Surface organometallic chemistry is a new field bringing together researchers from organometallic, inorganic, and surface chemistry and catalysis. Topics ranging from reaction mechanisms to catalyst preparation are considered from a molecular basis, according to which the "active site" on a catalyst surface has a supra-molecular character. This. the first book on the subject, is the outcome of a NATO Workshop held in Le Rouret. France, in May. 1986. It is our hope that the following chapters and the concluding summary of recommendations for research may help to provide a definition of surface organometallic chemistry. Besides catalysis. the central theme of the Workshop, four main topics are considered: 1) Reactions of organometallics with surfaces of metal oxides, metals. and zeolites; 2) Molecular models of surfaces, metal oxides, and metals; 3) Molecular approaches to the mechanisms of surface reactions; 4) Synthesis and modification of zeolites and related microporous solids. Most surface organometallic chemistry has been carried out on amorphous high-surf ace-area metal oxides such as silica. alumina. magnesia, and titania. The first chapter. contributed by KNOZINGER. gives a short summary of the structure and reactivity of metal oxide surfaces. Most of our understanding of these surfaces is based on acid base and redox chemistry; this chemistry has developed from X-ray and spectroscopic data, and much has been inferred from the structures and reactivities of adsorbed organic probe molecules. There are major opportunities for extending this understanding by use of well-defined (single crystal) oxide surfaces and organometallic probe molecules."

The aim of this book is, as its title suggests, to help sOilleone with little or no knowledge of what thermal analysis can do, to find out briefly what the subject is all about, to decide whether it will be of use to him or her, and to help in getting started on the more common techniques. Some of the less-common techniques are mentioned, but more specialized texts should be consulted before venturing into these areas. This book arose out of a set of notes prepared for courses on thermal analysis given at instrument workshops organized by'the S.A. Chemical Institute. It has also been useful for similar short courses given at various universities and technikons. I have made extensive use ofthe manufacturers' literature, and I am grateful to them for this information. A wide variety of applications has been drawn from the literature to use as examples and these are acknowledged in the text. A fuller list of the books, reviews and other literature ofthermal analysis is given towards the back of this book. The ICTA booklet 'For Better Thermal Analysis' is also a valuable source of information. I am particularly grateful to my wife, Cindy, for typing the manuscript, to Mrs Heather Wilson for the line drawings, and to Professor David Dollimore of the University of Toledo, Ohio, for many helpful suggestions.

Research on metal-containing polymers began in the early 1960's when several workers found that vinyl ferrocene and other vinylic transition metal TI -complexes would undergo polymerization under the same conditions as conventional organic monomers to form high polymers which incorporated a potentially reactive metal as an integral part of the polymer structures. Some of these materials could act as semi conductors and possessed one or two dimensional conductivity. Thus applications in electronics could be visualized immediately. Other workers found that reactions used to make simple metal chelates could be used to prepare polymers if the ligands were designed properly. As interest in homogeneous catalysts developed in the late 60's and early 70's, several investigators began binding homogeneous catalysts onto polymers, where the advantage of homogeneous catalysis - known reaction mechanisms and the advantage of heterogeneous catalysis - simplicity and ease of recovery of catalysts could both be obtained. Indeed the polymer matrix itself often enhanced the selectivity of the catalyst. The first symposium on Organometallic Polymers, held at the National Meeting of the American Chemical Society in September 1977, attracted a large number of scientists interested in this field, both established investigators and newcomers. Subsequent symposia in 1977, 1979, 1983, and 1987 have seen the field mature. Hundreds of papers and patents have been published."

This book is aimed at graduate students and research workers in aU branches of ohemistry, who wish to gain insight into what continues'to be one of the fastest growing areas of the subject. Aonding to a metal center may stabilize a ligand towards some reagents, activate it towards others, or modify its chemical behavior in more subtle ways. AU these effects have their uses, and aU invite understanding in terms of mechanism. Thus mechanistic insight is linked to control of reaction pathways. The detailed working out of this relationship provides the central theme of the book. The effect of the metal may be electronic or steric, and may involve the energy or the entropy of activation. It may depend on changes induced in the initial state of the ligand, or on those that only arise further along the reaction pathway. It may involve one coordination site or several, and the effects may be more, or less, specific to the metal involved and more, or less, amenable to control through the other Jigands. These remarks apply equaUy strongly to the carbon-bound ligands which occupy the major part of this work, and to those attached by other atoms. Thus the reactions discussed here are relevant in such diverse areas as bulk homogeneous catalysis, stereoselective stoichiometric synthesis, and bioinorganic chemistry.

