Probe molecules combined with spectroscopic methods

2 PROBE MOLECULES COMBINED WITH SPECTROSCOPIC METHODS 

FTIR and NMR spectroscopies have been used to study the surface basicity of solids by adsorbing different probe molecules such as pyrrole, but-l-yne, acetonitrile, chloroform, CO, C02 and thiols.[19,22] Limitations arise from the formation of various adsorbate structures leading to complicated patterns, or complete dissociation of the molecule with the disappearance of the signal, or polymerization of the molecule upon heating. 

Though scarcely used up to now, nitromethane has nevertheless been an interesting probe to obtain information about the strength and nature of basic sites. FTIR and NMR spectroscopies have been used in combination with this probe molecule.132,331 The 13C CP-MAS NMR has been the most interesting because it is a 

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Probe molecules combined with spectroscopic methods. 174... 

A number of modern physical techniques are used to characterize heterogeneous catalysts. These methods range from techniques probing the interaction of catalysts with probe molecules, to in situ surface characterization techniques as well as structural elucidation under both in situ and ex situ conditions. In general, interaction of catalysts with probe molecules is followed using some spectroscopic property of the probe molecule itself and/or the changes induced by the heterogeneous catalyst. The spectroscopic techniques used include vibrational spectroscopies, NMR spectroscopy, UV-Vis spectroscopy and mass spectrometry to name a few examples. Similarly, in situ techniques tend to use properties of probe molecules but also combined with structural techniques such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). In recent years XAS has been widely used in the characterization of catalysts and catalyst surfaces. 

Of all the numerous spectroscopic methods to probe molecular conformation (including NMR, circular dichroism, Raman spectroscopy, two-dimensimial IR spectroscopy, etc.), UV or IR action spectroscopy is most suited for the application to gas-phase molecules, either neutral or charged. Optical spectroscopy in the microwave, infrared or UVA is spectral range can provide detailed structural information, especially in combination with molecular beam techniques or other cryogenic methods (low internal temperatures), resulting in highly resolved spectroscopic signatures [73]. 

The special combination of orientational order and mobility possessed by liquid crystals, the wide variation in these properties in different liquid crystalline types, and the fact that bulk orientation can be easily controlled by a number of different methods have led to numerous applications in which liquid crystals are employed as anisotropic solvents for the study of various physicochemical properties of molecules. This chapter deals with the use of liquid crystals as solvents or supports in spectroscopic applications, in gas chromatography, and for chemical reaction. In each case, the emphasis is on studies in which the solute is of primary interest, and in which calamitic liquid crystals are employed as solvents to control orientation or mobility. Spectroscopic studies in which some property of the mesophase is of primary interest, whether they involve a study of probe molecules or the liquid crystal itself, are treated in earlier chapters and will not be dealt with here. In many cases, studies of this type are intimately re-... 

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