Cross-relaxation experiments

Besides the advantage of the high-temperature measurements for quantitative interpretation of NOESY spectra, fig. 6 also indicates a special role of the high temperature maximum (note that positive cross-relaxation rates increase downward) of u". If the NOESY spectrum can be recorded at several temperatures around the cr" maximum, than calculated cross-relaxation rates can be used to obtain simultaneously the correlation time and the interproton distances without the necessity of any other knowledge. A typical problem in the cross-relaxation experiments is that cross-relaxation rate depends on two parameters, Tc and r (eq. (la)), and to calculate one of them the other must be independently known. However, the position of the maximum uniquely determines correlation time, and its height uniquely determines interproton distances. 

Section 10.2.3.4 deals with cross-relaxation experiments. Before discussing NMR experiments, the thermodynamics of a blend is briefly outlined. 

Various methods have been proposed for (1) and (3). In Section 10.2.3.2, methods for (1) based on the Goldman-Shen experiment [8] are reviewed. In Section 10.2.3.3, 2D NMR experiments, which enable us to detect the region of specific interpolymer interaction and the domain size on a scale of 2 nm, are discussed. In Section 10.2.3.4, heteronuclear cross-relaxation experiments between — H, electron to etc., which... 

If one of the component polymers has a third spin other than and H, several cross-relaxation experiments become possible. Since spin diffusion occurs a few 10 nm before the polarization decays, only a small amount of the spins near the interface must be detected for immiscible blends. Section... 

F CROSS RELAXATION EXPERIMENTS AND PROCESS-INDUCED DEFECTS... 

The elucidation of the scalar coupling network by the correlation experiments is, apart from small molecules, not sufficient for the unambiguous, sequential and stereo-specific assignment. The complementary information of spatially adjacent protons is obtained via cross-relaxation experiments, the laboratory-frame nuclear Overhauser enhancement spectroscopy (NOESY) and the rotating-frame nuclear Overhauser effect spectroscopy (ROESY). These experiments provide also the distance restraints for the structure determination and help to recognize exchange processes. 

A study of chemical shifts in fluorobenzenes seeks to understand the so-called ortho effect 7" using the methodology and basis sets used previously by de Dios and Oldfield in their study of various fluoro-substituted benzenes. Unfortunately, these more recent caleulations have problems matching the chemical shift components that the authors deduce from the cross-relaxation experiments in solution. " ... 

Cross-relaxation The mutual intermolecular or intramolecular relaxation of magnetically equivalent nuclei, e.g., through dipolar relaxation. This forms the basis of nOe experiments. 

Fig. 9.1 The internuclear transfer of magnetization via NOE cross-relaxation in an isolated spin-pair. (A) Build-up curves for the cross-peak intensity in a 2D NOESY experiment for various internuclear distances r. The dashed line indicates a typical mixing time tm = 300rns used for drug-like molecules.

Fig. 9.2 Schematic representation of the three basic experiments useful for the determination of (A) transient NOE experiment, (B) 2D NOESY and (C) 2D ROESY. The gray-filled half-circle represents a frequency-selective inversion pulse which inverts the spin to which the cross-relaxation...

Early conformational studies by HOESY experiments are illustrated by the work of Batta and Kover54 who were able to access oligosaccharide sequencing and conformational distribution around the glycosidic bond in model compounds. These determinations make use of relayed proton-proton-carbon cross-relaxation. 

In the cross modulation experiments (Mentzoni and Row, 1963 Mentzoni and Rao, 1965), an electron plasma is briefly heated by a microwave pulse while a weak microwave signal probes the mean electron energy. Assuming no electron loss and insignificant ambient gas heating, these authors derived the following equation for the relaxation of electron Maxwellian temperature T.toward the ambient temperature T ... 

As with the COSY experiment, the sequence starts with a pulse followed by an evolution period, but now the mechanism that couples the two spins (which must be in close proximity, typically <6 A) is the Nuclear Overhauser Effect (NOE). The second pulse converts magnetization into population disturbances, and cross-relaxation is allowed during the mixing time. Finally, the third pulse transfers the spins back to the x-y-plane, where detection takes place. The spectrum will resemble a COSY spectrum, but the off-diagonal peaks now indicate through-space rather than through-bond interactions. 

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