Relaxation spin-lattice, time

The frequency ranges covered by these three relaxation times spin-lattice (T2) spin-spin (T2), and spin-lattice in the rotating frames (Tle), are determined by the following equations ... 

The crosslinked network formed during photolysis of polymers can not be identified by usual methods, such as FTIR or molecular weight measurements. nuclear magnetic resonance ( C NMR) is a useful technique for detecting new types of carbon atoms formed in the crosslinking. Some studies with NMR are based on the determination of relaxation times spin-lattice... 

There are two kinds of popular relaxation times, spin-lattice relaxation time and spin-spin relaxation time T2. Dipolar interactions between nuclear spins in polymers predominantly govern relaxation times. According to the Bloembergen-Purcell-Pound (BPP) theory [5], and T2 vary against the correlation time (t ) of the molecular motion, as indicated in Figure 8.1. Tj has the minimum at = 1/ , in which CO is the resonance angular frequency. If the Tj minimum is observed, the absolute correlation time of the molecular motion can be obtained. On the other hand, T2 decreases with an increment of From these behaviors of the relaxation times, the dynamics of polymer chain can be obtained. 

Bq = magnetic field strength [flux density] Tj = spin-lattice relaxation times T2 = spin-spin relaxation time = spin-lattice relaxation time in the rotating frame. 

Figure 2. Fluorine NMR relaxation times for a sample of Linde molecular sieve 13X containing about 6.6 molecules of SFg per cage O, spin lattice relaxation time , spin-spin relaxation time T2 characterized by exponential decay V and A, T2 characterizedby two relaxation times ticked O, decay as r2. Solid lines are theory to the left of 10Z/T = 6 based on molecular diffusion to the right of 10Z/T controlled by Tu. For dashed lines see text (20)...

A complementary article by Dais (Iraklion, Crete) addresses the theoretical principles underlying the phenomenon of carbon-13 nuclear magnetic relaxation, encompassing spin-lattice (Tt) and spin-spin (T2) relaxation times, the nuclear Overhauser enhancement, and their relation to the motional behavior of carbohydrates in solution. With examples broadly selected from simple sugar derivatives, oligosaccharides, and polysaccharides, the author shows how qualitative treatments have provided useful interpretations of the gross mobility of molecules in solution, but demonstrates how a quantitative approach may be of greater ultimate value. 

For carbons possessing a directly bonded proton, the carbon-13 spin lattice relaxation time,, spin-spin relaxation time,... 

After B, is turned off, nuclei can change their nuclear spin orientations through two types of relaxation processes. Spin-lattice (longitudinal) relaxation (governed by relaxation time 7, ) involves the return of the nuclei to a Boltzmann distribution. Spin-spin (transverse) relaxation (governed by relaxation time 72 or 7 ) involves the dephasing of the bundled nuclear spins. Normally 7 < T2 < 7,. 

If the motion of an acrylic polymer radical about the Cp bond is hindered, changing the temperature should lead to changes in the TREPR spectrum. This is indeed observed for all acrylic polymers we have examined to date. Simulation of the complete temperature dependence of TREPR spectra of acrylic polymer main-chain radicals should allow information regarding the conformational motion of the polymer in solution to be extracted, such as rotational correlation times, spin-lattice relaxation times (Ti), and activation energies for conformational transitions. 

The methods by which excited nuclei return to their ground state and by which the Boltzmann equilibrium is reestablished are called relaxation processes. In NMR systems there are two principal types of relaxation processes spin-lattice relaxation and spin-spin relaxation. Each occurs as a first-order rate process and is characterized by a relaxation time, which governs the rate of decay. 

Dynamic NMR studies of the silk fibroin from silkworms have been reviewed. The chain dynamics of B. ntori and S.c. ricini silk fibroins in aqueous solution were studied to determine the relaxation parameters spin-lattice relaxation time, nuclear Overhauser enhancement and line width. - C NMR observations were also made to study the chain dynamics of silk fibroin stored in the silk gland of living silkworms. The chain dynamics of the B. mori and S.c. ricini silk fibroin fiber were studied with "H and C solid-state NMR. The solid-state NMR spectra of S.c. ricini silk fibroins showed a gradual and monotonic slowdown of the overall molecular motions leading to molecular aggregation. The chain dynamics of silk fibroin absorbed solvent were detected by H and C solid-state NMR methods, and also using an NMR imaging method. ESR approaches were applied to the dynamic studies of Tyr side-chain in the silk fibroin. 

Temperature Dependence of Spin-Lattice Relaxation. The spin-lattice relaxation rate T ) is comprised of various contributions to the relaxation process, including homo- and heteronuclear dipolar interactions, quadrupolar interactions, chemical shift anisotropy, spin-rotation, and others (10). When the relaxation mechanism is dominated by inter- and intramolecular dipole-dipole interactions, the will increase with temperature, pass through a maximum, and decrease with increasing temperature. Since the relaxation rate is the inverse of the relaxation time, the Ti will decrease, pass through a minimum (Timin), and then increase with increasing temperature (77). The T lmin values are proportional to the internuclear distances. 

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