Additional methods

Ar= 2-halC6H4, 2-MeC6H4, 3-MeC6H4, 4-MeC6H4, 2-ACC6H4, 2-CO2HC6H4 

The protocol shown in Section 3.1.2.7 is also usefnl to prepare unsymmetrical diaryl tellurides.  

Irgolic, K. J. in Houben-Weyl-Methods of Organic Chemistry. 4th edn, Vol. E12b, p. 389. Georg Thieme, Stuttgart, 1990. 

Diorganyltellurium dihalides are often the primary products in the synthesis of organic derivatives of tellnrinm and are therefore immediate precnrsors of the corresponding tellnrides. 

As described by Williams et al. (1990), random amplified polymorphic DNA-PCR (RAPD-PCR) is a variant of PGR that utilizes oligonucleotide probes (9 to 12 base pairs or bp) to amplify several regions of the genome. The amplification products are then separated electrophoretically. Resolution depends upon the primer sequence and reaction conditions. RAPD-PCR can be made more specific by use of highly specific oligonucleotide probes. Holt and Cote (1998) applied this technique toward the identification of dextran-producing Oenococcus strains, and Esteve-Zarzoso et al. (1998) were able to identify Saccharomyces and Zygosaccharomyces species. Quesada and Cenis (1995) used the method to characterize wine yeasts. 

The microbial genome contains randomly interspersed repetitive DNA sequences that are strain specific and thus serve as a genotypic fingerprint (Versalovic et al., 1991 1994 1998). The value of the repetitive sequence-based polymerase chain reaction (REP-PCR) method is limited only to regions of compatibility (average 10 to 15 bp), and other polymorphic sites are missed. 

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The molecular electronic polarizability is one of the most important descriptors used in QSPR models. Paradoxically, although it is an electronic property, it is often easier to calculate the polarizability by an additive method (see Section 7.1) than quantum mechanically. Ah-initio and DFT methods need very large basis sets before they give accurate polarizabilities. Accurate molecular polarizabilities are available from semi-empirical MO calculations very easily using a modified version of a simple variational technique proposed by Rivail and co-workers [41]. The molecular electronic polarizability correlates quite strongly with the molecular volume, although there are many cases where both descriptors are useful in QSPR models. 

NMR spectra have been predicted using quantum chemistry calculations, database searches, additive methods, regressions, and neural networks. 

Y Fu and L Lai 1997. A New Atom-Additive Method for Calculating Partition Coefficients. mal of Chemical Information and Computer Science 37 615-621. 

Group additivity methods must be derived as a consistent set. It is not correct to combine fragments from different group additivity techniques, even for the same property. This additivity approximation essentially ignores effects due to the location of one functional group relative to another. Some of these methods have a series of corrections for various classes of compounds to correct for this. Other methods use some sort of topological description. 

Surface tension is usually predicted using group additivity methods for neat liquids. It is much more difficult to predict the surface tension of a mixture, especially when surfactants are involved. Very large molecular dynamics or Monte Carlo simulations can also be used. Often, it is easier to measure surface tension in the laboratory than to compute it. 

Completely ah initio predictions can be more accurate than any experimental result currently available. This is only true of properties that depend on the behavior of isolated molecules. Colligative properties, which are due to the interaction between molecules, can be computed more reliably with methods based on thermodynamics, statistical mechanics, structure-activity relationships, or completely empirical group additivity methods. 

An example of using one predicted property to predict another is predicting the adsorption of chemicals in soil. This is usually done by first predicting an octanol water partition coelficient and then using an equation that relates this to soil adsorption. This type of property-property relationship is most reliable for monofunctional compounds. Structure-property relationships, and to a lesser extent group additivity methods, are more reliable for multifunctional compounds than this type of relationship. 

The primary problem with explicit solvent calculations is the significant amount of computer resources necessary. This may also require a significant amount of work for the researcher. One solution to this problem is to model the molecule of interest with quantum mechanics and the solvent with molecular mechanics as described in the previous chapter. Other ways to make the computational resource requirements tractable are to derive an analytic equation for the property of interest, use a group additivity method, or model the solvent as a continuum. 

A similar technique is to derive a group additivity method. In this method, a contribution for each functional group must be determined. The contributions for the functional groups composing the molecule are then added. This is usually done from computations on a whole list of molecules using a htting technique, similar to that employed in QSPR. 

The development of group additivity methods is very similar to the development of a QSPR method. Group additivity methods can be useful for properties that are additive by nature, such as the molecular volume. For most properties, QSPR is superior to group additivity techniques. 