Chemical Synthesis: Gnosis to Prognosis (XTUllKtl ~uv8eoTr ana TT) rVWOT) OTT) npaYVWOT)) " . . . . other things being equal, that field has the most merit which contributes most heavily to, and illuminates most brightly, its neighbouring scientific disciplines[l] One hundred scientists, a blend of students, industrialists, and academics from twenty countries gathered to circumscribe, understand, and elaborate this topic in the magical setting of Ravello, Italy. The mandate of this workshop? To survey existing knowledge, assess current work, and discuss the future directions of chemical synthesis as it impinges on three exciting interdisciplinary themes of science in the 1990's: bioactive molecules, man-made chemical materials, and molecular recognition. This tempting but inexact menu summoned diverse students and scientists who wished to seriously reflect upon, dissect, and eject ideas and own experiences into open debate on this topic, which is at a crossroad in internal evolution and impact on the life and material sciences. The group arrived from many directions and in various forms of transportation, matters soon forgotten, when it found itself in the village which nurtured Wagner's inspiration and set to work immediately to ponder the question which has received extensive thought, prediction, and caveat from illustrious chemists over a period of time [2], two of which, to the delight of all, in presence among the Lectures.

The literature contains tens of thousands of publications and patents devoted to the synthesis, characterization and processing of polymers. Despite the fact that there are more than one hundred elements, the majority of these publications and patents concern polymers with carbon backbones. Furthermore, the limited (by comparison) number of publications on polymers that contain elements other than carbon in their backbones are typically devoted to polymers based on silicon, especially those with Si-O bonds. This disparity is partially a consequence of the dearth of low cost organometallic feedstock chemicals potentially useful for polymer synthesis. It also derives from the lack of general synthetic techniques for the preparation of organometallic polymers. That is, by comparison with the numerous synthetic strategies available for the preparation of organic polymers, there are few such strategies available for synthesizing tractable, organometallic polymers. In recent years, commerical and military performance requirements have begun to challenge the performance limits of organic polymers. As such, researchers have turned to organometallic polymers as a possible means of exceeding these limits for a wide range of applications that include: (1) microelectronics processing (e.g. photoresists) [1]; (2) light weight batteries (conductors and semi-conductors) [2]; (3) non-linear optical devices [3] and, (4) high temperature structural materials (e.g. ceramic fiber processing) [4,5].

Essential themes in the biochemical cycling of mercury are the relative importance of anthropogenic versus natural sources, transformation and migration processes at the local, regional and global scale, global emission inventories of different mercury sources (both point and diffuse) of both natural and anthropogenic origin. In this regard, Siberia, with its vast territory and variety of natural zones, is of special interest in the global mercury cycle and in terms of the influence of geographical zones on source and sink terms in regional budgets. Siberia contains large areas of mercuriferous belts; natural deposits that emit mercury into the atmosphere and water. Siberian gold has been mined with the use of mercury since the early 1800s. But there, too, huge forest zones and vast areas of tundra and wetland (bogs) can act as efficient sinks for atmospheric mercury. Audience: Environmental scientists, legislators, politicians and the interested citizen wishing to gain a clear picture of the biogeochemical cycling of mercury.

Biological inorganic chemistry is a field of research at the interface of inorganic and biological chemistry. The rapidly developing insights into the role of metals in biological systems has far-reaching implications not only for biological science but also for related disciplines, ranging from molecular medicine to the environment. In each volume the reader, whether engaged in chemistry, biochemistry, biology or molecular medicine, receives a comprehensive summary and critical overview of a topic of hight current interest written by leading international experts.

