Selected Examples

2 Selected Examples. - Pulsed magnetic field gradient (PFG) NMR is today a routine method for the determination of self-diffusion coefficients. However, a remaining goal is the improvement of the precision of the method. The best procedure for the determination of accurate diffusion coefficients by PFG NMR is a calibration with a sample of precisely known D value. Thus Holz et al presented temperature-dependent self-diffusion coefficients of water and six selected molecular liquids. The gained accurate self-diffusion data are suited for an elaborate check of theoretical approaches in the physics of molecular liquids. Price et al examined the translational diffusion 

Ludwig, in Nuclear Magnetic Resonance, ed. G. A. Webb, Specialist Periodical Reports, The Royal Society of Chemistry, Cambridge, 2000,30, p. 181-213. 

Kramer, P. Kranmier, T. Richter, M. Jung, H. Vogel and P. Zehner, Angew. Chem. Int. Ed, 1999,38,2998. 

Yamaguchi, N. Yasutake and N. Nagaoka, Chem. Phys. Lett., 2001,340,129. T. Tsukahara, M. Harada, Y. Ikeda and H. Tomiyasu, Chem. Lett., 2001,420. 

Yablonskiy, D. S. Gierada and M. S. Conradi, Magn. Reson. 

In order to demonstrate the convenient properties of lenses combined with an electrostatic energy analyser, three examples will be discussed first the important case in which an acceleration/retardation lens is placed in front of the analyser which allows an operation mode with a constant pass energy in the analyser second the case in which increased acceptance angles are used, which is important for coincidence experiments and third the case in which 0 eV electrons are handled and analysed with high resolution and high acceptance. 

An experimental set-up using a lens system for the acceleration/retardation of the electrons in conjunction with an electrostatic analyser is shown in Fig. 4.32. The electron spectrometer is a spherical analyser with a mean radius R0 of 101.6 mm combined with a three-aperture zoom lens at the entrance and the exit. Only the purpose and the characteristic properties of the entrance lens will be 

In Fig. 4.33 the relevant potentials and energies are shown for the case of acceleration. Because the electrode L, is grounded, and L3 is at the potential needed to accelerate/retard the incident electrons to the same pass energy in the spectrometer, one derives from equ. (4.40) the following relations for the voltages 

The energy resolution of the spherical analyser which has first-order focusing in A and all-order focusing in A(p will be evaluated first. According to equ. (4.7b) one has for a spherical analyser 

the modification of the angular range A by the lens is considered. At the entrance of the lens, A ( is restricted to Ai9y +3° by an aperture (cone C in Fig. 4.32). (Together with a counterpart in the image plane of the lens this aperture 

The main problem of mechanistic studies is the broad variety of synthesis systems even within the nonaqueous sol-gel approach, which makes it very difficult to establish generally valid rules and concepts. Every precursor-solvent combination behaves differentiy and slight variations of the synthesis conditions might change the reaction mechanism. Accordingly, it is impossible to exhaustively cover the whole field. Instead, we present selected ex2unples from the recent literature, which, in our opinion, provide a representative overview of the activities in the area of nanoparticle formation mechanisms in nonaqueous environment. 

20 min, (d) HRTEM top-view image on the edge of a stack, (e) overview image after 1 h, and (f) HRTEM top-view image on the edge of a stack after 1 h. 

We restricted our discussion of nanoparticle formation mechanisms to surfactant-free systems, because they are typically easier to study and more data are available. However, there are a few papers dedicated to surfactant-assisted nonhydrolytic processes as well. An overview is given in Ref. [176]. 

Sticky, polar molecules can cause practical problems, because they adsorb on the cell walls and the transfer lines and it takes quite a long time to get rid of them after the experiment. While this is normally no problem in the cell, which can be baked out over night, the transfer lines retain some amount of the reactants and do not have a bake out function. Consequently, there are limitations with respect to the reactants that can be used. 

In the following sections, the above methods will be illustrated with a couple of selected examples. The list cannot be comprehensive. In particular, mass spectrometric studies on the analytical characterization meanwhile number into the hundreds, while the other aspects described here are less prominent in the literature. 

Several earlier studies [3-6,8] clearly showed that enhanced metal ion effects on rates of ester cleavage are obtained in model substrates in which a covalently linked polyether chain holds the metal ion in close proximity to the ester function undergoing nucleophilic attack. 

