NMR of Probe Molecules

Recently, Peng et al. [Ill] have suggested making use of diphosphine molecules to study Bronsted acid sites via P MAS NMR spectroscopy of the probe molecules. It could be shown that these molecules are sensitive to the acidity of the acid sites as well as to the distances between the acid sites. 

Finally, Mildner and Freude [130] have studied the proton transfer between Bronsted acid sites in H - Y zeolites and adsorbed benzene molecules via two-dimensional exchange spectroscopy. This exchange can be considered to be the first elementary step of a Bronsted acid-catalysed reaction. The proton exchange rate turned out to depend on the strength of acidity of the Bronsted acid sites. In addition, activation energies of ca. 90-100kJ/mol could be observed for the proton transfer reaction by this method. 

NMR spectroscopy allows the determination of the geometry of adsorption complexes and of the mobility of the probe molecules. This will be demonstrated for CO molecules adsorbed on bridging hydroxyl groups. 

Rigid molecules should exhibit a broad signal with the typical shape of a line dominated by the chemical shift anisotropy. The principal values of 

If the NMR spectrum consists of two or more superimposed signals, it may be advantageous to apply the MAS technique in order to resolve the different signals (see above). Structural information, e.g. internuclear distances, may then be obtained by a munerical analysis of the spinning sideband pattern [70]. 

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Table 3 Diameters of Spherical Objects Determined by Cryofracture and the Diameter of Inner Water Core Determined by NMR of Probe Molecules...

The diameters determined by cryofracture are compared with those computed from an indirect determination method using F NMR of probe molecules (see below). It is interesting to note that the diameters determined by the two methods are of the same order of magnitude. The values determined by cryofracture were computed from the observed diameter from which the thickness of the interface (da) and in addition the length of a layer of the cosurfactant (dh) were subtracted. Note also that for bicontinuous systems such as sodium oleate-pentanol-water-benzene or sodium oleate-butanol-water, the ratio between the two types of diameters is equal to about 10 [20]. 

A wide variety of NMR methods are being applied to understand solid acids including zeolites and metal halides. Proton NMR is useful for characterizing Brpnsted sites in zeolites. Many nuclei are suitable for the study of probe molecules adsorbed directly or formed in situ as either intermediates or products. Adsorbates on metal halide powders display a rich carbenium ion chemistry. The interpretation of NMR experiments on solid acids has been greatly improved by Ae integration of theoretical chemistry and experiment. 

It is often said that the property of acidity is manifest only in the presence of a base, and NMR studies of probe molecules became common following studies of amines by Ellis [4] and Maciel [5, 6] and phosphines by Lunsford [7] in the early to mid 80s. More recently, the maturation of variable temperature MAS NMR has permitted the study of reactive probe molecules which are revealing not only in themselves but also in the intermediates and products that they form on the solid acid. We carried out detailed studies of aldol reactions in zeolites beginning with the early 1993 report of the synthesis of crotonaldehyde from acetaldehyde in HZSM-5 [8] and continuing through investigations of acetone, cyclopentanone [9] and propanal [10], The formation of mesityl oxide 1, from dimerization and dehydration of... 

The effect of probe molecules on the 27A1 NMR has attracted some attention recently. In particular, the determination of the quadrupole coupling constant, Cq, is a sensitive means to learn more about the bonding situation at the aluminum in acid sites, and how it reflects the interaction with basic probe molecules. If one of the four oxygen atoms in an AIO4 tetrahedral coordination is protonated, as in a zeolitic acid site, the coordination is somewhat in between a trigonal and a tetrahedral A1 environment [232]. The protonated oxygen decreases its bond order to A1 to approximately half of its size compared to an unprotonated zeolite. 

Many techniques have been developed to study coking, both chemical (adsorption of probe molecules having different sizes (2), solvent extraction (3)) and physical (X-ray diffraction (4), electron microscopy (5), IR (6), NMR (7)). Among these techniques 129-Xe NMR applied to microporous systems such as zeolites (8), coupled with HRADS techniques makes it possible to clarify the location of the coke and the blocking of the zeolite micropores. 

In Sections 2.2 and 2.3, it is described that the network structures of polymer gels, especially microscopic inhomogeneity, may be detected through the diffusion behaviour of probe molecules of different sizes, which can be obtained by using time-dependent diffusion NMR spectroscopy. In this section, it will be... 

Walther, B., Loren, N., Nyden, M., and Hermansson, A.M. (2006). Influenee of kappa-carrageenan gel structures on diffusion of probe molecules determined by transmission electron microscopy and NMR diffusometry. Langmuir, 22, 8221-8228. 

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. 

Spectroscopic Characterization of Heterogeneous Catalysts. Part A. Methods of Surface Analysis. Part B. Chemisorption of Probe Molecules, J.L.G. Fierro. Ed.. Elsevier (1990). (Part A on surface structure methods, surface groups on oxides. X-ray, Mdssbauer Part B on Infrared, NMR, EPR, thermal desorption,. ..)... 