The design, -synthesis, and selective pyrolytic conversion of organo metallic precursdrs to materials of high purity or specific morphology (for electronic or optical applications), high strength and/or high-temperature stability (for structural or refractory applications) represents a poten tial area of extreme growth at the overlap of chemistry and materials science (materials chemistry). Research in this area is likely to have considerable impact at both the academic and societal levels because it will require development of scientific expertise in areas currently not well understood. Examples include: (1) The thermodynamics of molecular rearrangements in organometallic molecules at temperatures above 200 DegreesC; (2) The electronic properties of amorphous ceramic materials; (3) The phys icochemical properties of ceramic molecular composites; and (4) The optical properties of multicomponent glasses made by sol-gel processing. The opportunity to establish the scientific principles needed to pursue useful research goals in "materials chemistry" requires communica tion between chemists, ceramists, metallurgists, and physicists. To date, there have been few opportunities to create an environment where such communication might occur. The objective of this NATO Advanced Research Workshop was to promote discussions between experts in the varibus disci plines aligned with "materials chemistry. " These discussions were intended to identify the scope and potential rewards of research efforts in the development of: Custom-designed precursors to common and exotic materials, methods of selectively transforming these precursors in high yield to the desired material, and methods of characterizing the final products.

This volume contains the papers and reports presented at the First International Conference on Dye-Protein Interaction, held 24-28 July 1988 at the University of Compiegne, France. This was the first international meeting dealing entirely with dye-protein interaction. The major focus of the conference was on the better understanding of the mechanism of interaction of proteins with different triazine dyes and the synthesis of novel structural dyes having good biomimetic activities. The potentials and limits of their use in biotechnology, mainly for purification, were stressed. Current contributions in developing dye-based affinity methods were highlighted in such areas as affinity partition, affinity precipitation and new support matrices for efficient affinity chromatography, etc. The interrelation between metal chelates and dyes in terms of their interactions with proteins was underlined. It is our belief that this proceedings volume will be a stimulus for broad and creative applications of dye affinity concepts in many fields of biomedical research and biotechnology. for understanding the In addition, a discussion session emphasised the necessity toxicological aspects of these dyes, their fragments and their metabolites. This helped to trigger plans for future work, and this topic will be one of the priorities in a future meeting on dye-protein interactions. C. R. The help of the International Scientific Committee, which included Drs Lowe (UK), G. KopperschHiger (GDR) , E. Stellwagen (USA), D. Thomas (France), G. Birkenmeier (GDR), S. Rajgopal-Narayan (USA), J. P. Dandeu (France), D. Muller (France) and E. Dellacherie (France), in organising this meeting is gratefully acknowledged.

Solid-State NMR is a branch of Nuclear Magnetic Resonance which is presently experiencing a phase of strongly increasing popularity. The most striking evidence is the large number of contributions from Solid-State Resonance atNMR meetings, approaching that ofliquid state resonance. Important progress can be observed in three areas: Methodological developments, applications to inorganic matter, and applications to organic matter. These developments are intented to be captured in three volumes in this series, each of them being devoted to more or less one of these areas. The present volume on Solid-State NMR III is devoted mainly to organic matter. The recent developments of deuteron NMR and their applications are reviewed in the first chapter. Crosspolarization, MAS, and dynamic angle spinning are being explored for enhancement of information and sensitivity. In addition to the analysis of classical relaxation times and modern 2D spectra, detailed dynamic information becomes accessible from investigations of the relaxation time anisotropies. The second chapter examines cross-polarization in static and rotating solids under conditions of spin diffusion and thermal motion. The underlying dipole-dipole interaction is further exploited by the techniques described in the third chapter for studies of polymer-polymer miscibility. Short range techniques are discriminated from long-range techniques based on spin diffusion. The use ofthese techniques is illustrated by a case study ofPMMAJPVF blends. The last chapter addresses novel z methods and applications of two-dimensional exchange NMR for investigations of relative molecular orientations, polymer morphology, molecular dynamics, and macroscopic molecular order."

The term biomimetic is comparatively new on the chemical scene, but the concept has been utilized by chemists for many years. Furthermore, the basic idea of making a synthetic material that can imitate the func tions of natural materials probably could be traced back into antiquity. From the dawn of creation, people have probably attempted to duplicate or modify the activities of the natural world. (One can even find allusions to these attempts in the Bible; e. g., Genesis 30. ) The term "mimetic" means to imitate or mimic. The word "mimic" means to copy closely, or to imitate accurately. Biomimetic, which has not yet entered most dictionaries, means to imitate or mimic some specific bio logical function. Usually, the objective of biomimetics is to form some useful material without the need of utilizing living systems. In a simi lar manner, the term biomimetic polymers means creating synthetic poly mers which imitate the activity of natural bioactive polymers. This is a major advance in polymer chemistry because the natural bioactive polymers are the basis of life itself. Thus, biomimetic polymers imitate the life process in many ways. This present volume delineates some of the recent progress being made in this vast field of biomimetic polymers. Chemists have been making biomimetic polymers for more than fifty years, although this term wasn't used in the early investigations."

