Intermediate spin relaxation

By measuring the proton relaxation times, and T,p, it is possible to estimate the mobility of polymer chains within the cell wall (11). Proton spin relaxation editing (PSRE) is a method of expressing these results. It separates the components seen in a conventional CP-MAS C spectra into low-mobility and intermediate-mobility components. If PSRE is applied to a experiment (12) the mobility of the... 

Also spin-lattice relaxation times T and spin-spin relaxation times T2 were measured as a function of pressure on different selectively deuterated DPPC (at C2, Cg and Ci3, respectively) by Jonas and co-workers (Fig. 14). The spin-latticed relaxation time T is sensitive to motions with correlation times tc near Uo i e., motions with correlation times in the range from 10 to 10 " s. In comparison with Ti, the spin-spin relaxation time T 2 is more sensitive to motions with correlation times near (e qQlh), i.e., in the intermediate to slow range (10 " to 10 s). The Ti and T2 values obtained showed characteristic changes at various phase transition pressures, thus indicating abrupt changes... 

As opposed to the previous examples, the rate of the pair substitution BRi BR2 BR3 can be varied by neither the reactant concentrations nor the solvent polarity because it is intramolecular and only involves neutral species. However, the ratio of polarizations of corresponding protons in Pi and P2 exhibits a pronounced temperature dependence, " which is shown in Fig. 9.8 and can be explained in the following way. Ideally, these opposite polarizations should have exactly equal magnitudes, but their ratio deviates from 1 if nuclear spin relaxation in the paramagnetic intermediates is taken into account. Biradicals with nuclear spin states that slow down intersystem crossing of BRi live longer, so their nuclear spins suffer a stronger relaxation loss. 

Analysis of the NMR parameters and the dynamic processes depends on the exchange rates. The basic one-dimensional band-shape analysis is best suited to intermediate rates (10—10 per s). Slow exchange rates (—0.1-10 per s) are most accurately measured by using magnetization transfer experiments and Ti-relaxation times. Fast dynamic processes (> 10 per s) can be elucidated by investigating the spin-spin relaxation times. 

The importance of different exit channels can hardly be overstressed. If both exit channels lead to the same product, the spin sorting is undone, and no S-Tq-t)q)e CIDNP results. However, a difference of reaction probabilities suffices for some CIDNP to remain. Another important factor avoiding a cancellation that would otherwise be perfect is nuclear spin relaxation in longer-lived paramagnetic intermediates, free radicals or triplet molecules. Even if there is a complete cancellation of the polarizations at long times but an imbalance of reaction rates, CIDNP occurs as a transient effect and can be detected in a time-resolved experiment. 

If the NH exchange rate is low, the NH peak is still broad because the electrical quadrupole moment of the nitrogen nucleus induces a moderately efficient spin relaxation and, thus, an intermediate lifetime for the spin states of the nitrogen nucleus. The proton thus sees three spin states of the nitrogen nucleus (spin number = 1), which are changing at a moderate rate, and the proton responds by giving a broad peak. In this case, coupling of the NH proton to the adjacent protons is observed. Such is the case for pyrroles, indoles, secondary and primary amides, and carbamates (Fig. 4.34). 

There are three mechanisms of spin flipping solvent-induced spin relaxation (spin-lattice relaxation), spin-orbit coupling (SOC) and hyperfine coupling (HFC) (2,31. While the first of these mechanisms is quite slow in the absence of paramagnetic impurities, HFC is important in biradicals in which the two radical centers are relatively far apart (1,6-biradicals and longer) [4], and SOC dominates in short biradicals, which are observed as intermediates in numerous photochemical reactions. For these systems, the order of magnitude of the SOC element is about 0.1-5 cm , which is much larger than that of the typical HFC, which is about 0.001 cm. We will therefore concentrate on the SOC mechanism only. 

---