Experimental Aspects

Four main aspects are addressed in turn (i) neutron sources and reflectometer operation (ii) choice of substrate, substrate preparation, and characterization (iii) cell design and assembly and (iv) data acquisition and analysis. 

Due to the experimental difficulties involved, there have been only three reports of XSW measurements at electrochemical interfaces. Materlik and co-workers have studied the underpotential deposition of thallium on single-crystal copper electrodes under both ex situU9 and in situ120 conditions. In addition, they report results from studies in the absence and presence of small amounts of oxygen. 

In the ex situ studies, the thallium layer was electrodeposited and the electrode was subsequently removed from solution and placed inside a helium-filled box where the XSW experiments were carried out. 

From an analysis of their data, Materlik and co-workers were able to determine that for the ex situ case and in the absence of oxygen, the thallium atoms are located at twofold sites at a mean distance of 2.67 0.02 A. For the in situ case and again in the 

Deutsche and Moscowitz, a number erf semiempirical models have been proposed to explain features in experimental spectra. Although none has enjoyed any but the most modest successes, experience is gradually accumulating on the limits of applicability. Some of these are reviewed below. It is readily shown that within the Born-Oppenheimer (BO) approximation, the electronic contribution to the magnetic dipole transition moment associated with a vibrational transition of a molecule in its ground electronic state vanishes. A non-BO theory of VCD intensities was independently developed by several groups.2° 2  

Ab initio computer codes have been available in the last five or six years to calculate VCD intensities by several variations of the theory. Agreement of simulated theoretical spectra with experimentally measured spectra for small molecules is often excellent and sufficiently accurate to establish VCD as the first spectroscopic method for the determination of absolute configurations of small molecules in the gas or liquid phase. Most of the available computational work is reviewed in the sections below. 

Standard IR cells with windows appropriate for the materials under investigation are used. To avoid erroneous VCD signals, absorption intensities should be kept in the range 0.2-0.8, and it is usually good practice to check instrument performance regularly with a compound for which the VCD spectrum is well documented, e.g., a-pinene. With an FT interferometer, VCD intensities must be calibrated, - a procedure in which the sample is replaced by a second polarizer. Samples are usually in solution in a suitable solvent, in a matrix of solid inert gas, or neat liquids. The spectra of a few selected gaseous materials have been reported.Solid samples of chiral compounds are not suitable for CD measurements because they also exhibit linear dichroism that generally dominates and obscures the circular dichroism. 

The fact that the molecules are adsorbed on a solid surface giv rise to a number of new effects compared to the gas phase situation. The experimental situation consists of a monolayer of molecules adsorbed on a metal surface, on which we shine infrared radiation and then detect the reflected light. The macroscopic theory for the electromagnetic response of such a system is reproduce in the previous reviews. A more microscopic treatment has been given by Persson, showing that the integrated infrared absorptance for p-polarized light is given by  

An important consequence of the presence of the metal surface is the so-called infrared selection rule. If the metal is a good conductor the electric field parallel to the surface is screened out and hence it is only the p-component (normal to the surface) of the external field that is able to excite vibrational modes. In other words, it is only possible to excite a vibrational mode that has a nonvanishing component of its dynamical dipole moment normal to the surface. This has the important implication that one can obtain information by infrared spectroscopy about the orientation of a molecule and definitely decide if a mode has its dynamical dipole moment parallel with the surface (and hence is undetectable in the infrared spectra) or not. This strong polarization dependence must also be considered if one wishes to use Eq. (1) as an independent way of determining ft. It is necessary to put a polarizer in the incident beam and use optically passive components (which means polycrystalline windows and mirror optics) to avoid serious errors. With these precautions we have obtained pretty good agreement for the value of n determined from Eq. (1) and by independent means as will be discussed in section 3.2. 

Over the last decade there have been great developments of the infrared spectrometers used in surface science. One has moved from simple, single 

Furthermore, if one uses the wavelength modulation technique, which is strictly not surface sensitive but only enhances sharp structures and as the bandwidth of a grating monochromator decreases with decreasing energy, the low frequency peak will appear broad for reasonable slit widths. The 

However, during the last three to four years, I have in several stages improved on my infrared spectrometer along the lines discussed above. 

A detailed description of the principles of operation of conventional Mossbauer spectrometers may be found in a number of excellent mono-graphs. Although some of these concepts will be reviewed here, most of the attention will be focused on those aspects which are of special importance to the design of in situ experiments. 

