Dipolar cross-relaxation

Quantitative evaluation of NOE magnitudes in terms of internuclear distance is problematic and often questionable, since there are influences (experimental and structural) in addition to distance. In particular, if a third NMR active nucleus X is close to nucleus I, reduced enhancements of the 1 signal may be observed because the dipolar cross relaxation of I is dominated by X and no longer by the irradiated S. Indeed, negative NOEs (signal intensity decreases) may occur212. 

The pulse sequence shown in Fig. 10.4 is sometimes used to study exchange, but more than 99% of the use of this sequence is to study the effect of dipolar cross relaxation via the NOE. As a result, this type of study is given the name nuclear Overhauser effect spectroscopy, NOESY, and the pulse sequence of Fig. 10.4... 

The polarization of the olefinic protons of the in-cage recombined product (1) was derived from the originally polarized cyclopropyl protons via the intramolecular dipolar cross relaxation. 

A slightly more complicated case of nuclear Overhauser effect was recently observed in our laboratory in the photoreduction of furil. Furil is considered as a weakly coupled AMX three-proton spin system and the dipolar cross relaxation involves the interplay of all three spins. The experiments were done in a light-modulation mode with a FT spectrometer and a detailed theoretical analysis of the experimental data will allow the evaluation of the various different cross-relaxation rates (138). ... 

In the double resonance experiments, the effect of the deuterium nuclear quadrupole transitions on the NMR signal50 or the NQR signal42 of another nucleus is observed. For example, an ingenious method of detecting pure quadrupole transitions by nuclear double resonance was suggested by Schwab and Hahn.50 In their experiment, the quadrupole transitions of deuterium in a 12 percent deuterium-enriched sample of p-dichlorobenzene were excited, and the effect on the proton resonance signal through dipolar cross relaxation was detected. The 35 Cl spins served to establish and monitor an ordered metastable spin state of the protons. 

NOESY, ROESY yield correlation signals which are caused by dipolar cross-relaxation between nuclei in a close spatial relationship. 

So far, only coherent Interactions, which can be represented in the Hamiltonian, have been considered. There are, however, also incoherent pair interactions, in particular in the form of dipolar cross-relaxation and of chemical exchange processes. 

NOESY (Nuclear Overhauser Enhancement Spectroscopy). This technique depends on the occurrence of dipolar cross-relaxation (Bodenhausen et al. 1984). This so-called nOe effect depends on the distance through space and is independent of direct bonding. In this way it is a powerful technique to establish the stereochemistry and conformation of molecules. In Fig. 6 the... 

NOE intensities can be reduced if alternative relaxation mechanisms compete with dipolar cross-relaxation. Paramagnetic relaxation may broaden the signals. Select diamagnetic analogs if possible. Nuclei with low gyromagnetic ratios (such as C) are less sensitive than protons... 

A cross peak in a NOESY experiment indicates dipolar cross relaxation between two nuclei that are spatially close to one another. The cross peak intensity is dependent upon the inverse sixth power of the distance between the two nuclei. Thus, for two protons, i and / separated by a distance r, which give rise to a NOESY cross peak, the intensity of that cross peak I is proportional to r. ... 

Dipolar cross-relaxation rates, and thus distances, can be determined through NOESY or ROESY experiments using various approaches. The measurement of build-up rates involves the recording of several NOESY spectra with different mixing times. To ensure equal conditions, the measurements should be made in succession. The integrals of cross peaks are determined, and the volumes are plotted as a function of mixing time ... 

Hyperpolarized noble gas atoms, like xenon, can interact with protons and other nuclei via dipolar cross-relaxation, a mechanism which is supported by stochastic processes of motion. By means of this interaction, hyperpolarized xenon can transfer its polarization to protons and other nuclei of interest. An efficient way to make use of this SPINOE mechanism is to freeze hyperpolarized xenon onto the surface of the sample. During the defrosting process large amounts of xenon can penetrate into the liquid sample, resulting in a large SPINOE enhancement. 

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