In order to gain insight into these deactivation phenomena, we have investigated the adsorption and diffusion of probe molecules on fresh samples with different Si/Al ratios and deactivated on samples on stream during the SCR of NOx- While the i29Xe NMR of physisorbed Xenon was useful to study the exchange of gaseous molecules between the main... 

The interaction of probe molecules with acidic OH groups is also studied, " even though the detail is not described in this report. Moreover, the spinning sidebands analysis in H MAS NMR offers the information on the distance between the bridging OH groups and aluminium ions in the zeolite framework. - ... 

Various spectroscopic techniques and probes have been used to investigate solubilization of probe molecules, mostly using UV/visible spectroscopy, fluorescence spectroscopy, ESR spectroscopy [64, 74, 217, 287] and NMR-spectro-scopy [367-369]. Fluorescence spectroscopy is particularly versatile [370], as various static and dynamic aspects can be covered by studying excitation and emission spectra, excimer or exciplex formation, quantum yields, quenching, fluorescence life-times, fluorescence depolarization, energy transfer etc. 

Farcasiu has suggested that shifts of probe molecules like mesityl oxide be used in an acidity scale based on NMR measurements (57). Farcasiu and coworkers showed that whereas the a olefinic carbon shifted only 2-3 ppm going from CDCI3 to 100% H2SO4, the p olefinic carbon moved downfield by nearly 50 ppm. Thus, in the in situ study of acetone conversion, the apparent chemical shift discrepancies are accounted for by protonation equilibria. 

However, time-resolved optical spectroscopy is perhaps the premier method for learning about the dynamics of a complex system, especially on nanosecond or picosecond time scales. Some DNA dynamics data from NMR spectroscopy are presented in Table 4.3. Time-resolved emission decays, time-resolved fluorescence anisotropy, and time-resolved Stokes shifts measurements of probe molecules in DNA have been described (and see below) and fast components in the time decays assigned to various DNA motions. The dynamics as a function of sequence are incompletely mapped and provide an exciting area for future investigations. 

Of course, there are many other methods of characterizing zeoHtes and zeolite-containing systems, in particular non-spectroscopic ones such as chemical analysis, thermal analysis, temperature-programmed desorption of probe molecules, Xe NMR, etc. These will be dealt with in one of the subsequent volmnes of the series. 

The acidity of sites in the walls for surfactant templated aluminosilicates, or silicates doped with other transition metals prepared via surfactant templating can be characterised using a range of techniques including temperature-programmed desorption (TPD) of ammonia, ° solid-state NMR with probe molecules such as trimethylphosphine which bind to acid sites or Fourier transform infrared and diffuse reflectance UV-Vis with ammonia or CO probes.These techniques show that... 

In order to study the interactions of molecules at these sites without the complication of subsequent reaction, much use has been made of probe molecules with different basicities, and therefore with different strengths of interaction at acid sites. The interactions have been monitored by many different techniques, including microcalorimetry and TPD, particularly of amines, and infrared spectroscopy and solid state NMR of the adsorbate-sorbate complex. This work has primarily aimed to establish a measure of acid strength that can be linked to catalytic activity in hydrocarbon transformations. 

Spin Labelling - In recent years there has been a great deal of emphasis on the preparation and study of various kinds of macromolecular probes. Probes may be defined as small molecules, some measurable property of which is altered as the result of non->fCovalent attachment to a blopolymer. Fluorescence, optical absorption, NMR, and electron-spin resonance have all formed the basis for the design of probing molecules. While all of these techniques have their... 

The location of the aromatic molecule, benzyl alcohol, in two structurally similar surfactants, AOT and sodium bis(2-ethylhexyl) phosphate (NaDEHP), in the absence and presence of water-soluble polymer, poly (vinylpyrrolidone) (PVP) is investigated by NMR. According to the variation of the chemical shift of respective protons of AOT and NaDEHP molecules in the presence of probe molecules, it is interesting to find that most of the benzyl alcohol molecules are located around the surface of AOT micelles, while they mainly incorporate in the hydrophobic chain or close to the core of NaDEHP micelles. 

In general, it is accepted [140,141] that Lewis acidity of zeolites and related catalysts (sUica-alumina, y-alumina) is related to the presence of aluminium species on the smface. Therefore, two possibUities exist for the study of Lewis acidity by NMR (i) an analysis of highly resolved Al NMR spectra similar to the MAS NMR spectroscopy described in Sect. 2.1 and (ii) the use of probe molecules such as pyridine or others. 

The ordering of probe molecules in liquid crystals was in fact known much earlier than 1968. It is widely used to determine various parameters of the solute and solvent liquid crystal using nuclear magnetic resonance (NMR), electron spin resonance (ESR), ultraviolet (UV), visible, and other spectroscopie teehniques. After the pioneering work of Saupe and Englert in 1963 [8], the NMR spectroscopy of molecules oriented in liquid crystals became very important in structural chemistry, as it provides the only direct method for precise determination of the molecular geometries in liquid phase. In addition to structural and confor-... 

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