The simplest empirical calculations use a group additivity method. These calculations can be performed very quickly on small desktop computers. They are most accurate for a small organic molecule with common functional groups. The prediction is only as good as the aspects of molecular structure being par-... 

Franke and co-workers evaluated a standard additions method for a voltammetric determination of Tl. A summary of their results is tabulated here. 

Bader, M. A Systematic Approach to Standard Addition Methods in Instrumental Analysis, /. Chem. Educ. 1980, 57, 703-706. 

Nimura, Y. Carr, M. R. Reduction of the Relative Error in the Standard Additions Method, Analyst 1990, 115, 1589-1595. The following paper discusses the importance of weighting experimental data when using linear regression Karolczak, M. To Weight or Not to Weight An Analyst s Dilemma, Curr. Separations 1995, 13, 98-104. 

An additional method for increasing particle size deserves mention. When a precipitate s particles are electrically neutral, they tend to coagulate into larger particles. Surface adsorption of excess lattice ions, however, provides the precipitate s particles with a net positive or negative surface charge. Electrostatic repulsion between the particles prevents them from coagulating into larger particles. 

Two additional methods for determining the composition of a mixture deserve mention. In multiwavelength linear regression analysis (MLRA) the absorbance of a mixture is compared with that of standard solutions at several wavelengths. If Asx and Asy are the absorbances of standard solutions of components X and Y at any wavelength, then... 

The generalized standard addition method (GSAM) extends the analysis of mixtures to situations in which matrix effects prevent the determination of 8x and 8y using external standards.When adding a known concentration of analyte to a solution containing an unknown concentration of analyte, the concentrations usually are not additive (see question 9 in Chapter 5). Conservation of mass, however, is always obeyed. Equation 10.11 can be written in terms of moles, n, by using the relationship... 

Raymond, M. Jochum, C. Kowalski, B. R. Optimal Multicomponent Analysis Using the Generalized Standard Addition Method, /. Chem. Educ. 1983, 60, 1072-1073. 

This experiment demonstrates the application of the generalized standard additions method for the analysis of mixtures of K2Cr207 and KMn04. 

Some additional methods of classification are under development that center on the use of lignite for combustion in utihty boilers or electric power generation. Correlations based on the sodium concentration in the lignitic ash (10), or soluble A1 concentration (11) are used. The classifications are often given in terms of the severity of boiler fouling. 

Refining to a High Purity Product. The normal yeUowcake product of uranium milling operations is not generaUy pure enough for use ia most nuclear appHcations. Many additional methods have been used to refine the yeUowcake iato a product of sufficient purity for use ia the nuclear iadustry. The two most common methods for refining uranium to a high purity product are tributyl phosphate (TBP) extraction from HNO solutions, or distiUation of UF, siace this is the feedstock for uranium enrichment plants. 

Standards used to constmct a cahbration curve must be prepared such that the matrix of the standard is identical to the sample s matrix because the values of the parameters k and b associated with a linear cahbration curve are matrix dependent. Many areas of chemical analysis are plagued by matrix effects, and it is often difficult to duphcate the sample matrix when preparing external standards. Because it is desirable to eliminate matrix effects, cahbration in the sample matrix itself can be performed. This approach is called the standard addition method (SAM) (14). In this method, the standards are added to the sample matrix and the response of the analyte plus the standard is monitored as a function of the added amount of the standard. The initial response is assumed to be Rq, and the relationship between the response and the concentration of the analyte is... 

Fig. 13. The standard addition method where MB is the confidence interval for the slope of the line = k, and represents 95% confidence interval (14).

Yullj Additive Method. No electrolytic plating step is used ia the fully additive process. The copper circuit is formed directly on the board without a continuous copper film. Heavy-build electroless coppers are used to iacrease the final thickness of the entire circuit. This process is much more difficult to control than the others. Additive processiag is becoming increasingly important ia high aspect ratio, very small diameter through-holes that caimot be easily electrolyticaHy plated. 

The two major methods of preparation are the cycloaddition of nitrile oxides to alkenes and the reaction of a,/3-unsaturated ketones with hydroxylamines. Additional methods include reaction of /3-haloketones and hydroxylamine, the reaction of ylides with nitrile oxides by activation of alkyl nitro compounds from isoxazoline AT-oxides (methoxides, etc.) and miscellaneous syntheses (62HC(i7)i). 

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