Other Structure Characterization Methods

Several non-rheological methods for probing molecular structure are described briefly here, as these often provide important information that complements rheological data or is required to verify or calibrate a rheological method for structure determination. 

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NMR is one method used for the compositional and structural characterization of cellulose ethers (41-47). Other structural characterization methods are based on various chromatographic techniques (48-51). The results show that the reactivity of the three hydroxyl groups can vary significantly depending on the alkylating reagent, the type of reaction, and reaction conditions. For most cellulose ethers, substitution occurs primarily at the C-2 and C-6 hydroxyl groups (51-53). 

In this chapter we review some of the most important developments in recent years in connection with the use of optical teclmiques for the characterization of surfaces. We start with an overview of the different approaches available to tire use of IR spectroscopy. Next, we briefly introduce some new optical characterization methods that rely on the use of lasers, including nonlinear spectroscopies. The following section addresses the use of x-rays for diffraction studies aimed at structural detenninations. Lastly, passing reference is made to other optical teclmiques such as ellipsometry and NMR, and to spectroscopies that only partly depend on photons. 

Other characterization methods are usually used to detect the changes in physical properties, which usually result from the changes in the morphology and structures of the substrates due to grafting. 

Polyesters are not different from other polymers, and any of the characterization methods commonly used in polymer science can obviously be applied to polyesters and provide information on their structure and properties. In this section, some data specific to polyesters—solubility information, COOH and OH endgroup titration, and infrared (IR) and NMR spectra assignments—are briefly summarized. Most of these data originate from the authors laboratory. References are provided on some particular points only. 

A Brief Review of the QSAR Technique. Most of the 2D QSAR methods employ graph theoretic indices to characterize molecular structures, which have been extensively studied by Radic, Kier, and Hall [see 23]. Although these structural indices represent different aspects of the molecular structures, their physicochemical meaning is unclear. The successful applications of these topological indices combined with MLR analysis have been summarized recently. Similarly, the ADAPT system employs topological indices as well as other structural parameters (e.g., steric and quantum mechanical parameters) coupled with MLR method for QSAR analysis [24]. It has been extensively applied to QSAR/QSPR studies in analytical chemistry, toxicity analysis, and other biological activity prediction. On the other hand, parameters derived from various experiments through chemometric methods have also been used in the study of peptide QSAR, where partial least-squares (PLS) analysis has been employed [25]. 

Chromatographic methods (essentially using ion-exchange resins) are especially useful for separating heparin from other glycosaminoglycans, for further analytical and structural characterization. However, it should be realized that recovery of heparin from resins is seldom quantitative,42 probably because of irreversible absorption of heparin fractions in the... 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

At shorter distances, particularly those characteristic of H-bonded and other charge-transfer complexes, the concepts of partial covalency, resonance, and chemical forces must be extended to intramolecular species. In such cases the distinction between, e.g., the covalent bond and the H-bond may become completely arbitrary. The concept of supramolecular clusters as fundamental chemical units presents challenges both to theory and to standard methods of structural characterization. Fortunately, the quantal theory of donor-acceptor interactions follows parallel lines for intramolecular and intermolecular cases, allowing seamless description of molecular and supramolecular bonding in a unified conceptual framework. In this sense, supramolecular aggregation under ambient thermal conditions should be considered a true chemical phenomenon. 

Smith et al. [1.127] reviewed the dielectric relaxation spectroscopy (DRS) as a method for structural characterization of polymers and proteins providing, among others, information about the water content and states of water. 

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

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