The sample kinetic data listed in Table 5.2 shows that the size of rate enhancement critically depends on the substrate-metal ion combination, and is markedly influenced by the solvent. The largest effect is displayed by 2-AcO-21 C6, which reacts with EtOBaBr half a million times faster than with EtONMe4. The conclusion was reached [6] that the huge rate enhancements observed in the ethanolysis reactions are a consequence of the fact that not only cation-anion electrostatic binding but also coordinative binding to the polyether chain in the metal-bound transition states are much more efficient in EtOH than in MeOH. 

An example of a more structured aryl acetate is offered by the monoacetyl derivative 2 ofp-tert-butylcalix[4]arene-crown-5 (1). The half-life for deacetylation in the presence of 1 mM MeONMe4 in MeOH at 25 °C is 34 weeks, but drops to 8 s upon addition of 1 mM BaBr2 [17]. Also, SrBr2 accelerates the reaction, albeit to a somewhat smaller extent. Kinetic and UV spectroscopic data show that in the absence of metal ions MeO reacts with the unionized form 2, but in the presence of metal ions the reactive species is the metal complex of the ionized form 3, which is present in significant amounts by virtue of the acidity enhancing effect of the metal ion complexed by the polyether chain. 

Rate enhancements brought about by metal ions, fesr/ o = 1.2x 10 ° and kga/ ko = 2.1 X 10 translate into transition state stabilization of 8.3 and 10.0 kcal moU, respectively. To the best of our knowledge, these are the most striking examples of electrophilic catalysis by Sr and Ba ions. 

Analytical ultracentrifugation methods have been employed to study the serine acetylransferase (SAT) of E. coli, in order to elucidate how many protein subunits are present in the functional enzyme. In bacteria, L-serine is converted to cysteine in a two-step process SAT catalyzes the first reaction in this metabolic pathway, described as sulfur fixation . The SAT subunit has been sequenced, and its MW is 29.3 kDa. 

Additional sedimentation velocity experiments were done using three SAT concentrations, at higher velocity (40,000 rpm), under otherwise similar experimental conditions. The cell was scanned every 30 min at 280 nm for 14 h. The data 

TABLE 13.1. Equilibrium Sedimentation Derived Molecular Weights  

Enzyme Concentration (mg/mL) MW Enzyme MW Subunit Apparent Quaternary Structure 

Retroviruses are viruses in which the genetic material is RNA. Several retroviruses are related with human diseases, like influenza or HIV. Some human retroviruses have glycoproteins as important surface structures these are usually of crucial importance for infectivity, and therefore for virus characterization. 

In contrast, the adsorption of hydrogen increases the reflectivity of Pt and Pd electrodes [4, 33, 81, 82]. The increase in the reflectivity by hydrogen adsorption is attributed to the increase in free-electron density of the surface [4, 86). In the case of cytosine adsorption on Au shown above, the charge transfer is from the electrode to the adsorbed molecule as is seen from the CV, and thus the reflectivity of the Au electrode decreases. The basehne is also sensitive to the applied potential and is minimal at the pzc of the electrode [15, 43]. 

Fernandez and G. Wider, Curr. Opin. Struc. Biol., 2003,13, 570. 

Pettersson, I. Furo, and P. Stilbs, Goncepts Magn. Reson., 2004,22A, 61. 

Branca, S. Capuani, and B. Maraviglia,Concepts Magn. Reson. Part A, 2004, 21A, 22. 

Johnson. 1996. Electrophoretic NMR. In D.M. Grant, R.K. Harris, editors. Encyclopedia of nuclear magnetic resonance. Chichester Wiley p 1886-1895. 

Most quantitative analyses are performed by coupling a chromatographic separation device with mass spectrometry. As enumerated in Chapter 5, the online 

The development of a robust and reliable online LC/MS combination via atmospheric pressure ionization (API) has greatly advanced the science of quantitative analysis of polar and thermally labile compounds. The speed of analysis, simplicity, and enhanced selectivity are the highlights of this combination. HPLC is more conveniently coupled to mass spectrometry via attnospheric-pressure chemical ionization (APCI) and electrospray ionization (ESI) interfaces. 

Quantification with LC/ESI-MS The use of LC/ESI-MS and SIM is demonstrated in the quantitative determination of a peptide drug, Ac-Arg-Pro-Asp- 

Pro-Phe-NH2, in human and rabbit plasma. In this procedure, after the addition of deuterated IS, the sample was desalted, proteins were precipitated, and the drug was extracted with solid-phase extraction (SPE). The HPLC separation was performed in the isocratic mode with a mobile phase that consists of 1% methanol and 20% acetonitrile in 10 mM ammonium formate (pH 5.2). A flow splitter was used to allow only 5% of the flow to enter the ESI source. 

