Theoretical Studies and Physical Properties

Theoretical Studies and Physical Properties.—HMO-7r-bond orders for C—S bonds in benzo[6]thiophen and other derivatives have been related to experimental bond lengths. A detailed rotational analysis of the 0—0 band of benzo[6]thiophen at 2936 A has been carried out. The effects of substituents in the 3-position of benzo[A]thiophens on u.v. and i.r. spectra and on the charge-transfer spectra with tetracyanoethylene have been measured. A linear correlation of the wavenumber of the first maximum 

Kucharczyk, B. KakiC, and V. Horak, Coll. Czech. Chem. Comm., 1969, 34, 2959. 

Theoretical Studies and Physical Properties.—Quantum chemical calculations on the reactivity of thienothiophens have been carried out. The e.s.r. spectra of the radical anions of carbonyl, nitro, and cyano derivatives of the two [b]-fused thiophens have been studied. The proton chemical shifts of 2-substituted thieno[2,3-b]thiophens have been correlated with the two-parameter equation of Swain and Lupton.  

The interaction of the sulphonyl group with an adjacent unsaturated centre has been studied in the sulphone (246) and in its differently fused isomers.  

Theoretical implications of the structure of l,6,6a/S -trithia-pentalenes are currently open to discussion. Classical theory of a bonds and tt orbitals does not explain all the properties of these compounds and various theoretical explanations have been put 

This discussion could be conducted in two ways. First, we could try to deduce structural information from experimental facts through unequivocal mathematical calculations. This would be most satisfying and certainly constitutes the ultimate goal for structural studies. Unfortunately, divergencies of opinions currently observed show that we are still far from such achievement. One important reason for these difficulties may be that quantum mechanical treatment of d orbitals has not reached the precision attained for s and p orbitals. So, in many cases, it cannot be said whether discrepancies between experience and theory are due to inadequacy of theoretical concepts or to shortcomings of mathematical tools. 

A less ambitious, but more practical, approach is to examine if and how known experimental facts may agree with a limited set of general assumptions, and this is what we shall try to do. 

Let us first consider structures such as 81 or 82. Whenever X-ray determinations have been made, it has been found that the three 

Moreover, structure 82 (or 84) should explain why usual thione properties are markedly diminished, though not entirely suppressed, for these compounds. 

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In the first chapter, devoted to thiazole itself, specific emphasis has been given to the structure and mechanistic aspects of the reactivity of the molecule most of the theoretical methods and physical techniques available to date have been applied in the study of thiazole and its derivatives, and the results are discussed in detail The chapter devoted to methods of synthesis is especially detailed and traces the way for the preparation of any monocyclic thiazole derivative. Three chapters concern the non-tautomeric functional derivatives, and two are devoted to amino-, hydroxy- and mercaptothiazoles these chapters constitute the core of the book. All discussion of chemical properties is complemented by tables in which all the known derivatives are inventoried and characterized by their usual physical properties. This information should be of particular value to organic chemists in identifying natural or Synthetic thiazoles. Two brief chapters concern mesoionic thiazoles and selenazoles. Finally, an important chapter is devoted to cyanine dyes derived from thiazolium salts, completing some classical reviews on the subject and discussing recent developments in the studies of the reaction mechanisms involved in their synthesis. 

The solvent used was 5 %v/v ethyl acetate in n-hexane at a flow rate of 0.5 ml/min. Each solute was dissolved in the mobile phase at a concentration appropriate to its extinction coefficient. Each determination was carried out in triplicate and, if any individual measurement differed by more than 3% from either or both replicates, then further replicate samples were injected. All peaks were symmetrical (i.e., the asymmetry ratio was less than 1.1). The efficiency of each solute peak was taken as four times the square of the ratio of the retention time in seconds to the peak width in seconds measured at 0.6065 of the peak height. The diffusivities obtained for 69 different solutes are included with other physical and chromatographic properties in table 1. The diffusivity values are included here as they can be useful in many theoretical studies and there is a dearth of such data available in the literature (particularly for the type of solutes and solvents commonly used in LC separations). 

In recent years, metal nanoparticles and thin films supported on oxides have become fundamental components of many devices as their small dimensions present structures with new chemical and physical properties, often enhancing the reactivity of these surfaces relative to their bulk counterparts. Numerous theoretical and experimental studies show that the metal particle size and shape as well as direct adsorbate interactions with the oxide support can each play a key role in enhancing the reactivity of these surfaces. Further investigations imply that the support material may... 

To develop a unifying view of iron center catalysis, properties of the iron center in individual enzymes must be determined. Obviously, the definitive solution for the structure is atomic resolution of the active enzyme and postulated intermediates determined by diffraction or nuclear magnetic resonance (NMR) spectroscopy. Just as obviously, these methods are limited by enormous time, effort, and instrumentation requirements as well as by practical and theoretical considerations. This point is emphasized by the paucity of available protein structures. In addition to the strictly structural details of the iron center, chemical and physical properties are required and, in some cases, these results augment diffraction or NMR structural studies. Discussed below are a few of the more common processes by which this information is obtained. 

Crystallinity and disorder are important structural parameters for understanding relationships between structure and physical properties. Flaws and distortions are the main features that limit the ultimate properties of textile fibers. Some of these crazes, cracks and voids are revealed under the electron microscope, either on the surface or in cross sections stained with heavy metals (J, 2). However, these staining techniques (that reveal the main morphological features) make it much more difficult to determine the degree of distortion of the crystalline fraction. Theoretically, line profile studies permit separation of effects due to crystalline size from those due to structural distortions. However, the lack of peaks in semicrystalline fiber x-ray patterns hinders that approach. 

Nitromethane is the simplest and perhaps the most studied prototypical energetic material. It has been subjected to most of the applicable experimental techniques and its chemical and physical properties have been investigated for the solid, liquid, and gas phases. Significant progress in theoretically modeling of nitromethane in the various phases is being made. It appears that the initial steps in the chemical decomposition mechanisms are the same in the liquid and gas phases [83],... 

A great number of investigations was dedicated to studying the physical properties of organic compounds of germanium, tin and lead. We discuss here only those that include a comparative study of these compounds as well as studies which verify or develop theoretical concepts of their electronic structure. 

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