In more recent studies, mass spectrometry has commonly been used to supplement or replace total pressure measurement as the means of directly 

How can Eq. (3.52) be verified For verification, the two variables — concentration and surface tension — need to be determined independently. One way is to use radioactively labeled dissolved substances. The radioactivity close to the surface is measured. /Temi tiers (3H, 14C, 35S) are suitable because electrons only travel a short range, i.e., any recorded radioactivity comes from molecules from the interface, or close below [48], 

Plots of surface tension versus concentration for n-pentanol [49], LiCl (based on Ref. [50]), and SDS in an aqueous medium at room temperature are shown in Fig. 3.7. The three curves are typical for three different types of adsorption. The SDS adsorption isotherm is typical for amphiphilic substances. In many cases, above a certain critical concentration defined aggregates called micelles are formed (see Section 12.1). This concentration is called the critical micellar concentration (CMC). In the case of SDS at 25°C this is at 8.9 mM. Above the CMC the surface tension does not change significantly any further because any added substance goes into micelles not to the liquid-gas interface. 

The adsorption isotherm for pentanol is typical for lyophobic substances, i.e., substances which do not like to stay in solution, and for weakly amphiphilic substances. They become enriched in the interface and decrease the surface tension. If water is the solvent, most organic substances show such a behaviour. The LiCl adsorption isotherm is characteristic for lyophilic substances. Most ions in water show such behaviour. 

In order to describe the influence of a substance on the surface tension, one could specify the gradient of the adsorption isotherm for c — 0. A list of these values for some substances dissolved in water at room temperature is shown in Table 3.2. 

Factors that affect enzyme activity, that is, the rates of enzyme-catalyzed reactions, include  

Some of the main experimental observations with respect to concentration effects, point (3) above, are  

The effect of extreme pH values can be similar to that of T in denaturing the enzyme. This is related to the nature of enzymatic proteins as polyvalent acids and bases, with acid and basic groups (hydrophilic) concentrated on the outside of the protein. 

Mechanical forces such as shear and surface tension affect enzyme activity by disturbing the shape of the enzyme molecule. Since the shape of the active site of the enzyme is specifically engineered to correspond to the shape of the substrate, even small changes in structure may drastically affect enzyme activity. Consequently, fluid flow rates, stirrer speeds, and foaming must be carefully controlled in order to ensure that an enzyme s productivity is maintained. 

For these reasons, in the experimental study of the kinetics of enzyme-catalyzed reactions, T, shear and PH are carefully controlled, the last by use of buffered solutions. In the development, examples, and problems to follow, we assume that both T and pH 

Craig and coworkers [50, 51] were the first to demonstrate the existence of 

7 Mossbauer-Active Transition Metals Other than Iron 

The 93 keV Mossbauer level is populated either by jS decay of Cu or by EC of Ga. The decay scheme, reproduced from [1], is shown in Fig. 7.19. The nuclear data of interest for Zn Mossbauer spectroscopy may be taken from Table 7.1 (end of the book). 

The sources most commonly used so far consisted of sintered disks containing about 100 mg ZnO enriched with 90% Zn. The disks were irradiated with 12 MeV deuterons or 30 MeV He particles, to yield the 78 h activity of Ga, and then annealed by heating in oxygen to 700-1,000 K for about 12 h and cooling down slowly (about 50 K h ) to room temperature. A Nal scintillation counter, 2-3 mm thick, is suitable for the detection of the 93 keV y-rays. Because of the relatively high transition energy, both source and absorber are generally kept at liquid helium temperature. 

Most of the Zn Mossbauer experiments so far have been carried out with ZnO as absorber. De Waard and Perlow [54] used polycrystaUine ZnO enriched to 90% in Zn with various pretreatments. They intended to determine (1) the quadrupole splitting in ZnO, (2) the influence of source and absorber preparation on the width and depth of a resonance, (3) the SOD shift, and (4) the influence of pressure on the source. 

Three different polyethylenes referred to as HD, LD, LLD have been investigated. They mainly differ in their molecular weight distribution and in the structure of the chains which can be either very hnear as in the case of HD (high density polyethylene) or short-branched in the case of LLD (linear low density polyethylene) or long-branched in the case of LD (low density polyethylene). The weight average molecular weight and the polydispersity index of the samples are siunmarized in Table la. 

Some terminal viscoelastic parameters have also been evaluated at 160°C using a Cole-Cole expression for the dynamic viscosity. Table Ic shows the zero shear viscosity (t]o) the characteristic relaxation time (ko, corresponding to the 

The extended Cox-Merz rule [49] can be successfully applied for HD and LLD. This nile states that the viscosity and elasticity coefficients for oscillatory and steady state shear flows are related, according to  

Whenever it is applicable, such a comparison can lead to a considerable widening of the shear rate range, this is especially interesting in the case of normal stresses which are generally difficvilt to measiu on a broad window of rates. However, significant differences were noted in the case of LD preventing the use of the previous rule and any enlargement of the data set. 