The 6th course of the futernational School of Solid State Physics was held in Erice, Sicily, at the Ettore Majorana Centre for Scientific Culture, 19-29 June, 1995. The course was organized as a NATO Advanced Study Institute and received generous support from NATO's Nanoscale Science Program. This volume is based on the lectures presented during the course. fudispensable for the planning of the summer school was the support of the Director of the Ettore Majorana Centre, Professor A. Zichichi. We wish to express our sincere appreciation to the center staff, Dr. Maria Zaini, Dr. Alberto Gabriele, Dr. Pinola Savalli and Dr. Jerry Pilarski for their expert assistance in all organizational matters. A special word of thanks must go to the director of the International School of Solid State Physics, Professor Giorgio Benedek, not only for his valuable advice in the planning stage, but also for his active participation in the program itself. I would like to thank my coworkers, Stefan Frank, Nikola Malinowski, Renee Stotz, Frank Tast and Kristin Wirth for their valuable assistance in preparing these proceedings of the meeting. The success of a school is, in the last analysis, determined by the inter- est and commitment of the lecturers and participants. I am very grateful to the lecturers for their carefully prepared formal presentations, to the par- ticipants for their contributions to the spontaneous evening "workshops", and to all for their inexhaustible enthusiasm.

The field of low-dimensional conductors has been very active for more than twenty years. It has grown continuously and both the inorganic and organic materials have remark able properties, such as charge and spin density waves and superconductivity. The discovery of superconductivity at high temperature in copper-based quasi two-dimensional conducting oxides nearly ten years ago has further enlarged the field and stimulated new research on inorganic conductors. It was obviously impossible to cover such a broad field in a ten day Institute and it seemed pertinent to concentrate on inorganic conductors, excluding the high Tc superconducting oxides. In this context, it was highly desirable to include both physics and chemistry in the same Institute in order to tighten or in some cases to establish links between physicists and chemists. This Advanced Study Institute is the continuation of a series of similar ones which have taken place every few years since 1974. 73 participants coming from 13 countries have taken part in this School at the beautiful site of the Centre de Physique des Houches in the Mont-Blanc mountain range. The scientific programme included more than forty lectures and seminars, two poster sessions and ten short talks. Several discussion sessions were organized for the evenings, one on New Materials, one on New Topics and one on the special problem of the Fermi and Luttinger liquids. The scientific activity was kept high from the beginning to the end of the Institute."

Chemistry of Powder Production focuses on the solid-state chemistry of powder materials and relates this to the structure, properties and preparation, and characterization techniques for these important industrial products. Additionally, the properties of the particles are discussed in relation to their surface structure and characteristics. This book describes the fundamentals of statistical methods for measuring the characteristics of particles. New advanced materials being developed in powder technology manufacturing techniques are also emphasised, including powdered materials for advanced ceramics as well as magnetic and pigment materials.

We have used in Vol.2 the same structural scheme similar they are organized alphabetically, just for as used in Vol. 1. convenience in consulting.The alphabetical order We used as the list of minerals the reference is provisional, it is not an important aspect of the book Mineral Reference Manual by Nickel and classification, and willtend to disappear. Nichols, edited by Van Nostrand Reinhold, New In Vol. 1 some condensed model sheets were York, 1991, and we first organized the minerals by presented to illustrate the simplicityof the patterns chemical formulas, from the simpler to the more of the packing layers of the A, AmB and ApBqC n r complex (Tables 73 to 172), as presented on page close-packed minerals (Tables lL to 17Lof Vol.1). 1of Vol.1.The results of the structural studywere The aim was to stimulate the complete systematic ordered by structural formulas (Tables 27S to derivation of the simple mineral close-packed 59S). Then we summarized the classified structure structures, as was tried by the author (Lima-de- types (not including the tentatively classified) in Faria (1965) Zeit. Krist., 122, 359-374). In Vol. 2 Tables 60S to 61S. Finally we presented some the layers are more complex and the correspond- global results (Tables 62S and 63S). ing condensed models sheets were not included. In certain cases the general chemical and the The reader should refer to the book Structural structural formulas may be difficult to compare.

