Carbon-13, Overhauser effect

Key Words Carbon-13 spin relaxation, T, Measurements, Nuclear Overhauser effect, Rotation-diffusion tensor, HOESY experiments... 

It is not usually possible to integrate routine C spectra directly unless specific precautions have been taken. However with proper controls, NMR spectroscopy can be used quantitatively and it is a valuable technique for the analysis of mixtures. To record C NMR spectra where the relative signal intensity can be reliably determined, the spectra must be recorded with techniques to suppress the Nuclear Overhauser Effect and with a long delay between the acquisition of successive spectra to ensure that all of the carbons in the molecule are completely relaxed between spectral acquisitions. 

An important first step in interpreting the C-13 spectra is to distinguish a-carbons from 3-carbons, i.e. methine from methylene. Observation of multiplicity when the proton decoupler is off is one way, but this is not always easy if the lines are broadened by chemical shift multiplicity. Measurement of has been used for this purpose since the 3-carbon with two bonded protons relaxes about twice as fast as the a-carbon with only one. A very positive way is by deuterium labelling. In Fig. 3 is shown the main-chain 25 MHz carbon spectrum of two styrene-S02 copolymers containing 58 mol% styrene, or a ratio of styrene to SO2 of 1.38 (7 ). In the bottom one, 3,3-d2-styrene has been used, cind all the 3-carbon resonances are distinguishable from the a-carbon resonances since the presence of deuterium has eliminated their nuclear Overhauser effect because of this eind the deuterium J coupling ( 20 Hz), they are markedly smaller eind broader than the a-carbon resonances. 

The line widths of C-l—C-S 13C resonance peaks in the gel are about 150 Hz, which are rather broad in comparison with those of alkaline solution (0.22M NaOH)(Fig. 2D) and also with those of the lower molecular weight fraction (Fig. 3A). By comparing the integrated peak intensities (with nuclear Overhauser effect sur-pressed) with those of the corresponding peaks in the 0.22M NaOH solution, in which the curdlan molecules are considered to have random-coil conformation, it was estimated that these observed 13C peaks account for only 20-30 and 60% for the C-1--C-5 and C-6 carbons of the total gel, respectively. Hence, the remaining invisible portion of the 13C should be ascribed to the junction zones of the gel net work and to residues located closely to these junctions. 

Nuclear Overhauser effect (NOE) experiments have been used to determine the stereochemical relationship between the groups on adjacent carbon atoms of the ring <1997JOC5953, 20020L1299, 2005SL1559>. For example, m-azetidinc 16 and /ra/M-azetidine 17 have been characterized using NOE experiments (Scheme 3) <2005SL1559>. 

We described the nuclear Overhauser effect (NOE) among protons in Section 3.16 we now discuss the het-eronuclear NOE, which results from broadband proton decoupling in 13C NMR spectra (see Figure 4.1b). The net effect of NOE on 13C spectra is the enhancement of peaks whose carbon atoms have attached protons. This enhancement is due to the reversal of spin populations from the predicted Boltzmann distribution. The total amount of enhancement depends on the theoretical maximum and the mode of relaxation. The maximum possible enhancement is equal to one-half the ratio of the nuclei s magnetogyric ratios (y s) while the... 

One of the main advantages of 13C-detected INADEQUATE is that, unlike 1H-detected INADEQUATE, it provides information about connectivity of quaternary carbons. However, quaternary carbons are handicapped by slow spin-lattice relaxation. In order to improve their sensitivity, the addition of relaxation agents has been proposed.9 It should be noted, however, that this procedure should be carried out with caution, as it may degrade signal strength by quenching the nuclear Overhauser effect.10 One study showed that a moderate... 

The serial publications listed in Section 1.3 contain a number of articles on various aspects of relaxation. The Encyclopedia of NMR includes 13 articles under titles beginning with the word Relaxation and several other related articles, including Nuclear Overhauser Effect," Carbon-13 Relaxation Measurements,100 Paramagnetic Relaxation in Solution,101 Brownian Motion,102 and Spin—Rotation Relaxation Theory.103 Many of these articles are based on density matrix formulations, which can readily be understood with the background provided in Chapter 11. 

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