Using the cone and plate geometry, stress growth experiments have also been performed using different temperatures and different shear rates. Correct tangential (X+(t,Y) Tj+(t,Y)) and normal stress (Ni(t,Y) Vi(t,Y)) data were 

The positioning of the sample relative to the mass analyzer is also important. Inaccuracy in the distance to the secondary ion extractor associated with mass analyzer affects mass resolution, but does not strongly affect spectral intensity. Tilting the sample relative to the 

Olt et al. introduced Na NMR micro imaging to map the sodium distribution in living plants. The experiments were performed at 11.71 T with a double resonant Na- H probe-head. The Na micro imaging promises great potential for physiological studies of the consequences of salt stress on the macroscopic level and thus may become a unique tool for characterizing plants with respect to salt tolerance and salt sensitivity. 

Surface coils (cf. Section 9.2) Ackl, Bosl J are applied to the object from one side so that the associated B field is inhomogeneous within the sensitive volume. Apart from symmetry, semitoroidal coils offer particular advantages in spatially resolved NMR. A semitoroidal coil is a solenoid which is bent into a semicircular arc, and the rf field 

2 Special surface coils, (a) Electron micrograph of a planar microcoil. Adapted from fWebI J with permission from Elsevier Science, (b) Drawing of a semitoroidal coil [Assl J. 

In inhomogeneous Bq and B fields the flip angle may be relatively well defined if the Bo gradient is strong. Then the B field is quite homogeneous over the sensitive 

4 Excitation with a surface coil of 9 mm diameter in a Bo gradient of the order of lOT/m by the NMR-MOUSE (a) Series train of CPMG echoes from a carbon-black filled SBR section of an intact car tyre with a steel belt. A fit of the echo envelope with an exponential decay function yields a transverse relaxation time Ti [Eidl]. (b) Variation of the pulse duration in an a — te/l — Ta — ts/ i- pulse sequence for different rf frequencies. For each frequency maxima and minima are observed which define the nominal 90° and 180° pulse widths. With decreasing rf frequency the distance of the sensitive volume from the rf coil increases. A frequency of 17.5 MHz correspond to depths of 0-0.5 mm, 16MHz to 0.5-1.0mm, and 16.5 MHz to 1.0-1.5 mm into the sample [Gut3], (c) Hahn- and solid-echo envelopes for a sample of carbon-black filled cross-linked SBR. The Hahn-echo decay is faster because of residual dipolar couplings which are partially refocused by the solid-echo [Gut3]. (d) Multi-echo excitation. 

A so-called spin-lock effect manifests itself in a dependence of the effective transverse relaxation time Tie on the echo time tn of the multi-echo sequence [Gut3]. 

The single-pellet electrochemical cells were prepared on 1 mm thick 6 or 8 mol% yttria-stabilized zirconia pellets (Zr02-FSZ, Metoxit 

They were sandblasted and then sonicated in 2-propanol before deposition of the electrodes. 

The working electrodes were usually deposited by thermal decomposition of suitable precursors (RuCls, H2lrCl6 or RhC ). The method consists of application of a dilute solution of the precursor in 

The reference electrode for EP experiments is usually prepared by deposition of a continuous porous gold film onto the solid electrolyte. In the case of YSZ (an 0 -conducting solid electrolyte), the potential of this reference electrode is determined by the redox reaction 

In order to obtain the true ohmic drop free potential of the catalyst (working electrode), the potential difference measured with respect to the reference, k wR. must be corrected from the ohmic drop in the solid electrolyte, This calculation requires knowledge of the potential distribution in the solid electrolyte, which may be obtained either with the primary or with a secondary current distribution. The approximation ofprimary current distribution isjustified when most ofthe resistance is 

Redfield described an apparatus that can pneumatically move an aqueous liquid NMR sample sealed in a 5 or 8 mm standard tube, from the center of a standard Varian 500 MHz spectrometer and probe out to any position in its fringe field, and back, under computer control. Applications to low-field relaxation and other measurements are discussed briefly, particularly using P, and and with proton detection, utilizing the full power of the commercial system for preparation before, and detection after, a relaxation or mixing interval at low field. Transverse relaxation, polarization transfer, and other transverse manipulations are virtually impossible with this apparatus while the sample is at the low (fringe) field, because of the tremendous inhomogeneity of that field. 