Fundamental QSARs for Metal Ions describes the basic and essential applications of quantitative structure-activity relationships (QSARs) for regulatory or industrial scientists who need to predict metal ion bioactivity. It includes 194 QSARs that have been used to predict metal ion toxicity and 86 QSARs that have been used to predict metal ion bioconcentration, biosorption, and binding. It is an excellent sourcebook for academic, industrial, and government scientists and policy makers, and provides a wealth of information on the biological and chemical activities of metal ions as they impact health and the environment. Fundamental QSARs for Metal Ions was designed for regulatory and regulated organizations that need to use QSARs to predict metal ion bioactivity, as they now do for organic chemicals. It has the potential to eliminate resources to test the toxicity of metal ions or to promulgate regulations that require toxicity testing of metal ions because the book illustrates how to construct QSARs to predict metal ion toxicity. In addition, the book: Provides a historical perspective and introduction to developing QSARs for metal ions Explains the electronic structures and atomic parameters of metals essential to understanding differences in chemical properties that influence cation toxicity, bioconcentration, biosorption, and binding Describes the chemical properties of metals that are used to develop QSARs for metal ions Illustrates the descriptors needed to develop metal ion-ligand binding QSARs Discusses 280 QSARs for metal ions Explains the differences between QSARs for metal ions and Biotic Ligand Models Lists the regulatory limits of metals and provides examples of regulatory applications Illustrates how to construct QSARs for metal ions Dr. John D. Walker is the winner of the 2013 SETAC Government Service Award.

"Catalysis is more art than science," probably all of you have heard and even used this expression. Whether it is true or not, it alludes to the experience that new catalysts are hard to find, and near impossible to predict. Hard work and a lifetime of experience is invaluable. However, a keen mind might give insight into where to search, but not necessarily about where to find the answers. Historically, "quantum leaps" have often arisen from serendipity - we all know the story about the nickel-contaminated reactor that triggered further research towards the first coordination catalyst for ethene polymerization. Taking advan tage of this event, Karl Ziegler became the first chemist to earn both a Nobel prize and a fortune for the same invention. A broken NMR tube helped Walter Kaminsky discover the effect of high concentrations of methylaluminoxanes as co catalysts for metallocenes. When air reacted with the concentrated trim ethyl aluminum solution, sufficient amounts of methylaluminoxanes were formed, and the lazy catalyst dormant in the NMR tube suddenly became sensationally active. Ziegler and Kaminsky were lucky and had the genius needed to take advantage of their luck."

Some years ago, I agreed to contribute a volume to the Academic Press 'Organo metallic Chemistry' series - the metals to be covered were rhodium and iridium. Initially, my plan was to discuss both the fundamental organometallic chemistry and applications in organic synthesis. When the first draft of the manuscript was complete, it was apparent that I had exceeded my allowance of pages by a huge amount. It was then that I decided that the catalysis section warranted separate treatment. I am grateful to Reidel for agreeing to publish this volume on Homogeneous Catalysis with Compounds of Rhodium and Iridium as part of their 'Catalysis by Metal Complexes' series. The material I had for the original Academic Press project covered the litera ture to the end of 1978. I decided to update this to the end of 1982 with a few key references from 1983. It is some measure of the rate of progress in this field that the number of references almost doubled during this revision."

A lively demonstration of the great vitality and the multidisciplinary character of cluster research and of the usefulness of synthesizing its various aspects was given at this symposium. This volume covers all aspects of the physical and chemical properties of free and supported clusters or small particles: static, dynamical, electronic, magnetic and optical properties, adsorption and chemical phenomena. It thus gives a complete overview of the status of the field and its development.

