POLARIZATION TRANSFER IN REVERSE

What are the differences between INEPT and DEPT experiments Why is DEPT considered a distortionless experiment  

Why do quaternary carbons not appear in polarization transfer experiments  

The inverse INEPT (Bodenhausen and Ruben, 1980) and inverse DEPT (Brooks et al., 1984) experiments utilize such an approach. In the inverse INEPT experiment, successive 90° pulses are applied to the C nucleus, followed by a H read pulse. Protons not coupled to the C nucleus are suppressed by presaturation of the entire H-NMR spectrum before the polarization transfer, so only those signals will be detected that are generated by polarization transfer from the C nucleus. 

17 Application of the reverse DEPT pulse sequence to monitor C-labeled glucose by mouse liver-cell extract. (A) Normal FT spectrum. (B) Reverse DEF T spectrum showing the a- and /3-anomeric proton resonances. (C) Two different CH2 proton resonances, a and b, appear after 1.5 h of metabolism. (D) Edited H spectrum confirming that the CH2 resonances arise from metabolic products. (Reprinted from J. Magn. Resonance 56, Brooks et al., 521, copyright 1984, Academic Press.) 

Transfer of polarization from C nuclei to H nuclei and their subsequent detection leads to a 16-fold increase in sensitivity because the C magnetization is being measured indirectly through detecting it via the nucleus with the higher magnetogyric ratio (i.e., H). Irradiation of the protons between the scans causes a further threefold increase in the population of the C nuclei due to nOe, so an overall 50-fold increase in sensitivity is achievable in contrast to direct C measurements. However, because of 

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Figure 13.7 Selectivity effected by employing different step gradients in the coupled-column RPLC analysis of a surface water containing 0.40 p-g 1 bentazone, by using direct sample injection (2.00 ml). Clean-up volumes, (a), (c) and (d) 4.65 ml of M-1, and (b) 3.75 ml of M-1 transfer volumes, (a), (c) and (d), 0.50 ml of M-1, and (b), 0.40 ml of M-1. The displayed cliromatograms start after clean-up on the first column. Reprinted from Journal of Chromatography, A 644, E. A. Hogendoom et al, Coupled-column reversed-phase liquid chromatography-UV analyser for the determination of polar pesticides in water , pp. 307-314, copyright 1993, with permission from Elsevier Science.

Solvatochromic pareuaeters, so called because they were Initially derived from solvent effects on UV/visible spectra, have been applied subsequently with success to a wide variety of solvent-dependent phenomena and have demonstrated good predictive ability. The B jo) scale of solvent polarity is based on the position of the intermolecular charge transfer absorption band of Reichardt s betaine dye [506]. Et(io> values are available for over 200 common solvents and have been used by Dorsey and co-%rarkers to study solvent interactions in reversed-phase liquid chromatography (section 4.5.4) [305,306]. For hydrogen-bonding solvents the... 

By reversing the order of polarization transfer steps, NOE contacts of signals hidden under the bulk of other resonances can be established in a ID TOCSY-NOESY [39] experiment (fig. 1(d)). The necessary requirement is that an efficient TOCSY transfer can be made to a particular proton from... 

Alternating-current and frequency effects. With an AC rather than a DC voltage applied to the electrodes, the processes above reverse themselves with the period of the alternating voltage. But each process proceeds at a different rate (with a characteristic relaxation time) so that their relative contributions to energy dissipation vary with frequency. As the frequency is increased concentration-polarization can be reduced or eliminated, particularly if the electrode reaction is reversible (fast electron transfer in both directions). 

Most NMR spectroscopists have on some occasion examined the Periodic Table of NMR-active nuclei with 13C, I70, etc., and contemplated that at first glance essentially the whole Periodic Table is potentially open to study by NMR. Solution-state NMR studies of nuclei with small magnetic moments exist, but often when papers report data concerning such nuclei the information is obtained not by simple direct excitation but indirectly using polarization transfer techniques such as INEPT and DEPT, and/or by using reverse detection methods.1 As many of these indirect methods are not available in the... 

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