitypes (and even more show a partial structure/antistructure relation). In this way we find that, in oxides for example, many cation arrangements are identical to the arrangements of the atoms in known (or plausible) alloy structures. (In some cases the cations and atoms are identical in the two cases.) This is particularly helpful in those oxide examples where it has not previously been possible to describe the structure in any simple terms, often because the anion array is not regular in any simple way, for instance in many metal sulphates. But it is also revealing in some cases that are describable in conventional terms, e.g. the humites. A long (but still incomplete) list of examples is given in Table 3. Sufficient specific cases are described and discussed in detail in the text in order to expose the principles and some of the advantages of this unfamiliar approach. One of these advantages is that, in favourable cases, it leads at once to complete quantitative descriptions of the structures (i.e. until cell dimensions and anion position parameters), e.g. for spinel, garnet, bixbyite etc. Another is that it throws a new light on possible martensitic mechanisms for phase transformations (e.g. β- → γ-Ca2SiO4, low- → high-temperature K2SO4), and on the possibility of anomalous substitution (e.g. of CO32− for PO43− in apatite). It also reveals an explanation for those phase transitions, due to high pressure, that are not attended by an increase in primary coordination number, e.g. olivine → spinel, zircon → scheelite. [ABSTRACT FROM AUTHOR]

A numerical method for solving the time-dependent Schrödinger equation for a one-electron atomic system in an intense, short-pulsed laser field is presented. An effective potential formalism is proposed and tested for representing the excitation of the valence electrons in rare gases. Results for ion production yields, photoelectron distributions and harmonic conversion are presented and compared to recent experimental results. [ABSTRACT FROM AUTHOR]

Using Rydberg atoms and microwave fields it has been possible to observe virtually all one electron strong field phenomena. The attraction of these experiments is that they can be more controlled than most laser experiments, with the result that more quantitative information can be extracted. The insights gained from these experiments can be profitably transferred to optical experiments. To demonstrate the latter point we demonstrate that apparently non-resonant microwave ionization, in fact, occurs by resonant transitions through intermediate states. These experiments demonstrated clearly the power of Floquet analysis of such processes, and the ideas were subsequently applied to the analogous problem of laser multiphoton ionization. [ABSTRACT FROM AUTHOR]

One-electron atoms subjected to a time-dependent external field provide physically realistic examples of scattering systems with chaotic classical dynamics. Recent work on atoms subjected to a sinusoidal external field or to a periodic sequence of instantaneous kicks is reviewed with the aim of exposing similarities and differences to frequently studied abstract model systems. Particular attention is paid to the fractal structure of the set of trapped unstable trajectories and to the long time behavior of survival probabilities which determine the ionization rates of the atoms. Corresponding results for unperturbed two-electron atoms are discussed. [ABSTRACT FROM AUTHOR]

To afford a basis for constructing Thomas-Fermi-like approximations to the electronic structure of atoms in intense applied fields, the canonical density matrix (or equivalently the Feynman propagator) for free electrons is first set up. This is done for both intense magnetic and intense electric fields: in each case, exact results are available for arbitrary static field strengths. Physical properties of atoms and ions in intense magnetic fields are hence obtained in the statistical limit of Thomas-Fermi theory. This discussion is then supplemented by the ‘hyperstrong' limit, considered especially by Lieb and co-workers. Chemistry in intense magnetic fields is thereby compared and contrasted with terrestrial chemistry. Some emphasis is then placed on a model of confined atoms in intense electric fields: the statistical Thomas-Fermi approximation again being the central tool employed. [ABSTRACT FROM AUTHOR]

In the presence of a homogeneous magnetic field the center of mass and internal motion of a neutral or charged two-body system cannot be separated. We review the effects and phenomena which occur due to the inherent two-body character of these systems. The most prominent effects that occur for neutral species are the chaotic classical diffusion of the center of mass, the intermittent near-threshold behavior and the appearance of weakly bound states with an extraordinarily large electric dipole moment in crossed electric and magnetic fields. For ionic two-body systems the regions of strong mixing of the center of mass and internal motion are identified and investigated. As major effects we observe the classical self-stabilization as well as selfionization processes of the ion. [ABSTRACT FROM AUTHOR]

When molecules are subjected to intense laser fields, they undergo multielectron dissociative ionization (MEDI), a process that is, as yet, little understood. This chapter gives a brief overview of some of the experimental techniques used to study the process. At present there are two models that attempt to explain the facts, inertial confinement (with rapid ionization at a critical inter-ion separation) and laser-induced stabilisation. In order to ascertain which, if either, of these models is correct, more definitive experiments are required, involving interferometric control of laser parameters, good laser characterisation, and a more careful analysis of the dissociation dynamics. [ABSTRACT FROM AUTHOR]