Magnetic resonance experiments

The relationship between the physical observables and molecular motions is thus rather more complicated for NMR than for depolarized light scattering. We follow here parts of the discussion of Dais and Spyros(28). A given chemical bond has a time-dependent orientation vector S2(t). Corresponding to the orientation vector are a series of Wigner rotation matrices which are related to the 

Gm(t)=S7t DUm0))Dr 0mt))), whose Fourier transforms are the reduced spectral densities 

Dais and Spyros note that the NMR measurables are related to the reduced spectral densities by a series of forms such as 

Early work on NMR of polymers in dilute solution was reviewed by Heatley(29). It was already clear in that early review that relaxation times of dilute polymers were independent of polymer molecular weight, at least for molecular weights above a few to ten thousand, and were nearly independent of polymer concentration for concentrations up to 100-150 g/1 or so. A revealing exception to this rule was provided by polymers plausibly expected to rotate as nearly rigid bodies, for which Ti continued to depend on M up to much larger M. From these observations, it was plausibly inferred that local chain motions are primarily responsible for the observed relaxation times. Dependences of Ti on solvent temperature and viscosity were concluded to scale linearly with solvent viscosity, at least in most systems, a matter treated in more detail below. Heatley also considers correlations between Ti and chain structure. 

As examples of this approach, Zhu and Ediger examined atactic perdeutero-1, 2-polybutadiene(30) and backbone-deuterated polystyrene(31) in multiple solvents, using NMR to obtain Ti and thence correlation times for the C-D bond 

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Carr H Y and Purcell E M 1954 Effects of diffusion on free precession in nuclear magnetic resonance experiments Rhys. Rev. 94 630-8... 

Aqueous GPC can also be semiprepped in manner just like nonaqueous GPC. In this case one must consider carefully the buffers, salts, and biocides used in the eluant. If the fractions are destined for nuclear magnetic resonance experiments it will be imperative to either reduce the salt concentration in the eluant or remove salt after the initial fractionation. Likewise, if the collected samples are destined for infrared (IR) analysis, it is important to choose salts and buffers that have good IR transparency in the wavenumber ranges of interest. 

D. I. Hoult, R. E. Richards 1976, (The signal-to-noise ratio of the nuclear magnetic resonance experiment), J. Magn. Reson. 24, 71. 

In a magnetic resonance experiment, we apply not only a static field B0 in the z-direction but an oscillating radiation field Bx in the xy-plane, so that the total field is ... 

In a continuous wave (CW) magnetic resonance experiment, the radiation field B is continuous and BQ is changed only slowly compared with the relaxation rates (so-called slow passage conditions). Thus a steady-state solution to eqns... 

Although relaxation measurements have been widely used in nuclear magnetic resonance studies of solid catalysts and adsorbed molecules, they have not found such favor in similar ESR work. Relaxation phenomena, however, do play a very important role in any magnetic resonance experiment, whether or not this aspect of the problem is studied. In fact, the temperature at which most ESR experiments are conducted is dictated by the relaxation process. Furthermore, even qualitative data on relaxation times can be used as supporting evidence in the identification of a paramagnetic species. 

An illustrative example is the work of Clark et al, on the conformation of poly(vinyl pyrrolidone) (PVP) adsorbed on silica 0). These authors determined bound fractions from magnetic resonance experiments. In one instance they added acetone to an aqueous solution of PVP in order to achieve theta conditions for this polymer. They expected to observe an increase in the bound fraction on the basis of solvency effects as predicted by all modern polymer adsorption theory (2-6), but found exactly the opposite effect. Their explanation was plausible, namely that acetone, with ability to adsorb strongly on silica due to its carbonyl group, would be able to partially displace the polymer by competing for the available surface sites. 

Y. Pan, P- F rotational-echo double resonance nuclear magnetic resonance experiment on fluoridated hydroxyapatite. Solid State Nucl. Magn. Reson. 5 (1995) 263-268. L. Wu, W. Forsling, P.W. Schindler, Surface complexation of calcium mineral in aqueous solution, surface protonation at fluorapatite surface, J. Colloid Interface Sci. 147 (1991) 178-185. 

Nerdal, W. and Andersen, 0.M., Intermolecular aromatic acid association of an anthocyanin (petanin) evidenced by two-dimensional nuclear Overhauser enhancement nuclear magnetic resonance experiments and distance geometry calculations, Phytochem. Anal, 3, 182, 1992. 

Inverse-Detected 15N Nuclear Magnetic Resonance Experiments... 

The models in Figures 2 and 3 show that a part of the low molecular weight liquid obviously separates the polymer chains from each other, thus facilitating segment mobility. Another part of it fills the cavities and displays almost liquid state behavior in them. This rather simplified model of the glass structure has been verified in some by nuclear magnetic resonance experiments. 

D. The table below shows signal-to-noise ratios recorded in a nuclear magnetic resonance experiment. Construct graphs of... 

If we transform the problem to a frame rotating with the microwave field, it is static and cannot induce transitions. The transformation to the rotating frame, often used to describe two level magnetic resonance experiments, is discussed by Salwen37 and Rabi et al.3H. 

Lewis acid centers, which were thought to be the primary catalytic sites. Boreskova et al. (51) studied the poisoning effect of quinoline on the cracking of cumene over Na, H—Y zeolite and observed a linear decrease in activity with the amount of quinoline added until a constant level of activity was reached. The catalytic activity was attributed to trivalent aluminum centers (Lewis acids), which were poisoned by coordinately bound quinoline. In a similar study of cumene cracking, Turkevich et al. (50) also concluded on the basis of magnetic resonance experiments that Lewis centers were the active sites. 

This chapter is intended to recall the principles of magnetism, the definition of magnetic induction and of magnetic induction in a vacuum which is referred to as magnetic field. Readers may not recollect that the molar magnetic susceptibility is expressed in cubic meters per mol Some properties of electron and nuclear spins are reviewed and finally some basic concepts of the magnetic resonance experiments are refreshed. In summary, this chapter should introduce the readers into the language used by the authors. 

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