Nuclear Overhauser enhancement pulse sequence

Fig. 2.51.. /-Modulated spin-echo sequence with gated proton decoupling for acquisition of -/-resolved two-dimensional 13C NMR spectra, and the CH magnetization vectors in the x y plane controlled by pulses and. /-modulation. During the preparation period between successive experiments, nuclear Overhauser enhancement of 13C magnetization is retained by minimum proton decoupling.

NMR observations basically contain spin relaxation processes which are associated with molecular motions with different specific frequencies in a given system. For quantitative measurements to determine the compositions of the system or selective measurements of particular components with different relaxation parameters, it is essential, therefore, to understand the principle of the relaxation mechanism. When our interest is focused on molecular motions, spin relaxation parameters such as spin-lattice relaxation time T, spin-spin relaxation time T2, and the nuclear Overhauser enhancement (NOE), are directly measured as a function of temperature or field frequency by using appropriate pulse sequences. Such temperature or frequency dependencies of the spin relaxation parameters are analyzed in terms of appropriate models to obtain detailed information of molecular motions with frequencies of Hz in the system. In this chapter, the basic theories and analyses... 

Signal enhancement from INEPT and DEPT pulse sequences differs substantially from nuclear Overhauser enhancement. While polarization transfer enhancements are dependent on the number of protons coupled to... 

Figure 1.7 The pulse sequence used to observe a decoupled spectrum without the nuclear Overhauser enhancement (inverse gated decoupling).

CP measurements on static samples use of spectral simulation based on line shapes obtained from model compounds Optimization of sensitivity for analysis of coals, with judicious choice of pulse flip angles and use of a special pulse sequence to correct baseline problems Use of advanced NMR methods (nuclear Overhauser enhancement, DNP, 2D correlation and other 2D methods, spectral editing, etc.)... 

Figure 1.8 C NMR spectrum of an anionically prepared polybutadieDe at 25 °C in CDClj at 60 MHz. The labels correspond to the p numbers and triad sequences of Table 1.1. In this study [25] extrone care was taken in obtaining quantitative information avoidance of the nuclear overhauser enhancement was achieved by decoupling only during the signal acquisition pulse angle 90°, 40000 scans, 33 s pulse delay. Reproduced from [25] with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK...

Figure 5.43. Gated decoupler method for recording 2D J-resolved C-NMR spectra. The upper portion shows the pulse sequence while the lower portion represents the effect of the pulse sequence on magnetization of a CH group decoupling during the preparation period provides nuclear Overhauser enhancement. The position of the two magnetization vectors in (d) will be dependent on 11 the sum of the two vectors which is detected will depend both on and the magnitude of the coupling constant, J. The signals are then said to be J-modulated. ...

Judeinstein et al have conducted direct measurement of through-space NMR interactions that provide definitive evidence for spatial proximity of different species. Dipole-dipole interactions can be measured in principle between any NMR active nuclei with heteronuclear correlation experiments in the liquid or solid state." The dipole-dipole interactions decay quickly with the internuclear distances (r ), and are difficult to evaluate for long-range distances and even more difficult when exchange, conformation, or motion phenomena are present. However, the measurement of the nuclear Overhauser method" based on the dipole-dipole-induced crossrelaxation, was proposed to successfully measure intermolecular interactions" and the formation of ion pairs." " In agreement with recent studies, the pulsed field gradient enhanced inverse HOESY (heteronuclear Overhauser enhancement spectroscopy) sequence is usually preferred because it is more sensitive for isotope pairs H- Li and also improves the digital resolution in the H crowded spectrum." ... 

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

Discussion of the these three methods is outside the scope of this book, but in later chapters we consider other methods for producing much less dramatic non-Boltzmann distributions. By using rf irradiation to alter spin populations, the nuclear Overhauser effect results in signal enhancement (Chapters 8 and 10). Several techniques use pulse sequences to transfer polarization from nuclei with large y to nuclei with small y in solids (Chapter 7) and liquids (Chapters 9 and 12), hence to provide significant signal enhancement. 

The NMR signals of insensitive nuclear spins can be enhanced by transferring polarization from a more sensitive species to which they are coupled. The well-known pulse sequences as the polarization transfer techniques are insensitive nuclei enhanced by polarization transfer (INEPT), distortionless enhancement by polarization transfer (DEPT), and reverse insensitive nuclei enhanced by polarization transfer (RINEPT) The INEPT sequence is an alternative to the nuclear Overhauser effect. The INEPT experiment does not require any particular relaxation mechanism and therefore a better enhancement factor can be obtained. Furthermore it is demonstrated that INEPT sequence can be used to determine the multiplicity of each signal in a NMR spectrum. More recently, the INEPT and DEPT experiments were used for the coherence transfer via heteronuclear J-coupling between spin-1/2 and quadrupolar nuclei in the solids. " Fyfe et showed that coherence transfer via the scalar coupling between spin-1/2 and quadrupolar nuclei can be obtained in the solid state by using INEPT experiment. 

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