Relaxation mechanism gyromagnetic ratio

Figure 2 The four-level diagram for a system of two interacting spins, in this case an electron (S) and nucleus with a positive gyromagnetic ratio (/). The intrinsic electron and nuclear spin relaxation are given by p and w°, respectively, and the dipolar and/or scalar interactions between the electron and nuclear spin are represented by p, w0, w, and w2. The transition w0 is known as the zero-quantum transition, while w, is the singlequantum transition and w2 is the double-quantum transition. Nuclear and electronic relaxation through mechanisms other than scalar or dipolar coupling are denoted with w° — 1/Tio and p — 1/Tie, where Ti0 and T1e are the longitudinal relaxation times of the nucleus and electron in the absence of any coupling between them. Since much stronger relaxation mechanisms are available to the electron spin, the assumption p>p can be safely made. Adapted with permission from Ref. [24].

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... 

From this point of view, nitrogen-15 exhibits a peculiarity which is characteristic of nuclei with a negative gyromagnetic ratio and is frequently emphasized. Indeed, it is possible for the relative contribution of the dipolar mechanism to be reduced to about 20% and therefore to give values of n = -1 and I/Io = 0. The signal is then cancelled by the Overhauser effect. This phenomenon should be kept in mind, in particular, when using relaxation reagents, since the electronuclear interaction may compete with the nuclear dipolar effect to yield approx-imatively n j g = 1 [Eq. (2.6)] (H 13). 

This relaxation mechanism is important when NMR is used to study protonated samples and samples that contain paramagnetic impurities like oxygen. As the gyromagnetic ratio of an electron is about 1000 times greater than that of a proton, a small amount of paramagnetic impurity can drastically reduce the values in a sample. 

In an A- X experiment (i.e., irradiate X, observe A), the NOE is dependent on the gyromagnetic ratio of A and X nuclei, provided that the intramolecular dipolar relaxation mechanism is the predominant pathway through which nuclei A are relaxing. Thus the maximum NOE factor /a(X) for the A signal in an A- X experiment is given by... 

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