Carbon-13 spin-lattice magnetic relaxation

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. 

Figure Bl.13.2. Spin-lattice and spin-spm relaxation rates (R and/ 2> respectively) for a carbon-13 spin directly bonded to a proton as a fiinction of correlation time at the magnetic fields of 7 and 14 T.

At the same time the proton magnetization is being partially transferred to the carbon spins it relaxes to the lattice proton spin lattice relaxation in... 

For effective spin-lattice relaxation, the tumbling magnetic nuclei must be spatially close to the resonating nucleus. For attached protons provide effective spin-lattice relaxation. A carbonyl carbon or a carbon attached to four other carbons thus relaxes very slowly and is more easily saturated because the attached atoms are nonmagnetic ( C and... 

Here, Hz is the Zeeman term, Hq is the quadrupolar interaction term for nuclei with 1 1, Hd is the dipolar interaction term for nuclei with 1 = 1/2, Hs is the electron shielding term and Hj is the J-coupling term. Spin relaxations will be induced by the time fluctuations of these interaction terms. For example, H spin-lattice relaxation behaviour is dominated by Hq, whereas Hq mainly determines the relaxation process of the H or magnetization in organic materials. In some cases without significant contributions from Hq and Hq, the time fluctuations of Hs and Hj also induce spin relaxation for example, the magnetization of a carbonyl carbon with a large chemical shift anisotropy relaxes due to the contribution from the time fluctuation of Hs. Nevertheless, since the main interest of polymer scientists is NMR, we focus on the description of the relaxation process in this chapter. 

P(3HO) produced by P. oleovorans from sodium octanoate as the sole carbon source is a copolyester composed of 85% octanoate monomer units (with n-pentyl side chains). The remaining monomer units are roughly equally distributed between valerate, caproate and decanoate units [88]. This sample has a T of — 36°C and T of 61°C [88]. The chain dynamics of the amorphous part of this P(3HO) sample have been studied by measuring variable-temperature NMR spin-lattice relaxation times Ti and the nuclear Overhauser enhancements at two magnetic fields [90, 91]. Well-resolved spectra of... 

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