Quantification with LC/ESI-MS/MS The use of LC/ESI-MS/MS for quantification is exemplified by a procedure for the determination of amoxycillin and its metabolites (5R,6R- and 55,6/J-amoxycilloic acid and amoxycillinpiper-azine-2,5-dione) in animal tissues [11]. Amoxycillin is an antibiotic that is prescribed for the treatment of a respiratory, gastrointestinal, and urogenital infection. In the procedure reported, drugs were extracted from animal tissues by liquid extraction and subjected to SPE. The IS ampicillin was added to the tissue sample prior to the extraction step. The samples were analyzed with LC/ESI-MS/MS. MS signal was recorded in the SRM mode by monitoring the transitions m/z 365.8 207.9 (for amoxicillin), m/z 383.8 189.0 (for 5R,6R- and 5S,6R-amoxycilloic acid), m/z 365.8 160.0 (for amoxicillinpiperazine-2,5-dione), 

The nine water molecules present in the asymmetric unit are arranged in a cluster-like fashion. They establish hydrogen bonds to other water molecules within the cluster and to the surrounding carbohydrate molecule. They are arranged to fill up an empty space in the crystal packing. Six and seven water molecules, respectively, are involved in the hydration of the two trisaccharides. 

The analysis of the packing indicates a well defined hierarchy of intermolecular contacts, some of them suggesting how Le Le interactions may occur in biological conditions providing the molecular basis for cell-cell recognition event. 

The least-squares superposition of the C and O-atoms of these two structural investigations, has a root mean square deviation of 0.05 A for the non hydrogen atoms. Some several discrepancies are noticed in the positions of O-H hydrogen atoms and consequently in hydrogen bonding scheme. The way H atom positions were located in the two investigations may explained such discrepancies as in one case they were located in theoretical positions whereas in the other one, they were located from differences Fourier analyses. 

When a subcell is constructed of the two central D-glucopyranoses in the two independent molecule, the obtained cell dimensions are identical to those of the cellulose II allomorph. Based on this subcell, a new model for cellulose II has been proposed [8]. 

One can found information about simple disaccharides (maltose, sucrose, ) 

MgO(lOO) single crystal from Barth. 2003 [26], (b) Large-scale and atomically resolved (top. positive bias bottom, negative bias) images of the surface of a NiO(lOO) single crystal  

While air annealing of NiO(lll) single crystals is an appropriate way to stabilize the p(2 X 2) reconstruction with Ni termination, similar sample preparation of MgO(lll) single crystals yields a (1x1) structure that results from an unreconstructed, 0-terminated (111) surface covered by hydrogen [42]. On annealing of MgO(lll) single crystals to above 1450°C, partial reduction and the occurrence of a ( 30 X y30)R30°, a (2x2), and a (2 30 x 2v 30)R30° structure was observed. 

Finally, it should be mentioned that attempts to prepare thin MgO(lll) films on Ag(lll) by alternating deposition of magnesium and oxygen resulted in unreconstructed MgO(lll) layers that show semiconducting or even metaUic behavior [43, 44]. 

2 Corundum-Type Surfaces-AljOj, CrjOj, FejOj.VjOj 

Batyrev, 1999 [46]. (b) Model of the relaxed Al20j(0001) surface according to GIXS measurements from Renauld, 1998 [13]. 

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Table 5.28 gives the modifications in physical/chemical characteristics resulting from deeper and deeper hydrotreatment (Martin et al., 1992). The sulfur contents could thus be reduced to first as low as a few hundred ppm, then to a few ppm. The level of aromatics in the selected example drops from 39% to 7% while the cetane number increases from 49 to 60. Note here that such a treatment, possible through experimental means, does not correspond to current industrial practice because of its high cost and its very high hydrogen consumption. 

This section deals with some selected examples of typical SFA results, collected from various research areas. It is not meant to be a comprehensive review, rather a brief glance at the kind of questions that can be addressed with the SFA. 

Only a very few selected examples have been discussed. The number of processes based on shape-selective catalysis by zeolites is ever increasing, particularly in the field of speciality and fine chemicals and quite a few have been... 

The first example of enantioselective catalysis of a Diels-Alder reaction was reported in 1979 . Since then, an extensive set of successful chiral Lewis-acid catalysts has been prepared. Some selected examples will be presented here together with their mechanistic interpretation. For a more complete... 

The hydroxyl groups of glucose (and, of course, other saccharides) must be regio- and stereo-selectively attacked, if this most abundant natural carbon compound is to be used as starting material. We shall first show with a few selected examples, how this can be achieved (A.H. Haines, 1976 J. Lehmann, 1976 L. Hough, 1979). 