Harris et described a numerical methodology to obtain more efficient relaxation filters to selectively retain or remove components based on relaxation times. The procedure uses linear combinations of spectra with various recycle or filter delays to obtain components that are both quantitative and pure. Modulation profiles are calculated assuming exponential relaxation behavior. The method is general and can be applied to a wide range of solution or solid-state NMR experiments including direct-polarization (DP), or filtered cross-polariz- 

This type of reactor has been used for the study of the pyrolysis of normal hexane under supercritical conditions, at temperatures of between 290 and 365 °C and at pressures of between 210 and 15 600 bar (the critical values for hexane are 234 °C and 29.3 bar). These studies are interesting for understanding the evolution of oil-fields. 

A reactor consisting of a gold tube of 3.8 (or 3) mm diameter, of 0.2 mm thickness and of 40 (or 16) mm length is used. The tube is sealed at one extremity, then filled with hexane under an atmosphere of argon to avoid having oxygen present, then electrically soldered at the other extremity. A reactor contains about 100 to 150 (or 20) pL hexane. 

The reactor is placed in a vessel filled with water, and is itself inside an autoclave which is put in an oven the temperature of which is regulated. 

At the instant t = 0, the reactor is put under pressure by the water, the pressure being applied by a movable piston in a cylinder. At the instant t = tj., lying between 14 h and 1 month, the gold capsule is removed from the apparatus, placed in a recipient which is emptied the ampoule is then pierced, the products are taken out and analysed by gas-phase chromatography and GCMS (Gas Chromatograph coupled to a Mass Spectrometer). 

The batch reactor is convenient for the study of the sensitivity of the reaction to the nature and the relative extent of the reactor walls. Three types of experimental tests are carried out. 

It may be concluded that the use of a continuous, porous gold fdm as reference electrode in the given solid oxide electrochemical cell is an appropriate choice. It was shown to be catalytically inert and to have a reasonably stable potential. The estimated error of the latter is less than 20 mV under typical conditions of electro-chemical promotion experiments. The potential distribution in the solid electrolyte was shown to be highly sensitive to the exact position of the electrodes. Nevertheless, estimation of the catalyst potential remains reliable because the ohmic drop correction is almost negligible, e.g., at 375°C it is in the order of 1 mV pA.  

All Stable chlorine, bromine and iodine nuclei possess an electric quadrupole moment and in non-symmetric environment their NMR spectra are usually broad and cannot be studied by ordinary high resolution techniques. Notable exceptions are halide ions in aqueous solution where the NMR signals become reasonably narrow (approximate line widths at half height are = 8 Hz for Cl and 5 Hz for Cl 

81 / and Br spectra of aqueous ions have been obtained both with wide- 

The NMR sensitivity of the quadrupolar halogen nuclei is low (cf. Table 1.1 of Section 1.1) and without signal averaging techniques it is difficult to study halide ion spectra at [Cl ] and [I ] 0.1 M or bromide ion spectra at [Br ] 0.05 M. 

For covalent chlorine, bromine and iodine compounds the relaxation times of the halogen nuclei are extremely short and problems of sensitivity considerable even for pure liquids. Direct observations of NMR signals have therefore been reported only for chlorine compounds. Narrow signals are obtained when the nucleus is at a site of tetrahedral symmetry in covalent compounds (cf. Chapter 9). 

Relaxation data on quadrupolar halogen nuclei may be obtained either from pulsed NMR studies or from line width measurements. The lower limit of T.j and T2 values that can be measured with a pulse spectrometer is set by the effective dead time of the receiver system after a pulse and by the width and amplitude of the pulse. At present 

JOHN D. KENNEDY Department of Inorganic and Structural Chemistry, University of Leeds, Leeds LS2 9JT, England. W. McFARLANE Department of Chemistry, Sir John Cass School of Science and Technology, City of London Polytechnic, London EC3N 2EY, England. 

Nuclide Spin Natural abundance (%) Receptivity Resonance frequency (MHz)  

In principle direct observation of Si, Sn, or Pb will provide the best sensitivity for the measurement of couplings to carbon, but there will often be problems of dynamic range due to the low abundance of C and it may be difficult to assign the splittings in molecules of any complexity C observation is therefore normally the preferred approach for such measurements. 

Two-dimensional homonuclear /-resolved and COSY Si spectra have been used to study polymeric species in aqueous potassium silicate solutions enriched in Si, and a two-dimensional chemical exchange Sn spectrum has been reported for a five-coordinate ditin compound. Much work has been done on Si NMR in the solid state using magic angle spinning (MASS) and cross-polarization (CP) methods, and this technique has potential also for Sn and Pb. 