Chemists have been aware of the existence of coordination compounds con taining organic macrocyclic ligands since the first part of this century; however, only during the past few years have they expanded research into the chemistry of these compounds. The expansion was initiated in the early 1960s by the synthesis and characterization of compounds containing some new macrocyclic ligands. The synthesis of compounds which may serve as model systems for some natural products containing large rings as ligands provided the main goal for the early expansion of research effort; indeed, a recurrent theme behind much of the reported chemistry has been the analogy between synthetic macrocyclic compounds and many natural-product systems. More recently, the emphases of reported research have ranged over the whole spectrum of chemistry, and the number of publications that discuss macrocyclic chemistry has increased at a dramatic rate. The completed research has been reported in a variety of journals throughout the world but there has been no previous attempt to bring the major developments together under one cover. This book, therefore, attempts to satisfy the need for a single source in which there is both a collection and a correlation of information concerning the coordination chemistry of macrocyclic compounds. The chapters in this book discuss various aspects of macrocyclic chemistry, and while these chapters as a whole constitute an in-depth survey of the state-of the-art of the field, each chapter is written as a complete unit."

This volume reports the main lectures and seminars given at the NATO Advanced Study Institute on Vibronic Processes in Inorganic Chemistry held at Riva del Sole, Tuscany, Italy between 7th and 18th September 1988. In addition to the about 40 hours of lectures repres ented by this volume, a further fifteen lectures on current research topics were given by the other participants. Many factors contributed to the decision to hold this ASI but the final trigger was given at a meeeting in Padova when Marco Bettinelli, Lorenzo Disipio and Gianluigi Ingletto asked me to recommend a text where the diverse conceptual, spectroscopic and structural consequences of the impossibility of treating the motions of the electrons and nuclei independantly in inorganic compounds were presented. There seemed to be no suitable comprehensive text where the relationship between the relatively simple theoretical ideas and the huge range of their application in inorganic chemistry and physics was developed. The Institute and this text are a contribution to filling this gap. Seventy-nine participants from fifteen countries attended the Institute. Topics raised in the lectures and from the participants own research frequently led to discussions which went on long into the night."

Over the past fifteen years the Commission on Equilibrium Data of the Analytical Division of the Inter national Union of Pure and Applied Chemistry has been sponsoring a noncritical compilation of metal complex formation constants and related equilibrium constants. This work was extensive in scope and resulted in the publication of two large volumes of Stability Constants by the Chemical Society (London). The first volume, edited by L. G. Sillen (for inorganic ligands) and by A. E. Martell (for organic ligands), was published in 1964 and covered the literature through 1962. The second volume, subtitled Supplement No. 1, edited by L. G. Sillen and E. Hogfeldt (for inorganic ligands) and by A. E. Martell and R. M. Smith (for organic ligands), was published in 1971 and covered the literature up to 1969. These two large compilations attempted to cover all papers in the field related to metal complex equi libria (heats, entropies, and free energies). Since it was the policy of the Commission during that period to avoid decisions concerning the quality and reliability of the published work, the compilation would frequently contain from ten to twenty values for a single equilibrium constant. In many cases the values would differ by one or even two orders of magnitude, thus frustrating readers who wanted to use the data without doing the extensive literature study necessary to determine the correct value of the constant in question."

The Joint Meeting comprisIng the 3rd International Symposium on Clathrate Compounds and Molecular Inclusion Phenomena and the 2nd International Symposium on Cyclodextrins was held on 23-27 July, 1984, in Tokyo, Japan. It was organized by the Japan Association for Inclusion Chemistry together with the International Organization Committee, with the auspices of sixteen societies and associations in Japan. This event was the first joint meeting with the hope of unifying the above two symposia. The program of the symposium consisted of 142 papers, including 14 invited papers. The invited papers and some selected topics were presented verbally, and all the other 118 papers were displayed in poster sessions. The symposium was held at Hoshi University in Tokyo. Due to the multidisciplinary nature of the subjects treated, the scope and subjects were grouped into two parts. In the first group, the chemistry of cyclodextrins, synthetic organic hosts, inorganic and metal complex hosts and layered hosts were treated. In the second group applications in various fields, biomimetic aspects, physicochemical aspects, selectivity, stereo-specificity and other aspects were discussed. The scientific sessions were carried out in a really vivid atmosphere. The number of participants viz 50 from 19 overseas countries and 253 domestic partici pants exceeded our expectation."

The essential introduction to the understanding of the structure of inorganic solids and materials. This revised and updated 2nd Edition looks at new developments and research results within Structural Inorganic Chemistry in a number of ways, special attention is paid to crystalline solids, elucidation and description of the spatial order of atoms within a chemical compound. Structural principles of inorganic molecules and solids are described through traditional concepts, modern bond-theoretical theories, as well as taking symmetry as a leading principle.