Selected Examples of the Application of Distillation to the Separation of Inorganic Ions... 

Selected Examples of Organic Compounds of Biochemical, Pharmaceutical, and Environmental Significance That Show Natural Fluorescence or t Phosphorescence... 

Flow injection analysis has also found numerous applications in the analysis of clinical samples, using both enzymatic and nonenzymatic methods. A list of selected examples is given in Table 13.3. 

This article also aims at estabHshing the stmcture—property—appHcation relationships of aerogels. Selected examples are given to show what some desirable properties are and how they can be deHvered by design based on an understanding of the preparation and preservation of a gel s microstmcture. 

Divalent molybdenum compounds occur in mononuclear, dinuclear, and hexanuclear forms. Selected examples are shown in Figure 6. The mononuclear compounds are mostiy in the realm of organometaUic chemistry (30—32). Seven-coordinate complexes are common and include MoX2(CO)2(PR3)2, where X = Cl, Br, and I, and R = alkyl MoCl2(P(CH3)3)4, heptakis(isonitrile) complexes of the form Mo(CNR) 2 (Fig. 6d), and their chloro-substituted derivatives, eg, Mo(CNR)3CR. The latter undergo reductive coupling to form C—C bonds in the molybdenum coordination sphere (33). 

Aledicine. A wide variety of alkaloids (qv) contain the quinoline ring system this fact accounts, in large measure, for the extensive synthetic research reported (107). In addition to the naturally occurring compounds, a large number of synthetic quinolines, eg, (22) and (23), have been prepared and studied for use in medicine. Table 3 presents selected examples. 

A survey of known types of rhenium compounds is presented in Table 2. The selected examples include a number of commonly encountered compounds and emphasize the diversity in stmctures. 

Make a list, based on your own observations, of selected examples of components and structures made from cement and concrete. Discuss how the way in which the materials are used in each example is influenced by the low (and highly variable) tensile strength of cement and concrete. 

Table 10.6 Selected examples of compound specific instruments...

Table 10.12 Selected examples of sampling techniques for air contaminants...

FIGURE 8.11) Application of tfie strategy in system selection examples. 

Selected examples for specific reactions sequences which will illustrate possible combinations for the preparation of 19-norsteroids via 19-functional-ized intermediates are given below. 

The fundamental objective in this section is to describe the factors and procedures to select the right material for a specific structural application. The right stuff for a material, as for a fighter pilot or an astronaut, is a complex combination of characteristics. To select the proper material requires being able to characterize and evaluate various composite materials (or metalsl) and to compare their attractive characteristics with the behavioral features required for a particular structure. Finally, a materials selection example of a space truss design problem will be addressed. 

Cyclic ethers having perfluorinated side chains or perfluorinated rings attached in the a-position with respect to oxygen are remarkably stable toward cobalt trifluoride and can be successfully fluormated to perfluoro ethers at 440 °C without being cleaved to a considerable extent at the oxygen [16, 17], Selected examples are shown in Table 4. 

Table 4. Selected Examples of Preparation of Perfluoro Ethers [16, 17] by Heating with Cobalt Trifluoride and Calcium Fluoride at 440 "C...

This chapter makes no distinction between gel-permeation chromatography (GPC) and size exclusion chromatography (SEC). We make mention of specific analysis conditions wherever possible. We have attempted to include a variety of conditions but by no means should this chapter be considered a comprehensive review of conditions for analyzing polyacrylates. We have drawn extensively from our own experience in selecting examples. 

The primary objectives of this chapter are to detail the methods by which enamines (a,/3-unsaturated amines) (I) can be synthesized and the mechanisms of enamine formation. The enamines discussed are those in which the nitrogen is tertiary and, with the exception of a few selected examples, Contain no other functional groups. The term simple enamines might be used to describe the majority of enamines noted in this chapter. 

A comprehensive treatment of the oxoacids and oxoanions of P is inappropriate but selected examples have been chosen to illustrate... 

Polymer-supported BINOLs thus prepared were treated with Zr(Ot-Bu)4 to form polymer-supported zirconium 20. In the presence of 20 mol% of various zirconium 20, the model aza Diels-Alder reactions of imine Id with Danishefsky s diene (7a) were performed results from selected examples are shown in Table 5.8. Whereas the 4-t-butylphenyl group resulted in lower enantiomeric excess (ee), higher ee were obtained when 3,5-xylyl, 4-biphenyl, 4-fluorophenyl, and 3-tri-... 

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