NMR technique

This chapter focuses on the application of solid-state NMR techniques for the characterization of oxidation catalysts. Initially, a brief introduction to these techniques is provided (Section 5.2), within which methods suitable for the study of both bulk structure (Section 5.2.1) and surface characteristics (Section 5.2.2), are described. Examples of the application of these techniques are then provided in Section 5.3, for bulk oxides, and Section 5.4, for surface properties. Finally, Section 5.5 provides an outlook as to future directions in this area. 

Solid-state NMR has become a fundamental part of the toolkit to characterize catalysts and other solid materials. It is now routine to employ one of the many NMR techniques available to extract information about both the surface properties and the bulk properties of catalysts. Additional information on redox properties and 

The principal difference between the NMR spectra of liquid samples and those acquired in the solid state is that linewidths of solid samples are very much broader than those for liquids. This difference arises because, in liquids, molecular tumbling averages anisotropic interactions and therefore removes them from the spectrum. To be in a position to overcome this problem it is necessary to understand the properties that give rise to an NMR spectrum. 

The key properties of nuclei that allow them to be probed by NMR are nuclear magnetism and nuclear spin. Quantum mechanics dictates that the angular momentum of any system is quantized and can take only the values  

The relationship between the magnehc moment and angular momentum, P, is given by  

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Derome A E 1987 Modem NMR Techniques for Chemistry Research (Oxford Pergamon)... 

Because model colloids tend to have a ratlier well defined chemical composition, elemental analysis can be used to obtain detailed infonnation, such as tlie grafted amount of polymer in tire case of sterically stabilized particles. More details about tire chemical stmcture can be obtained using NMR techniques (section B1.13). In addition, NMR... 

Quantum chemistry methods allow the prediction of the ultraviolet transitions in good agreement with the experimental values in the case of thiazole and its three methyl derivatives (Table 1-18). A very weak absorption has been indicated at 269.5 nm that could correspond to an n- TT transition given by calculation at 281.5 nm (133). Ultraviolet absorption spectroscopy has been investigated in connection with steric interactions in the A-4-thiazoline-2-thione (74) series (181). It was earlier demonstrated by NMR technique that 4-alkyl-3 isopropyl-A-4-thiazoline-2-thiones exist in solution as equilibrium mixtures of two conformers (75 and 76), the relative populations of which vary with the size of R4 (182) for R4 = rBu the population of rotamer A is 100%, whereas for R4 = Me it is only 28%. Starting from the observed absorption wavelength for... 

Richard R Ernst of the Swiss Federal Institute of Technol ogy wonthe 1991 Nobel Prize in chemistry for devis ing pulse relaxation NMR techniques... 

A number of 2D NMR techniques are available for a variety of purposes They are especially valuable when attempting to determine the structure of complicated natural products and the conformations of biomolecules... 

Section 13 19 2D NMR techniques are enhancements that are sometimes useful m gam mg additional structural information A H H COSY spectrum reveals which protons are spin coupled to other protons which helps m deter mining connectivity A HETCOR spectrum shows the C—H connections by correlating C and H chemical shifts... 

COSY (Section 13 19) A 2D NMR technique that correlates the chemical shifts of spin coupled nuclei COSY stands for... 

DEPT (Section 13 18) Abbreviation for distortionless enhance ment of polarization ttansfer DEPT is an NMR technique that reveals the number of hydrogens directly attached to a carbon responsible for a particular signal... 

HETCOR (Section 13 19) A 2D NMR technique that correlates the H chemical shift of a proton to the chemical shift of the carbon to which it is attached HETCOR stands for heteronuclear chemical shift correlation Heteroatom (Section 1 7) An atom in an organic molecule that IS neither carbon nor hydrogen Heterocyclic compound (Section 3 15) Cyclic compound in which one or more of the atoms in the nng are elements other than carbon Heterocyclic compounds may or may not be aromatic... 

For a growing radical chain that has monomer 1 at its radical end, its rate constant for combination with monomer 1 is designated and with monomer 2, Similady, for a chain with monomer 2 at its growing end, the rate constant for combination with monomer 2 is / 22 with monomer 1, The reactivity ratios may be calculated from Price-Alfrey and e values, which are given in Table 8 for the more important acryUc esters (87). The sequence distributions of numerous acryUc copolymers have been determined experimentally utilizing nmr techniques (88,89). Several review articles discuss copolymerization (84,85). 

A. E. Deiome, Modem NMR Techniques for Chemical Research, Peigamon Press, Oxford, U.K., 1987. 

Figure 1 shows the decomposition sequence for several hydrous precursors and indicates approximate temperatures at which the activated forms occur (1). As activation temperature is increased, the crystal stmctures become more ordered as can be seen by the x-ray diffraction patterns of Figure 2 (2). The similarity of these patterns combined with subtie effects of precursor crystal size, trace impurities, and details of sample preparation have led to some confusion in the Hterature (3). The crystal stmctures of the activated aluminas have, however, been well-documented by x-ray diffraction (4) and by nmr techniques (5).

The thermal glass-transition temperatures of poly(vinyl acetal)s can be determined by dynamic mechanical analysis, differential scanning calorimetry, and nmr techniques (31). The thermal glass-transition temperature of poly(vinyl acetal) resins prepared from aliphatic aldehydes can be estimated from empirical relationships such as equation 1 where OH and OAc are the weight percent of vinyl alcohol and vinyl acetate units and C is the number of carbons in the chain derived from the aldehyde. The symbols with subscripts are the corresponding values for a standard (s) resin with known parameters (32). The formula accurately predicts that resin T increases as vinyl alcohol content increases, and decreases as vinyl acetate content and aldehyde carbon chain length increases. 

The secondary structure elements, formed in this way and held together by the hydrophobic core, provide a rigid and stable framework. They exhibit relatively little flexibility with respect to each other, and they are the best-defined parts of protein structures determined by both x-ray and NMR techniques. Functional groups of the protein are attached to this framework, either directly by their side chains or, more frequently, in loop regions that connect sequentially adjacent secondary structure elements. We will now have a closer look at these structural elements. 

Also, new areas for applications are opening up. A most recent development has been the successful demonstration of three-dimensional imaging of ceramic and polymeric materials by solid state NMR techniques. This area is most likely to expand considerably. 

H. Eckert. Ber. Bunsenges. Phys. Chem. 94,1062,1990. A recent review of modern NMR techniques as applied to various Materials Science problems. 

X-ray studies indicate that the vinyl chloride polymer as normally prepared in commercial processes is substantially amorphous although some small amount of crystallinity (about 5% as measured by X-ray diffraction methods) is present. It has been reported by Fuller d in 1940 and Natta and Carradini in 1956 that examination of the crystalline zones indicates a repeat distance of 5.1 A which is consistent with a syndiotactic (i.e. alternating) structure. Later studies using NMR techniques indicate that conventional PVC is about 55% syndiotactic and the rest largely atactic in structure. 

The rates of removal of axial and equatorial protons from 4-t-butylcyclohexane in NaOD/dioxan have been measured by an NMR technique. The rate of removal of an axial proton is 5.5 times faster than for an equatorial proton. What explanation can you offer for this difference ... 

The case for the generality of the o-complex mechanism is further strengthened by numerous studies showing that benzenium ions (an alternative name for the o-complex) can exist as stable entities under suitable conditions. Substituted benzenium ions can be observed by NMR techniques under stable-ion conditions. They are formed by protonation of the aromatic substrate ... 

NMR Techniques in Catalysis, edited by Alexis T. Bell and Alexander Pines... 

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