Spin lattice relaxation configuration

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

The proton spin-lattice relaxation-rate (R,) is a well established, nuclear magnetic resonance (n.m.r.) parameter for structural, configurational, and conformational analysis of organic molecules in solution. " As yet, however, its utility has received little attention in the field of carbohydrate chemistry,... 

Each of the physical phenomena - , including spin-lattice relaxation and NOE experiments and the resulting NMR parameters, is discussed. By presenting selected examples, it is shown how information from these parameters can be used for the determination of relative configuration. The practice of recording NMR spectra in the presence of auxiliary reagents is also demonstrated (Section 4.1.1.4.). 

The efficiency of spin-lattice relaxation of a given nucleus is also dependent on the number of nearby nuclear dipoles. In fortunate cases this can be used to obtain structural information. For example, there are more protons spatially close to the methine carbons in the c/.v-configurated tricyclic compound 3 than in the trans-isomer220. Thus, the 7 value of the methine carbons in the (Tv-isomer is considerably smaller. 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

Spin-lattice relaxation (time TJ results from dissipation of the excited-state energy among vibrational modes of the matrix. In mobile liquids, vibrational fluctuations are spread over a very wide frequency range. This configuration decreases the probability of spin-lattice coupling. As a result, Tl is long and thus makes a negligible contribution to the line width. 

Figure 4a. Electronic part Tle, dipole-dipole part Tld and the total spin-lattice relaxation times T. assuming disordered configuration of H0-atoms at all temperatures.

Since the first report (5) of a pronounced configurational dependence for the anomeric protons of monosaccharides in aqueous solutions, studies in this laboratory have shown (6), (7 ), (8), (9) that proton Ri-values have a variety of stereospecific dependencies and reflect the overall steric environment of a proton. In the following discussion, a brief introduction to the phenomenon of spin-lattice relaxation and to the measurement of proton Ri-values by pulse Fourier transform methods, will be... 

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