Sensitivity to local environments

The limitations of NMR include the inability to directly distinquish enantiomers the addition of a chiral shift reagent or chiral solvating agent is necessary. Also, polymeric materials, where a large distribution of molecules are present, must be treated as a lumped parameter. At the moment, the biggest limitation of NMR seems to be that that the chemical shifts are perhaps too sensitive to local environment. Consequently, the signals are too non-stationary in position, and Eq. (2) becomes intractable in many, but not all, cases. 

Tryptophan fluorescence is the most sensitive to local environment of the fluorophore. When the tryptophan is sunoundcd by a non-polar environment, the position of its fluorescence maximum is located at 320 nm It shifts to 355 nm in the presence of a polar cnvironnKnt. This is why denaturation of a protein induces a shift of the tryptophan fluorescence to 355 nm (Figure 3.2)... 

Apart from these four main techniques there are many other techniques but their applicability is often limited to some elements. MAS-NMR technique turns out to be very useful for determining if non zero nuclear spin elements are incorporated into the lattice because of its sensitivity to local environment symmetry. Also in favorable cases techniques as Mossbauer, diffuse reflectance UV-Vis and ESR spectroscopies are very helpful while high resolution electron micrographs and simulated imaging are of great interest (38) with in addition the possibility of specific chemical analysis at nanometer scale by EDX-STEM. At last XPS may be informative to determine if surface and bulk compositions are identical or not, although the technique by definition is only sensitive to the first top layers (1 to 2 nm) while zeolitic features concern the whole material. Some examples will be given below. 

As with all elec trochemical studies, the environment must be electrically conduc tive. The corrosion rate is direc tly dependent on the Tafel slope. The Tafel slope varies quite widely with the particular corroding system and generally with the metal under test. As with the Tafel extrapolation technique, the Tafel slope generally used is an assumed, more or less average value. Again, as with the Tafel technique, the method is not sensitive to local corrosion. 

Aside from the general thermal state of the body, a person may find the thermal environment unacceptable if local influences on the body from asymmetric temperature radiation, draft, vertical air temperature differences, or contact with hot or cold surfaces (floors, machinery, tools, etc.) are experienced. The data for local thermal discomfort is mainly based on studies of people under low activity levels (1.2 met). For higher activities it can be expected that people are less sensitive to local thermal discomfort. The relations between dissatisfaction and local discomfort parameters are found in CR 1752. 

Also, one would be remiss if one did not at least mention the various analytical and diagnostic NMR techniques that have come into prominent use over the last decade, particularly with respect to their applications to drug discovery. Because of the sensitivity of its chemical shift to local environment, 19F NMR has increasingly been used as a probe in the study of structure and dynamics of small molecules, proteins, and other biological systems.32-36... 

Nevertheless, C02 is an extremely valuable probe molecule because the infrared spectra of the chemisorbed species respond very sensitively to their environments. Thus, the frequency separation of the typical band pairs of the carbonate structures may be taken as a measure of the local asymmetry at the chemisorption site. The application of 13C-FT-NMR should be extremely valuable for a still more extensive study of the nature of sites by C02 adsorption. Due to the very detailed information on the structure of sites on oxide surfaces that can be obtained by C02 chemisorption studies, this compound should in some cases also be applicable as a specific poison. A very careful study of the type of interaction with the surface, however, has to be undertaken for each particular system before any conclusive interpretation of poisoning experiments becomes meaningful. 

Solid-state NMR is one of the most powerful spectroscopic techniques for the characterisation of molecular structures and dynamics.1 This is because NMR parameters are highly sensitive to local chemical environments and molecular properties. One advantage of solid-state NMR is that it enables dealing with quadrupolar nuclei, which most of the NMR-accessible nuclei are in the periodic table. Moreover, it provides an opportunity to obtain information regarding the orientation dependence of the fundamental NMR parameters. In principle, such NMR parameters are expressed by second-rank tensors and it is the anisotropy that is capable of yielding more detailed information concerning the molecular properties. 

In spite of a high number of levels, only a few characteristic transitions are necessary to identify Sm(III) in solution. The visible region is devoid of transitions which are sensitive to the environment of Sm(III). Some transition to 6F multiplet are localized in the infrared region. The transition 6Fi/2 <—6Hs is hypersensitive and occurs at 6400 cm-1. The intensities of the bands have been analyzed [82,103],... 

Solid-state NM R is perhaps unique in its ability to probe each of these characteristics. In particular, its sensitivity to local geometry and coordination environments provides advantages over many more traditional characterization methods. However, no single NMR experiment can furnish the investigator with data in each and aU of these areas. A number of distinct, innovative, approaches have been developed to probe different structural features. A number of these are outlined below. 

Electron spin resonance spectroscopy (ESR), also known as electron paramagnetic resonance (EPR), is based on the property that an unpaired electron placed in a magnetic field shows a typical resonance energy absorption spectrum sensitive to its environment. Recently, this technique, which was primarily developed for biological studies of membrane properties, has been adapted for the study of adsorbed polymer/surfactant layers. The mobility of the ESR probe (stable free radical incorporated into the polymer or surfactant molecule) depends of orientation of the surfactant or polymer and the viscosity of the local environment around the probe. 

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