The corresponding effects upon other group IV resonances are less well studied but can be expected to be greater for Pb, and less for Si and Ge. Other isotopic substitutions such as C/ C have much smaller effects owing to the smaller relative-mass change, and can generally be ignored. The 0/ 0 effect upon 5( Si) has potential for mechanistic studies.  

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Perhaps the best discussions of the experimental aspects of the capillary rise method are still those given by Richards and Carver [20] and Harkins and Brown [21]. For the most accurate work, it is necessary that the liquid wet the wall of the capillary so that there be no uncertainty as to the contact angle. Because of its transparency and because it is wet by most liquids, a glass capillary is most commonly used. The glass must be very clean, and even so it is wise to use a receding meniscus. The capillary must be accurately vertical, of accurately known and uniform radius, and should not deviate from circularity in cross section by more than a few percent. 

A nice collection of review articles dealing with various experimental aspects of modern NMR spectroscopy several papers on relaxation. 

Hesselink W H and Wiersma D A 1983 Theory and experimental aspects of photon echoes in molecular solids... 

The Molecular Weight Dependence of Viscosity Experimental Aspects... 

In order to carry out an experimental study of the kinetics of crystallization, it is first necessary to be able to measure the fraction d of polymer crystallized. While this is necessary, it is not sufficient we must also be able to follow changes in the fraction of crystallinity with time. So far in this chapter we have said nothing about the experimental aspects of determining 6. We shall now briefly rectify this situation by citing some of the methods for determining 6. It must be remembered that not all of these techniques will be suitable for kinetic studies. 

First, we consider the experimental aspects of osmometry. The semiperme-able membrane is the basis for an osmotic pressure experiment and is probably its most troublesome feature in practice. The membrane material must display the required selectivity in permeability-passing solvent and retaining solute-but a membrane that works for one system may not work for another. A wide variety of materials have been used as membranes, with cellophane, poly (vinyl alcohol), polyurethanes, and various animal membranes as typical examples. The membrane must be thin enough for the solvent to pass at a reasonable rate, yet sturdy enough to withstand the pressure difference which can be... 

Surface Area and Permeability or Porosity. Gas or solute adsorption is typicaUy used to evaluate surface area (74,75), and mercury porosimetry is used, ia coajuactioa with at least oae other particle-size analysis, eg, electron microscopy, to assess permeabUity (76). Experimental techniques and theoretical models have been developed to elucidate the nature and quantity of pores (74,77). These iaclude the kinetic approach to gas adsorptioa of Bmaauer, Emmett, and TeUer (78), known as the BET method and which is based on Langmuir s adsorption model (79), the potential theory of Polanyi (25,80) for gas adsorption, the experimental aspects of solute adsorption (25,81), and the principles of mercury porosimetry, based on the Young-Duprn expression (24,25). 

H. B. Bebb and E. W. Williams, in Semiconductors and Semimetals. (R. K. Willardson and A. C. Beers, eds.) Academic Press, vol. 8, 1972. An extensive review of PL theory and technique, with emphasis on semiconductors. Some of the experimental aspects and examples are becoming outdated. 

It is the purpose of this book to afford a critical review covering both theoretical and experimental aspects of some of the most important and... 

The first substituted thiirene dioxides21 and thiirene oxides22 (e.g. 4 x = 2 and x = 1, respectively) were synthesized and characterized by Carpino and coworkers, while the parent thiirene oxide and dioxide are not known to date. However, the successful syntheses of the substituted unsaturated systems 4 opened the door to an extensive research involving the theoretical and experimental aspects of this class of intriguing compounds2, particularly as far as the unique role and characteristics of their sulfone and sulfoxide groups are concerned. 

Vetter, K. J. (1967). Electrochemical Kinetics Theoretical and Experimental Aspects , Academic Press. 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

Ultrastructural properties of human enamel apatite. In Lazzari, E., ed.. Handbook of Experimental Aspects of Oral Biochemistry, Florida, CRC Press 159-179. 

Johnson, M.K. (1992). Molecular events in delayed neuropathy Experimental aspects of neuropathy target esterase. In B. Ballantyne and T.C. Marrs, (1992). Clinical and Experimental Toxicology of Organophosphates and Carbamates 90-113. 

Verbrugge MW, Tobias CW (1987) Some experimental aspects of the cadmium-teUurium electrochemical codeposition process. J Electrochem Soc 134 3104-3109... 

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